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@ Bajaman,
I've been doing some simulations on this circuit and you are spot on about the ZTX951 pulling down the frequency response above 1khZ while most other BJT's don't.

But interesting that all the transistors (available in my sim) including the ZTX951 do seem to improve the waveform (more rounded) when compared to just using 1N914 or similar. Diodes show harder edges on the output waveform, especially when the signal is large.

My point being; Maybe any BJT will at least change the waveform and the addition of that larger 470pF mentioned earlier to help osculations might still sound good, better or just plain old different.

For those that don't have the XTX951 you might find it useful to try other transistors.
Of course I've not built this and sims don't always tell it like it really is so suck it and see.
Phil.

I was wondering why Friedman and others have Vbias going through an op amp? Is there an advantage to this like less noise or is this just something you do with unused op amps?

mwelch55 wrote:I was wondering why Friedman and others have Vbias going through an op amp? Is there an advantage to this like less noise or is this just something you do with unused op amps?

screenshot.642.png

It provides a low impedance bias rail that is very stable. Plus it's something to do with that left over opamp. Who can really know what the designer was thinking, all we can do is surmise.

mwelch55 wrote:I was wondering why Friedman and others have Vbias going through an op amp? Is there an advantage to this like less noise or is this just something you do with unused op amps?

Both.

It puts to good use a spare opamp and it supplies a bias voltage at much lower source impedance (aka as a stiffer bias voltage) than using 2 resistors. Ok the decoupling capacitor reduces the source impedance (of a simple potential divider) at higher frequencies but as you approach DC the source impedance is equal to the 2 voltage divider resistors in parallel, so if you used 100k then the source impedance near DC would be 50k.

With an opamp the source impedance will be somewhere between 100 ohms and 1k (depends upon which opamp), this will reduce possible interaction between stages due to bias voltage fluctuations.

You may ask does it matter that the bias voltage is low impedance or not. Well sometimes it does, sometimes it doesn't but IMO it is good practice and is one of several things you can do at the design stage to make a circuit that will behave as well as possible under differing environments.

/rant
Every time I read of a pedal that squeals at certain settings, or picks up radio signals, or clicks, or exhibits other undesirable traits that indicates to me that the "designer" has not taken due care to eliminate potential problems by using good design techniques, and/or has not tested it under real life situations and/or doesn't care.
rant/

My pedal is sounding a bit dark. As in I feel the treble and pres needs to be on max to sound normal.
All my component values check out. Is anyone else finding this?
There are a lot of caps to ground, I guess I'll start reducing them and see what happens...

mwelch55 wrote:I was wondering why Friedman and others have Vbias going through an op amp? Is there an advantage to this like less noise or is this just something you do with unused op amps?

screenshot.642.png

This is a more stable bias point than a capacitively decoupled resistive divider. Output impedance is actually Zero, as it is an "active ground" and achieves zero output impedance due to negative feedback. It is limited by the maximum output capability of the op-amp however.

I wrote this earlier in the thread, but it is worth noting again. Not only is a resistive divider high impedance, but no amount of capacitance will make it a stable bias point if you are creating assymetrical signals. More capacitance will just create longer and longer time constants before stability is achieved. This won't always be a problem, but bias shifts usually don't sound good, especially with multiple DC coupled stages. Worth knowing, as it may help you debug a circuit someday.

If you were to wish to only use a resistive divider/capacitor as your bias point you would want to take some things into account:

1) Eliminate DC gain on all stages. (first stage has DC gain)
2) Eliminate DC coupling between gain stages (Last LED clipping stage has DC coupling)

It would actually get more complex than this, as transient signal and asymetrical signals are quite complex. Beyond the scope of many very thick textbooks.

The question is: Will my powerful circuit overpower the stability of my wimpy Vref?

DC gain and DC coupling has much more current capability than a capacitively filtered resistive divider, so It won't be a very predictable reference voltage to rely on R's and C's. Hope that is helpful.

bmxguitarsbmx wrote: Output impedance is actually Zero, as it is an "active ground" and achieves zero output impedance due to negative feedback. It is limited by the maximum output capability of the op-amp however.

+1

Ignore my figures above, it's a while since I did any analogue design and I forgot about the effect of feedback!

Thanks for the detailed response. I have been wondering about this for a while.

bmxguitarsbmx wrote:

Jan1966 wrote: Just sounds like spending more money on something that won't make a difference for most people. Depends on how you play. I play chunk palm muted etc and never had any problems.

Yes. Definitely don't experiment. It could cost you many cents or even whole dollars.

Yeah right https://www.bitsbox.co.uk/index.php?mai ... p55fvj3mq6

Not cents

Jan1966 wrote:

bmxguitarsbmx wrote:

Jan1966 wrote: Just sounds like spending more money on something that won't make a difference for most people. Depends on how you play. I play chunk palm muted etc and never had any problems.

Yes. Definitely don't experiment. It could cost you many cents or even whole dollars.

Yeah right https://www.bitsbox.co.uk/index.php?mai ... p55fvj3mq6

Not cents

My sarcasm still stands

Finally got some time to complete my build - I call it "pink eye" It fired from the first try without any issues. I put NE5332P for U1 and U2 because I had those in my bin (shouldn't be any worse than TL072) and TL073 for U3. Other than that, it's a stock circuit, but I put sockets to be able to play with all clipping diodes and with the 120pF capacitor. I'll play with this version for a few days and then I'll experiment with ZTX clippers.

Didn't have time to thoroughly test it this morning, but I got to plug it into the mini Blackstar Fly3 and it sounds promising. It's got that raw hot-rodded Marshall vibe. Knobs do less than I'd expect in a way that they don't really change the character of the pedal, but they do tweak the sound nicely.

Will try to make few recordings over the weekend and follow up with the clips.

Looks great.
Are those 1/4w resistors? I should have done it like you instead of getting the small ones
The switch looks great. Where can I get those?

bancika wrote:Finally got some time to complete my build - I call it "pink eye" It fired from the first try without any issues. I put NE5332P for U1 and U2 because I had those in my bin (shouldn't be any worse than TL072) and TL073 for U3. Other than that, it's a stock circuit, but I put sockets to be able to play with all clipping diodes and with the 120pF capacitor. I'll play with this version for a few days and then I'll experiment with ZTX clippers.

Didn't have time to thoroughly test it this morning, but I got to plug it into the mini Blackstar Fly3 and it sounds promising. It's got that raw hot-rodded Marshall vibe. Knobs do less than I'd expect in a way that they don't really change the character of the pedal, but they do tweak the sound nicely.

Will try to make few recordings over the weekend and follow up with the clips.

[ Image ]

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Does the shielded cable for the jacks make any difference ?

@andlord yes, those are 1/4w resistors. I can't find 1/8w where I live so these were the smallest available.

The switch is from musikding.de, they have few colors. One of my favorite shops

@picatostas Haven't compared shielded vs regular, but I always use shielded in high gain pedals and amps. As I was testing the pedal all finished except for jack wiring, I used alligator clips to connect inputs and outputs to guitar/amp and it was squealing and humming like crazy, so I thought that it would be beneficial to shield the leads.

bancika wrote:@andlord yes, those are 1/4w resistors. I can't find 1/8w where I live so these were the smallest available.

The switch is from musikding.de, they have few colors. One of my favorite shops

@picatostas Haven't compared shielded vs regular, but I always use shielded in high gain pedals and amps. As I was testing the pedal all finished except for jack wiring, I used alligator clips to connect inputs and outputs to guitar/amp and it was squealing and humming like crazy, so I thought that it would be beneficial to shield the leads.

As the jacks are grounds, so the pots , you practically reduce all that noise considerably

bancika wrote:Finally got some time to complete my build - I call it "pink eye" It fired from the first try without any issues.

Had me thinking Friedman Pink Taco but, Pink Eye is ok if you like it.. I've had both and I much prefer the Pink Taco over Pink Eye any day!

J0K3RX wrote:

bancika wrote:Finally got some time to complete my build - I call it "pink eye" It fired from the first try without any issues.

Had me thinking Friedman Pink Taco but, Pink Eye is ok if you like it.. I've had both and I much prefer the Pink Taco over Pink Eye any day!

If you had designed a taco sticker ... Would be aweeeesome , you still have time, order one at redbubble or stickermule haah

yeah, pink taco sounded a bit...aggressive

Here's a very quick and messy demo of my build


plugged into interface and added Bias Amp (plexi clean) and a touch of reverb. All controls at noon, both on the pedal and in the amp sim.

Cheers,
Bancika