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Tim Stinchcombe's Synth Stuff


This page details modifications to the M38 ADSR main module—if you are looking for the mods for the M38A ADSR Expander module (which adds voltage control to the ADSR), they are here.

Whilst writing up the mods for the M38A Expander module I decided it would be worth adding some comments here about the behaviour of the 'sustain' voltage level, which is not quite what one would normally expect of an ADSR.

Plan B M38 ADSR modification

It doesn't take long playing around with an M38 to realise just how completely intractable the control range is—most of the useful action from the attack, decay and release pots takes place in about 5% of their available travel. The following is a plot of the attack time versus the pot wiper voltage, over much of the range which produces a change:

That's a change from about 200μs to over 7secs in a little under a 0.6V change in the pot wiper—re-scaling the wiper axis to encompass its entire sweep from 12V down to ground (which more or less equates to turning the pot from fully CCW to fully CW) emphasizes just how little of its travel is being used:

(The 200μs is about as fast as mine will go; it will go slower than the 7secs plotted, but seeing how vertical that line is becoming, it gets even harder to control it, so I didn't bother taking any further readings.)

It is so hard to control it is difficult to understand how Plan B even thought that in this configuration it was a viable module, and indeed, an old thread at the (now defunct) modularsynth.net forum acknowledged that it was problematic, and that around 25% of those sold at the time had resulted in comments from concerned buyers. The module I have is marked 'Rev 1' and dated 2008; they began shipping around the middle of 2008, and by Jan 2009 message #2412 at the Plan B Yahoo group further acknowledged that changes were needed (though the proposed change of pot type would have left the problem substantially unresolved), and since Plan B effectively ceased-to-be around mid-2009, I doubt there are any other later revisions or changes to the module out there. (It is perhaps also worth noting that twiddling the trimpot doesn't have a great deal of effect on the issue—all it does is move whereabouts in the total pot travel that the 5% effective bit actually sits.)

The problem is caused by a missing resistor, which means that the voltage applied to the input of the exponential converter driving the main timing circuitry, instead of having a range of the usual 300 or 400mV, turns out to be more like 6V or so, with the consequence that over most of the range the expo converter is being over- or under-driven. Adding the resistor (detailed below) attenuates the voltage, and we can choose either a smaller value to give a narrow range of envelope times, but have fine control over the changes, or a larger value to give a wider range of times at the expense of increased difficulty of 'fine control'.

There is also another not-so-pleasant effect of the original design set-up: if no 'release' phase is required, and so the release pot is simply 'parked' at its fully CCW position, then there is a fair chance that the resulting envelope has a huge 'step' at the start:

It affects both short and long envelopes, and is essentially caused by the same larger-than-normal voltages presented to the expo converter in conjunction with the latter's natural lag—when switching from 'release' to 'attack' it takes a finite amount of time for the expo converter to react, with the effect that the large release voltage continues to be passed through the expo converter during the initial stages of attack, with the result we get a really fast attack until the expo catches up and changes to the actually-desired much smaller attack voltage (necessary for the 'reasonable' envelope time). It can be partially ameliorated by dialling in some release in order to lower the large voltage change upsetting the expo converter, but the downside is because of the difficulty in controlling all the pots, it is hard to reduce this initial step-up completely and still not have any release—the green trace here shows how the step is absent, but compared to the red there is clearly more release (the blue trace is the 'gate' signal of course—I was using 'trigger' in the oscilloscope sense!)

The modification. After some calculation and experimentation, as well as adding the missing resistor at a suitable value, I decided to increase the value of the main timing cap to 100nF (the standard fit appears to be 10nF), and halve the current-limiting resistor feeding the CA3080 OTA in order to double the maximum current output by the exponential converter. The extra resistor needs to be added at the input of the expo converter, down to ground—this photo shows the locations on the top of the board:

...and this photo the bottom:

(Note the factory-fitted mod correcting an error in the PCB!)

At first I tried a 3.3kΩ—this proved the feasibility of it all, but the envelope range was a bit restrictive, so I upped it to 4.7kΩ; I experimented with a few other combinations, but the 4.7k felt best. Here is a photo showing the extra bits added—the 4.7kΩ to ground, a second 27kΩ across the existing one to halve its value, and the additional 100nF cap across the existing 10nF one, so giving about 110n in total (of course the latter two could be replaced completely if desired):

The 4.7kΩ with 100nF gives a shortest envelope of a couple of milliseconds, and a longest of around a minute and a half:

(Note the difference in amplitude is because these measurements come from different scopes, and their calibrations are clearly a little in disagreement!) It should also be noted that since the voltages at the input of the expo converter are now changing by much smaller values, the problem of the large 'step up' at the beginning of the envelope in the original configuration has disappeared.

Repeating the initial plot at the top of the page, of the attack time versus the attack wiper voltage, and we can now see that the complete sweep of the attack pot gives some useful effect (the original trace is again included, for comparison purposes)—the sweep is from a minimum attack time of about 750μs to about 14.5 seconds:

To adjust the trimpot: set all the pots fully counter-clockwise, including the trimpot—this will give the fastest envelope. Now turn the trimpot clockwise until the envelope just starts to slow down (probably best done on a scope if available, but feeding the envelope to a VCO will probably do as well). Setting the trimpot any further CCW will simply mean that the front-panel pot travel is wasted, so this method should give the biggest sweep of envelope times from fastest to slowest.

Other combinations I tried were the new resistor at 6.8kΩ with a 68nF cap in parallel to the existing one. This gave a huge range: the quickest attack was then about 500μs, with the whole envelope about 3ms; the longest was long—I miscalculated how many readings I needed to make with my Picoscope so I couldn't capture the whole thing, but just the attack was about 25 minutes, and I abandoned the readings a few minutes later! Naturally enough these values suffer from the original problem (but perhaps to a much lesser extent), in that it was starting to get a little fiddly to get the pot positions just right.

I also tried 5.6kΩ plus the 100nF cap—this gave a shortest envelope of around 3ms again, and the longest was around 10 minutes or so, and again it is clear that small pot movements end up giving quite largish changes to the envelope.

Another thing of note perhaps is that the attack phase is always fairly linear. It should be possible to make it more log-like by increasing 100kΩ resistor 'R7' near the attack pot (it is standing up on its footprint due to another PCB error...)—this will of course lengthen all envelopes.

Sustain voltage level behaviour: in most normal envelope generators we expect that once the peak of the envelope is reached at the end of attack, then the envelope would start falling away (we are in 'decay' after all!), often down to some (non-zero) 'sustain' level, where it remains until the gate is released. In the M38, the circuitry controlling the sustain level doesn't actually limit the sustain voltage to be below the peak reached at the end of attack (about 6V); consequently if the sustain pot is above about setting '6', then the sustain level is actually above the level of the (would be) peak at the end of attack, and so the envelope continues to rise even though we are nominally 'in decay'! The follwing set of traces illustrate the different envelopes generated as the sustain pot is set at '0', '2', '4' and so on, up to fully clockwise at '10':

[Page last updated: 5 Sep 2017]