First of all, let me start by saying I was made aware of some of the following concepts in an interview with guitarist & producer DW Norton, of Australian band Superheist. DW Norton specialises in recording heavy bands, and it was one line in the interview that caught my attention. To paraphrase (since I can’t find the article anywhere): the typical recording process goes drums / bass / guitars / vocals, in that order. Since a lot of metal bands use detuning, what happens is that the bass ends up being over-written by a lot of the guitar tone. If you want to actually hear the bass, it should be recorded after the guitars, so you can find gaps in the audio spectrum to fill with bass guitar.

Hang on a minute, says I – there are gaps in the audio spectrum? Where are they, and why are they there?

The answer to this goes all the way back to the early days of electric guitar and a certain Mr Leo Fender. Without getting into the engineering details, the short version is this – the early open-backed guitar cabinets suffered from weak bass and treble response, and the cheapest fix was to compensate with a few capacitors and resistors in the amplifier to change its frequency response. The result was Fender’s passive tone stack, with cut-only mids and a boost to the bass and treble. The Fender stack was adapted by Marshall, Vox, Alembic amongst others, and is to many people’s ears the sound of the electric guitar revolution.

Speaking of ears, they don’t work the same at all frequencies. If you have a look at the equal-loudness curve graph to the left, it turns out we are most sensitive to frequencies in the high-mids, around 1k – 5k, and are decreasingly sensitive to both bass and treble. As an example, look at the 20 phon line – to our ears, 20 dB at  1 kHz sounds as loud as 50 dB at 80 Hz,and 35 dB at 10 kHz.

In order to hear a smooth response, we naturally turn up the treble and bass on our stereos and amps. Notice that the difference tends to flatten out a bit at higher volumes, ie: we are relatively less sensitive to mids at a typical live band level of 100 – 110 dB. This is why a heavily mid-scooped bass or guitar tone that sounds great for solo practice at typical television viewing volume (60dB)  can become both muddy and shrill at gig volumes (the dreaded ‘bedroom tone’ I opened with). Inversely, if you have a mid-heavy tone, it can sound dull or nasal when played in isolation.

Do yourself a big favour and download Duncan’s Amp Tools TSC 1.3, a brilliant little calculator that shows you the frequency response curves based on several tone stacks. You can even edit component values to see what happens when you modify the tone stack, handy for amp modders.

The first graph shows the frequency response of Fender, Vox and Marshall tone stacks with the knobs set to halfway. Note that the Fender & Vox tone stacks have a 12 dB notch centred around 500 and 800 Hz respectively, while the Marshall stack has both a higher gain and a milder 5 dB notch at 700 Hz (as well as rolling off the bass at a higher frequency).

If you go all early Metallica on your amp, you get something a bit like the curves to the left – this is the classic ‘mid-scoop’ beloved by the palm-muting set. By some accounts, they actually used to cut the mids all the way to zero until they started working with Bob Rock (who also fixed their bass tone issues!). Notice how that not only does the mid-scoop become more pronounced, but the centre frequency shifts to a lower position due to the increased treble.

A quick note on wah pedals couldn’t hurt at this point. As almost everybody knows, wah pedals provide a strong boost at a narrow band of frequencies. The reason they can be so effective a tool in making solos come alive is that those frequencies typically range from 400 Hz up to 2 kHz. The lower half of the wah travel is right in the typical mid-cut zone of most amps, so the wah really leaps out in comparison to the rhythm guitarist. Fixed-wah soloing a la Michael Schenker takes advantage of this effect, by finding the most noticeable centre frequency for the wah and leaving it there.

Finally, what if you want to flatten your amp eq? Well, it requires a pretty drastic positioning of the eq knobs that few would come up with by themselves. With the three-knob eqs, you get a reasonably flat response with the bass on 1, mids anywhere from 5 to 10 and treble at 0. As to why you’d buck the mid-scoop trend: you may have access to a good rack eq or floor pedal that you prefer, or maybe you’re the second or third guitarist in the band and need to differentiate your tone from the other guitars. Try it out next time you plug in and you’ll be hearing more of the natural guitar tone and speaker colouration.

Now, on to a subject close to home for me: bass. Not every amplifier has a variation of the passive tone stack, but the trick is that most guitarists have been trained to hear that scoop as ‘good tone’, and will replicate it anyway! It’s a safe assumption that almost every electric guitar you play alongside will have a fair amount of mid frequency content removed somewhere between 300 Hz to 1 kHz. This is further enhanced in the various metal genres, where mid-scooping helps accentuate palm muted riffing.

Some bass amplifiers have a Low Mid control; for example, GK amps centre their low-mids on 250Hz, MarkBass around 350ish. Some have a graphic EQ, such as my old BBE 383 Pre-amp with a 6-band graphic (mids at 250 Hz and 600 Hz),  and the Ibanez Promethean head (mids at 400 and 800 Hz); these are all good frequencies to boost to help add definition and punch to a bass sound. Even better is a semi-parametric Mid with a variable centre frequency – these can be used to dial in the most appropriate centre frequency for the band you’re with and the room you’re in.

How? Finding the right spot is a little like hunting feedback in a PA system – you apply a large boost to the EQ level, and sweep the dial until the feedback leaps out at you. That tells you what frequency you need to cut to avoid feedback. In this case, it’s a bit tougher, as you need to make adjustments while playing, but at least you know that you need to be somewhere between 200 Hz and 1 kHz at the extremes. I suggest starting low, apply a large boost to the parametric mids and gradually shifting the frequency up during a couple of songs at sound check. At some point, you may find that your bass tone is actually beginning to drown out the rest of the band – this is your centre. Dial the boost back to a more subtle setting, and you’ll be able to hear yourself and compliment the mix.

So, in summary:

• most guitar amp tone controls are NOT flat when everything is set to the midpoint,
• excessive mid scooping can rob your tone of definition and sound bad at gig levels,
• both bass and lead guitar can benefit from filling the mid-cut of the other guitar amps.

Needless to say, this article only covers the basics. I recommend searching for articles on ‘recommended equalization frequencies’ and having a good read through some of the results – you’ll get lots of ideas for both recording and live sound. Combine that with doing some research on the various amps you and your band mates use (download every manual you can get your hands on!) and you’ll be able to create a cohesive blend of tones that will impress your audience, and perhaps more importantly, keep your soundguy happy!

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This was the stock wiring; random pots with 6mm shafts, and a .1 uF capacitor (versus the more typical .047 uF used in Precision basses). The pots were okay, but the .1 uF capacitor is just a bit too woolly for my tastes – you don’t get a lot of signal left over 400 Hz when you roll the tone back all the way.

First of all, thoroughly clean all the dust and buffing compound out of the cavities – this helps the tape stick. Before cutting out random shapes, rub the foil over a control or pickup cavity to get the perfect fit. Cut outside the line, so there’s a bit of overlap.

I don’t try to get the sides done in one piece as you can never keep the alignment through the corners. Cut the foil slightly deeper than you need, and cut some slits in the bottom edge so it doesn’t bunch up. Watch out that you don’t go over the top to the extent that the foil will peek out from under the pickguard/bridge plate/control plate etc. Finally, run a strip up the side to one of the control plate screw holes to ensure a good tight connection.

A digression: on the left we have an Electrosocket jack system of random Asian manufacture. On the right is the standard cup & jack arrangement used by Fender. One of these will slip at a random moment on a gig, and will be a huge pain to fix. Note that I’ve used a stereo jack (it’s a fairly accurate clone of the Switchcraft ones, but costs about 1/4 the price) – this prevents the plug becoming loose once the jack wears out over time. Can’t remember where I picked up this little tip, but it’s a keeper.

Sandwiching the bridge ground lead between a strip of foil & the bridge itself ensures a good connection – if you’re paranoid, you could tape over the wire too.

It’s important to electrically connect all the cavities together – normally I use the shielding braid from some good quality coaxial antenna cable; it’s great as you can run at least two wires through it to shield them, so long as the hole is straight. I got a little creative this time and went under the pickguard. Needless to say, a multimeter (even a cheap one)  is essential to check continuity – try and get one with an audio tone so you’re not always watching a display.

Note that I shielded the entire pickup cavity – the copper foil reflects the black of the pickup cover, so it looks stock. I then ran a foil tab up and over to where the pickup cover was going to be installed – I decided I might as well make it functional & help to shield the pickup from interference.

Here we can see the finished wiring – I’m not sure if the twisted wire actually ‘rejects’ EMF noise, but it’s worth a shot, especially as I didn’t shield the jack cavity or the connection between pickup and control cavities.

I like to be as economical as I can with my wiring, while leaving just enough wire to re-do the job easily later. Of note is the .047 uF paper-in-oil capacitor – it doesn’t do much to the tone, but it looks cool. You’ll see I used one leg to ground it to the volume pot, via the volume ground lug. The bridge, jack and pickup grounds are all hooked up to the wire between the volume ground lug and the pot casing – just loop them over and crimp them tight with a set of needle-nose pliers, and you can solder the whole lot in rapid succession. Always make a good mechanical connection before you pick up the soldering iron if you can!

As an example of a bass where the pickup is surrounded by the pickguard, here’s my Aria Pro II. It is shielded with my older batch of foil without the conductive adhesive. Note the tack soldering; it’s a major pain in the ass. I went a little over-the-top with this one, you need just enough solder to make an electrical connection.

Don’t forget to shield the underside of the pickguard too – I’ve used conductive foil again here. You’ll only get the full benefit of the shielding once every screw is in place to hold down the pickguard & controls plates. I was chasing a bad connection for half an hour when I shielded my Jazz bass, before I thought to install every screw. Doing so immediately put the clamp-down on the buzz as well!

Points to note:
- hookup wire &  shileding foil in small amounts is usually cheapest on eBay.
- I love vintage style cloth covered wire – it’s a piece of cake to work with, no wire stripping required. Rub it in some dirt or ash if you want it reliced.
- if you shield the jack cavity, make sure the jack doesn’t touch the sides when you plug in. It’s far better to have some EMF noise than to short-circuit the bass entirely – if in doubt, give it a miss.
- CTS pots are overrated in this case, as they are a 1/4″ shaft, slightly over 6mm. The original MIJ ‘chrome dome’ knobs won’t fit, nor will any other metric (6.0mm) knob. Although purists may sneer at using anything other than CTS pots, Alpha pots are metric (and smaller) and practically as good in terms of quality. I’ll be using them from now on – since most of the time I’m replacing 16mm pots with metric shafts, I may as well re-use the knobs.

As I mentioned in my long-winded post about ‘Miss Blue’, my old Jazz bass, shielding is one modification with no discernible downside. It can make a P.O.S. instrument gig-worthy, and a good player just that little bit more special!

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This time around, I skipped the BB-1 and true bypass units, concentrating instead on testing every single brand  of pedal I had access to at the time. The graph turned out quite differently, with a couple of surprises (*cough* Ibanez and Dunlop *cough*). I owe Boss an apology (even if their pedals do start rolling off the highs around 3.5 kHz) – sorry guys!

Final update: A poster on TalkBass theorized about the effect of hooking up 10 Boss pedals (and their associated patch leads), so I HAD to step up to the challenge! The overall attenuation is huge, bass is rolled off below 50 Hz (bassists beware) and treble starts rolling off gently around 3 kHz (of more interest to guitarists, of course).

For the record, the pedals used were:

• BF-2 Flanger
• CE-2b Bass Chorus
• CEB-3 Bass Chorus
• DD-20 Giga Delay
• LS-2 Line Selector
• MT-2 Metal Zone
• NS-2 Noise Suppressor
• OC-2 Octave
• PS-2 Pitch Shifter/Delay
• RV-3 Digital Reverb/Delay
• TR-2 Tremolo

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Currently residing on my workbench is an old Guyatone BB-1 Flip Valve “Bass Driver” pedal, on loan from OzBass forum user Crayzeebass. It’s fairly heavily influenced by the BK Butler / Chandler Tube Driver, (made famous by a Mr David Gilmour of a little band called Pink Floyd you may have heard of) and runs on 9 volts. He’d purchased it from another forumite, but commented to me that it lost a bit of his tone. I’d been curious to try it out (it has a neat semi-parametric mid control), and he’s local, so he dropped it off to me for to have a look-see.

First up, let me say this: it’s an awesome little tube distortion pedal for guitar – if you see one cheap, GET IT. Upon plugging one of my Precision basses into it and running it through my Fender Pro rig (TB600 head and 215 Pro cab), it does seem to lose some bass frequencies. I’d always wanted to do a proper frequency analysis of some of my preamps & pedals, so this seemed the perfect opportunity.

First of all, you’ll need the following:

• PC with USB recording interface (I use a Line 6 Toneport)
• Headphones (plug into the monitor output of recording interface)
• Multi-track DAW software (I use Audacity, free download)
• Spreadsheet software (Excel, Open Office Calc)
• Pink noise soundfile (just Google for one)
• 1/8″ stereo to 1/4″ mono adaptor (from PC line out to guitar cable)
• 3′ guitar lead (from PC to unit being tested)
• 6″ patch lead (from test unit to USB interface)
• Something to test – FX pedal or preamp, mic’d guitar cabinet

A couple of quick notes: it’s possible to test passive circuits (and pickups!), but not with this method exactly. I’ll do a followup post later, as I’ve been meaning to do some further investigations regarding passive tone controls, cable length and suchlike. Also, through repeated tests I discovered that my resolution is limited to +/- 0.2 dB – your hardware may be better or worse, the only way to find out is to perform the following test on the same audio several times and compare your results. I won’t do anything like this again until I’ve updated some of my hardware!

Safety note: Pink noise is usually at or close to the 0 dB mark – ie, LOUD. It’s not essential to listen to the samples being played, so I keep the headphones off (or around my neck) so I can hear the audio is coming through okay.

First of all, open Audacity & import your pink noise file. At this point I save the file with an obvious name, like “BB-1 pedal test”. You then need to run the Line Out from your PC straight into your USB interface. The purpose of this is to re-record the pink noise and establish a base level frequency profile for your particular combination of soundcard, cables and USB interface. Adjust your playback and recording levels until your recording is not clipping, but is as close to the original pink noise file as possible. Once you’re satisfied, make a recording of this new (pink) noise, and label the track ‘Soundcard’ or something similiar.

Step two is to hook up the pedal or preamp in question, then re-record your original pink noise file with the pedal bypassed.  Another obvious label will come in handy; if you’re testing one pedal, label it ‘Bypass’. You already have enough data to make a meaningful statement about the effect the pedal is having on your signal chain. It was at this point I decided I was going to test the bypass mechanisms of a bunch of different pedals, so I quickly hooked them up in various combinations and blasted some pink noise through them.

In Audacity, select the ‘Soundcard’ clip, then open Analyze > Plot Spectrum. This will generate a funky purple graph – leave the Algorithm as ‘Spectrum’, and Function as ‘Bartlett Window’. Adjust the Size to 2048 samples or higher, as this gives us enough resolution to cover the low end, and make sure the Axis is ‘Log frequency’. You can then Export this data, which will give you a text file of Frequency (Hz) vs Level (dB) measurements. Open this in your spreadsheet as a tab or space delineated file, and you should have two seperate columns.

Having fun yet? Then let’s continue! If you haven’t already guessed, you then repeat this Export process for your ‘Bypass’ recording (and any others), and bring the new data into your spreadsheet. Remove subsequent Frequency (Hz) columns, as you only need it once for your  X axis. Column A contains the Frequency, Column B contains the Level data for your Soundcard measurements, and Column C onwards contain the Level data for your bypassed pedal(s). Most of your data will be in the negative scale, so you’ll need to use a simple equation to generate some nice lines:

Relative level = (-1 * B) – (-1 * C)

Relative levels

So, for the first row of data at A3, Level = (-1 * B3) – (-1 * C3). Simply enter the equation into a free cell at say, G3, then copy and paste that equation down the column. It is then a simple matter* of graphing Column A (Frequency, Hz) as the X axis (logarithmic) versus Column G (Relative level, dB), as the Y axis (linear). If your graph looks a little squashed, you can adjust the X axis to only show data from 20 Hz to 20kHz, and the Y axis probably only needs to go from -9 dB up to 9 dB unless you have a serious boost pedal to test! Consider also focusing on a smaller picture – if you’re worried about lost low end, then restrict your data to 400 Hz or lower, for example.

* Unless you’re having the same issue with labelling the X axis in Open Office Calc that I am! In that case I highly recommend downloading Datascape from Tucows.com. Select the columns you want to graph, hit Copy, then Paste into a blank txt file in Notepad. Save as, then open the file with Datascape.

I ended up testing the following:

• Boss buffered bypass
• SansAmp buffered bypass
• True bypass
• Three linked true bypass pedals
• Three linked Boss pedals

The graph shows the results, which I find quite interesting. That’s a Log scale on the X axis from 10 Hz to 20 kHz, apologies for the bad labelling – I had issues with Open Office Calc, but Datascape is not as good at showing this much information. First of all, please note that I appear to have hit that snag with resolution I mentioned earlier, so any differences less than 0.2 dB should be taken with a grain of salt. You’ll notice straight up that the single True Bypass signal is actually very slightly above unity gain – this is obviously a spurious result (probably caused by the cheap gear I’m using!), but the other characteristics of the line are still useful – note that it’s pretty flat in the lower end.

There’s a big difference between the SansAmp buffer, the Boss buffer and the mechanical but non-true bypass BB-1 switching. The SansAmp buffer has none of the crazy harmonics of the Boss and hardly any signal loss – it basically looks like a true bypassed signal. The Boss buffer is not at unity gain, and stacking three of them accentuates both the low- and high-end rolloff. The BB-1′s vintage bypass clearly has to go – it’s nothing but insertion loss & tone suckage! Next step is to get my hands on a decent Tube Screamer, (it’s supposed to have a good buffer as well), and a selection of other common pedals to test.

Once you’ve got the process down pat, you can repeat it with the subject pedal(s) on different settings. This way you can compare what’s going on as you change the amount of distortion, or adjust the EQ. This is particularly handy for pedals or pre-amps with poorly marked controls. For example, if you know a parametric Eq has a mid range from 100 Hz to 2.5 kHz, you can find out what frequency is boosted at the halfway point and determine if the control is linear or logarithmic.

For those with preamps that use the passive Fender tone-stack (also Alembic, Marshall, Vox etc), you can get a better handle on what the controls are actually doing to your sound – you might be surprised. In this case, you can also measure amplifiers with a pre-amp out or FX-send section – just make sure you’re not running a tube amp with no load, okay?!

Oh, and for those of you wondering about the Guyatone BB-1, it’s a counter-intuitive pedal: the clean signal that can be blended back in has LESS bass than the distorted signal with the bass boost engaged. Go figure I’ve been commissioned by CrayzeeBass to mod the pedal, so it will be the subject of a follow-up post. This may seem like a lot of effort to go to, but not all of us have the ears of a recording engineer. Knowing what’s happening to your audio at various points along your signal path will help you get the best tone possible, and that’s something we should all strive for.

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]]> http://rudemechanicaloz.wordpress.com/2010/07/15/whats-the-frequency-analysis-kenneth/feed/ 2 rudemechanicaloz guyatone-bb-1-flip-valve Audacity pink noise Spectrum analysis Spreadsheet Relative levels Pedal bypass methods http://rudemechanicaloz.wordpress.com/2010/07/14/upcoming-project-blogs/ http://rudemechanicaloz.wordpress.com/2010/07/14/upcoming-project-blogs/#comments Wed, 14 Jul 2010 10:39:32 +0000 rudemechanicaloz http://rudemechanicaloz.wordpress.com/?p=187 ]]> Coming soon: how not to make a guitar case! Okay, a little pessimistic, but I certainly went the hard way about it. Also, I’ll finally finish my MIDI bass pedals and I have a job to refurbish an old Guyatone BB-1 Flip Valve pedal.

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Anyway, Bill Wyman of the Rolling Stones is generally accredited with being the first guy to defret a fretted electric bass, and his bass playing is pretty much as solid as you could ever want. There’s also some sweet and subtle fretless work in Pink Floyd (“Hey You” “Learning to Fly” “A Pillow of Winds”), but probably THE reason for me getting the fretless bug was Pino Palladino’s utterly killer bass playing in “I’m Gonna Tear Your Playhouse Down” by Paul Young. Bakithi Khumalo’s bass lines on Paul Simon’s “Graceland” certainly helped fuel the fire.

I’d been taking an SX Precision copy as a backup bass to gigs, but it got bumped out of rotation when I scored a 1976 Aria Pro II ‘Professional Bass’. I’d wanted to defret a bass for a while, so the one I paid a whole $103 for seemed to be the perfect candidate!

One of my secret shames is having brutally defretted an old Gibson EB-0 copy, back in my uni days, using a pair of nail-clippers. Never finished that job, so I vowed I’d get over the line with this one. OzBass forums user PilbaraBass mentioned using timber veneer from Bunnings as fret markers; sure enough, I was able to pick up 5 meters of iron-on Maple veneer for under $10. So, time to begin the defretting process again, with a little more care this time around! I removed the neck prior to starting, and I’d oiled the fretboard earlier, just in case it was a little dry. Supposedly it may help the frets ease out if you really saturate it before the job.

It’s worth mentioning the tools used – this is a cheap project:

• cheap pair of end-nippers, tips filed down
• soldering iron
• old coping saw blade (you could also use a hacksaw or craft knife)
• scraper
• scissors
• F-clamp
• tack hammer
• mineral turpentine
• 3/4″ wood chisel
• 120, 180, 240, 320, 400 & 600 grit wet & dry sandpaper
• 0000 grade steel wool
• lemon oil based ‘fretboard conditioner’

For each fret, I’d hold the soldering iron on the end I was going to pull first for long enough to transfer some heat (this helps the wood release the fret more easily), then move the iron along a bit while I levered the fret up. I basically walked the nippers along behind the iron until the fret popped free, then repeated the process another 20 times. Took maybe half an hour. For this fretboard (rosewood), it worked a treat.

There was the usual chatter from the fret tangs and maybe half a dozen little (<2mm) chips that started pulling out, but no clean breaks.  I wicked a bit of superglue into the chips to hold them down during sanding, then knocked it back with some 180 grit. By the way, for those playing at home, it’s the little rips from the fret tangs that make the wood filler method look so nasty. I cleaned out the slots with a coping saw blade that just happened to be the perfect width.

In order to use the timber veneer for position markers, you first need to remove the adhesive from the back. I cut off a length that would be about enough to do 21 frets (and some spares). Clamped one end down to the ‘workbench’ (my old outdoor table, seen better days!), got a green scourer and some turps and loosened up the adhesive. Scraped off the gunk with a blade, wiped down with some paper towel & repeated the process a second time to my satisfaction.

It was then just a matter of cutting the strip to the widths needed (with enough to hang over the sides), and checking the fit. It turns out that these fit the slots perfectly – nice and snug, but not much force to get them in. The thing to check is whether they’re sitting flush or not – most manufacturers radius the fret slots so there’s less void space in the fretboard. I’d figured that this was  such a cheap instrument that I could get away with it, but that turned out not to be the case. I ended up giving the underside of the markers a quick and nasty radius, just enough to stop them rocking in the slot.

A brief diversion: this is Martin Koch’s recipe to build a radius sanding block. If you’ve got enough scrap timber to put one together, you can save yourself a bit of cash. Alternately, make one 3 feet long and you can do the whole neck in one hit!

To install the fret markers, I ran a line of glue over the slot, quickly seated the veneer, tapped it in with a  hammer and then wicked more glue along the slot. I didn’t get perfect placement of all the markers, but it looks good enough for an SX!

Once the glue had dried overnight, I took to the veneer with a wood chisel (very carefully – haven’t used one since Shop A in junior high) to knock the worst of it down. Just take your time, keep the chisel at a flat angle relative to the fingerboard and NEVER position your hand in front of the chisel!

I then used 120 grit on a plain sanding block to lightly work the bumps out, making some effort to follow the existing radius.All you’re doing here is getting the worst of the veneer ridges and superglue overflow off; don’t go crazy with the paper at this stage.

As per Martin Koch’s plans, I made a mongrelised radius block from scrap wood. It didn’t work out too well for a 12″ radius as the wood pulls into a v shape when screwed all the way down. As is, it’s about 18″ radius. Ideally, you’d make one about 3′ long and clamp it to your bench, as I mentioned earlier,  but there’s just no way I could get it plumb with my limited tools.

I had a few issues with sanding, but there’s plenty of meat left on the fingerboard so I’ll leave it alone for the time being and see how I get on. It’s a tad lopsided down at the bridge end, maybe elsewhere too. I worked up from 180 grit to 600 grit with my radius block, then rubbed in lemon oil with some 0000 grade steel wool. Incidentally, I rolled the edges with a screwdriver shaft, then gave it a very light sand before the oiling, just to take the harshness out of the fingerboard edge and make it feel more ‘played in’.

After the oil had dried, I gave the fretboard a quick buff with a clean rag & reinstalled the neck. Lowered the nut a fraction to make the action feel the same as when it was fretted, then popped a well-worn set of Fender SS Flats (40 – 95) on. Final step was to adjust the truss rod to lower the relief, then plug into a Boss OC-2 and chorus, and crank out the Paul Young! I did get a little more contemporary with some Roni Size “Brown Paper Bag”, I must confess.

I may not be Jaco’s biggest fan, but I _do_ have the urge to repeat the process with a cheap Jazz bass, and epoxy (or superglue) the fretboard…

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VGA vs Parallel cables

I mentioned in Part I that my bright idea of using a VGA cable fell over due to the cable only having 8 conductors plus shielding ground. The solution was to go back to a world of pre-USB comms cables; the humble parallel cable has plenty of conductors for the job at hand.

I scored a couple for free at a local computer store, and it was a simple matter of cutting the ends off & connecting one end up to my VGA plug, and the other to the foot pedal assemblage. Easy peasy. For the record, parallel cable has 19 conductors – this means you could use an octave-and-a-half bass pedal unit to extend your range.

However, I must fess up to having done something pretty effing stupid: during the final construction of the bass pedal assembly, I put a nail through the comms cable. That severed 4 of the conductors, requiring its removal & re-wiring. Notice I said nail, not screw – this is the other part of my stuff-up: it’s impossible to easily unassemble the case I made for the bass pedals.

Given the amount of work required to salvage this project, I’m thinking of going back to my original plan… that’s right: the DX21 is going under the knife. I already have all my jumper leads wired up ready to connect to the bass pedal contacts – the question is whether or not the DX21 parts will fit in the case as built. I don’t think they will, so a rebuild is probably in order.

Nearly done

So, to re-iterate: the method I used in Part I is absolutely sound. During the pre-assembly testing, my re-wired parallel cable + VGA plug work exactly as planned, enabling the playing of the bottom octave on both the keyboard & the bass pedals. The picture here shows the almost completed bass pedals, just before I attached the lid & decided to nail in a piece of wood to protect the contacts from the cable…

I have a bit of work on the go (my latest endeavour involves making true bypass loop pedals), but I’m determined to knock this on the head ASAP!

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]]> http://rudemechanicaloz.wordpress.com/2009/05/18/midi-bass-pedals-addendum-%e2%80%93-on-the-horns-of-a-dilemma/feed/ 0 rudemechanicaloz para-vs-vga nearly-there http://rudemechanicaloz.wordpress.com/2009/03/13/midi-bass-pedals-part-i-a-crash-course-in-dx21-surgery/ http://rudemechanicaloz.wordpress.com/2009/03/13/midi-bass-pedals-part-i-a-crash-course-in-dx21-surgery/#comments Fri, 13 Mar 2009 01:44:05 +0000 rudemechanicaloz http://rudemechanicaloz.wordpress.com/?p=94 ]]> This is inspired by the 1976 Moog Taurus I bass synthesizer – a 13 note synth that you play with your feet, used notably by the Police & Rush amongst others. I have a tendancy towards progressive music, so these appeal to me. I’d found a few alternatives like the Roland PK5 MIDI pedals, which can be hooked up to any MIDI synth, but those were all just about as expensive as the Moog in the first place. Then I found that a few people had made their own, using old organ bass pedals and cheap keyboards, and the hook was well and truly set.

I knew that in order to make a set of MIDI bass pedals I would need a MIDI capable synth to hack up, but one without velocity sensitive keys. That is, there is only note on/note off (anybody else get Mr Miyagi there, or Yoda?) signals. I asked around, and posted a request on the Ozprog.com forums. I eventually got a reply, and for AUS $50 I picked up a Yamaha DX21 with two sticky keys but in otherwise good condition.

The DX21 is a poor man’s DX7 – it’s a 4 operator FM synth, with 8 note polyphony and enough presets & user savable patches to be interesting. Like all FM synths, they seem to inspire a love/hate relationship here on the internet. It comes in a nice steel case, except for the plastic end caps.

One neat feature is that you can split the keyboard and use two separate patches, or stack (and detune!) two patches – this works a treat with a good bass or lead sound! Needless to say, given my fondness for strays, I didn’t want to sacrifice the whole keyboard just to make my bass pedals. But I still didn’t want to shell out US $100 for the BASYN MIDI kit… what to do?

I opened up the DX21 to have a look inside, and fix the sticky keys. There’s a lot of screws to remove, as the circuit boards, keyboard contact boards & keys are all attached to the frame separately. Keep a container handy – I use inverted CD spool cases. There’s a few different sizes, so if you’re in danger of getting confused it wouldn’t hurt to make a few notes, or even better, take some pictures.

In the picture titled “Innards”, you can see the main circuit board which will need to be removed. This exposes the contact board (brown, with the diodes visible on the underside) which is the main point of interest. You’ll want to unplug the jumper leads to make it easier to work with the board, they’re easy enough to re-attach later.

Note that the keyboard contacts are on two separate boards, which are attached to the main circuit board by jumper leads. It would be very easy to disconnect the larger (treble end) board if you wanted to cut this synth down to fit it into a smaller casing. Simply cut through the plastic bracket where indicated, and unplug the jumper leads from the larger board.

Mounting everything else into the new case is going to be trickier; the easiest thing to to would be to leave the synth on a basic patch & never change it. That way, the only thing you need to surface mount is the MIDI out & power switch. The total size of the box is then constrained only by the largest circuit board.

Regarding the sticky keys, all I had to do was remove the keyboard assemblage from the casing & pop out the offending keys. It turned out that the rubber guides that fit inside the keys and keep them in place had deformed a little over time. I took a stanley knife and shaved away about a millimetre of the side; this freed them up enough that they now spring up as normal when released. This was also the time to clean out all the cobwebs and cockroach shit that was in there…

Hey, while I’m thinking about it, how about a SAFETY WARNING? Avoid, where possible, touching anything relating to the power supply. Any large capacitors (see those honking great black ones?) can hold voltage for quite some time after the power is removed. You wouldn’t want to get a shock just because you were being cheap, after all.

This sort of keyboard works by the keys manually pressing down on a conductive strip, which links a common ground to the positive end of the circuit, letting current pass through the diode you can see on the PCB.

I had a brainwave: it doesn’t matter how that connection is made, so what if I ran some jumper leads from the common grounds, and from the downhill side of the diodes? All I have to do is make those two ends touch, and the circuit is completed. By breaking those leads out to a socket, I can make similiar arrangements on the bass pedals.

Being that the DX21 groups it’s notes in 12s, I needed 2 common ground wires (one for the bottom C, one for the next 12 notes) and 13 hot wires, making 15 conductors in all. But where to find an easily available 15 conductor cable? BINGO: the common VGA monitor lead. Durable, easily found & replaced and best of all, cheap!

Edit: well, the VGA sockets worked great. VGA cable, however,  has 5 common pins… don’t worry though, I found a replacement that was just as cheap. More detail in the next post.

So, after firing up the soldering iron, it was a slow but simple process of wiring different coloured hookup wire jumper leads between the diode & positive contacts of the lowest 13 keys, and black leads from each of the two ground pads. Where I double up on colours, I used a loop of tape to differentiate. If you’re doing something like this, allow plenty of length in case you have issues routing the leads later. In this picture you can clearly see the conductive strips, like interlocking fingers, and the two ground traces in the bottom right (under the bit of foam).

The real pain in the ass came when I had to then wire those 15 jumper leads into the back of the VGA socket. Make sure you don’t over-tin the ends, or they won’t fit in the lugs. Once that was soldered up, I made a note of which lead was soldered to each lug – this is so you can match up the corresponding leads in the bass pedals. I’m a big fan of cable ties; I used three to keep all the jumper leads together (and out of the pitch bend / mod wheel assemblies) and managed to get one under a chassis screw to keep them from flopping around.

Next up came a task I was not looking forward to: drilling a hole in the chassis for the VGA socket. The DX-21 has a lovely steel case, and I suspect I may have dulled my favourite step-bit (normally used for drilling aluminium cases) trying to make a decent sized hole or two. In the end, I could see that it was going to be easier to jigsaw a slot into the edge of the chassis & slide the socket in. Well, the jigsaw blade bounced about so much that it dislocated the mechanism. I opened up the jigsaw & popped everything back into place, and was barely able to finish the job… kind of.

I then spent the best part of an hour filing the slot to shape, removing burrs and blowing/vacuuming all the metallic dust out of the DX21 (shorts are bad, mmmkay?). I used a pair of self-tapping metal screws to hold the socket in place, then realised I need to vacuum the thing out _again_ before I reassembled everything.

Hmm, well – it’s not as tidy as I would have liked, but it’s functional. If I were doing this again, I’d probably just wire a VGA cable straight in rather than use the socket. It’s a lot easier to drill a circular hole than to make a parralellogram! You can always use an extension cable if the one you wire in isn’t long enough… anyway, enough regrets, it’s time for the moment of truth.

A quick cross-your-fingers moment ensued as I fired the DX21 up for the first time. Mostly I was concerned that I’d caused a short somewhere with some steel filings, or fried one of those diodes while soldering. No worries though, the synth plays as normal on the lowest octave despite my butchery modifications.This is great – I’m now using it as an interface for my Novation A-Station rack synth.

Next step: the bass pedals.

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A while back I needed a bass with that “old-school feel” to support a local indie singer-songwriter. After trying every Jazz bass I could get my hands on (around a dozen), I settled for the aforementioned Mexican Fender in a pristine Agave Blue finish.

Like almost all Fender instruments, and MIM versions in particular, she had a few issues. I spent a lot of time with the bridge pickup soloed (or with a hint of neck dialled in), which meant there was always some single coil hum. Not so bad live, but terrible in the studio. Also, there was no shielding of any sort in the bass (something that has been rectified since 2007 I believe). Touching the strings lessened noise somewhat, but not completely.

The stock MIM bridges are a bit rubbish, in that the saddles can easily move laterally. On USA models, the height adjustment screws sit in little grooves in the baseplate, and are usually strung through-body, making for a very firm coupling of strings to bridge to body. I found a strong tendancy for the strings to buzz around the saddle – in a band setting, it was mostly inaudible, but you could feel it – and it felt terrible!

So, in order of what was done, here is a list of common modifications that can be done to cheaper Fender basses, and what I think about them!

1.) Shielding & Star Grounding: I used adhesive-backed copper foil in the cavities, and aluminium foil on the back of the pickguard. All ground leads went to a single lug screwed into the side of the control cavity. If you’re doing this, use a multi-meter to check your connections – make sure the back of the pickguard is connected to your shielding with a tab of foil over one of the screw holes.

Effect: almost zero noise from electrical interference, even when not touching the strings. No negative aspects that I could detect, making this an essential mod.

2.) Changing the Bridge: I installed a Gotoh 201B, which is a direct drop-in for Fender instruments. It’s a higher-mass bridge, with grooves for the saddles – the 2008 USA Standard Fenders come with a bridge that looks a LOT like this one. The Gotoh bridge was cheaper than a Bad-Ass, and not as ugly either.

Effect: no more saddle movement, but tonally I found it bit darker – lost a little high end clang, more of a bronzey bell like tone. If you’re really enamoured with the idea of the brightest bass possible, maybe use a traditional bent-plate bridge with saddle grooves.

3.) Changing Pickups: there is a ridiculous number of after-market pickups available for Fender style instruments – choosing the right one for _you_ can be difficult. Based on reviews online & the tone charts at Seymour Duncan’s website, I chose a set of SD Basslines Stk-J2 “Hot Stacks for Jazz”.

Effect: The stacked coil arrangement makes each pickup hum-free, while the combination of pickup materials makes for higher-output with more mids (and less highs). I dug it at first, but then I found that a lot of what I’d liked about the original tone was now gone. In hindsight, the “Classic Stacks” may have been what I was after.

4.) Changing Volume & Tone pots: Okay, so I’d taken my bass too far towards the dark side – fortunately, some of the easiest (and cheapest) mods involve changing the wiring of your instrument.  The stock pots on a MIM Jazz are 250K – I got my hands on some 500K Volume pots, and a Fender “No-Load” tone pot. 250K pots lose more highs to ground than 500K pots; this is why they tend to be used for single coils.  Since I was using humbuckers, it made sense to me to use 500K pots as that’s what normally gets used in humbucker equipped instruments. The No-Load is neat, in that winding it all the way to 10 takes it out of the circuit completely, something you normally need a switch to do.

Effect: This had a huge effect on the tone – it wasn’t the same as the stock pickups, but there was a very defined edge to it. With the No-Load fully engaged, there was a touch more output from the bass and some extra brightness. With both pickups engaged, the slap tone was like a razor! I’d probably not do both of these mods again in the same instrument, but it definitely got me closer to what I originally liked about the bass.

5.) Series/Parallel modification: This is a very, very cool thing to do in a Jazz bass. Parallel mode is the standard Jazz wiring, with the output from both pickups reaching the jack. Series mode runs the hot from one pickup into the ground of the other, making a huge humbucker with only one volume control. Swapping one of my  volume pots for a Gotoh 500K Push-Pull, I used this diagram from the Seymour Duncan site.  Fender achieved the same thing with their “S1″ switching, but have since pulled it from their premium models.

Effect: With the Stk-J2 pickups in, there was an immense output in Series mode, with increased low mids & even more treble cut… it was too big a sound, muddy & ill-defined.

6.) Out with the new, in with the old: I removed the Hot Stacks,  and replaced them with the originals . I also took the No-Load tone pot out.

Effect: My favourite fingerstyle bridge tone has returned – I think the 500K volume pots offset the darker sounding Gotoh bridge just enough. The tone in Series mode is huge but well defined, and snarly with a pick – very similiar to a Precision bass. This a useful mod to anybody looking to get tones that a J normally doesn’t deliver.

The moral of the story is that if you like the way an instrument sounds, be very careful if you change the pickups. – sometimes you might have to pass up on convenience in favour of tone.

A bit of back-story here: said indie singer-songwriter dumped her band via email halfway through recording an LP (it took all year). I didn’t have as much need for a retro looking bass, and I have since obtained a G&L Tribute L-2000 which buries all other Fender-like instruments I’ve played. Sadly, no matter what I did to the bass,  it couldn’t find a niche in my collection, and had such a negative vibe attached to it that I let it go on eBay.  Sold it for enough to pick up a MIM Precision, which will be the basis of a few more mods!

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