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Old 29th January 2001, 09:07 PM   #1
Nisbeth is offline Nisbeth  Denmark
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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
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Old 30th January 2001, 01:29 PM   #2
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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)
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Old 30th January 2001, 03:33 PM   #3
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You might want to check out the Crystal ( http://www.crystal.com ) CS3310. It looks like a pretty neat chip and seems to be used in some pretty high end equipment. Do a search on CS3310 and see.
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Old 30th January 2001, 04:49 PM   #4
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Nice little thing, BUT there are a few flaws:

1 : Input is limited to +/-3,75 Volt, cutting any peaks brutally.

2 : The frequency-response is -3dB at @200kHz at full output, introducing an unnecessary extra pole in the signalpath.

Having said so, I must admit it`s a much simpler solution to the problem.
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Old 30th January 2001, 08:07 PM   #5
Nisbeth is offline Nisbeth  Denmark
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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
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Old 30th January 2001, 08:47 PM   #6
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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.
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Old 4th February 2001, 10:09 PM   #7
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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 . I would part with one of them for $100 if you're interested.
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Old 5th February 2001, 05:39 AM   #8
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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.
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Old 5th February 2001, 09:24 PM   #9
Nisbeth is offline Nisbeth  Denmark
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>>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 :-)
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Old 6th February 2001, 04:05 AM   #10
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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...
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