# Reducing gain on fixed gain op amp (Integral Preamp / TC Integrated Pre)



## mnemonic (Apr 21, 2022)

Hi guys, I have an Integral Pre (TC Integrated Preamp) that I built a while ago, the only problem I have with it is I get some op-amp clipping with my loudest guitar when I wang on the strings hard (which is most of the time).  I know the build is fine as my original TC integrated preamp, as well as my veroboard clone from back before the pedal was hyped, both exhibit the exact same behavior, and otherwise sound the same.

From what I understand the pedal is just a fixed gain op amp amplification stage and a two-band shelving EQ, with some additional fluff around the input for anti-pop when plugging/unplugging.  I was hoping to reduce the op-amp gain slightly as I tend to have plenty of range left on the volume knob to make up for any loss, However it doesn’t appear to be a typical inverting op amp stage. 

I’ve done some googling and the closest I can find to what the schematic looks like would be an integrator op amp (maybe makes sense based on the original pedal’s name?) but it doesn’t look like integrator op amp circuit is meant for audio. 

Below is the schematic off the build sheet:






Does anyone know how I would reduce the op amp gain of this circuit?


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Also as a side note, I use this pedal as a boost to tighten the guitar sound into a high-gain amp for metal, which means I always have the bass turned quite low.  The sweet spot tends to be quite small, between zero (very tight) and about 9:00.  Stock bass pot is c50k, I replaced for a50k and this greatly extends the sweet spot, up to about noon.  I tried a linear pot first, but very little difference.  Would not recommend this if you ever boost the bass, however.


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## jesuscrisp (Apr 21, 2022)

Looking at the schematic, I feel like the Vref resistors are chosen awkwardly, there's not dedicated Vref filter cap and thus the whole op amp clipping you hear is due to misbiasing.


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## mnemonic (Apr 21, 2022)

Sorry I'm a bit of a noob when it comes to biasing (and much other stuff), I have just measured VCC as 31.40v, and VREF as 27.96v.  Should VREF then be lower, closer to 15v, to avoid clipping?


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## jesuscrisp (Apr 21, 2022)

Usually yes but also the whole design has some weirdness to it.

But that's apparently intentional, unless the trace is wrong. Still, at 33V headroom I really don't expect it to clip.


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## mnemonic (Apr 21, 2022)

Ok, must be a weird circuit indeed.  The voltage divider for VREF is 100k/1m, so I paralleled a 120k resistor on the 1m so the divider is now 100k/107k.  VREF now 15.45v, VCC still 31v.  Sound is very gated, distorted, and quiet.  

changed to 500k parallel with the 1m (value now 333k), now somewhere between the stock circuit and the gated/distorted/quiet.  Still quiet and distorted but I can hear notes now. 

changed to 1m5 parallel with the 1m (value now 600k), almost sounds stock but slightly more distorted.  

are the large values to blame for this? I notice the tube screamer for instance uses 10k/10k divider. 


The original schematic is slightly different in this area, 100k/820k (rather than 100k/1m in the integral).


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## jesuscrisp (Apr 21, 2022)

The distortion plus for example has a 1M/1M divider. The integrated preamp is a weird design tbh so maybe standard biasing/vref rules do not apply?
Maybe someone smarter can join in and give some clarity. In terms of reducing gain though, have you tried reducing R6? 

Also, how much of an issue is op amp breakup anyway in case you (most likely) play this through a high gain amp anyway?


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## mnemonic (Apr 21, 2022)

jesuscrisp said:


> The distortion plus for example has a 1M/1M divider. The integrated preamp is a weird design tbh so maybe standard biasing/vref rules do not apply?
> Maybe someone smarter can join in and give some clarity. In terms of reducing gain though, have you tried reducing R6?
> 
> Also, how much of an issue is op amp breakup anyway in case you (most likely) play this through a high gain amp anyway?



potentially not a problem at all... I have been using this pedal (or the original) for years, but it was more about whether the sound could be improved by reducing clipping.  

I will try fiddling with R6 tomorrow and see what that does.


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