Inductor/Cap Mid Range Boost in Vox amps

[mth5044]

Looking at the VOX Super Beatle amp's tone stack on the Brilliant channel (might be huge):


http://elektrotanya.com/vox_super_beatle_v1141.pdf/download.html

Focusing on the MRB part. The frequencies boosted on the three different settings are reported to be 750, 600 and 450Hz. With the mid boost on and using the 0.5H and switch caps, it looks like the frequencies are 1038, 711 and 480Hz. Perhaps 480Hz is close enough to 450, but the 600 to 711 and 750 to 1038 don't seem right. Does the addition of R124 and the additional tone controls play part in determining the frequency?

Seems the same thing is in the MKI Vox Supreme (and probably the others in this line) ~2/3 down on this page:

http://www.backfromthesixties.co.uk/vox_supreme

4th schematic. In the 3rd MKIII preamp it is set up a bit differently, but with the same cap values.

What's the dealio? Also, has anyone played such an amp? Does the MRB actually do anything you'd want to play with? Seems strange as it's not even paired with the distortion.

[mth5044]

How do you determine how much the frequency is boosted? I had read that these amps provide 6db boost at the frequencies. I feel like the word resonance is going to be key.

[teemuk]

Here's a simulated frequency response of that HUGE schematic (people, please learn to resize images).


The "peaky" curves plot circuit's response with those three different capacitance values, the other curves in pretty much the same group plot the frequency with the Mid-Range Boost feature off.

I'm not entirely sure about potentiometer tapers but overall they are not really that important since they don't affect the response peaks, just the magnitude of low and high frequency parts. Dialing different settings from both controls "at noon" would just as well. You get the overall idea from the graph. As you might have guessed already the circuit is passive so any "boosting" is derived from attenuating all other frequencies.

So how much boost is there? The answer is that it's actually quite hard to determine. The non-mid-range-boosted response of that circuit is already radically non-linear with a huge mid-range notch and the response is also widely interacting with tone control settings. Naturally.

All in all, There really isn't a solid line of reference to evaluate how much gain those resonant peaks have. 6 dB? In some sense maybe yes. That would be somewhat close if we started from the assumption that mid-range response was rather linear or that we merely focused on amplitude of those resonant peaks alone. But it really doesn't work like that in practice so in another sense 6 dB would be ridiculously too low figure and about 28 dB would be more in the correct ballpark.

BTW, the resonant frequencies seem to be about 500 Hz, 700 Hz and 1kHz. Since the scale is closer to hundreds of Hz few Hz deviations here or there are really not that important, IMO. I'm pretty sure the component tolerances alone will cause that much "scatter".

[mth5044]

Thanks for the charts! You mention of the huge schematic (how does one resize an image hosted by someone else?) - is the simulation just the tone section from the treble to C118?

Your comments make a lot of sense and are quite helpful. I've begun to look up ways to do this with opamps for more precision, rather than emulating what an amp did, just to see what kind of differences in sounds are found.

Thanks again!

[Lurco]

For resizing, download the image to your computer, and open it with Irfanview. http://www.irfanview.com/

[bluebunny]

how does one resize an image hosted by someone else?

When you insert the image in your post, you get these tags: [img][/img].  Change the first tag to look something like this: [img height=150].  F'rinstance:

[mth5044]

For resizing, download the image to your computer, and open it with Irfanview. http://www.irfanview.com/

Impossible on work computer, but...

When you insert the image in your post, you get these tags: [img][/img].  Change the first tag to look something like this: [img height=150].  F'rinstance:

That's what I was looking for! Danke both.

[R.G.]

What's the dealio?

It's a neat effect. There were a number of 60s records with this as a noticeable effect.

Also, has anyone played such an amp?

I own three amps with this efffect. Yes, I have.

Does the MRB actually do anything you'd want to play with?

Yes. It's not a mainstream effect these days, but it has its uses.

Seems strange as it's not even paired with the distortion.

A lot of the "architecture" of the Thomas Organ Vox amps is odd. Still, it's interesting all on its own.

The Small Bear "Wild Mouse" replicates this almost exactly in a footpedal, by the way.