Ethos TWE-1 - 1.8v Zeners?

[MAO]

Can someone explain the purpose of the zeners in this circuit?

I can't detect an audible or visual change with them in or out of the circuit as they are shown in the schematic

If I connect one end to ground rather than Vref, then the top of the waveform is altered a bit and the amplitude is reduced as well.

They don't appear to have any influence connected as shown in the schematic.

Are they some kind of circuit protection rather than an audible influence?

Or maybe a schematic error?

Here's what the waveform looks like taken at the top of the zeners using a 1Khz input signal. It does not change with the Zeners in or out of the circuit. (zeners connected to Vref)



Here is the waveform with the bottom of the zeners connected to ground rather than Vref. The tops of the peaks are altered a bit.  



The pedal sounds awesome regardless, just trying to understand the zeners function here.

Thanks

[Frank_NH]

Looking at the schematic, I would speculate that the diodes downstream from the gain stages are creating some sort of signal compression.  It could be that the zeners don't affect the signal unless the gain is all the way up or you have a really hot input from your guitar.  Try increasing the amplitude of your test signal and see if the diodes affect the signal peaks more.  If you're handy with Spice, you could simulate the compressor circuit and see what it does for different input signals.

[MAO]

Thank Frank,
Probably will get spice as I'm sure it will help me understand lots of other designs too.

As for the TWE, I couldn't detect any change regardless of settings or input level. Seems like the 1n4148s kick in before the zeners get a chance to.  We'll see what Spice has to say about that.

On another note, since you probably have looked at many things with Spice, when there is a 1uf coupling or output cap, for instance after the output of an opamp, what's the effect if I were to use a 2u2 instead? (or something in between)

Reason I ask is most of the MLCC 1uf caps I have measure well under 1uf, more like 750-800nf. I usually double them up with a 220nf to get close to 1uf. Wondering if I could just use 2 of the 'out-of-spec' 1uf instead and save all the measuring.

   

[Frank_NH]

"On another note, since you probably have looked at many things with Spice, when there is a 1uf coupling or output cap, for instance after the output of an opamp, what's the effect if I were to use a 2u2 instead? (or something in between) "

Using a large cap for audio coupling allows more low frequencies to pass without attenuation.  The corner frequency (the frequency below which the signal becomes attenuated) is given by:

fc = 1/(2*pi*R*C)

In series, the larger C is the lower the corner frequency and thus more lows are allowed through.

Having said all this, depending on the circuit, you may not notice a huge difference between 1 uF and 2.2 uF coupling caps (I usually don't hear much difference).  That's often because the fc is already low (e.g. 10 Hz) and making it lower won't affect the tones we care about (at least with guitar).