# Integral Preamp unpleasant distortion



## kahles (Jan 30, 2021)

Hi everyone and at first thanks to all writing here! I built my first three pedals, ran into some problems and learned so much by fixing them.

But now I think I need some more help 

I built an Integral Preamp and it does what it should, except it seems a bit overstrained by my passive, but very hot Häussel pickups ... When using the pedal for clean sounds there's an unpleasant flattering distortion that disappears, if I turn down the volume poti of my guitar. I thought this is an easy one, socketed R1 and tried different resistors - without success ...

Has anyone a suggestion, what I can do to make this pedal more rugged to higher level input signals?


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## kahles (Feb 1, 2021)

Ok, this might be the same problem as described here: https://forum.pedalpcb.com/threads/help-please-debugging-how-to.3047/
I will audio probe it, and see where the distortion appears ...


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## Mcknib (Feb 1, 2021)

Perhaps post some pics of your build more eyes might spot something I know I've checked problem boards multiple times and still missed something someone else has spotted


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## kahles (Feb 2, 2021)

When I disassemble it (today or tomorrow) I will make photos!

Btw: I have a triangulum built by a professional pedal maker and it has the same problem but by far not so intense. I tested changing my BC549C (I read it's an equal replacement for a BC548C) against the BC550 he used, and couldn't hear a real difference ... but that was just some quick experiment yesterday.


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## music6000 (Feb 2, 2021)

kahles said:


> When I disassemble it (today or tomorrow) I will make photos!
> 
> Btw: I have a triangulum built by a professional pedal maker and it has the same problem but by far not so intense. I tested changing my BC549C (I read it's an equal replacement for a BC548C) against the BC550 he used, and couldn't hear a real difference ... but that was just some quick experiment yesterday.


While we wait for PCB photo's, Häussel Humbuckers or Single Coils. what K's are we talking?


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## music6000 (Feb 2, 2021)

kahles said:


> Ok, this might be the same problem as described here: https://forum.pedalpcb.com/threads/help-please-debugging-how-to.3047/
> I will audio probe it, and see where the distortion appears ...


When the pedal is in bypass & you are playing Guitar at full volume through a very Clean Amp setting, Set the pedal to Unity so that it is the same Level when On & in Bypass mode with Bass & Treble at 12 O'Clock.
Do you get a Distorted signal when pedal is On?
If Yes, then there is an issue with the Pedal, not the Pickups.
This pedal by Original design is known to Distort with a standard strat if not getting enough voltage .
The Original can take up to 32v. 
This has an internal Charge Pump so it should be OK!


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## temol (Feb 2, 2021)

kahles - check this thread


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## kahles (Feb 2, 2021)

music6000 said:


> While we wait for PCB photo's, Häussel Humbuckers or Single Coils. what K's are we talking?


The most crazy one he has 😜
It's a Humbucker, named TOZZ B Fat XL. Sounds awesome, but is almost a bit too loud. I had to increase the distance to the strings because it even drove my Diezel Hagen into a fuzzy distortion.



music6000 said:


> When the pedal is in bypass & you are playing Guitar at full volume through a very Clean Amp setting, Set the pedal to Unity so that it is the same Level when On & in Bypass mode with Bass & Treble at 12 O'Clock.
> Do you get a Distorted signal when pedal is On?


Yes, unless I turn down the guitar's volume poti.

The guitar was plugged into the Integral (Vol ~11:00, Bass&Treble 12:00) and I went from the Integral directly into my Palmer Macht power amp.

@temol Thanks, I will read that later!


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## music6000 (Feb 2, 2021)

kahles said:


> The most crazy one he has 😜
> It's a Humbucker, named TOZZ B Fat XL. Sounds awesome, but is almost a bit too loud. I had to increase the distance to the strings because it even drove my Diezel Hagen into a fuzzy distortion.
> 
> 
> ...


As temol has raised, the Triangulum is basically the Integral Preamp but with fixed EQ.
I would say that it is *definately* not being helped by the Pickup you have but it is also by design that this is not a Clean Boost as one would expect!
We can still look at your PCB to make sure, just take a good Gut shot of the components while still in the Enclosure to see if anything looks wrong to start with!
It would be good if you could get  some BC549's to try out, hfe is less than half BC549C's!


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## Chuck D. Bones (Feb 2, 2021)

It's a clean boost until you saturate the output with either hot pickups or cranking the BASS & TREBLE.  Since the gain is not adjustable, you get what you get.  Messing with Q2's hFE will move the bias point around, it will not affect the gain of the circuit because that is controlled by the feedback loop.  The simple solution is don't turn the TREBLE & BASS up too high.  We should check the bias just to make sure that's good.  IC1-6 should be close to 16V.  Measure that and IC1-7 (Vcc).

Oh yeah, one more thing...
This pedal is capable of outputting roughly 25Vp-p before it clips. That's more than enough to saturate any pedal or amplifier.


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## kahles (Feb 3, 2021)

Hello again!

I'm a bit confused now ... it seems like I have two problems and the guitar ouput is none of them.

Audio probing showed me, that the distortion starts at Q2. What goes into pin2 of IC1 is already really dirty. So in my eyes the BC549C seems already to be a problem. I removed Q2 and measured 31V at pin 1 and 14V at pin 2.
I tested the transistor to be able to show you some values:





I'm also far away from the values Chuck told: I removed IC1 and measured at the socket: IC1-6 is 28,6V and IC1-7 33,4V.
I measured Vref (pin 2 of R4) at 27,7V and Vcc (pin 2 of R5) at 32,7V.

This is the pedal before disassembling it:




I found something that looked like a cold sold solder joint at R103, resoldered it and removed the socket of R1. Now it looks like this (and doesn't sound better):


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## Chuck D. Bones (Feb 3, 2021)

Make all measurements with all parts installed and power on.  With IC1 out of the circuit, all bets are off.

There should be zero signal at IC1-2 because that is a summing node.

Can you measure the voltage at IC1 pins 2, 3 and 6?

Pins 2 & 3 should be at Vref.

Pin 6 should be between 15V and 16V, depending on Q2's hFE.

If those voltages check out, then Q2 and IC1 are working as they should.

Do not try to measure the voltage on Q2-B.  The impedance there is too high and your DMM will load down the circuit.


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## kahles (Feb 3, 2021)

Tested it and it looks good ...?

IC1:
Pin 1: 150mV
Pin 2: 28,3V
Pin 3: 28,0V
Pin 4: 0V
Pin 5: 150mV
Pin 6: Jumping around between 2 and 20V
Pin 7: 31,5V
Pin 8: counting down against zero

I now also think it works as designed - I re-tested it against my Triangulum and finally I can at least set them up to sound exactly the same - which wasn't possible before.
I think it was a voltage problem caused by this soldering joint mentioned above.
It would be nice to have it a bit cleaner, but I can work with what I have now 🤩

Thank you all for your advice!


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## Chuck D. Bones (Feb 3, 2021)

kahles said:


> Tested it and it looks good ...?
> 
> IC1:
> Pin 1: 150mV - *don't care*
> ...



Pin 6 should not be jumping around.  You should measure this with no signal going in.  Short the input or plug in your guitar and set the guitar volume to zero.


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## kahles (Feb 4, 2021)

Ah ok - had the looper connected from audio probing.

Now with my guitar plugged in and volume set to 0, I measure exactly 20,6V at pin 6.

I compared to my Triangulum: Vcc and Vref are nearly the same, pin 6 of TL071 only has 10,6V.

Replacing the TL071CP through the TL071IP from the Triangulum did nothing, replacing the BC549C through the "CTBC 550C" from the Triangulum does that: Now I have 10,6V what is also a bit away from the 15-16V Chuck mentioned ...

Playing deep hard-hit power chords the distortion seems to be a bit more intense using the 550 instead of the 549. My ultra-professional transistor-tester measures nearly same values for hfe, Ube and Ic. Is the tester so crappy or what makes this difference?


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## kahles (Feb 6, 2021)

Got some other BJTs from my local electronics dealer:

hfeV IC1-62x CTBC548427/42819,2/19,42x BC549B28318,92x BC549C495/52319,6/19,7
Strange, that the 550 mentioned above leads to 10,6V at IC1-6, having nearly the same hfe as the 549C ...
For the sound these all make no great differences.


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## music6000 (Feb 6, 2021)

kahles said:


> Got some other BJTs from my local electronics dealer:
> 
> hfeV IC1-62x CTBC548427/42819,2/19,42x BC549B28318,92x BC549C495/52319,6/19,7
> Strange, that the 550 mentioned above leads to 10,6V at IC1-6, having nearly the same hfe as the 549C ...
> For the sound these all make no great differences.


It is not going to change in your situation with the Hot pickup you have.
I have a Tele style Guitar that I modded that has S.D Little '59 's in it with Split Coils.
I can be playing Clean as with a Pedal  in Single coils & as soon as I switch to Humbucker mode it is distorted in Neck or Bridge!.
Around 8.5K Split & 15K in Humbucker mode.


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## kahles (Mar 11, 2021)

Hello again,

finally I reached what I wanted by modding the Integral Pre with a simple voltage divider.
As Chuck wrote, there are some obsolete parts in this circuit and their footprints are quite helpful:







First I bridged R2+R3 and mounted a B5K pot between 1 and 3 of Q2. At 5 Ohms it already reduces gain but the distortion is still there. For my guitar/pickups a value of 2,7K turned out to be optimal - the distortion is gone (having BASS and TREBLE at noon!) and there is still enough headroom for boosting the signal for a reasonable amount using LEVEL.
I use this pedal to tighten up low end and push treble a bit. The only purpose of LEVEL is for me to be able to "restore" the input level.
Since I already had a socket mounted for Q1, i now have plugged a resistor there, but to be able to tune it for other guitars I will replace it with a C5K pot after my next parts order.

Thanks to all for your help!


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## music6000 (Mar 11, 2021)

As Winston once famously said ''Never Give Up, Never Give Up, Never Give Up ''
That's what Chuck & I like to see from all Members on this Forum!!!


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