# Triangulum - schematic error?



## Chuck D. Bones (Mar 17, 2019)

Looks like Q2 is wired wrong.  There is no source for base current, so it never turns on.  Maybe the bottom end of R10 is supposed to go to Vref?  If so, then it's a current sink that acts as an anti-pop network.  A 1Meg resistor would have done the same thing.

And what's with all of the series resistors?  R12 + R13 + R18 = 2.04K, which is plenty close to 2K.  On closer examination, the feedback network resembles a Baxandall tone stack with the Bass set near zero and the Treble set at 9.

I fail to see what all of the hype is about; why anyone would pay several hundred $ for an original is just crazy.  It's a clever circuit, but at the end of the day it's just another opamp treble boost.  Thanks to PedalPCB we can have one for about $25 in parts.

- Chuck


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## Robert (Mar 17, 2019)

Q2 is correct.   It's a form of high impedance pull-down.... see the TC Integrated Preamp schematic.

I knew it was a matter of time before someone asked about the strings of series resistors.    

These were "precision" values that aren't available from some parts vendors.   Rather than round up (or down) to the nearest E12 value, or indicate values that might prove hard to find from the usual sources, the equivalent series resistances were created.

Also, just an FYI,  R12 + R13 + R18 would be 2.84K, not 2.04K

Why there are resistor + cap + resistor series networks I can not explain, but I've seen this in more than one "similar" circuit so I decided not to re-engineer it.


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## Chuck D. Bones (Mar 17, 2019)

Right, I should know how to add by now.  
I ran simulations with the nearest 5% values and there is no noticeable difference in circuit performance.  We're talking tiny fractions of a dB.  Also, C15 may as well not be there, R2 & R15 are so small that they short C15 out at audio frequencies.  

I had not looked at the TC Integrated Preamp before.  Looks like Fortin copied it nearly verbatim, minus the pots in the tone stack and a few slightly tweaked resistor values.  Some schematics online carry the same typo.  The correct schematic has R10 wired to a contact on the input jack (see below).  I suspect that contact is the sleeve; Q2 turns on when the input jack has nothing plugged in.  There is no way Q2 will ever conduct (other than Icbo) the way it is wired in the Triangulum.  

As for all the series-connected parts...
From what I have gathered from conversing with some pedal designers, they fiddle the circuit until it sounds the way they want and never bother to clear out the extraneous stuff that accumulates along the way.  I'm not saying they all do that; some pedal designers think like engineers.


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## Robert (Mar 17, 2019)

The wiring of Q2 (or Q1 in the schematic above) would definitely make more sense the way it's drawn there, enough so that I took apart the original to double check.

The Triangulum schematic is definitely "correct" with respect to the original, whether or not it actually makes sense. 

I don't make the news, I just report it.


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## temol (Mar 19, 2019)

Is TL071 also used in the original pedal (F33)?

T.


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## zgrav (Mar 19, 2019)

interesting exchange.


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## Robert (Mar 19, 2019)

temol said:


> Is TL071 also used in the original pedal (F33)?
> 
> T.



Yep, and the _other_ one too.....   ⚙⚙


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## Chuck D. Bones (Mar 19, 2019)

Just my opinion, but...
This circuit is supposed to be "clean" in the sense that it doesn't introduce any distortion.  That means we don't depend on the opamp's overdriven behavior to produce a characteristic sound.  Any opamp that is low noise and sufficiently fast (>2V/us) will work in the Triangulum or Integral Preamp.  TL071 is an obvious choice, but there are plenty of others.


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## mnemonic (Mar 20, 2019)

My original TC integrated preamp has a UA741 in it. I know there were a few versions though, some had TL071, and some other single op amp I forget.

I have an OPA604 in my clones (the integral, and an old tagboard one I made a while back) and I can’t really say there’s any difference. I want to say the OPA604 might be slightly clearer and louder sounding but it’s so close that it might as well be placebo effect.


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