low audio, voltage at output

yakimicki · « **on:** 6 Mar 2006, 06:43 am »

I put together a preamp following the bugle schematic. One channel sounds amazing.kudos jim...the other channel...well it is too quiet to tell how it sounds.
The channel that is very quiet has another symptom, if I test for voltage at the RCA out (between post and ground) I have zero (well nearly) when I switch it on and then it slowly rises to about -4 or -5 volts after a minute or so. Let me point out that this is the right channel. I've looked it over and haven't found any misplaced parts, but it is late, so I'm not ruling that out tonight!
Does this lack of volume and creeping voltage give anyone any ideas to where i could look for the problem:?:

yakimicki · « **Reply #1 on:** 6 Mar 2006, 02:36 pm »

a note: I pointed out that its the 'right' channel because the voltage that comes in on this side of the opamp is positive whereas my voltage at the signal output is negative. I stated that under the (newbie) assumption that one channel used the -9V and the other the +9V. I've found out from someone with more experience in circuits that this is not the case, that the opamp just uses the 18V total.. my bad.. either way, I still have the problem and I will have some time tonight to scratch my head while staring at it. It only takes one hand to scratch my head, so if anyone has any suggestions for me as to what components i should poke at with the other hand, let me know.

-Joe

hagtech · « **Reply #2 on:** 6 Mar 2006, 07:29 pm »

Yeah, make sure opamps all have stable +/-9V on their power pins. And that everything is soldered good. Then track down the dc error.

jh

yakimicki · « **Reply #3 on:** 6 Mar 2006, 08:25 pm »

sounds good. I checked for the stable +9/-9 at each amp last night, it was fine. I won't know for sure until I get home, but I'm thinking that there is a possibility that the 1.43k resistor in the voltage divider between the last op amp's (-)B in and the 13k resistor that connects to the 'B Out' may have not made it to ground (R19 between U3B pin 6 and R20). I made this on a generic prototype board and don't recall making the bridge to ground from the pad that r19 lands on. I noted it on the layout I drew, but I think that I may have overlooked it when it came time to solder.
If thats not it then I'll double check for any cold solder joints.

I'll post the results here in case it helps anyone in the future.

yakimicki · « **Reply #4 on:** 7 Mar 2006, 12:46 am »

both channels are working now and t he stray voltage on the output is gone

turns out my guess was on the right track. R19 and also the last resister that the signal hits before it comes in to pin 5 on the last op amp were both supposed to be connected to ground but instead both were effectively dangling in the wind. to conserve space on the board I had those two resistors tied to a set of connected pads which I'd planned on jumping to ground but overlooked it when assembling.

Once the wife and kid go to bed I'll get to try it out a little more, sounds good so far!