Lightspeed Attenuator a new passive preamp

[udailey]

My new DAC is on the front step. So excited I cant hardly stand it. Running out to get it now.
Uriah

[sepolansky]

David,

Is there room in the preamp to add an output buffer? A very, very good one can reduce the phono stage output impedance to 200R or less. The lower shunt resistance of your LDR attenuator will then be a plus, reducing noise somewhat.

Stuart

[sepolansky]

Uriah,

ANOTHER new DAC? Wow!

Stuart

[udailey]

Well, the last one was so good I couldnt help myself. Is that what they call 'chasing the white rabbit?'

[AndrewT]

Originally Posted by David Garretson

A 50K volume control with a fixed series resistor and a variable shunt resistor that attenuates -10db to -63db, requires LDR shunt resistance up to around 25K. This is inside the range of reasonably close tracking for matched LDRs.

what about a switchable series resistor?
100k for the low volume setting and 100k//22k for the higher volume setting.
If this used a two pole normally open relay then the attenuator could sit at the lower volume setting at power on and revert to the lower volume setting at power off. That way you need to action the higher volume setting.

Because you are using just a shunt LDR you can very easily add a balance trim without upsetting the LDR matching.
Using the series/shunt LDRs makes balance trim much more difficult.

[David Garretson]

Sepolansky, The Atma-Sphere MP-1 is an all-tube balanced integrated phono/line stage. It would be possible to squeeze a SS buffer in there. However I'm inclined toward toob minimalism. The output impedance of the stock 12AT7 phono section was something like 10K. I've brought this down to around 2K by subsituting ECC99 after a rework of CCS.

AndrewT, Thanks, interesting idea.

[udailey]

Or you could use LDRs as series resistors and use them just as you were going to use the other resistors. Set them for a specific resistance and leave them there. Use 2 in parallel with each other, series with signal. Switch THEM on and off as Andrew suggested but without a relay in the signal. You dont have to have matched LDRs this way as you can 'set em and forget em.'

[AndrewT]

Originally Posted by udailey

Or you could use LDRs as series resistors and use them just as you were going to use the other resistors. Set them for a specific resistance and leave them there. Use 2 in parallel with each other, series with signal. Switch THEM on and off as Andrew suggested but without a relay in the signal. You dont have to have matched LDRs this way as you can 'set em and forget em.'

I doubt very much that you can maintain tolerance matching for Balanced Impedance connection.
0.1% match is suggested as the minimum standard for maintaining the rejection capability of the balanced link.
0.01% will give better rejection.

[David Garretson]

I agree with AndrewT on this point. In the phono load application I found that even if left on constantly, the LDRs drift 5%-10% from time to time depending on room temperature.

[sepolansky]

Originally Posted by David Garretson

Sepolansky, The Atma-Sphere MP-1 is an all-tube balanced integrated phono/line stage. It would be possible to squeeze a SS buffer in there. However I'm inclined toward toob minimalism. The output impedance of the stock 12AT7 phono section was something like 10K. I've brought this down to around 2K by subsituting ECC99 after a rework of CCS.

David,

Outboard of the phono stage, add a cathode follower, 6SN7 or 6CG7/6FQ7. (of course a pair for balanced operation, one tube per channel).

Couple the input with an 0.01uF V-Cap/10Meg grid leak.

Remove the much larger output coupling caps from your existing preamp (or bypass, add a second set of jacks, etc.).

Under the cathode follower goes a servo, sampling the output DC voltage and comparing it to 1/2 the negative power supply voltage. This allows DC coupling of the output. The servo op-amp controls a MOSFET in the cathode circuit of the CF. The (-) return is to a -12 to -24V supply.

Stuart Yaniger and Gordon Rankin have variations on this theme. SY's applies the corrective signal to the grid.

I've heard and used both Yaniger's and this version (modified Broskie circuit) and they both are superb and transparent.

The signal path is therefore only modified by replacement of a cap with one probably 1/100 (or even smaller) the size, the addition of one triode section of a nicely linear tube and a small value (47R-100R) Mills NI wirewound resistor.

Economizing, even an IRC resistor and good quality PP cap can sound very nice.

Ideally they are used as an integral part of a source, but they can be used as stand alone. The modified Broskie version I use can be used without an input coupling cap if you choose not to modify your existing preamp. However, the elimination of a big coupling cap at the output of the preamp is a nice sonic improvement.

Stuart