PassLabs Pre-Amp transistors Volume ?

jleaman · 2005-06-29 7:13 am

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 · 2005-06-29 7:20 am

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 · 2005-06-29 7:46 am

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 · 2005-06-29 8:51 am

analog_sa said:
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 · 2005-06-29 9:47 am

Lower cost? Absence of annoying clicks?

metalman · 2005-06-29 4:23 pm

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

jleaman · 2005-06-29 5:11 pm

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

I don't want that..

Nelson Pass · 2005-06-30 10:26 pm

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 · 2005-06-30 10:28 pm

Nelson Pass said:
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 · 2005-07-09 9:10 pm

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..

analog_sa · 2005-07-09 9:42 pm

i dont know what the digital ic is to turn each transistor on one at a time..

Any microcontroller with enough i/o pins will do. A 90S8515 off the top of my head will be fine.

Nelson Pass · 2005-07-09 11:53 pm

jleaman said:
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 ?

The pot has its own problems, of course. When we used relays
we got a lot of whining about relays clicking. The solid state
solutions that existed sucked (and still do), but Wayne developed
a circuit that had about 1/10 the noise and distortion and could
handle about 10 times the voltage, so we used it. No complaints
about sound, although there is still a slight "zipper" noise as you
spin the volume control.

jleaman · 2005-07-10 3:04 am

Nelson Pass said:
The pot has its own problems, of course. When we used relays
we got a lot of whining about relays clicking. The solid state
solutions that existed sucked (and still do), but Wayne developed
a circuit that had about 1/10 the noise and distortion and could
handle about 10 times the voltage, so we used it. No complaints
about sound, although there is still a slight "zipper" noise as you
spin the volume control.

Ive got the "schematic" i'm working on it right now.. Just need help now to desifer the chip that turn's each transistor on one at a time. I presume the Voltage ? audio signal goes through this chip and is then gone through a ladder resistor then to the output.

The reason i wanted to build this was to A gain experiance in SMD part's and more eagle board layout time. B to build my OWN pre-amp board's..

Nelson Pass · 2005-07-10 6:14 am

The chip is a digital latch, and does not see analog signal.

jleaman · 2005-07-10 6:23 am

Nelson Pass said:
The chip is a digital latch, and does not see analog signal.

ok... Nelson May i email you personally please..?

J'

wayne325 · 2005-07-10 4:54 pm

volume control logic

I'm right now doing the same thing you're talking about.

I'm going to use a PIC16F684 I think as the controller.
You can get a kit from Microchip called the "PICkit1" Flash
Starter Kit. It's $35 from Digikey or Mouser in the US.
The kit has software to program the part, I think the part
has enough program memory to do what you want to do.
THere is a programmer in the kit too so you would have
everything you need to make the controller with the PICkit1.

I'm going to go the relay route because I believe it to be
lower noise than transistors and it's simple enough to implement.
Clicking noises when changing volume wont bother me.

I'm going to use encoders for the volume and balance and
selector inputs - that's 6 which will use up port A of the PIC.

The PIC only needs 3 outputs to drive 74HC595 shift register
with parallel load. I'll use a 12V supply for the relays and use
VN2222LL n-channel FETS to switch the relay coil current.

I will use TQ2-12V relays and metal film resistors.

That's the plan. I haven't tested it yet and I haven't started
programming the PIC yet either but I do have the kit and
if you're a programmer it will be very easy for you to program
it. It took me about 2 hours to go thru the part manual to
figure out the 16F684. If I had a hex file I'd just mail it to
you.....

Anyone have any input on my choice of parts and the overall
plan ?

W.

jleaman · 2005-07-10 6:51 pm

Re: volume control logic

wayne325 said:
I'm right now doing the same thing you're talking about.

I'm going to use a PIC16F684 I think as the controller.
You can get a kit from Microchip called the "PICkit1" Flash
Starter Kit. It's $35 from Digikey or Mouser in the US.
The kit has software to program the part, I think the part
has enough program memory to do what you want to do.
THere is a programmer in the kit too so you would have
everything you need to make the controller with the PICkit1.

I'm going to go the relay route because I believe it to be
lower noise than transistors and it's simple enough to implement.
Clicking noises when changing volume wont bother me.

I'm going to use encoders for the volume and balance and
selector inputs - that's 6 which will use up port A of the PIC.

The PIC only needs 3 outputs to drive 74HC595 shift register
with parallel load. I'll use a 12V supply for the relays and use
VN2222LL n-channel FETS to switch the relay coil current.

I will use TQ2-12V relays and metal film resistors.

That's the plan. I haven't tested it yet and I haven't started
programming the PIC yet either but I do have the kit and
if you're a programmer it will be very easy for you to program
it. It took me about 2 hours to go thru the part manual to
figure out the 16F684. If I had a hex file I'd just mail it to
you.....

Anyone have any input on my choice of parts and the overall
plan ?

W.

Well i do have the schematic for how it all exactly should be.. Me i don't want to use relays. they draw to much current there slow and they make to much noise.. That is why im going the surfact mound transistor way.. Email me and we can talk more..

rsbay · 2005-07-10 6:56 pm

I use an Atmel AT89C51RC2 with a 10 Bit relay volume control in my
Aleph P preamp (not yet finished). Have a look at my homepage

http://www.r-stens.de/diy/AlephP/diy_aleph_p.php

Sorry, up to now it's only available in German language, but you can
have a look at the schematics and photo's.

I use a I2C bus with the PCF8574 I/O port and ULN2803A as the power
drivers for the relay's.

Ralph

RCBandwidth · 2005-07-10 7:19 pm

How about a different approach

Jleaman:Here's a link to an alternate way of volume control-a voltage controlled amplifier from Analog Devices.

http://www.analog.com/UploadedFiles/Data_Sheets/6819137SSM2018T_b.pdf

You can also check out some of their Digital Pots,If you want to experiment you could use a "Basic Stamp"microcontroller with built in I2c support..A few of the Digital Pots from Analog Devices have the I2c interface..Bob C.

Nelson Pass · 2005-07-10 7:20 pm

jleaman said:
Nelson May i email you personally please..?

Why not, everybody else does.

PassLabs Pre-Amp transistors Volume ?

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