Bass compressor

[nosamiam]

I searched the archive for this one, but I couldn't find and answer.  I'm sure it's out there, but I couldn't find one.

I'm a bass player and I am looking to build a compressor that will work well with my instrument.  I have played through a couple of of "guitar-specific" pedals with less than satisfactory results.  I want to build a compressor and I was hoping someone with some experience could give me a nudge in the right direction.

I'm looking for a pedal that allows for variable attack time.  Does anyone have any suggestions for builds that include this, or a way to mod an existing project to include an attack pot?

Any help would, of course, be very helpful.

[Ringwraith]

Ever try a Boss CS-2?
I've never used it live with bass (I'm a guitarist) but I've used it on many recordings with bass & it was great. It has the attack knob.
I especially liked it when I was doing a lot of funk bass stuff.
The one I have is from the mid eighties.
Not sure if there's a build for it but I'm sure you could pick one up off ebay for cheap.

Hmm, just checked ebay & there's lots of CS-3's & CS-1's but no 2's.

HTH
Sean

[MartyMart]

Tech 21's "Bass compactor" is fantastic !!
worth a try in a music shop for sure  

Marty.

[David]

I'm assuming you're also considering DIY.  Should that be the case, check out this thread:

http://diystompboxes.com/sboxforum/viewtopic.php?t=24935&highlight=flatline+bass

[Joe Kramer]

I searched the archive for this one, but I couldn't find and answer.  I'm sure it's out there, but I couldn't find one.

I'm a bass player and I am looking to build a compressor that will work well with my instrument.  I have played through a couple of of "guitar-specific" pedals with less than satisfactory results.  I want to build a compressor and I was hoping someone with some experience could give me a nudge in the right direction.

I'm looking for a pedal that allows for variable attack time.  Does anyone have any suggestions for builds that include this, or a way to mod an existing project to include an attack pot?

Any help would, of course, be very helpful.

For bass, and DIY, I would highly recommend the Anderton compressor.  It doesn't have variaible attack but I doubt you'll need it.  Optical compressors, because of their gentle attack and release slopes, are very friendly toward bass, and this one is no exception.  The only changes I would make to the stock circuit would be larger input/output caps, preferably film, no smaller than 1uF.  And never mind trying to scare up a CLM6000--NSL32 works as well if not better.  Good luck!

Joe

[rubberlips]

For bass, and DIY, I would highly recommend the Anderton compressor.  It doesn't have variaible attack but I doubt you'll need it.  Optical compressors, because of their gentle attack and release slopes, are very friendly toward bass, and this one is no exception.  The only changes I would make to the stock circuit would be larger input/output caps, preferably film, no smaller than 1uF.  And never mind trying to scare up a CLM6000--NSL32 works as well if not better.  Good luck!

Joe, I did a quick search for the anderton compressor but couldn't find a schematic. But what I was wondering, I have a photocopy of some projects which I think was the anderton book (Should have taken more notice when I copied it). The compressor, does it use
* 3 opamps (RC4739, LM201)
* CLM6000 opto
* has two controls - output volume and compressor
* runs off +/- 15V
* has a GODBOUT board from 1979
?

Pete

[Rob Strand]

Up for a week:

[nosamiam]

Thanks, folks!! Big help! I think I'll give the Anderton build a shot as my first project on the breadboard I just bought!

[nosamiam]

One question:  What is IC1 in the schem above? I can't make out the first digit too well.

[rubberlips]

Thanks Rob,

Yep that's the one.

I thought there was something fimiliar about it

One question: What is IC1 in the schem above? I can't make out the first digit too well

Its an RC4739 14 pin dual opamp. I've never been able to find them and they have a completely different pinout to most opamps. Most of the projects I've had to vero board up, it's just as much stuffing around cutting tracks.

Cheers

Pete

[David]

Thanks, folks!! Big help! I think I'll give the Anderton build a shot as my first project on the breadboard I just bought!

He did another, simpler one later.  IIRC, it has a TL072 and a LM301 with an opto.  Constantin Necrasov built this one and really liked it, as I recall.  Perhaps you could PM him for the schematic.

[Mark Hammer]

LM748 for the op-amp driving the optoisolator.

IIRC, there is a PCB layout for this circuit posted over at www.generalguitargadgets.com.  JD Sleep has a collection of PCB-mask layouts that support standard dual op-amps instead of the much rarer, more costly, and unessential 4739.  It was good for its time, but op-amps have moved on.

How critical is the 748 to making it "work"?  Probably not very, but some changes would be needed to do so.  For instance, the 20pf copensation cap would be unnecessary with a great many op-amps.

I noted on another compressor-related thread that a common complaint of compressor users is the noise during the silent passages.  Part of this stems from the way that compressors work.  You will note that in this one, in the absence of any input signal, there is a gain of roughly 4.5 applied in the first stage (1m divided by 220k) and a subsequent maximum gain of roughly 12.2 in the second stage, yielding a combined gain of around 55.  Take that noise presented at the input of the first op-amp and crank it up 55 times, and clearly it will be obnoxious.  This, of course, depends partly on where you set the output-level pot, but still it is a worst-case scenario.

The low-end rolloff throughout the circuit is kept more than low enough to suit bass adequately: 3.2hz at the input, 1.9hz between IC1a and IC1b, and 1.5hz at the output.  Of course, as a device intended to be applicable to anything, there is essentially little bandwidth limiting imposed at EITHER end.  There is no high-end rolloff on the first stage, and the maximum rolloff on the second stage (gain maxed) is around 80khz.  Clearly there is room for some improvement in noise management when it comes to bass.  A 47pf cap in the feedback loop of IC1a will provide a rolloff at around 3.4khz under no input conditions.

The second gain stage has a pot which allows gain adjustment from LESS than 1 at zero ohms of pot resistance (one of the nifty properties of inverting stages) to much more than 1.  With a 220pf cap in place of the 20pf value shown, you'd have a second highend rolloff starting at 7.2khz when the gain is maxed, which that rolloff moved upwards as the output-stage gain is reduced.  For instance, with the pot set halfway (R = 50k), the rolloff with C = 220pf starts around 14.4khz.

The upshot is that, as a bass player, you have means available to you to filter out some of the hiss that normally occurs under no input conditions (i.e., BETWEEN notes or songs)  without having to sacrifice and punch or clarity in your tone.

A paradox here is that while hiss tends to be exaggerated, one of the common properties of compressors is a loss of crispness or bite.  We can offset this using a common trick.  The gain of the second op-amp is given by the ratio of the resistor in the feedback loop (a pot here)and ther input resistor.  The low-end rolloff is given by the joint action of the input resistor and the cap just ahead of it.  As noted above, this is quite low in the stock circuit.

One of the neat things about such op-amp stages, though, is that (within limits) they don't particularly *care* how many such cap/resistor combinations are sitting at its input.  That is, after all, how they can be used for mixers.  So, what if we selected a second parallel resistor/cap combo that had a *different* rolloff and produced a *different* gain for the frequency range?  Voila, pre-emphasis and treble boost.

So, what would be an appropriate mod to restore punch and clarity?  The LOWER the input resistor value, the more gain is produced as the feedback resistance is increased.  The SMALLER the input cap valueis, the higher up in the spectrum any gain will be applied.

If we insert a .033uf cap in series with a 4.7k resistor in parallel with C5 and R2 as shown in the schem, you get a slight bump in gain in the upper mids, starting around 1khz.  With the 100k pot maxed, you have a gain of 12.2 for the full spectrum, and a gain of 21 for content over 1khz.  You can work out what sorts of emphasis sound best to your own ears but this example shows how to do it.  Best of all, because the added feedback loop caps keep hiss under control, the extra boost is offset by the lowpass filtering.  This should provide a bit more presence without the accompanying hiss.

Finally, the envelope follower circuit is designed to be essentially insensitive to frequency content.  If a person wanted it to provide less compression for higher notes than for lower notes - something I suspect a person playing 5 or 6-string bass might find useful - a small value cap placed in parallel with the 1M feedback resistor could do the trick.  For example, a 100pf cap in conjunction with the 1M feedback resistor (R10) would provide less response to notes above 1.6khz, and a 150pf cap would drop that to just over 1khz.  This would not *eliminate* compression of higher notes, merely reduce how much is applied.

There.  I quite enjoyed that little "circuit stroll".  I hope you find it useful in customizing your pedal to your needs.  One of the joys of Craig's circuits is that they are usually VERY moddable.  I learned a lot from him, and hope you do too.

[Joe Kramer]

Right on Mark!  Nice post, one for the archives.  There's nothing I could say to improve on it, but I would like to add a little.

I'm not the guy to ask about the freq response math, and I can't argue with the numbers--they don't lie after all.  But I will say, even for guitar, I can definitely hear the hi-pass effects of a .22uF input cap.  I suggested 1uF at least for bass, but I would honestly go lower.  In a rackmount stereo version of the Anderton compressor I built, using Holco resistors, AD op amps, toroidal power supply, etc, I used 10uF polypropylene, and would have used bigger if I could have afforded it (the whole project ended up costing in the neighborhood of $600!).  

Anyway, all I'm saying is, if you can try bigger values on a breadboard (and you can fit them in your enclosure), don't hesitate to try.  The math doesn't lie, but I'd trust my own ears before anything else.  

Thanks for the (usual) wealth of info Mark, and good luck Nosamiam!  Keep us posted on your progress with that compressor. . . .

Joe

[Mark Hammer]

Right on Mark!  Nice post, one for the archives.  There's nothing I could say to improve on it, but I would like to add a little.

I'm not the guy to ask about the freq response math, and I can't argue with the numbers--they don't lie after all.  But I will say, even for guitar, I can definitely hear the hi-pass effects of a .22uF input cap.  I suggested 1uF at least for bass, but I would honestly go lower.  In a rackmount stereo version of the Anderton compressor I built, using Holco resistors, AD op amps, toroidal power supply, etc, I used 10uF polypropylene, and would have used bigger if I could have afforded it (the whole project ended up costing in the neighborhood of $600!).  

Anyway, all I'm saying is, if you can try bigger values on a breadboard (and you can fit them in your enclosure), don't hesitate to try.  The math doesn't lie, but I'd trust my own ears before anything else.  

Thanks for the (usual) wealth of info Mark, and good luck Nosamiam!  Keep us posted on your progress with that compressor. . . .

Joe

Thanks for the support, Joe.  Obviously, I can't argue with your ears, any more than you can argue with the math.  You hear what you hear.  Personally I don't know how you could hear it, given that the rolloff is well below what most speakers can provide, but so be it.  Hell, we used "rumble" filters with vinyl disks for decades to filter out stuff in the under 20hz range, because that content interfered with reproduction of frequency content above that range, so the same or analogous things could apply in the case of how your bass sounds, right?

On the other hand, I can't help wondering how much of what you seem to have heard is more a question of capacitor TYPE than value.  T'wer me, I would have probably gone for multiple paralleled small value plastic caps to achieve the larger value with minimal phase distortion issues, than used a larger cap.

Again, how this all plays out with YOUR bass, YOUR amp, and YOUR speakers, is another matter.  Me, I'm the kind of cheap S.O.B. who doesn't like to have his entire rig - guitar, amp, and effects - cost more than $500, so I think we're in different leagues with respect to expectations and sonic requirements.

[Joe Kramer]

Thanks for the support, Joe.  Obviously, I can't argue with your ears, any more than you can argue with the math.  You hear what you hear.  Personally I don't know how you could hear it, given that the rolloff is well below what most speakers can provide, but so be it.

I know!  I don't know how I could hear it either, and I don't blame you at all for saying that.  Probably a better choice of words on my part would have been "perceive" or just simply "feel."  I think the feel thing comes from playing guitar for so many years.  I haven't compared notes with anybody else on this, but I can feel the delay when using a univibe type effect in the "wet" or vibrato mode.  The amount of delay must be micro-seconds I'm sure.  Something about the touch sensitivity changes with smaller input caps, and this is especially true with full-freq devices like overdrives and compressors.

Hell, we used "rumble" filters with vinyl disks for decades to filter out stuff in the under 20hz range, because that content interfered with reproduction of frequency content above that range, so the same or analogous things could apply in the case of how your bass sounds, right?

Right: intermodular distortion.  Bad for hi-fi, mostly good for guitar (and maybe bass)!

On the other hand, I can't help wondering how much of what you seem to have heard is more a question of capacitor TYPE than value.  T'wer me, I would have probably gone for multiple paralleled small value plastic caps to achieve the larger value with minimal phase distortion issues, than used a larger cap.

Don't get me started!  I'll bore you to death with my capacitor obsessions.  Although it's a whole 'nother thread, cap types make all the difference in the world and parallel caps are a great idea for super fidelity.  I'm not going to make more high-flying claims for myself here, but if you're saying types of input caps "feel" different, I heartily agree. . . .    

Again, how this all plays out with YOUR bass, YOUR amp, and YOUR speakers, is another matter.  Me, I'm the kind of cheap S.O.B. who doesn't like to have his entire rig - guitar, amp, and effects - cost more than $500, so I think we're in different leagues with respect to expectations and sonic requirements.

Not so fast!  I'm so cheap I use those plastic clips off the bread loaf bags as picks.

I wasn't too happy about having sunk such a large chunk into that compressor (though I'm happy with how it sounds), but that's used for studio recording with mics and so forth, not my guitar rig, and therefore a different set of standards applies.  The low freq response in that case wasn't a "feel" thing, but most certainly a "hear" thing: even with the math showing almost DC in the lows, .22 uF chopped-off important musical info.  As far as recording/reproducing music goes, you obviously want every last drop of sound, unaltered and unfiltered.  But unfortunately "how low can you go?" equals "how much can you spend?"

Overall, my first and only rule of thumb is, If it sounds good, it is good.   If it happens to be cheap too, all the better!

Joe