Noise on mixer channels, please help!

[paulrichards7]

And #5 could indeed be the culprit. From the pics you posted, it's definitely 2 revisions of the module...the cap on the back was there because they didn't include the traces for it in the older revision...the layout and silkscreen differences give it away, as well.

So, try swapping the blue resistors with their counterparts from a noisy channel. The blue ones are metal film, which are less noisy than carbon. It's very suspicious that the module has 4 of them, I'm guessing in specific, strategic locations. What are the part #s for those resistors? Where in the circuit do they sit?
The metal film resistors are off of the mic input and off the phantom power

[The Scum]

So you say you've got an oscilloscope - have you learned to use it yet?

There are some good turotials on the web.

Then, once you know how to use it, set up one noisy channel, and one clean one, so they can be powered up side-by-side, and walk through the strip, probing the opamp outputs, and comparing channels. When they diverge, you'll be much closer to your noise.

[paulrichards7]

Hi, the latest
Ive replaced all TL072, All transitors and the diodes
Re-soldered every connection
And guess what?
Still noise
Anymore ideas
Thanks so much for all your help
Paul

[paulrichards7]

Hi,
Whilst comparing the 2 different channels, Ive realised that there are different resistors elswhere
These are the ones in the pics with red rings around them
http://i390.photobucket.com/albums/oo34 ... -green.jpg
http://i390.photobucket.com/albums/oo34 ... -schem.jpg
They seem a lot smaller than ordinary resistors
They are off of the auxes and the L/R and buss switches
I was wondering why these are different and if it had to do with noise issues?
Also seen in the pic with green rings around them are jumper wires.
However these are 0 ohm metal film resistors compared to the other channels which are just strips of wire
Would these pick up noise?
Thanks
Paul

[suppositron]

Generally, the smaller the resistor, the smaller the wattage rating. Meaning they are not meant to dissipate as much power as bigger resistors. I talked to an engineer I work with and he is no expert on audio circuits but very smart in analog and digital electronics. I think because of the nature of the noise you are hearing (static) it has to be a mechanical problem like with those switches on the schematic. He thought that it could come from a faulty low pass filter due to a bad capacitor, but I know you've already swapped a great deal of components. Really, you should look into any switches or the summing amplifier that adds the channels together at the stereo buss. If you didn't see a problem with individual channels it could be else where in the summing amplifier that ties the channels together. You can try removing the switches and just hard wiring in a short to make the connections to rule them out, or look at the circuitry involving the summing amplifier in the mixer.

[paulrichards7]

hey suppositron
thanks for that, i really appreciate it
right now, im just monitoring off each board individually
so there is no summing going on
but maybe the switches are a problem
it might just be easier to buy a few new ones and change em out to see if that does the trick
thanks again
paul

[suppositron]

Hey no problem. I can't believe how much trouble you've gone through with this. I would try swapping out those switches with the channels that aren't noisey!

[The Scum]

right now, im just monitoring off each board individually

Off the direct out, or insert point?

And exactly how are the resistors different between the boards?

[paulrichards7]

hey the scum
im monitoring either off the direct out or various places on the circuit with a crystal earpiece
the resistors are really small, probably about a 1/2 or 1/3 the size of normal ones
hope this helps
paul

[The Scum]

My opinion is that those resistors are a red herring. They're the buss feed resistors used in the auxes and stuff. If you have the noise at the direct out, then you're picking it up before them.

So tell me: if you bypass the EQ, patch into the channel insert, and listen to the insert send, are both channels similarly noisy?

Also, I believe you'd do you troubleshooting skills a HUGE favor if you took the time to learn to use your oscilloscope.

[paulrichards7]

hi the scum
yes the noise is the same on both channels, meaning direct out and insert send and return
I can use my oscilloscope to see the noise, but I dont understand how that benefits me when my earpiece can hear the noise anyway
What is it that i should be seeing that makes the scope so beneficial
Ive used it to see a tiny bit of noise on PSU, but scope is only 1 channel and only goes down to 30mv
Ive read all kinds of books on scopes, but cant seem to find or understand how to use them to locate the inception of noise
Thanks for all your help and patience
paul

[nclayton]

Hi, the latest
Ive replaced all TL072, All transitors and the diodes
Re-soldered every connection
And guess what?
Still noise
Anymore ideas
Thanks so much for all your help
Paul

Hi. So does this mean you haven't yet tried replacing the resistors in the mic preamp as Scum was suggesting?

I really think you should try this if you haven't. Bad resistors can definitely make a kind of intermittent crackly noise somewhat similar to what you posted. Usually resistors that get crackly/noisey are carbon comps. Yours are carbon films, and they tend to be better, but can be similar. The resistors in the mic preamp have to be very quiet since any noise they generate gets amplified by every amplifier in the strip.

Does the noise by any chance seem to get worse as you turn down the channel's input gain? If so, then I'd suspect the emitter resistors. If the noise gets louder as the gain goes up, I'd suspect the phantom power resistors. If the noise stays the same at all levels of gain (input gain only, not depending on fader position or anything else) then I'd suspect the collector resistors. In any case, all of these could stand to be replaced with good quality 1% metal films.

Ned

[The Scum]

yes the noise is the same on both channels, meaning direct out and insert send and return

So, wait a second. You're contradicting yourself, and I'm lost.

A while back, you said that one type of channel was noisy, and the other was not.

But now you say "the noise is the same on both channels"

Can you clarify?

I've been trying to help you pinpoint the source of the noise, but you keep getting 3 steps ahead of me. I've love to be able to tell you "look closer at the EQ/Preamp/fader buffer," but you aren't exactly allowing me to be Socratic about it.

I can use my oscilloscope to see the noise, but I dont understand how that benefits me when my earpiece can hear the noise anyway
What is it that i should be seeing that makes the scope so beneficial
Ive used it to see a tiny bit of noise on PSU, but scope is only 1 channel and only goes down to 30mv
Ive read all kinds of books on scopes, but cant seem to find or understand how to use them to locate the inception of noise

There are a couple of problems with the earpiece:
-I don't know a thing about it. I don't know how it behaves or the circuit behaves when you use it.
-There are phenomena that can result in noise that you can't hear...low frequency crap on the power supply, high frequency crap from oscillating opamps, etc. A scope sorts them out pretty quickly.

But a scope is a tool that rerquires some expertise on the part of the user - you need to know how to get it to synch to an incoming signal, and what the limits of it's resolution are. I'm currently sitting in my design lab where I've got 3 different scopes in arm's reach...each one has different strengths and weaknesses, and I'll pick which one to use based on the specifics of a particular problem. But I can't really teach you to drive your scope over the Internet.

Tektronix had a decent downloadable beginners guide. Above that, get a copy of Bob Pease's "Troubleshooting Analog Electronics." Bob is THE MAN, and he writes truth. He's also probably more in depth that you'll care to try to read.

In general, noise on the scope means that the trace gets wide, and you can't synch to it. With a functional channel and a noisy one, you should be able to probe the opamp outputs, and compare the channels.

But a scope is only one tool, and sometimes it doesn't telly you anything very helpful. a lot of times, using the scope just gets me closer to something that a careful visual inspection of the system would have revealed. A really precise eyeball is invaluable.

So, have you looked over the resistors on a noisy channel and a clean one, and compared that the values are all the same? And that they match the schems?

[paulrichards7]

hey nclayton
yes i have just recieved resistors but havent tried em yet

the scum
when you posted questio i was unsure whether you were enquiring about the 2 different channels or two output channels(meaning the direct out and the insert)
thats where the confusion was
as of noise on noisy channels
some are more noisy than others
and after tracing the circuit with my earpiece
http://www.maplin.co.uk/Module.aspx?ModuleNo=3256
the noise is in difeerent parts of channel
some are around the switches and some go right back to the transistors
after thursday i will try the resistors and relube switches and pots
then i will compare quiet and noisy channels on my scope and see if i cant figure out what your talking about
thanks once againg for all of your help and patience
paul