Noisy JCM800 4010

[ricothetroll]

Hi !
I recently built a 19" rack unit with 5 channel of these, to work as an analog interface with Ableton Live (for looping purposes) :
http://img855.imageshack.us/img855/2692/livelooperinterfacesche.png

It works well ! It's pretty quiet on my Fender Hot Rod Deluxe and also on my bass player's Ashdown ABM 500 EVO III Head. But on my other guitarist's Marshall JCM800 4010 (50W combo), it's very noisy ! We switched between with and without the module I built and there's a great noise contribution of it. By the way, everything plugged into that amp seems to have a greater noise contribution than in any other amps. While the amp is quiet when nothing is plugged in, even a cable plugged in it makes a slight but still hearable noise ! Also, a - yet noisy - FV-1 unit I built turns out very noisy on the Marshall (also with TL072 in/out).

The amp's schematic is here :
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/marshallamps/jcm800_50w_4010.pdf

He told me that the amp was stocked in pretty bad conditions for a while (maybe freezing temperatures) and it is about 13 years old (compared to my 40 years old Ampeg B25B, that's pretty quiet and never been recapped, that seems pretty young !).

I guess some parts inside got old or damaged, but I'm not a tube amps specialist (sometimes I wonder what specialist I am  ). Any help would be appreciated.

Best regards.

Eric

[Thomeeque]

 Hi Eric,

Were "those" designed by you or is it some proven design? I don't understand, why

1. is C11||R2 grounded to GNDA (isn't goal of GND vs. GNDA to keep them disconnected DC-wise?)

and

2. is C10 value so small (compared to C7)

I have no idea if there is any connection with JCM noise issue, it just puzzles me..

Cheers, T.

[drewl]

What kind of noise?
60 or 120hz hum?
Hiss?

The Marshall is a higher gain amp then the other two you mentioned, that could be causing problems.

[mremic01]

I have a 2010 that does something similar. The hum doesn't get louder when the volume is turned up. I never had a problem with it at band level volumes, but when playing in my bedroom it's just kind of always there and it's annoying, but not really all that bad.

[ricothetroll]

Hi,
Thanks a lot for your answers !

What kind of noise?
60 or 120hz hum?
Hiss?

What I have here is mainly hiss (with also very slight/hardly noticeable hum).

Were "those" designed by you or is it some proven design? I don't understand, why

1. is C11||R2 grounded to GNDA (isn't goal of GND vs. GNDA to keep them disconnected DC-wise?)

and

2. is C10 value so small (compared to C7)

The design is my own actually, but I've been given great help by electronic guru John Roberts at Prodigy Pro Forum. Here is the link with the discussions about it :
http://www.prodigy-pro.com/diy/index.php?topic=43873.msg547787#msg547787

1. From JR's post at PP (a much better explanation than what I could give  ) :

Many opamp circuits can be configured as "differential" amplifiers. What that means is the amplifier can literally subtract two signals from each other and amplify the "difference"...  In your application, we have two different powered chassis. The guitar amp, and the send/return, so these two local grounds will be different. A fact of life.

Ideally we want to compute the difference between the ground on the send return board and the ground at the guitar amp input, and add (or subtract) that difference from the signal at the send/return output, so the signal arriving at the guitar amp is is just pure signal, relative to the guitar amp ground.

This will work even without a hundred ohm, (or whatever value) between the guitar amp ground and send return ground, but this compliance will keep stray currents from flowing between the two grounds if both connect back to safety grounds, and/or have stray rouge currents trying to find the easiest path to earth ground.

The signals being applied to the differential amp all are important. At the plus input with three equal value 3.3k resistors we are getting +1/3rd signal A, +1/3rd signal B, and +1/3rd guitar amp ground. The actual gain of these at the output of the differential amp depends on the feedback values too. The general equation for gain applied to the plus input is  1+(Rf/Rg), where Rf is resistor from output to - input, and Rg is resistor from - input to ground.  So with Rf= 3.3k and Rg= 1.65k, the gain for the three signals applied to the plus side is gain A= 1/3 x (1 + 2)= 1
Gain B= 1/3 x 3= 1
and gain for guitar amp ground is likewise = 1/3 x 3= 1, so the guitar amp ground is present in the output at 1:1. Note: this ground is not added but present at unity so ignored by the guitar amp input that is sitting at that same potential.

The gain for the send/return ground, coming into the - input is simply -Rf/Rg or -2x. So - 2 times the send return local ground voltage. The good news is the Signal A and Signal B, that are also in that output each contain +1x the send return ground each.

So the total result in the output is

+!x signal A (with +1x send/return ground)
+1x signal B (with another 1x send/return ground so +2x send return ground total)
+1x guitar amp ground
-2x send/return ground.
--------------------------
What is left is 1x signal A + 1x signal B, all relative to guitar amp ground, with send/return ground subtracted out.

The actual values of the resistors used don't matter but the ratios need to be correct, so 3.3k resistors can be any values, but all need to be the same value.  Rf and Rg can be any value as long as Rg=Rf/2

This circuit may not seem obvious but this simple differential amp, with it's ability to forward or back reference signals between different local  grounds is the glue that makes consoles and many products possible (without needing transformers everywhere).

JR

2. C7 is the output coupling capacitor. R8/C10 is a compliance between the guitar amp's ground and my local ground (as seen in many ground lift circuits, for RF reasons).

Best regards.

Eric