output from rcas preamp problems [Archive] - Audio & Video Forums


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03-01-2008, 07:07 PM
The potentiometer on my preamp is getting scratchy, and the source selection is getting a little bit worn as well. I would like to get a replacement for it, but it has a strange setup for the outputs that match my amps.

On the back of the preamp there are two rca's for each channel that go to the amps.

One is marked with a plus, the other a minus and an indication that the minus is used only for "balanced" operation.

The two ways to use the preamp are as follows:
If you have only one stereo amp you use the rca's marked with a plus for each of the respective channels and hook up the speaker cable as a normal +-left +-right

if you have two stereo amps that you would like to use as monoblocks you use a positive and a negative rca's for each amp, so left channel gets +rca to the right rca on the amp and the -rca goes to the left rca on the amp.. then the speaker for the left channel is wired ++left

The question is what would I need to do in the wiring of an interconnect to get this to work with either balanced connectors, or with a single rca.. next question have you ever seen anything like this before, and do you know of any preamps that are still made that use this setup?

The preamp is a Kinergetics KPA-2; both amps are KBA-75's

A diagram is attached:

03-02-2008, 04:38 PM
more information... I tried to hook it up with a y-cable.. figure that worst case it would blow up.. instead it did nothing. no sound.

then I said well hell lets hook it up backwards and see if I can get anything to happen.. it played, but sounded very hollow and kind of freaky.. I still have no idea what that second cable is, I'm thinking that it could be another negative and a chassis ground, like the sheilding on ballanced cables.. but I'm not sure and I'm not ready to ground out an RCA, as that usually involves very loud pops and with a class A amp it would more than likely going to pop the speakers or the amp. I guess it's time to take the preamp to someone who knows what they are looking at under the hood or get out the multimeter and see if any of them are grounded.

03-07-2008, 09:15 AM
In all my years of audio i have never seen that.

I could suggest you clean the pot on the pre amp (using contact clean/lube available at Rat Shack)...

03-07-2008, 06:49 PM
Thanks for the reply, I didn't think anyone was going to offer any ideas.. I'll check out some repair options, although the pot is very sealed from what I can see.

the preamp is a strange beast.. really a dual mono setup, these Kinergetics guys went overboard with the idea of channel sepperation - check out this photo:

Glen B
03-07-2008, 07:03 PM
What you have appears to be balanced output signals on RCA sockets versus XLR. I've seen less than a handful of amps with balanced inputs on RCAs. The matching Kinegetics amp probably was like that. With a single-ended amp, use a single RCA cable from the right and left + outputs to preamp to the amp's inputs. To use the preamp with an amp that has XLR inputs, you would have to wire up an adapter cable that has the + and - signals from the preamp going to pins 2 and 3 respectively of the XLR connector. Pin #1 is ground.

03-07-2008, 07:10 PM
wife just left with the camera, so I can't upload another photo tonight.. but I can assure you that on the rear on the amp (also a kinergetics) it notes that the left channel RCA is for the negative signal from the preamp if in balanced mode.

If this is a balanced connection without the usual connector, what signals do you think are going over the negative RCA line output, so incase I ditch the preamp?

Glen B
03-07-2008, 07:38 PM
A balanced signal has + and -, also called hot and cold. With your preamp the positive RCA carries the positive half of the signal and the negative RCA carries the negative half of the signal. Balanced connections provide the benefit of cancelling out noise pickup, usually for long cable runs, however the amp should work just fine in single-ended mode (i.e. signal to the + input only).

03-07-2008, 07:47 PM
so, if I disconnect the negative it should still work as hooked up(mono blocked), I just lose the benifits of a balanced connection, which should be pretty minimal considering it's a 1 meter cable.

on the off chance I pickup a preamp with banalced connectors it should be hooked up like this?

positive on the center pin/ negative sheilding on the right RCA
negative for the center pin/ground for the sheild on the left channel

03-07-2008, 07:56 PM
heads up, I just tried the right channel only in monoblock config.. it makes more of a pop when starting and the back ground has some hiss - it was pitch black before, so it's not a lot of difference.. it plays, and at full volume (I was expecting some drop).

Thanks a ton for helping me out, as you may know kinergetics folded about 5 years ago, so we are left to the charity of others for support, so thanks again.

Glen B
03-09-2008, 01:09 PM
If by "a pop when starting" you mean when the amp is turned on, this could be an entirely different matter unrelated to hookup. A small pop or thump at turn on could be normal for the amp or it could be due to DC offset. Bias and DC offset can drift in old amps; any significant DC offset appearing at the output can cause a pop or thump at turn on and also contribute to distortion.

DC offset can be checked with a digital multimeter (DMM) set to the DC millivolt range. With speakers disconnected and no input signal, place the probes of the DMM across the outputs. Anything between 0 and 20mV is great. Anything greater, up to a few hundred mV is cause for concern and you may want to consider adjusting it to zero if the amp has the means. Some amps don't have adjustment means in which case the solution is to replace the input differential pair transistors. The amp will thank you with better sound..

03-09-2008, 08:46 PM
I'll look into this, but I already figured out how to wire up a fake balanced connection using a y-cable and that took care of the pop and hiss..

I just put negative to both the positive and negative pins of the second rca.. works great.

03-13-2008, 09:25 PM
XLR to 2x RCA

A 3-pin XLR with a stereo signal can be split into left and right by wiring pin 2 of the XLR to the tip of one RCA plug, and pin 3 of the XLR to another RCA tip. Pin 1 of the XLR connects to the sleeve of both RCA plugs.


03-15-2008, 10:35 PM
Thanks, I can use this info for sure.

03-16-2008, 01:37 PM
XLR to 2x RCA


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