PassLabs Pre-Amp transistors Volume ?

[jleaman]

I've been looking and i see a few products now with a digital volume pots and stuff. I'm wondering how they do it.. i can see from pictures they use surface mount transisters one transistor per step with a resistor to change the volume out put. Does any one know how this could be done. Maybe with a rotoray control of some how ect ect. to turn on each transistor. Id like to try and build one.. So far i have seen the pass labs pre-amp and the conrad johnson pre-amp's and a few others.. Any one got any idea's. ? I don't want to step on any toes but it would be a good project and be able to hear silence when turning up the volume instead of relay's..

[jleaman]

Quote from a friend..


it's "just" a ladder of resistors, you change the "bottom" resistor in the ladder by turning on or off a transistor in series or parallel with a resistor..... simple system really - in theory.... the problem is getting it to work without noise from the switching..and getting rid of the noise from all the large resistors... you should go SMT IMHO to minimize the size of the thing

[analog_sa]

getting rid of the noise from all the large resistors

Why large resistors? I find it difficult to believe that a bipolar transistor switch can sound better than relays although it's obviously much cheaper.

[jleaman]

Originally posted by analog_sa



Why large resistors? I find it difficult to believe that a bipolar transistor switch can sound better than relays although it's obviously much cheaper.

Then why does pass labs do it ? and ill use smd resistors..

[analog_sa]

Lower cost? Absence of annoying clicks?

[metalman]

Have a look at http://electronics.dantimax.dk/

[jleaman]

Originally posted by metalman
Have a look at http://electronics.dantimax.dk/

I don't want that..

[Nelson Pass]

Well, there's a trick to getting the low distortion and noise
from such a technique, but currently it resides under trade
secret, until the patent office sees the light.

[jleaman]

Originally posted by Nelson Pass
Well, there's a trick to getting the low distortion and noise
from such a technique, but currently it resides under trade
secret, until the patent office sees the light.

I guess ill have to wait ? I dont want to patent any thing i just want to build it and learn
What do you think of this type of circuit nelson. What would you prefure a pre amp with all them transisters or a nice alps / DACT pot ?

[jleaman]

Ok I found it. Id like to build this.. SMD i know what the small signal transistors are but i dont know what the digital ic is to turn each transistor on one at a time..