CJ

Peavey "Randomizer" Circuit
« on: April 11, 2019, 05:41:04 PM »
ran across this while trying to debug a noisy channel on a Peavey Stereo Chorus 212 (rf fuzz on left channel)

this looks like a phase shift circuit, thought it might be involved in effects processing, but it looks like it was designed to add some funk to the overdrive wave form,

here is the waveform before and after the "randomizer" circuit

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


CJ

Re: Peavey "Randomizer" Circuit
« Reply #1 on: April 11, 2019, 05:42:11 PM »
here is the circuit, waveforms taken from A and B

that is all i have at the moment.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey "Randomizer" Circuit
« Reply #2 on: April 11, 2019, 06:15:13 PM »
and then there is a companderer after that,
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey "Randomizer" Circuit
« Reply #3 on: April 11, 2019, 06:28:20 PM »
i give up.

looks like algorithm for black hole.  :D
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Re: Peavey "Randomizer" Circuit
« Reply #4 on: April 11, 2019, 07:07:41 PM »
Jeez a black hole, imagine getting pulled  into one of them , even your crys for help on the radio wouldnt escape the vortex.

What does the random  fuzzler sound like anyway ,anything of note or just bland transistor waffle ?

From what I gather this new space observation seems to show  the genious of Einsteins work as more relevant than ever , Im still not sure knowing how the universe works really matters to most people in the grind  everyday life though .   

PRR

Re: Peavey "Randomizer" Circuit
« Reply #5 on: April 11, 2019, 11:31:42 PM »
The "companderer" is about the BBD after this.

The "randomizer" has a large phase-shift but sorta-flat amplitude response.

These appeared in broadcast audio processors. An organic "peak" may be tall and narrow. The transmitter clips (splatts) on the peak amplitude. The ear hears total power, not peak amplitude. Shifting the phase of the several partials of the peak spreads it out, more modulation before clipping.

Here it *only* drives the BBD, which is likewise a peak-limited channel (even with compansion). So I guess Peavey thought it worth a buck of parts for a few dB higher level through the BBD. Oddly I have not seen this on any other BBD box.

Re: Peavey "Randomizer" Circuit
« Reply #6 on: April 12, 2019, 12:42:17 AM »
There was a few of the higher end rack mounted boss chorus with comp /expand pre post bbd ,CE-300 I think did it, the earlier version DD-3's I think had it too ,Philips chip I think for the compression ,but that was digital delay . NE570 I think

PRR

Re: Peavey "Randomizer" Circuit
« Reply #7 on: April 12, 2019, 03:19:56 PM »
Oh, compansion was popular enough on high-end BBDs.

I have never seen a phase-twister in that application.

CJ

Re: Peavey "Randomizer" Circuit
« Reply #8 on: April 12, 2019, 03:24:46 PM »
well since we did not understand the circuit, the only choice left to fix the noisy channel was a can of de-oxit,

viola! no more voice coil rubbing sound out of the left channel.

reverb got stronger, chorus started working again and no noise the amp sounds wonderful, take the preamp outs and plug it into a tube pwr amp and you would have a nice sounding system,

red arrows where we sprayed, i think the 16 pin 072 was the main offender.

now if we could just get China to start making 5 dollar de-oxit...

thanks for the info PRR!

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey "Randomizer" Circuit
« Reply #9 on: April 12, 2019, 03:30:32 PM »
maybe that phase shifter is part of the chorus,

here is the total schematic>

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


CJ

Re: Peavey "Randomizer" Circuit
« Reply #10 on: April 12, 2019, 03:37:53 PM »
Peavey has the est wave soldering i have seen in an amp, who ever set up the tanks knew what they were doing,
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

JohnRoberts

Re: Peavey "Randomizer" Circuit
« Reply #11 on: April 12, 2019, 04:30:19 PM »
Peavey has the est wave soldering i have seen in an amp, who ever set up the tanks knew what they were doing,
Thats an interesting compliment? I don't know what tanks you are thinking of but there may be a trim for height of the wave... too high and liquid solder can spill on top of a sagging PCB making a serious mess.. Boards like that go through the wave the narrow way (less sag) and sometimes wide boards get stiffeners clipped on to prevent/reduce sag...

The PCB design has a lot to do with solder integrity as well as several other process related factors (solder temperature, composition, flux, board preheaters, feed speed, etc). High volume layouts can even factor in direction of board movement through the wave to reduce solder bridges that require expensive manual rework.

I notice that a bunch of traces in the power amp section do not have solder mask on them... the extra solder is left there on purpose to reduce trace resistance. Doing that to the entire board would waste solder that cost money, but widely done on Peavey power amps (cheaper than heavier copper). 

I am pretty sure there is 100% inspection and hand touch up on boards with that many parts before going on to final assembly (that may be in different buildings). The wave soldering and machine assembly area was in Decatur (near where I still live) and the guitar amp was likely final assembled on a production line in Meridian (20+ miles away).

I was surprised to learn that Peavey put a BBD inside a guitar amp... (they didn't even call me...  :o). The Adverb chips from Peavey's digital group is less surprising to see.  I won't speculate how they sounded but replacing springs was a long time desire of Hartley's. I was saddled with some similar digital efx in powered mixers from my group and we made them work.  ::)

One trick I did was to take the digital overload output designed to light an LED so the operator would reduce the input level (probably some internal digital accumulator overflow flag??). Instead of lighting an LED I used the overload line to trigger a JFET shunt limiter, reducing the input level but only as needed. The different reverb algorithms had widely varying headroom so this helped a bunch and was pretty much invisible to the end users.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

PRR

Re: Peavey "Randomizer" Circuit
« Reply #12 on: April 12, 2019, 06:48:05 PM »
>> part of the chorus,
>> here is the total schematic
> surprised to learn that Peavey put a BBD inside a guitar amp...


He pulled the whole wish-list for this thing. The star of the show is a digital reverb with as much DRAM as an Apple ][ (of a decade earlier). Maybe the BBD _is_ just a Chorus.

Ah, this is intended to one-up the R-brand "Jazz Chorus". Similar power, stereo, spacey. But made in USA and maybe at competitive price/feature point.

CJ

Re: Peavey "Randomizer" Circuit
« Reply #13 on: April 12, 2019, 08:09:37 PM »
yes the reverb is amazing, and there are 16 presets )A through P check my math)  which include slap back and long exchoplex type delays, combine this with the stereo chorus and overdrive and you need no stomp boxes for most hick bars.  steel players like this amp too. we are the good ol boys.  :D
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

abbey road d enfer

Re: Peavey "Randomizer" Circuit
« Reply #14 on: April 14, 2019, 07:41:28 PM »
here is the circuit, waveforms taken from A and B

that is all i have at the moment.
This part of the circuit has a rising frequency response starting at 250Hz, ciulminating with a 16dB peak at 6.2kHz. The interesting point is the NFB circuit shifts phase more than 180°, so it is applied to the non-inverting input!
Unless there's a mistake in the schemo...
EDIT: LTspice simulation led me to believe that, but it's wrong, well, actually, the schemo is wrong, with the + and - inputs of the second opamp cross-reversed. But the frequency response is correct.
« Last Edit: April 15, 2019, 11:15:30 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

JohnRoberts

Re: Peavey "Randomizer" Circuit
« Reply #15 on: April 15, 2019, 10:48:54 AM »
i give up.

looks like algorithm for black hole.  :D
I have been trying not to look at this, and the schematic is cut off in the middle of the noise reduction expander section but I do not believe R13 is a 100 ohm R to ground from pin 2 of 572 (expander side).  :o

Any R value there would reduce low level output and act like a downward expander/gate, but 100 ohms to ground there would pretty much shut it off...  I can't see the fast attack cap (pin 4) that might support some sound output but not much sustain... with 100 ohm discharging 10uF. Normally I would expect a 1uF fast attack cap at pin 4 to mirror the compressor, and if anything a much higher value R to ground at pin 2.

It is generally useful in BBD delay designs to filter out clock frequency from the audio, when sending that filtered audio into a 1:2 expander the filter rolloff will get expanded 1:2 also so perhaps they used extreme pre-emphasis before the BBD to compensate, but BBDs are notorious for limited headroom (I never used a 3007 in one of my designs).  I've used as much as 12 dB of HF pre/de-emphasis in a BBD path to improve apparent S/N. HF boost before the BBD, symmetrical HF cut after, which has the added benefit of attenuating clock component.

JR

Visit https://circularscience.com to hear what properly "cleared" drums sound like.

CJ

Re: Peavey "Randomizer" Circuit
« Reply #16 on: April 15, 2019, 04:49:13 PM »
thanks for all the posts!

John, there is a full schemo pdf upstream about 6 posts,

looks like James Brown checked it, i might drop him an email and see if he has anything to add,
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

JohnRoberts

Re: Peavey "Randomizer" Circuit
« Reply #17 on: April 15, 2019, 05:30:07 PM »
thanks for all the posts!

John, there is a full schemo pdf upstream about 6 posts,

looks like James Brown checked it, i might drop him an email and see if he has anything to add,
Funny that full schematic has a 100k R130, not a 100 ohm R13   :o  :o  So that would actually work as a noise gate to cut off low level poop.

If you talk to James at Amptweaker tell him I said yo.... 

We have done multiple wrist curls together.  8)

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

CJ

Re: Peavey "Randomizer" Circuit
« Reply #18 on: April 16, 2019, 03:21:06 PM »
email from James Brown!  :D

"Hi,
Tell John I said hey!

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. 

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"



and be sure to visit

https://amptweaker.com/     

for all your pedal needs!
« Last Edit: April 16, 2019, 03:33:35 PM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

abbey road d enfer

Re: Peavey "Randomizer" Circuit
« Reply #19 on: April 16, 2019, 03:33:14 PM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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