Great! I put the 4558 in there,and fired it up. It appears to be working.
No DC on the output,but if i turn the gain pot up/down I can see the cone on the test speaker move in/out a bit,but it quickly settles. Not sure if that's normal for this amp or not.
I haven't connected a source yet (waiting for the iPod to charge) but doing the 'finger test' on the input jacks,I do get a nice hum...and at higher gain settings a bit of a HF 'squeak' along with the hum. I'm assuming it's just HF crap coming from the amp itself,but we'll see once I feed it a real signal.
Idle current is ~2A.
Thanks again Perry! I'll post more findings in a bit.
Edit: Rail voltage is about 90Vdc. Still perhaps a bit high,but not cap-exploding high. 🙂
No DC on the output,but if i turn the gain pot up/down I can see the cone on the test speaker move in/out a bit,but it quickly settles. Not sure if that's normal for this amp or not.
I haven't connected a source yet (waiting for the iPod to charge) but doing the 'finger test' on the input jacks,I do get a nice hum...and at higher gain settings a bit of a HF 'squeak' along with the hum. I'm assuming it's just HF crap coming from the amp itself,but we'll see once I feed it a real signal.
Idle current is ~2A.
Thanks again Perry! I'll post more findings in a bit.
Edit: Rail voltage is about 90Vdc. Still perhaps a bit high,but not cap-exploding high. 🙂
I don't know if it will be a problem for this amp but many amps require that the signal source be grounded to the amp ground to work properly. The floating ground that exists when using ipods and other portable players has caused many hours of troubleshooting problems that didn't exist.
I don't remember having the low frequency voltage pulse when changing the gain setting but I've never looked at the speaker when changing the gain setting on one of these amps.
I don't remember having the low frequency voltage pulse when changing the gain setting but I've never looked at the speaker when changing the gain setting on one of these amps.
Ahh,thanks for the tip on the floating source ground.
The movement in the speaker cone from turning the gain knob almost completely disappears when I turn the subsonic filter on.
There seems to be some kind of oscillation issue though. Above a certain signal level there's a 'squeaking' in the signal,and on the scope it shows some HF oscillation on the peaks of the sinewave. I'm wondering if this might be a grounding issue,as I have the board removed from the heatsink,and there are grounding pads around the 5 mounting screws on the PCB.I might screw it back down into the heatsink,and see if that helps anything.
Other than these two issues,the amp appears to be working okay. (but looks can be deceiving.)
I'll keep poking around,and see if I can find anything else.
Thanks for your help Perry!
The movement in the speaker cone from turning the gain knob almost completely disappears when I turn the subsonic filter on.
There seems to be some kind of oscillation issue though. Above a certain signal level there's a 'squeaking' in the signal,and on the scope it shows some HF oscillation on the peaks of the sinewave. I'm wondering if this might be a grounding issue,as I have the board removed from the heatsink,and there are grounding pads around the 5 mounting screws on the PCB.I might screw it back down into the heatsink,and see if that helps anything.
Other than these two issues,the amp appears to be working okay. (but looks can be deceiving.)
I'll keep poking around,and see if I can find anything else.
Thanks for your help Perry!
Okay,It appears that there is a bit of grunge/overshoot on the gates of one set of output FET's,the ones being driven by U10. The gate signal on the other half is nice and square,and looks ok. I tried to probe the input pin of U10 with my scope,but apparently the amp doesn't like that,because it starts squealing alot and draws excessive current. I'm going to go over the resistors and stuff surrounding U10 and U9 and make sure that they are all within tolerances.
As an FYI, I had a 1000bd with the exact same issue. U21 was a TL072 in a SOIC package. At first when I read this post I wasn't sure why Perry thought the TL072 was the issue based on how a comparator worked until I looked at the schematic. Pin 6 should have ~+7.2V on it. I noticed the part was not an automotive grade part (only rated 0 to 70C). I replaced it with a TL052 because I could find it cheaper, but I made sure to get a -40 to 85C part. I'm hoping this lasts a little longer if it is indeed a temperature rating problem.
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