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I'm having a strange problem with a newly built PP 12V6 amplifier. In lieu of the ability to post a schematic, it's simply an actively loaded 12sn7 cap-coupled to a pair of 12V6s with a CCS in their shared cathodes. The CCS's are cascade DN2540, and the one on the 12SN7 plate is configured with the mu stage output.The amp sounds great, is eerily quiet, and all the voltages check out OK, but I'm concerned the power-down cycle is putting my speakers at risk. There is some hum (not particularly loud) and I can see the woofers contract. I know from my measurements that the DC voltage bleeds in about 2 seconds (due to CCS in OP stage I assume), and music stops playing immediately. I've been through half a dozen on-off cycles so it's not a catastrophic failure, but could there be any risk in the long term?
My guess would be that the noise is due to the Dyna Z565 transformers releasing stored energy through the secondaries, but I could be way off. Any suggestions or illumination would be very helpful.
Thanks in advance,
Michael
Follow Ups:
That does make sense, thanks guys. I will measure the voltage tonight and post the results.Here is a schematic for the amplifier. As I mentioned above, each CCS is a DN2540 cascade. This being my first experiment with a differential output stage, I am very encouraged by what I'm hearing so far.
Thanks for any general comments or suggestions.
Best,
Michael
OK, here's the schematic:
Michael
Its being caused from the DN2540 which is voltage operated like an FET and the residual charge that is left over as the supply is bleeding down is letting the transistor conduct putting out a slight DC offset to the output..Take your dvm and put it on DC volts with correct polarity and see how much DC is at the output when it powers down..It should be a negative voltage and if its 1 volt or less dont worry about it..
Michael,I've measured a few times now, and it seems to be a small positive voltage of 30mv at most. The charge is so brief that it's difficult to measure. Does this make sense to you?
Is that a long-tail pair driving push-pull outputs? It's getting unbalanced somewhere as the B+ drops...
The 12v6's act as a sort of long tail pair themselves, with the CCS in the cathode attempting to force balance. With 'matched' pairs, they are within 1 or 2 ma in both channels.Since I am using a separate transformer for the heater supply, it would be easy to switch the B+ off separately. I wonder if the brief imbalance would still occur as it bleeds though?
Do you expect this is any danger to the speakers, or now that I think about it, the output transformers themselves?
Won't hurt the transformer, probably won't hurt the speakers, but I would try to fix it. Got a schematic?
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