Thank you Stephen for your insight. The blocking cap on the amp's input has been removed. How does that change your advice? Still not enough DC to matter?? Remember, this is a Tube amp with (of course) output transformers. Also, note that the measurements were taken from the output jacks with a CD playing.
I second Mooly's advice. I think it would take at least 100mV dc to even begin to alter the bias of the input stage of any decently designed tube amp w/direct coupled input to any meaningful degree. This is part of why it is VERY common to see amp's w/tube input stages that have no input dc blocking cap.
You guys have all been so helpful, I hate to just drop this discussion. I set up Mooly's filter and I'm not sure it is working correctly. Had to use a 450V 47uf cap with 47K resistor. Ridiculous for a few mv, but it was all I could scrounge up. I'll go to the Rat shack for the right value if needed. Should it matter? DC with filter is 2.5mv at rest, but climbs when a CD is playing, then drops, climbs, etc. I've pretty much decided not to test fate and only use this player through a cap coupled preamp. Two other players here measure no DC. and never had a problem with them driving the Fisher amps directly. Thank you all so much, a good discussion and a little more learned.
The cap should be OK on the face of it but against that, high voltage parts may not be the best when it comes to low ESR and impedance.
The voltage will alter when playing music as the cap is hanging on to asymmetric low frequency signal voltage.
2.5mv is way low enough to say there is simply not an issue here.
The voltage will alter when playing music as the cap is hanging on to asymmetric low frequency signal voltage.
2.5mv is way low enough to say there is simply not an issue here.
Any reason why one can't connect an output cap into an interconnect? I know it may be (and look) a bit messy but it would allow for the situation where you have both a source and an amp with no dc blocking. And presumably 1 good cap in the middle is better than 1 in each of the source and amp.
Pete
Pete
No confusion, its just that the typical DVM will not accurately record the offset in the presence of the low level hash from the DAC/filter stages.
Some players do actually have a DC offset null facility and you will find that averaging and trimming the offset by eye on the scope (so you continually switch between AC and DC coupling and trim so that the trace doesn't shift one iota) will give a measurable DC offset when read on the meter alone. Using the filter I outlined reduces that effect dramatically.
I have a strange problem with this configuration.
I have an Aura AUC50 CD player on the worckbench.
It has passed into the hands of a saguin who has done more harm than good, but now it works, BUT, because there is but, the player cd refuses to recognize the cd if it is connected to an amp " dc coupled ".
the cd turns turns turns again without ever reading the "TOC", but if I unplug the cinch, it reads the cd right away.
Has someone already had this problem ?
I have an Aura AUC50 CD player on the worckbench.
It has passed into the hands of a saguin who has done more harm than good, but now it works, BUT, because there is but, the player cd refuses to recognize the cd if it is connected to an amp " dc coupled ".
the cd turns turns turns again without ever reading the "TOC", but if I unplug the cinch, it reads the cd right away.
Has someone already had this problem ?
I have trouble explaining myself in English.
in fact, only my amp that does not have an input capacitor generates the problem.
my other amps with a condenser dc do not pose a problem on this CD player and I can not understand why.
and the phenomenon and clear and clear, it is enough that I debranche cable rca so that it reads the cd.
I have never had this case in 20 years of troubleshooting that's why I ask the question.
in fact, only my amp that does not have an input capacitor generates the problem.
my other amps with a condenser dc do not pose a problem on this CD player and I can not understand why.
and the phenomenon and clear and clear, it is enough that I debranche cable rca so that it reads the cd.
I have never had this case in 20 years of troubleshooting that's why I ask the question.
If it only does this when connected to the amp then there are two things to try.
1/ See if the player has a DC offset present. This is very important. If you are running the amp and player off a limited current supply (such as an isolation transformer) then the DC offset could be causing the power amp to pass DC to the speaker and that in turn will draw current... maybe enough current to pull the mains voltage down.
2/ Instead of connecting to the amp, try terminating the players RCA output (left and right) with a fixed resistor that is similar to the input impedance of the amp. Does that effect the player?
1/ See if the player has a DC offset present. This is very important. If you are running the amp and player off a limited current supply (such as an isolation transformer) then the DC offset could be causing the power amp to pass DC to the speaker and that in turn will draw current... maybe enough current to pull the mains voltage down.
2/ Instead of connecting to the amp, try terminating the players RCA output (left and right) with a fixed resistor that is similar to the input impedance of the amp. Does that effect the player?
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