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I have a CD player in which the output impedance of the tube output stage is around 12Kohms at 20Hz, but is otherwise "tame" (650 ohms at 1KHz 622ohms at 20K). These are Stereophile's measurements, but without a graph. My speakers go down to 45Hz, no more. How big an output cap can I stick in there to lower the impedance without affecting the bass i hear from these speakers? The caps are now 0.22uf.
Observe, before you think. Think before you open your yap. Act on the basis of experience.
Follow Ups:
The measured OP impedance seems to be mainly a function of the impedance of the OP cap for the frequencies mentioned except 20Hz. But the cap cannot possibly be 0.22uF and provide 12K at 20Hz as the impedance of the cap alone works out to a whopping 36K!
You would need something like 12uF keep impedance fairly constant at 20Hz but in reality it depends on the input impedance you are driving. Assuming 50K you'd want at least 2 - 4uF IMO even with 45Hz speaker limitation.
Naz
Thanks for the help. Those are big caps you are talking about!
Observe, before you think. Think before you open your yap. Act on the basis of experience.
OTOH, we still don't know what input impedance it's driving so if it happens to be 100K plus then extreme LF reduction may not be so bad.
Naz
CDP: ouput impedance 650K-600 Ohms 1-20K Hz.
Pre: input impedance 50K Ohms; output impedance: 1K Ohms.
Amp1: input impedance 470K Ohms; output impedance 470K ohms
Amp 2: input impedance 100K; don't know the output impedance.
Speakers: > 8 Ohms and flat
Observe, before you think. Think before you open your yap. Act on the basis of experience.
> > CDP: ouput impedance 650K-600 Ohms 1-20K Hz < <
Hmm, can you clarify that 650K is not a typo?
Also, are both amps connected permanently and if so, how ... or did you mention them because you swap amps from time to time?
Cheers
No, it's not. JA pointed it out in the review in Stereophile. The output stage is an anode follower (with 2 ECC188); that was his explanation, as I recall.I guess that's a cathode follower on the anode, instead of the cathode? You can tell me why that raises the impedance in the bass. I'm an economist. I can tell you what upper hemi-continuity of a utility function is!
:)
Observe, before you think. Think before you open your yap. Act on the basis of experience.
Edits: 02/18/15
That makes all the difference and starts to make some sense. In fact, depending on the amount of feedback utilised a coupling cap of 0.22uF could be quite adequate because the feedback compensates for what would normally result in reduced LF. It's even possible to design for slight LF boost.
It pays to know exactly what we are dealing with and what your specific goal is. If we can see a schematic we can tell you precisely what to do to achieve your goal. This may not entail swapping OP caps at all.
Naz
Thanks, but there isn't one and the circuit is on a board and i can't follow it.
I am wondering if you can tell me what would happen if I hooked up a TVC to this Dac, and whether I am not better off simply attenuating at the preamp output (because of high gain in the DAC and amp) with my one pair of 95dB sensitive speakers.
Observe, before you think. Think before you open your yap. Act on the basis of experience.
Is excessive gain your only problem & is this the only reason for wanting to hook up a tvc?
It would be possible but there are other cheaper & possibly better ways to accomplish the same thing. You may need some technical expertise though.
Naz
What's the input impedance of your preamp (or integrated if applicable).
0.22uF is insanely small for the output of a tube CDP or other source, but if you are driving a 100K impedance, it isn't going to matter all that much.
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