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In Reply to: RE: A dangerous game posted by Brick527 on April 21, 2018 at 07:50:06
Although neither of us can be sure, it is probable that the DC voltage on the line between the output of the DAC and the input of whatever is less than 50V. Because the output stage is likely to be a cathode follower, which means that the output is taken from the cathode of the last tube. The VDC between the cathode and ground is typically way less than the plate voltage. However, I share your concern in a general sense.
One other thing that I wonder about: The OP says that the input capacitor is 10uF value. I wonder why the value would be so large. This suggests that the input impedance, largely determined by the resistance between the input jack and ground, is rather low. If one knew more about THAT circuit, one might justify removing the input capacitor, rather than the output capacitor from the DAC, especially if the input capacitor is an electrolytic. But don't, without knowing what's up with the input stage of the driven component. It's possible that the input capacitor is blocking grid bias voltage from the upstream component, in which case it's not safe to remove it (unless and only possibly if you re-install the output cap).
Follow Ups:
Hi Lew-- Input impedance of the C2200 is spec'd at 47K ohms at the balanced inputs.
Those 10uf/50v caps are in series on each of the four (stereo pair) balanced signal lines just as the balanced connectors come into the discrete balanced input/output board on a McIntosh C2200. Each cap is closely followed by a 267K resistor going to ground. This same value and configuration of C's and R's is mirrored right before the signal leaves the balanced input/output board to go off to the main PCB. I had earlier upgraded each of those 10uF electrolytics with Silmic IIs of the same value and voltage rating.
When the balanced signal lines come onto the main PCP, they each encounter a 47.5K resistor in series before they go into a balanced to unbalanced converter to become single-ended signals for amplification. I had assumed it was these resistors that are setting the input impedance. I also had earlier replaced each of these resistors with Caddock TF series.
I would post PDF with schematics, but not sure I can here?
Those 10uf 50v Silmics are just begging to be bypassed by your .01uf 63v KP1830's.
"The needs of the many outweigh the needs of the few or the one"
Vulcan Proverb
Most tube B+ supplies are 300 volt.Most directly coupled cathode followers are approx. 50% of B+ so about 150 volts.
He could easily had that kind of voltage had the designer used that kind of output stage.
Additionally the tube after this "tweak" now had DC on the feedback loop going to the grid totally skewing the operating parameters of the circuit.So who knows what the correct voltage readings of this circuit are.I'm sure they're different than the 50 volt reading.
Really a moot point though.The parts in his dac(output realays)and the input switching and volume controls in his preamp aren't built or designed to have DC going through them ,whether its 10 volts or 1000 volts.
They will fail.Never mind 50 volts through his interconnects.
Edits: 04/23/18
I hadn't accounted for the fact that most CFs don't have a plate resistor between plate and B+, so the B+ at the plate is that produced by the PS, which makes DCV at the cathode higher than for a typical gain stage. But we agree on the basic point for the OP's benefit.
Your thoughts on the removal of the input cap with the output cap in place in the dac. Not trying in any way to be insulting but how do you come about your knowledge, are you a trained electronics tech? Thanks for the responses. T456
"The needs of the many outweigh the needs of the few or the one"
Vulcan Proverb
"are you a trained electronics tech"I am and I can tell you, you do not advise someone to remove a coupling cap without first knowing all the details of the source and destination components.
It can be very dangerous.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/24/18
Building with tubes have been a hobby of mine for over 30 years now.Enough info out there now that if you want to put the time in you could learn alot from guys like Morgan Jones who goes step by step in his books to build amps,preamps,etc.
You can learn alot of the basics involved reading his work.Goes step by step very concisely and thoroughly.
Back when I started it was whatever you could get ahold of at the library or take a course.Times have changed.
Have a coworker who is absolutely incredible with woodwork.So he builds chassis and I do the audio circuits in some preamps we've been doing.Latest is a 7193/2c22 resistor plate loaded direct coupled to a 6sn7 cathode follower with a Mosfet constant current sink. Kinda a CONRAD JOHNSON warmth voltage amp with a Audio Research output stage.
Power supply is 165 mfd poly power supply with choke feeding another constant current source to a pair of 0d3 shunt regulators.Really happy with the results. Does what I set out for,warm with great drive.
Sorry to ramble on.
As far as the input cap removal unless you have a schematic to know exactly what going on with the circuit I would leave it in,to many variables.Could very well be the circuit input is raised a few volts to function and the cap is to keep the dc volts confined to the circuit.
Some solid state circuits bias the transistors or mosfets up to 10 or 20 volts to get them to operate correctly.Check pic.
Edits: 04/23/18 04/23/18 04/23/18
Seems like an excellent and highly useful reply Lew. Your explanation of the voltage makes me feel a little better. As to the 10uf on the input couldn't that be a case of "overkill" in a concern for phase shift in the bass?? Love the suggestion of getting rid of the input cap if appropriate and safe and especially if it is true that there is feedback from the dac output. It sounds like the OP may like the sound better with his new cap configuration? Maybe it's the feedback back in that helps the soundstage since I can't , in general see how caps would sound better than no caps?? At the very very least that likely electrolytic input cap should be done away with and replaced with a film or Silmic if not removed. T456
"The needs of the many outweigh the needs of the few or the one"
Vulcan Proverb
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