Cabsims... and even more cabsims

Stompboxes circuits published in magazines, books or on DIY electronics websites.
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george giblet
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Post by george giblet »

Changed 510R to 1k, replaced 470k with 249k. Now, the oscillation apperars on power-on and fades out after 2 or 3 seconds (most of the times :) ).
That means we are on the right track but haven't quite got there yet.
Maybe I should try the same mod for the next gyrator - lower the 470k value to 220k and increase 4n7 to 10n.
I have this circuit in lstpice now. From what I can see modification of he gyrator values takes around 1dB from the notch.
My apologies. I checked everything except that because I didn't think it would have an effect since 470k is so much higher than the 10k (near the 3.6k). I tried to fix it but then I realized those gyrators are not working like you would expect. The 470k appears across the inductor not across the resonant circuit. That has a large impact on the notch. The resonant circuit looks like the minimum impedance is 1020 ohms however the parallel resistor messes things up and it's only going down to 2.7k. When we change the 470k to 220k it misbehaves even more, the minimum impedance only gets down to 4.5k.

The high Q of the resonator is what is causing the problem. I can't see an easy solution which will stop the oscillation and not affect the circuit.

One idea is to change the type of gyrator. The idea is to change from the gyrator on the left hand side to the one on the right.

https://sound-au.com/articles/gyrator-f6.gif

Ignore the Rs part.

The mods would be, for the existing cab sim circuit
- short the top 510 ohm resistor
- keep the 10n cap
- Replace the existing gyrator 510 ohm, 33n, 470k with the gyrator on the right hand side in the link.
- Use part values:
R1 = 1.5k, R2 = 1.2k, C1 = 4.35µ

Unfortunately we have to use a larger cap with a crazy value. 2 x 2.2uF in parallel would do.

From what I can see there should be no change in the notch depth and that circuit should be a little less susceptible to oscillation. It's better but I still don't know if it is good enough. The only option I can see is to reduce the Q of the resonant circuit. That will increase the 10nF cap which then helps the stability.

Not so easy to solve!

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temol
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Post by temol »

I'll try to incorporate those modifications and report here.

Thanks!

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george giblet
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Post by george giblet »

It's not clear from those gyrator schematics but the grounds need to be connected to Vref not ground.

Something which might also help the problem is large caps across Vref and across the power rail.

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temol
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Post by temol »

Original circiut uses bipolar supply so GND to Vref is obvious for me. There's already 100u across power rail and 47u for the Vref.

IMG_20210214_143826_1.jpg

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george giblet
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Post by george giblet »

I had some more thoughts about the gyrator oscillation issue. The ADA microcab uses a different form
of gyrator. I derived the design equations and played around with it. This mod has the best gain margin
for the opamp out of all the different methods.

Please plug it into your spice simulator. The response should be extremely close to the original.
Gyrator Stability Mod V11.png

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george giblet
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Post by george giblet »

Something I'd like to add to the thread about the ADA varicab.

We see the ADA varicab posted a lot as a cab sim. When we look at the response it always looks weird.
The circuit also looks weird, notably there is an absence of low and high-pass filters.

After researching this a number of times it finally clicked that the ADA varicab is *not* a cab sim.

The ADA varicab feature appeared on the ADA Cobra and ADA Viper amplifiers. The amps are combos.
I believe the varicab EQ modifies the sound of the amp + speaker as a whole to sound like it has
different speakers. The line-out is not intended to be recorded. You record a mic'd speaker.

That explains why there's no low and high-pass filter. The idea relies on the existing speaker
to provide some of the response shaping.

From what I could find out:
Viper: One 12-inch Celestion G12-T75 driver. Open back cabinet.
Cobra: Two 10-inch ADA Custom British Classic drivers. Open back cabinet?

On the other hand, the ADA microcab *is* a cab sim.

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temol
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Post by temol »

Thank you very much for your help george.

I have to digest it all :)
I was hoping for some non-invasive corrections, but it looks like it will be necessary to make a new PCB in the end. But first, some breadboard tests.
Btw.. I'm almost sure that I've tested 2410 on a breadboard before making a PCB and haven't noticed any issues.

You are probably right about ADA Varicab. There's no evidence of a stand alone unit.
I've just fonund Cobra manual, and there's a diagram with a Varicab module.
varicab.jpg
varicab.jpg (8.58 KiB) Viewed 422 times

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george giblet
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Post by george giblet »

I was hoping for some non-invasive corrections, but it looks like it will be necessary to make a new PCB in the end. But first, some breadboard tests.
Btw.. I'm almost sure that I've tested 2410 on a breadboard before making a PCB and haven't noticed any issues.
Some circuits ride on a thin line from oscillation and it only takes one minor issue to push it over the edge.

The least invasive mod is reduce to the Q of the resonant circuit. That would mean only part value changes. It's actually a good thing to try as it lets you know you can at least stop the oscillation. The question is how far to back-off the Q before the oscillation stops. Then the question is when that is done can we tell the difference compared to the original circuit.

It's possible there's some other problem and messing with the gyator is just helping to hide that problem. On the other side of the coin, there is a reason why the gyator is high on the list. We know the oscillation is at the same frequency as that band. An oscillator is just a high Q resonator. When I look at the gain margin, a thing which indicates how stable something is, the original circuit is something like 0.03dB. The last gyrator mod has about 0.14dB gain margin. Normally we would target a 10dB or more gain margin which would be unrealistic for a circuit like this one.

FWIW, currently I've been trying to duplicate your cabsim results using LTSpice.

I've set-up the system like this,

Take wav file for non-cabsim signal (like the first sample in your sample sets). Use that as input into the cab sim circuit in LT spice. Apply an input or output gain factor to adjust the level. You need to make sure the cab sim circuit isn't clipped by the inputs signal level. Write the output waveform to a wavefile using LTspice. From that I can get a set of wave files to listen an compare. It's good because the input is exactly the same. The only tricky thing is adjusting the level.

That method would be a good way to see if lowering the Q makes a difference to the tone.

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