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Bought a new Samson C Valve..it adds hum when connected to my EMX312 Yamaha powered mixer. I've tried narrowing down the source. I unplugged its power supply, then unplugged my mic from back of C Valve, still hums. From my chair it looks like it would have to be the unit itself, or the short TRS cable from C Valve to PA.

Before adding the preamp, I had virtually no hum (that I can hear), which actually surprised me, considering I have a laptop with an additional video monitor for karaoke lyrics, a desktop PC connected to "rec out", and a couple of mics, all plugged into the Yamaha 312.

Any ideas or suggestions before I return this thing? I can just touch the case of the preamp with my hand and the hums gets worse. This is with NO power connected to the preamp, and NO mic plugged into the preamp. Just the TRS cable connecting the preamp to the PA.  :shock:

Thanks, Jim.

Increase in hum by touching indicates interference from flourescent lights or a neon transformer. With no power applied chances are you can ground the amp to the rack and get rid of it. (sounds like a lifted ground internal or in the cable) Is the trs cable using 1/4" plugs?

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Yep, you've got yourself an "antenna" now connected to your system......Make sure to try locating it farther away from computers and monitors first.

I hop you get it working cause I'm always looking for new outboard gear.

TRS is XLR on one end, and 1/4" balanced on the other.

Not in a "rack mount" config, so I'm not sure how to do that.

There ARE alot of 4' flor lights in the shop, but I've been able to eliminate virtually all hum so far by lifting grounds (using  to PC's, monitors...ect...( yea I know  :shock:  not the best resolve!)

Thanks....Jim

Hmmm...I may not be able to overcome the hum issue, in its current location. I don't have any alternatives as far as all my sound/PC equipment!  :(

Sure you can....just patch in a longer cable temp and move the pre a few feet away to see if it clears up....If it does, then you are half way there.

XLR to 1/4"?....you might have a swapped lead working against you. Re-eximine your pin assignments.

XLR on one end & 1/4" on the other?  Where is the 1/4" being plugged into?  Try a cable with XLR on both ends.

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In that case its just like powered speakers The tip of the (mono ? !/4") should go to 2  on the xlr back on the 1/4" the other lead other than the shield sholuld go to the ring (ground) It takes 2 conductor with a shield A single conductor will do the same thing on powered speakers Lifting the ground on them eliminates the hum also but other than the hum the impedance match is wrong also. If you can unscrew the !/4 "plug see if it has 2 conductors If not that is your problem.
Are you going into mic in on the mixer?

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Looks like he went outboard without any inserts on his mixer strips.....ALL MIXERS NEED INSERTS!, just wish they included them.

that means he connected his mic via XLR to the pre, then went 1/4" from the pre to XLR srtip input.....If that's the case he has an outboard pre inputting to his mixer strip preamp......Not good.

Lonnie have you hooked up any tube pre to a dfx12 or cfx12?

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I have........I connect my tube Art Pro Channel via insert jack......Works perfectly and silently.....it patches in before the channel pre and eq so there is no conflict.

Not all inserts are alike, some patch in before strip eq.

I do nt know why but on a dfx12 (i have checked the gain with real time ) and a 58 I have to turn the pre up tp 3 oclock well just about. I can hook up a cheap mic and run it lower. Bugs the hell out of me.

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You described the exact scenario! What do I need to do? Or is there a feasible fix?

Thanks, Jim.

You need a line level input have a local tech put a 10k resistor in series with the hot lead in the xlr and a 470 ohm to ground If you can do it its a $2 or less fix. (voltage divider)

Actually put the 10k in series to to pin 3 and 470 to ground on pin 3.

This knocks the line level back to mic input level A pure resistive net does nothing to freq response etc

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Output from outboard preamp is line level(I think, don't make me look it up).......so jack it into line input on mixer channel strip using plain 1/4" TRS cable from preamp to mixer...1/4" on each end.....Try that first.

....or does he need a DI box in there to convert from low Z to line?

No the reason I would go with the voltage divider is going into the input amp summing amp + & _  the impedance is not a factor.. In fact balanced audio does not have an impedance factor although this is not strictly balanced. And to think I used to charge $100 an hour for this Iwould have him going in 15 minutes.

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$100/hour?.....I do it fer a squeeze  or beer if they's a guy-type.

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JimW Wrote:





Ok Jim, you gotta gives us a little more to go on here. Where are you plugging it into, i know your mixer..but exactly where?  an effects loop, a channel insert, a channel mic imput????


Lonman Wrote:   he can't, the Samson C Valve, has only a 1/4" TRS "Tip/Ring/Sleeve output. No need to adapt it to make matters worse.

JimW. Let us know what's going on, and for heaven sakes, if you want to live to be a ripe old age Don't Lift Any Grounds Or Defeat Any

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