Good news, I had my meters set wrong. Sometimes the best case scenario is that one [ME] is a bit of a dope and makes a simple mistake.
It is difficult to overvalue a good night's sleep and 12 hours of letting the problem percolate in the back of the noggin.
Right channel:
Left channel:
And the amp is coming up to temp with the cover on etc:
I know this info is all over the place, but to reiterate in vocabulary that makes sense to me:
- These 20 turn trimpots have a faster value change toward the center of the turn range
- This bias circuit is interactive in the following ways:
- more bias to either N or P means more bias to both channels
- achieving 0mv DC offset on the speaker outs is like mixing hot and cold water to
get the perfect temperature. For instance: if DC offset is too positive, bring the P
channel down a bit and the N channel up a bit.
It is difficult to overvalue a good night's sleep and 12 hours of letting the problem percolate in the back of the noggin.
Right channel:
Left channel:
And the amp is coming up to temp with the cover on etc:
I know this info is all over the place, but to reiterate in vocabulary that makes sense to me:
- These 20 turn trimpots have a faster value change toward the center of the turn range
- This bias circuit is interactive in the following ways:
- more bias to either N or P means more bias to both channels
- achieving 0mv DC offset on the speaker outs is like mixing hot and cold water to
get the perfect temperature. For instance: if DC offset is too positive, bring the P
channel down a bit and the N channel up a bit.
Back again.
Ok so my amp is fully biased and set. With a phone and a passive attenuator and small speakers it sounds great. This is good
When I use a either one if my DACs as an input into the same chain, the amplifier makes staticky noises and a smell comes from the amp, so I turn it off immediately.
Is there a reason this might be happening? The passive attenuator is wired to connect all of the input grounds to the negative side of the power amp inputs
Ok so my amp is fully biased and set. With a phone and a passive attenuator and small speakers it sounds great. This is good
When I use a either one if my DACs as an input into the same chain, the amplifier makes staticky noises and a smell comes from the amp, so I turn it off immediately.
Is there a reason this might be happening? The passive attenuator is wired to connect all of the input grounds to the negative side of the power amp inputs
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Rayma, can you help me understand why a USB fed DAC unit might oscillate when a smart phone would not?
How do I check for HF oscillation?
Plug-in 1: On the first plug-in I used a DAC. there was noise and smoke. I immediately returned the amp off, moved it to the bench and checked every part carefully -- no signs of scorching or capacitor bulge. Looked carefully again and found nothing.
Plug in 2: Cautiously powered it in with no load. No problems. Attached speaker load but not any inputs. No problems. Attached attenuator. Good. Attached phone, great, played a song or two.
Plug in 3: move it back to the stereo rack, attach speaker load and attenuator with no input. Power on, everything is good. Stable, no noise. Attach one channel of a different DAC. Attenuator turned all the way down. Power on and there's noise and a smell only from the channel with the DAC plugged in, no smoke this time though. Power off immediately and post here.
Unplug everything and post here like a sweet helpless dum-dum
How do I check for HF oscillation?
Plug-in 1: On the first plug-in I used a DAC. there was noise and smoke. I immediately returned the amp off, moved it to the bench and checked every part carefully -- no signs of scorching or capacitor bulge. Looked carefully again and found nothing.
Plug in 2: Cautiously powered it in with no load. No problems. Attached speaker load but not any inputs. No problems. Attached attenuator. Good. Attached phone, great, played a song or two.
Plug in 3: move it back to the stereo rack, attach speaker load and attenuator with no input. Power on, everything is good. Stable, no noise. Attach one channel of a different DAC. Attenuator turned all the way down. Power on and there's noise and a smell only from the channel with the DAC plugged in, no smoke this time though. Power off immediately and post here.
Unplug everything and post here like a sweet helpless dum-dum
RIP modi, burned an SMD resistor at the USB input. Doesn't power up.
My audio GD DAC doesn't have any signs of scorching and is still recognized by my PC. I'll have to check it later.
If I understand correctly, you are suggesting that I need to check to see if the RCA ins on the F5 are shooting DC to my inputs? Which would suggest that the F5 is oscillating?
Maybe this is a dumb question but isn't DC closer to low frequency? 0hz?
My audio GD DAC doesn't have any signs of scorching and is still recognized by my PC. I'll have to check it later.
If I understand correctly, you are suggesting that I need to check to see if the RCA ins on the F5 are shooting DC to my inputs? Which would suggest that the F5 is oscillating?
Maybe this is a dumb question but isn't DC closer to low frequency? 0hz?
No, you are checking to see if the dac or other sources are outputting DC. You could do this without them being plugged into the F5. They are not likely to be outputting dc but it’s possible. They should measure in the single digit millivolts for dc.
You have other problems though…not sure how to troubleshoot from here but I’d make sure your attenuator is wired correctly. I would also check for conductivity between the mosfets and chassis and speaker/rca jacks on the f5.
You have other problems though…not sure how to troubleshoot from here but I’d make sure your attenuator is wired correctly. I would also check for conductivity between the mosfets and chassis and speaker/rca jacks on the f5.
Audio GD dac measures 11mV on both channels, pins and shields. This DAC still works fine and is playing now through a different amp. It's noise was louder and a little deeper than the Modi.
Phone does not measure any DC offset.
Modi is of course toast.
Is it unreasonable to conclude that my amp was built fine but my inputs have been shooting DC?
If this is the case then here are my takeaways:
- I've always used tube amps with coupling/blocking caps so DC was never a problem for me before
- I need to find a source with no DC offset
- I should consider building a preamp with a coupling cap somewhere in the circuit to insure that DC is not sent to my power stages
- It could make sense that with the F5 accepting DC voltage, the Modi began flowing more supply from its power source and burned a resistor - I believe the resistor that burned is on the 5v supply of the USB so this seems to fit
Am I on or off here?
Phone does not measure any DC offset.
Modi is of course toast.
Is it unreasonable to conclude that my amp was built fine but my inputs have been shooting DC?
If this is the case then here are my takeaways:
- I've always used tube amps with coupling/blocking caps so DC was never a problem for me before
- I need to find a source with no DC offset
- I should consider building a preamp with a coupling cap somewhere in the circuit to insure that DC is not sent to my power stages
- It could make sense that with the F5 accepting DC voltage, the Modi began flowing more supply from its power source and burned a resistor - I believe the resistor that burned is on the 5v supply of the USB so this seems to fit
Am I on or off here?
yes, full compliment of new cables. DAC still has noise, smart phone still does not.
To clarify: With identical cables and associated component chain, my F5 is playing happily with a phone, albiet without much gain, and making a buzzing noise with the DAC.
Could this be a grounding issue?
To clarify: With identical cables and associated component chain, my F5 is playing happily with a phone, albiet without much gain, and making a buzzing noise with the DAC.
Could this be a grounding issue?
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The DAC connections are AC in / USB in from PC / Single ended audio out.
I suppose it doesn't hurt to try another USB out from my desktop.
I am flipping the amp off immediately upon hearing the buzz, so there hasn't been any smoke. I am assuming there would be smoke if I didn't flip it off ASAP.
Here is what I believe to be a correct schematic for my attenuator, which I've recently learned is a voltage divider. The chassis is tied to only one channel to avoid ground loops.
I suppose it doesn't hurt to try another USB out from my desktop.
I am flipping the amp off immediately upon hearing the buzz, so there hasn't been any smoke. I am assuming there would be smoke if I didn't flip it off ASAP.
Here is what I believe to be a correct schematic for my attenuator, which I've recently learned is a voltage divider. The chassis is tied to only one channel to avoid ground loops.