[Pete Schumacher]

Hi Steve,

Have you ever thought about adding an electronically controlled passive volume control to your Final Drive?  That unit would make the basis for one heck of a nice passive preamp using an optical volume control.

[audioengr]

Hi Steve,

Have you ever thought about adding an electronically controlled passive volume control to your Final Drive?  That unit would make the basis for one heck of a nice passive preamp using an optical volume control.

Pete - actually, yes.  I was planning to prototype something maybe later this year.  The problem with many systems is the drive from the DAC however, so I planned to put a single stage of active gain in it. I also wanted it to be isolated, input and output, so no ground-loops.  I was planning to use a stepped transformer for gain.  Making this remote controlled is a bit of a challenge.

Steve N.

[Pete Schumacher]

Pete - actually, yes.  I was planning to prototype something maybe later this year.  The problem with many systems is the drive from the DAC however, so I planned to put a single stage of active gain in it. I also wanted it to be isolated, input and output, so no ground-loops.  I was planning to use a stepped transformer for gain.  Making this remote controlled is a bit of a challenge.

Steve N.

What's the problem with other DACs?  I mean if a DAC can provide signal for the input of a preamp, the proper optical volume control would simply put a variable resistive ladder between the output of the source and the input to the amp?  That resistive ladder, using light sensitive resistors, can be set up to maintain the proper load impedance.  http://www.newark.com/silonex/nsl-32sr2/optocoupler-resistor-2000v/dp/71C0279 

Controlling volume by modulating the LED current would be something that would be fairly straightforward to handle remotely.

Or do you need the active buffering to use the transformer gain since that presents a lower impedance to the source?

[audioengr]

What's the problem with other DACs?  I mean if a DAC can provide signal for the input of a preamp, the proper optical volume control would simply put a variable resistive ladder between the output of the source and the input to the amp?  That resistive ladder, using light sensitive resistors, can be set up to maintain the proper load impedance.  http://www.newark.com/silonex/nsl-32sr2/optocoupler-resistor-2000v/dp/71C0279 

Controlling volume by modulating the LED current would be something that would be fairly straightforward to handle remotely.

Or do you need the active buffering to use the transformer gain since that presents a lower impedance to the source?

Pete - the problem is dynamics.  When you use resistors in series with the signal path, this reduces the curent available to drive the amps.  This is why I use only passive transformer attenuation, never resistors.

Because most DACs use opamps for output drivers, these have relatively high output impedance, which means they dont have the current drive needed to get the most dynamics out of amps.

Steve N.

[G Georgopoulos]

what about transformer distortion isnt it a problem?

[audioengr]

what about transformer distortion isnt it a problem?

Not with the RIGHT transformers.  The ones I use are really transparent and high-bandwidth.

Steve N.