n_maher
Resistorous Conflagorous
- Joined
- Nov 20, 2004
- Posts
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- 34
Ok, so I'll freely admit that I'm a bit out of my league here. I helped my friend Ian build a PPA V2, he did all the board work and I built him a power supply and helped with the case work and wiring. So, two weeks ago we had a working PPA that wasn't in a case.
Fast forward to the other night when we decide to put the PPA into its home. Everything goes in and the only hiccup is that while checking the switch I accidentally put my meter probe across the positive LED pad and one of the switch pads (I think) causing a spark. Nothing explodes (LED included) and the only casualty appeared to be my nerves. So, we boot the amp up and everything still seems to be going fine so I check DC offset. The right channel has 25mV so we decide to swap out that 637 for a fresh one and chalk it up as the nights only badness. Recheck all the channels and we're back to everything below 3mV so we hook up a set of test cans. Holy hiss! We then futz around checking this and that for god knows how long and decide to see if it's just the test cans. Hook up a set of HD650s and viola, very nice sounding amp. Plug in SR325is and blech, hiss city. So we put the PPA aside (it's after midnight and we've been at this for 5 hours) and figure we'll revisit this another day. It seems to be a grounding issue as when you touch the wires leading to the headphone jack the hiss disappears.
Now jump to last night when Ian decides to try and see if the hiss is at all related to cranky buffer bias and starts checking the buffer bias. Ruh roh rorge, the bias is jumping all over the place and when he finally gets a steady reading poof, the ground output resistor R33G smokes - big time. So Ian brings the amp back to me with his head hung low and asks if I'll replace the resistor. "No big deal my friend", I say.
So tonight I swap out the toasted resistor for a new guy, recheck all the solder joints on the bottom of the board, double check the wiring, etc., etc. Hook everthing back up, press power and blamo, another toasted R33G. Here's the kicker though, tonight I took the opamps out figuring that I don't really want to shell out another $48 or whatever for [2] 637's and 627 (I gave the opamps to Ian as a "get your PPA started gift"). I thought that this meant that the output resistors shouldn't be seeing anything.
So, anyone have any idea what the heck is going on here? I've got some spare parts so if something thinks it'd be a good idea to swap out this or that I'm all ears. I can't even check the voltage at the sockets cause the resistor will go Mt. St. Helens on me. I have checked the STEPS and it's pumping out a healthy 34.7V with no apparent issues.
HELP, please, I'm trying to get this thing up and running for the Boston folks this weekend so they can hear the glory that is the PPA.
Thanks in advance,
Nate
Fast forward to the other night when we decide to put the PPA into its home. Everything goes in and the only hiccup is that while checking the switch I accidentally put my meter probe across the positive LED pad and one of the switch pads (I think) causing a spark. Nothing explodes (LED included) and the only casualty appeared to be my nerves. So, we boot the amp up and everything still seems to be going fine so I check DC offset. The right channel has 25mV so we decide to swap out that 637 for a fresh one and chalk it up as the nights only badness. Recheck all the channels and we're back to everything below 3mV so we hook up a set of test cans. Holy hiss! We then futz around checking this and that for god knows how long and decide to see if it's just the test cans. Hook up a set of HD650s and viola, very nice sounding amp. Plug in SR325is and blech, hiss city. So we put the PPA aside (it's after midnight and we've been at this for 5 hours) and figure we'll revisit this another day. It seems to be a grounding issue as when you touch the wires leading to the headphone jack the hiss disappears.
Now jump to last night when Ian decides to try and see if the hiss is at all related to cranky buffer bias and starts checking the buffer bias. Ruh roh rorge, the bias is jumping all over the place and when he finally gets a steady reading poof, the ground output resistor R33G smokes - big time. So Ian brings the amp back to me with his head hung low and asks if I'll replace the resistor. "No big deal my friend", I say.
So tonight I swap out the toasted resistor for a new guy, recheck all the solder joints on the bottom of the board, double check the wiring, etc., etc. Hook everthing back up, press power and blamo, another toasted R33G. Here's the kicker though, tonight I took the opamps out figuring that I don't really want to shell out another $48 or whatever for [2] 637's and 627 (I gave the opamps to Ian as a "get your PPA started gift"). I thought that this meant that the output resistors shouldn't be seeing anything.
So, anyone have any idea what the heck is going on here? I've got some spare parts so if something thinks it'd be a good idea to swap out this or that I'm all ears. I can't even check the voltage at the sockets cause the resistor will go Mt. St. Helens on me. I have checked the STEPS and it's pumping out a healthy 34.7V with no apparent issues.
HELP, please, I'm trying to get this thing up and running for the Boston folks this weekend so they can hear the glory that is the PPA.
Thanks in advance,
Nate