What would cause one channel of an amp to drop in output?

[Mind Riot]

Hi all. Here's my situation:

 

I've been using a Carvin DCM150 to power my studio monitors for a few years. Recently, I got back into playing live, so I pressed it into service to power my guitar cab. This was a 4x12 with three matching speakers and one mismatched one, long story.

 

I was running Channel 1 into a 5.3 ohm load (three 16 ohm speakers in parallel) and Channel 2 into an 8 ohm load (one 8 ohm speaker by itself). Both ohm loads are safe for this amp. The three matched speakers are very efficient, and Channel 1 ended up carrying most of the volume. I had it pushed to about 3/4, whereas Channel 2 never really got ran past half.

 

I got a replacement that matches the three speakers today, so I wired up the cab to be 8 ohm stereo. I tested it out with the amp, and it seems like one side is much quieter than the other.

 

I quadruple checked the wiring in the cab, and it's correct. I switched out the speaker cables, only running one channel at a time through the same speakers and the same cable. So I'm sure it's not a bad cable or mismatched ohms.

 

Channel 2 on the amp, the one I barely pushed, seems about half as loud at the same settings as Channel 1. There's a slight amount of hash or distortion on that channel that's sometimes there, sometimes not, and isn't audible when playing something through it. The sound isn't cutting out or badly distorting, it's just about half as much output as the other channel, all else being equal.

 

It's never done this before now. When I'm playing through my guitar cab, I put the amp on two paperback books on top of the cab so air can move underneath it, and I run an indoor fan directly behind the rig.

 

Any of you power amp gurus have any enlightenment for me? I appreciate any thoughts!

[agedhorse]

Double-chech to see that if it has a bridged mono switch, it's NOT in the bridged mono position. Depending on how the source is derived for the inverting channel, there can be a substantial difference in level. (in an ideal world this is indicitave of a poor design).

[Mind Riot]

Originally posted by agedhorse Double-chech to see that if it has a bridged mono switch, it's NOT in the bridged mono position. Depending on how the source is derived for the inverting channel, there can be a substantial difference in level. (in an ideal world this is indicitave of a poor design).

 

It does have such a switch, but it's not engaged. I engaged it just to be sure, and it seems to work fine, as do all the other switches, but the output difference remains when it's run in normal stereo mode.

[agedhorse]

Looks like it may be time for a shop service then...

 

Double check that one input is not unbalanced while the other is balanced. For example, if you are using balanced XLR inputs and one cable was wired with either pin 2 or 3 jumpered to pin 1, you will experience a 6dB loss of signal level.

 

Same applies to a 1/4" tip sleeve and tip-ring-sleeve cable driven from a balanced (or impedance compensated) source.

 

Check to see that the input cables are really identical in config.

[Mind Riot]

Originally posted by agedhorse Looks like it may be time for a shop service then... Double check that one input is not unbalanced while the other is balanced. For example, if you are using balanced XLR inputs and one cable was wired with either pin 2 or 3 jumpered to pin 1, you will experience a 6dB loss of signal level. Same applies to a 1/4" tip sleeve and tip-ring-sleeve cable driven from a balanced (or impedance compensated) source. Check to see that the input cables are really identical in config.

 

I'm using 1/4" TRS cables to run from my PODxt to the amp. They're Belden cable with Neutrik connectors that I wired up myself, I make my own cables.

 

To be sure, I paralleled the inputs using only one cable, same deal. I also hooked the amp back up to my recorder and studio monitors so that every connection was different; different patch cables running into it, and bare wire connected to the binding posts on the amp and monitors. Same deal.

 

I can't really think of any other factors that could be at work. I guess I'll have to look into getting it fixed. I hope I can find a shop that can do it around here, I live in a pretty rural area.

 

Well, thanks very much for your help, at least we've eliminated all the possible human errors that could have been at work. I shot off a message to Carvin, and I got a response with a phone number and the names of a couple of their techs, maybe I'll see what they have to say.

 

Thanks again.

[Mind Riot]

UPDATE: So I called the tech at Carvin, and he told me to take the cover off the amp and check the ribbon cables' connections at the preamp, possibly blowing things out with some compressed air. I did so, disconnected, blew them out, reconnected, and it worked. Both the channels seem to be equal in output now. I guess channel 2's preamp just wasn't letting enough signal through.

 

Whew! I was getting worried for a while there, I couldn't really afford to buy a new amp at the moment.

 

Thanks again for your help, agedhorse, I appreciate it. Oh, I was wondering what part of Northern California you're from, if you don't mind me asking?