Peavey "Randomizer" Circuit

[CJ]

this is a house amp so we could open it up again,

 

[JohnRoberts]

email from James Brown! 

"Hi,
Tell John I said hey!

kool... I have not talked to him for a while...

I had him test out my GFCI drop with cap coupled ground lift in his personal guitar rig, because I really respect his ears/guitar chops... he blessed it so it can't be that bad.  8)

Well, my biggest contribution to the Stereo Chorus 212 was the SuperSat design and figuring out how to implement the digital reverb in a nice way….most of the Chorus circuit was already done on the previous Stereo Chorus 400, but I did tweak some things. 

James is co-inventor on the patent(s) along with Jack Sondermeyer for the "transtube" solid state mimic for tube overload. I don't know if this amp used transtube technology or not.

The circuit around U3A is an anti-aliasing filter, and was commonly used back in ‘the day’ on many bucket brigade delay and chorus pedals and circuits….it’s right out of the MN3007 spec sheet.  It’s basically just a 4 pole low pass filter(the spec sheet used a 3 pole), used to roll off any highs above the frequency that the bucket brigade is operating at, to avoid what we call aliasing.  So it’s not specifically designed to provide a phase shift, although it certainly does shift at the cutoff frequency and above.  Aliasing happens when you run higher frequency content through the bucket brigade than it’s maximum sample frequency, and you end up creating artifacts that are down in the audio range.  It’s hard to explain, but easy to hear when you run a distorted signal through there, because of course a square wave has a lot of high frequency content.  If you bend notes that are up high on the B and E strings, you can hear the aliased note sort of going down when you bend up.  It’s much worse on delay circuits, because the sound is delayed further and more distinctly heard. 

Eventually digital circuits got around this need by using what’s called oversampling, which means they sample at a frequency much higher than what they’ll be passing through.  That way the aliasing that does occur is way outside the audio range.  In hindsight, it would have been a good idea at the time to also put a similar filter driving the digital reverb in that Stereo Chorus, but the aliasing it does was only noticeable at high volumes with the presence on the lead channel cranked way up.  And on later digital products, we’d increased the sample rate way up so it wasn’t an issue any more.

Hope that helps,
James"

I won't pick nits with his mash up of sampling theory et al...  ;D

James was actually a college professor before being hired to work at Peavey in Sondermeyer's "analog group" designing guitar amps, his true love.

and be sure to visit

https://amptweaker.com/   

for all your pedal needs!

Seriously James is well respected for his guitar amp design chops and his pedal company seems to keep growing year after year.

I hear nothing but good stuff about his pedals.

JR

 

[JohnRoberts]

So, if it's just a basic one-opamp 4th-order LPF, the values on the schematic don't seem right. I suspect teh schemo has been doctored in order to provide enough info for troubleshooting, but enough errors to prevent copy.

Or maybe it is accurate... If the digital group did that part I wouldn't be too surprised, but no need to flog a dead horse.

If competitors were looking for Peavey tech to copy it wouldn't be the BBD delay....  ;D

JR

 

[PRR]

> this looks like a phase shift circuit

This may be the granddaddy patent on peak phase shifting:
http://www.bobgroome.us/1-Documents/SymmetraPeakPatent.pdf

This seeks mostly to get peaks symmetrical, but it also shifts peak energy on symmetric input.