Musical Soft Clipping Circuits - Tri-to-Sine Waveshapers & the Differential Pair

[niektb]

niektb,

Please help me to understand how wav files are mapped in Faust

What is the range of the wav file data when converted to decimals ?

is highest possible data in the wav mapped to 1.00000000000 ?

As far as I know, no normalization is happening. So the possible input and output range is between -1.0 and 1.0 but that is only reached when your WAV file actually has peak levels at 0dBFS

[...]

That browser IDE is quite powerful. What kind of dedicated hardware can run Faust programs?

Often there is a conversion step needed so not native but working implementations are in JUCE (so VST plugins), SHARC DSPs, ESP32 and Teensy (and the last is the one I plan on using )

[Vivek]

Opamp will be high impedance and only sense voltage signals

Transformer will be low impedance and will draw current, needing constant current sink.

Transformer will reduce high frequency response

And might also saturate, creating a different kind of limiting / distortion by itself.

[R.G.]

I wonder how the behavior of a differential amplifier changes hooked up to a transformer vs. opamp?

For some quick insight, replace the transformer with the linear model of a transformer.
The primary side has the winding resistance and primary leakage inductance in series with the parallel combination of the primary inductance and the reflected secondary load. The secondary side has the secondary resistance and leakage inductance in series with an ideal voltage source that is Ns/Np times the voltage across the primary inductance and reflected secondary load.
In the diffamp, the two collectors see the winding resistance at DC, increasing with frequency as the inductive reactance of the leakage and primary inductances rise, in parallel with the Np/Ns squared ratio times the secondary load. The output impedance of the diffamp across the transformer primary is effectively the collector resistors, so there is a low frequency rolloff determined by the collector resistors, the secondary reflected load, and the primary inductance. This flattens out at frequencies where the primary inductor reactance increases above the reflected secondary loading and the driving impedance from the collectors.
The leakage inductance becomes important as frequency rises until the series primary and secondary leakages start limiting response at high frequencies. There is a little bridging effect of the winding to winding capacitance at quite high frequencies.
On top of that, there are any nonlinearities of the  core hysteresis. This can be quite small or large, depending on how hard the primary is driven toward saturation. It's generally quite small unless you're driving the transformer hard, and is mostly at the lowest frequencies the primary inductance lets through. The effect of DC imbalances on the transformer may make this worse at lower levels. It depends on how much offset there is.This can give asymmetrical distortion, but it's a lower frequency effect unless you're really abusing the transformer primary.

[Steben]

In comes the inevitable question whether a more common architecture without transformer yet with high pass EQ and clippers is that much different in result.

[teemuk]

I believe it is not that much different in result. Also, transformer saturation with these (low) working voltages and limited bandwidth of guitar signal is not a likely occurance.

[Rob Strand]

For the IVP preamp.  At one point there was uncertainty if the diff-amp current source resistor was 43 ohm or 430ohm.   I traced this thing back in the 80's and I got 430 ohm but the factory schematic had 43 ohm.   Finally I found a clear pic of the PCB which left no doubt the value was 430 ohm.

https://www.diystompboxes.com/smfforum/index.php?topic=121884

As part of that exercise I did a whole lot of calculations on the transformer.   Even with 43 ohm it was pushing getting to saturation so with the actual value of 430 ohm it seems unlikely the transformer is near saturation.   A lot of small transformers have incidental gaps between the laminations so that's going push saturation even further away.  HF roll-off is still possible.

https://www.diystompboxes.com/smfforum/index.php?topic=119132.0

[Vivek]

You guys who were born 20 years before me had all the fun with new discoveries, new schematics, new tracing, new ideas.

I'm so jealous !

[Steben]

You guys who were born 20 years before me had all the fun with new discoveries, new schematics, new tracing, new ideas.

I'm so jealous !

Yes, but always getting slapped back to reality with its constant pointing finger towards the actual very very little differences.

[amptramp]

That's an interesting circuit, lots of building blocks you don't see too often in pedals. I wonder how the behavior of a differential amplifier changes hooked up to a transformer vs. opamp?

Clipped portion of the IVP Preamp Schematic:

I'm curious as to why the designer didn't just take the centre tap of the transformer primary and connect it to V_A.  That would save a couple of 1100 ohm resistors.  I feel there should be a capacitor across R37 just to keep the input signal off the far side of the differential pair.

[R.G.]

I'm curious as to why the designer didn't just take the centre tap of the transformer primary and connect it to V_A.  That would save a couple of 1100 ohm resistors.

Possibly the designer just thought that running signal through a transformer magically made the circuit sound like a tube amp. 

[Vivek]

Or he felt he could use that for the Mojo factor "Transformers hand wound in Milwaukee to special secret specifications for that perfect warm analog sound"

[Steben]

Well. It's not rubbish. Just perhaps unnecessary.

[Vivek]

Clipped portion of the IVP Preamp Schematic:



What's to learn from the next stage after the transformer ?

Looks like a mini power amp, but we dont need any kind of power output from that stage

Wow, this is my 1000th post !!!

[PRR]

...I'm curious as to why the designer didn't just take the centre tap of the transformer primary and connect it to V_A.  That would save a couple of 1100 ohm resistors.  I feel there should be a capacitor across R37 just to keep the input signal off the far side of the differential pair.

One could think that resistor-coupling avoids DC through the transformer. (One could be wrong.)

Letting some signal leak-through the base network is OK because we need a very small differential signal and this will help some. And it keeps common-mode signal large which may flavor the distortion (or distort the flavor).

[PRR]

You guys who were born 20 years before me had all the fun with new discoveries, new schematics, new tracing, new ideas. I'm so jealous !

On a somber note: I was just reading a book written in my youth. A key character has died of paralytic Polio. Which you kids will never see. (One of the three types of Polio went extinct in 1999; the others are down in the dozens of cases instead of many thousands per year.) Rheumatic Fever and Whooping Cough were rampant when I was young and a friend has heart damage. Complicates medical and dental work. That friend has friends who had the whooping cough. 100 days of whoop-whoop, exhausting vomiting, breaking ribs.

Not to mention the unfiltered Camels we smoked around leaded gasoline ($0.34/gal) in cars without seatbelts. It seemed fun at the time.

[Steben]

Clipped portion of the IVP Preamp Schematic:



What's to learn from the next stage after the transformer ?

Looks like a mini power amp, but we dont need any kind of power output from that stage

Wow, this is my 1000th post !!!

It's everywhere in the circuit by the way, not just after the transformer.

[Vivek]

To add crossover distortion ?

[Steben]

To add crossover distortion ?

hardly, it is not designed to give a lot of that.

[Vivek]

to waste current so that the power supply sags ?

[Rob Strand]

The tube sound on the IVP preamps used to sound pretty good on bass.

Many bass cabs in that era were loaded with EV15L's which removed a lot of the fizzy high end.   With some of the cabs today you would probably want to add a low pass filter, somewhat more roll-off than anything from a small transformer.   Although shaving of above 9kHz never hurts anyway.

to waste current so that the power supply sags ?

You might be interested in the Lab series L5, L7 etc amps.   They fed the signal into the IABC pin of an OTA to get second order distortion.

Large schematic link at the bottom of the page,
http://www.netads.com/~meo/Guitar/Amps/Lab/schem.html