BillFitzmaurice.info

Bruce Weldy wrote:You didn't mention where the gain on the mixer channel is set. Also, what about the output of the playback device? Has it been changed? There are a lot of places along the line that have an effect on volume.

Everything was set half-way execpt the amp gain which was turn-up all the way. My pc sound card and ipod were up all the way too (ok, ipod at ~90%).

byacey wrote:If you're running balanced line inputs, check the solder joints on the amplifier ch 2 input connector.

Loss of gain could also be caused by a shorted feedback loop cap as I mentioned, or a defective op amp, assuming the level control is good.

Now that I think about it, my speakon cables have been a little finicky to remove from the amp..........
I really don't wanna crack open the amp, but I am a tinkerer and very curious. I'll try this next.

jimbo7 wrote: I really don't wanna crack open the amp, but I am a tinkerer and very curious. I'll try this next.

Before you do that, just take off the speakon connectors and attach the wires directly to the binding posts. If there is a change, then you have the culprit.

But, I gotta' tell you - that would be so far down the list of things that could be wrong that I doubt you'll find what you are looking for there.

Following byacey's line of reasoning . . It appears the Carvin amp is balanced input only via XLRs or 1/4" TRS jacks. Without a schematic to verify, I'll assume it's an active balanced circuit via a 2 section opamp. You could eliminate half the opamp's section by rigging up an unbalanced connector and feeding only pin 2 of the XLR or the tip of the 1/4" with the grounds attached to pin 1 & 3 on the XLR or the ring and sleeve on the 1/4". You will loose some level overall, but it will help determine if it's in the input stage of the amp. If it passes that test and both channels perform equally, then move the input signal to pin 3 of the XLR with ground to pin 1 & 2 or ring of the TRS with ground tied to tip and sleeve and try it again.

I should mention that you would need to lift the non-feeding wire on the mixer side so that it does not become grounded on the amp side of your test cable.

It's preferable to float the unused pin 2 or 3 of the mixer output, but it shouldn't hurt anything should it become grounded. There are almost always build out resistors on the opamp output to protect against shorted lines and high capacitive loading.

I can't see the speakons creating a drop in level or HF rolloff; either they pass signal or they don't.

Hackomatic wrote:Following byacey's line of reasoning . . It appears the Carvin amp is balanced input only via XLRs or 1/4" TRS jacks. Without a schematic to verify, I'll assume it's an active balanced circuit via a 2 section opamp. You could eliminate half the opamp's section by rigging up an unbalanced connector and feeding only pin 2 of the XLR or the tip of the 1/4" with the grounds attached to pin 1 & 3 on the XLR or the ring and sleeve on the 1/4". You will loose some level overall, but it will help determine if it's in the input stage of the amp. If it passes that test and both channels perform equally, then move the input signal to pin 3 of the XLR with ground to pin 1 & 2 or ring of the TRS with ground tied to tip and sleeve and try it again.

Most common is a dual opamp input like a TL072 or a 5532, 1 opamp section for each channel input. One balanced input would drive pin 2 and 3 of the 1st. opamp, and pin 5 and 6 of the 2nd. opamp section driven from the channel 2 input. Typically the output of the opamps would then be terminated on the high end of the respective input level controls.

If this is the topology, then driving either pin 2 or 3 would yield the same problem if that particular opamp section is defective. If you get signal driving one pin of 2 or 3 the XLR and none on the other pin, then it could be an input connector problem or bad solder joints, coupling cap, etc.

However, the original poster indicated the problem changed states after moving the parallel mono / bridging switches. These switches would be generally be located after the input balancing opamp outputs but before the level controls. Because he indicated the HF was rolled off on one channel, I think it's something more than just an open input pin.

If cleaning the switches doesn't clear up the problem and you don't have any experience, I would suggest you get it in to a technician to have a look.

PROBLEM SOLVED!!!!

Finally figured out what was causing it. It's a loose connection on my mixer's output. I bumped some wires and it came on for a second. Jiggled them around and it was cutting in and out. Traced it to amp input wires and then to the mixer.

So relieved that it's not my amp and just a 2nd hand mixer I paid $30 for.

Excellent!
It might be fixable, then?

Tom Smit wrote:Excellent!
It might be fixable, then?

Maybe. I'm guessing a loose solder connection. I'm working outta town and only home on the weekends, so my motivation puts this on the bottom of the list. I have another bare-bones mixer I can use in the meantime. I'm just so relieved I figured it out and that it wasn't the amp.

Cool, on the scale of things it's much better than the amp being the problem.

Here's hoping you get it sorted without to much trouble...