6-way volume control

Hello there!

I would like to design a preamp for a surround system. My idea is to use my dvd, which has build-in decoder and couple it directly to six poweramps using just a simple volume control (which can then be remote-controlled). This means I need to make a 6-way volume control that can be operated with just a single (stereo) potentiometer as the common regulator (I have a spare 10kohm Alps pot with motor that I would like to use). Any ideas on how to accomplish this?

/Uffe Nisbeth
 
One way of doing it is the digital way.
At the moment I`m designing a fully ballanced preamp, so therefor I`ve given it some thought, since I`ll be needing a 4-way attenuator. So here`s my idea:
You take a potentiometer, with a reference-voltage at the one end, and ground at the other end. The voltage on the slider is fed through a low-pass filter to an 8-bit A/D-converter. Of the 8-bit output you only use i.e. the six MSB, and feed them to 6x6 DPST-switches. The analog signal is then fed via the switches to an R-2R-network. The output of this network can then be fed to a buffer as an output of your preamp.
Note that the analog signal will NOT be converted into a digital one.
God fornøjelse ;o)
 

Nisbeth

Member
2001-01-29 9:58 pm
Denmark
Thanks for your replies. I dl'ed the datasheet for CS3310 and that looks promising. But question #1, Mr. Hoffmeyer: Why is -3dB @ 200 kHz that much of a problem? And, could the limitation to +/- 3,75 V be "removed", if the volume control was placed before the actual premamp-stage. That way, the IC would only handle the line-out signal from the DVD, and I doubt that exceeds 3,75 V?

What I am essentially trying to achive is a cheaper version of the Harman/Kardon Signature 1.0 preamp, which is a stereo preamp with an added 7.1 channel in/out that enables you to use for surround applications as well.

/Uffe
 
Just call me Ole ;o)
1: If the frequency-response is -3dB at @200kHz, there`ll be a phaseshift at 20kHz, not much, but ............
It may not mean anything in Your case, and if You can live with it, it`s an easy solution.

The same goes for Q# 2.

In my own case, my ballanced preamp-project, neither of the two are acceptable.
My point is only to bring light upon those "flaws", in case You`d like to use the chip elsewhere. Any limitations in the signalpath are best placed at the end of the signal-chain, and not in the start.
 
Hi there! I am new to this forum.

I am doing exactly this kind of project, and I am to the point where all I have left to do is etch the circuit board and build it!

For the volume control, I had to special order custom made 6-ganged volume pot. It should arrive soon.

I thought at first that I was going to build two units, but I am only going to build one now.

These suckers cost me $130 a piece :eek:. I would part with one of them for $100 if you're interested.
 
Two possible slutions

1 - manufacture your own ganged pot. It is not easy, but I figure soemone willing to build their own equipment has the savoir-faire. I have done this before when there was no alternative. Within a year of doing this I found some 5 channels on surplus and bought a few.

2 - use a single potentiometer, and drive the control input of 6 VCAs. I have done this approach too.
 
>>For the volume control, I had to special order custom >>made 6-ganged volume pot. It should arrive soon.

>>I thought at first that I was going to build two units, but I am only going to build one now.

>>These suckers cost me $130 a piece . I would part with one of them for $100 if you're interested.

Thanks for the offer, but my project is still pretty much on the drawing board, so not at the moment. If you could, however, give some information about where you ordered the pots from, how they loo and behave in a circuit etc. I would be very grateful :)

/Uffe

PS: Speakerguy, just to be sure, a VCA is a voltage-controlled amplifier or ? How would you implement that in a circuit (could you possibly provide an example for me? - TIA :)
 
Sure, I ordered it through http://www.state-elec.com

It is a custom made unit, and I don't know what it will look like yet, as I have not received it yet.

One thing though, is that it is a 1/8" shaft, as opposed to the usual 1/4". The guy I talked to there said that's the only way to get it done. No biggie. Works the same.

There are 6 100Kohm sections to it. All are linear taper. I will be connecting a 15Kohm resistor from the wiper to ground to obtain an audio taper, as described at http://www.sound.au.com. (better volume control project)

I prefer this method to the VCA method, because there are no VCA's that have the same performance as a really good OpAmp like the Analog Devices or Burr Brown...
 

Jimmy S

Member
2001-02-09 5:58 pm
Nisbeth, I was also trying to do the same thing. Michael Percy Audio (http://www.bainbridge.net/percyaudio/) has a kit for a remote controlled volume pot, using either an Alps or Panasonic motorized stereo pot. I figured that I would buy 2 more motorized pots, all driven by the same signal for the motor control, to control 6 I/0s. Cost would be around $250, not including a chassis, etc. The only question would be if all 3 motors "tracked" the same, given the same voltage.
I eventually am planning to do it another, non-DIY, not so elegant way, just to see how it will sound. Since I already had 2 Audio Alchemy DLCs collecting dust, I just bought another one used ($190), & am planning to use the 3 DLCs as my 5.1 volume control, since they can all be controlled by one remote, and they track exactly the same (volume is attenuated in the digital domain).
Good luck.
 

Matti

Member
2001-01-29 8:01 pm
Sweden
Hello all, and especially byu,

this is my first post on this forum and I'd better start by saying that I'm not an EE (not that I think anyone would have thought so anyway ;-), I'm only trying to learn this electronics stuff.

I'm thinking of building the Pass Balanced Line Stage and need a (balanced) volume control for it. After I saw byu's post about the volumne control on the Aleph P I became interested in that, I think it's a better way to solve it with relays than with an 4-gang pot.

Since I don't know much (if anything) adout d/a or digital circuitry my question are:

It seems like the ADC 0804 on the schematics of the Aleph P 1.0 have two unused channels. Is it possible to use these to have two more "steps" on the attenuation circuit? As I understand the attenuation circuit only have 6 steps, will that be enough? Or will the volume attenuation be in to big steps (I guess not since Aleph P 1.0 is a commercial product, isn't it?)?

Any thoughts would be very much apreciated. Even on better ways to solve the attenuation of a balanced circuit.

Thanks

/Mattias Wijk

*******
Later:

Hoffmeyer: After reading your post above I think that you are truying tho acheieve the same thing aren't you? How are you doing? Any input would be very much apreciated.

[Edited by Matti on 02-19-2001 at 08:38 AM]
 
Hi Matti,

The the Aleph P 1.0 use 6-bit of the ADC for a 64 steps resolution. you can change the input ( removing the 1K and 10K resistor ) to allow the entire 5V reference for a total of 8-bit 256 steps. However, you have to added two more relays like the one on 1.7. I think 64 steps are good enough unless you want very fine adjustment. I am thinking about building this to replace the two dual 10K pot on my balanced line stage ( 256 steps with digital display, not binary code but with 3 digit 7-seg LED ).

You should consider building the Aleph 1.7 instead of the balanced line stage, it has the same design with better performance. It's not much of a different both in cost and degree of difficulty. Just make sure you isolate the digital and analog section.

My summer projects are rebuilding my Balanced line Stage and a new Aleph 4 ( My current amp is a 8W SOZ ).
 

Matti

Member
2001-01-29 8:01 pm
Sweden
Hi byu, hej Hoffmeyer,

thanks, to the both of you, for your reply. You are so right 64 steps will probably be enough (I guess I had a brain-fart (I really love that expression!) or something not thinking of 6 _BIT_ as 64 steps.

Byu: I think I need to take a closer look on the schematics for the Aleph P 1.7 before I go any further on building a pre amp. At the moment I don't have the accuired knowledge to understand all the circuits completely, but I'll try to. I like the simplicity of the Balanced Line Stage, I tend to believe that simple is good. You say that Aleph P 1.7 has better performance. In what ways? Have you listened to them both? I don't relly need all the bells and whistles of the Aleph 1.7, but since I'm thinking a little bit of remote controled volume on the pre amp maybe I should go with the Aleph 1.7.

Thanks for you input anyway.

Matti
 
I am not an EE too, but it's my spare time hobby for the last 20 years. The 1.7 is basically the same as the balanced line stage ( OP-AMP, also the same as the SOZ ) with a stable constant current source ( instead of resistor bias ) that in turn lower overall distortion. Matching MOSFET also help. Also, the bias resistors of the balanced line stage generate a lot of heat ( you cannot touch the resistor ). The only thing I don't like about the 1.7 is the use of micro-controller and optical encoder, micro-controller generates RF-noise and if you are not careful on the PCB and grounding, it can get pretty bad. Also, I cannot afford the programmer and the $60 optical encoder. I am planning to replace it with the ADC ($6.00) of 1.0.