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In Reply to: RE: BTW- Vintage Power Amp with Passive Pre posted by Neff on July 14, 2012 at 06:14:54
The standard output for CD players is 2 volts; but the output impedance varies a lot. My Rotel CD player has a 75 ohm output impedance. 75k is a decent input impedance and you could use a 10k pot in between and not load down the output and have only a 12% drop across the pot with that player. On the other hand, I had a couple of Sony CD players that had output impedances around 1100 ohms. They would still work with a 10k pot; but the drop across the player's output impedance and the pot could be as much as 25%. That's still enough to drive the amp to full output, as it would take a 25% drop from 2 volts down to 1.5 volts to drive the amp.
More later, I have to go.
Jerry
Follow Ups:
I'm surprised by that Sony CDP's 1100 Ω O/P impedance. Under 1 KOhm is what's appropriate into the IHF "standard" load that all commercial products are supposed to be capable of driving. Perhaps the slogan should be: Sony, a lot of baloney. ;> ) The company has gone downhill, since Morita died. :> (
It is possible to do a JFET buffer with a unipolar PSU, but caps. in both the I/P and O/P circuitry are needed. With their capability for self bias, JFETs, depletion mode MOSFETs, and tubes can have DC coupling on the I/P side of a voltage follower setup, if a bipolar PSU is used.
FWIW. I would use a buffered volume control in combination with a Sony TA-3200F power amp. I believe strongly in adhering to the 1:10 impedance rule. A 10 KOhm passive setup would work, provided VERY low capacitance cabling (as in extremely short, braided, and unshielded) is used between the control and the power amp. Why handcuff yourself?
Eli D.
Eli,
With the buffer in an existing active preamp, there is already an output cap for the preamp, so all the buffer needs is one additional cap for the output.
The two Sony CD platers were an ES model and one of the regular line. Both were measured at 1100 ohms at 1 kHz.
Both were also sensitive to the capacitance of the interconnects, so I used 12" interconnects made from low capacitance coax. Capacitance was in the 25-30 pF range. With typical 3' interconnects with a capacitance of 100 pF, they both developed a hard, bright edge. My old, and dear departed Onkyo DX230 CD player and my Rotel RCD971 don't seem to care about cable capacitance; but the earlier Rotel RCD955AX does. With short, low capacitance cables the RCD971 and 955AX sound identical; but use 3' long, 100 pF cables with the 955AX and the difference is quite noticeable.
I have no idea what the root cause is; but my friend, the analog circuit guru, thinks it might be oscillations in the output circuit caused by the extra capacitance.
Jerry
IF a DC blocking cap. is present at the O/P of the driving unit, an I/P cap. is unnecessary. However, an awful lot of stuff has an opamp and no cap. at the O/Ps. A BJT buffer with a unipolar PSU can't work with 1 of those cap.-less opamp O/Ps, even though a DC offset is not present.
The reason some units get into trouble with "high" capacitance cables, even though they have respectably low O/P impedances, is that they lack O/P current capability. An ability to charge and discharge the cable capacitance is absolutely necessary. Even driving line level interconnect cables requires a measure of cojones.
Eli D.
Eli,
Thanks for that tip about current capability in regard to cable capacitance.
I know enough to check for an output coupling cap, when using a single ended buffer. Plus, I have run into some other stuff when adding buffers. Stuff like muting circuits which work by shorting the output to ground. In my Onkyo TX870 receiver's preamp they put a 2500 ohm resistor in series to limit current from the linear IC at the output, so it could stand the muting circuits downstream of it. The power amp is nicely done; but the preamp is sort of a dog's breakfast of circuit elements. Don't even get me started on the CD input circuit - my rant on that atrocity could last past midnight.
Jerry
Just less electronics in the way- likely a moot point. I have a ceramic Centralab CV-9000 series 7 amp button contact selector switch. I could place the switch solder lugs 1/2 inch from RCA jacks & two 10K pots right next to the switch with one for each channel. The center of switch would seperate left channel from right or I am saying a symetrical layout.
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