DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

I have two preamps racked in one case. They both work very well and sound as they should, but for some reason, one of them is quieter than the other. I measured the resistance for a few points with the Grayhill all the way down and the trim pot all the way up. I put the multimeter probes on the pads of the SC-1 so it can't be the cable I'm using to run from the SC-1s to the XLR and TRS connectors...

Between output pin 2 and input ground = 4.4 ohms for preamp #1

Between output pin 3 and input ground = 4.4 ohms for preamp #1

Between output pin 2 and input ground = 22.4 ohms for preamp #2

Between output pin 3 and input ground = 30+ish ohms for preamp #2


They both still sound great and work fine it's just annoying because I can't get full gain on one of them. What is a healthy resistance I should be reading? I'm guessing there is a resistor discrepancy somewhere that is causing this resistance drop and therefore volume discrepancy.

What do you think Owel? Thanks in advance.

Some things to try....

1. Maybe it's a cold solder joint somewhere?

2. Try swapping the 1510 chips. Did the problem moved to the other channel?

3. Try swapping the 1646 chips. Did the problem moved?

4. Try removing IC2 and remove the jumper JP1. Compare levels with each other.

5. Sure it's not somewhere down the signal chain? i.e. monitor volume?

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Okay, I THINK I got rid of the volume difference by getting rid of my cables that were connecting my inputs/outputs connectors to the SC-1s. I replaced it with new cable that I braided. Great! BUT, now the second channel has a lot of shhhhhhhhhhhhhhhhhh. The first channel sounds great but the second has a shhh noise. It's a loud noise that isn't so much a hum as much as a noise. Sounds similar to white noise, maybe RFI noise? The only thing I can think of is the second channel is the one with the transformer cables running over the SC-1 audio cables. Would this create that shhhh noise? The other thing I can think of is the XLR input's ground pad on the SC-1 got heated up a bunch of times because I tried replacing the ground cable to see if it would get rid of the noise. Maybe the noise is from the fact that the input isn't actually grounded because the conductive plating was stripped? Thanks so much,

Will

Generally, it's a good idea to keep the AC power lines away from the mic preamp input and output cables. ESPECIALLY the input XLR jacks and cables. Any tiny amount of noise coupled to the inputs (via magnetic or RF interference) will get amplified a lot when it enters the preamp. Your preamp can do 1000x, 2000x amplification of a tiny amount of signal (or for that matter, noise too).

See that your case and XLR jacks are grounded well too.

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Oh an important thing to say would have been that the noise is there regardless of how high the trim or gain knob is. Even all the way down... That has got to narrow it wayyy down. Any idea?

If the hiss is always present, regardless of the gain/trim setting on the preamp, then I would think the noise is not coming from the preamp board... but after the preamp board...

Wiring? i.e. from the output XLR going to your monitor/signal chain?

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Can't be the monitor chain... I plugged an SM57 into the first SC-1 and the output goes right to the first channel on my power amp which goes straight to my PA speakers. This sounds great. Then switched to the same SM57 and same XLR cable into the second channel of my SC-1s directly into channel one (the same channel) of my power amp into the same speakers. I suppose it does have to be the output wiring... I'll have to try and resolder those output points going to my TRS output jack. Thanks,

Will

Do you have a recording of the noise? Checking grounding on the XLR jacks.

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New update: When I turn the trim knob, far left creates loud noise. Far right creates same loud noise. The center creates a slightly less loud and less harsh loud noise...

There's many types of noise.. can you describe it? Is it a hum? A hiss? Can you email me a recording? Thanks.

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It sounds a lot like the first of the results, "TV STATIC."

http://www.soundsnap.com/search/audio/t ... atic/score

White noise, basically.

You didn't record that coming from the SC-1, right?

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No, certainly not. Sorry for not clarifying that. That's just a very, very similar sound.

Owel: just sent you an email of the noise. Thanks,
Will

Hi,
I am curious to know what the cause of the noise in this case came from.
Corgan4321, did you finally solve your noise problem? Owel, after listening to the noise, what did you think of it.

The reason I am asking is that I am also having similar (white noise) problem with my SC-1 and I am trying to debug it based on several suggestions on old posts (i.e checking solder joints, grounding XLR jacks, and checking for any short circuit.)

Thank you very much,
Nirand