Search for volume control

[Tenson]

Hi,

I have a Behringer DCX that I have modified with a transformer output stage. It has ~650R output impedance. My amps have an input impedance of 27K.

Now I would ideally like to have an analog volume control but I would rather not add active stages, so I was considering a shunt resistive unit from Welborn Labs. - http://www.welbornelabs.com/remote.htm

My question is, with a 650R output impedance, and 27K input impedance on the amps, is the system going to work okay with a resistive passive volume control or is this likely to mess up the impedance matching? If I used a 50K series resistor in the volume control then I guess the DAC would be happy but the amp would not and vice-versa.

If that is not going to work then I think I would go for a digital volume control, as all my sources are digital. Does anyone know of a unit that can simply take a digital input, dither to 24bit and apply digital attenuation and output a digital signal again for the DAC?

Thanks for your help!

[paulb]

A low output impedance feeding a high input impedance is fine. Impedance matching is only really necessary at high frequencies or where maximum power transfer is important (not here).
You could use a 10K potentiometer (audio taper) in the usual fashion for your volume control.

[Tenson]

By impedance matching I meant getting a good match for this purpose, not making them equal.

If I used a 10K pot then the DAC would see 10K which would be just about acceptable but wouldn't that be a little high to drive a 27K amp input. It might work but it seems like its pushing the limits IMO and I might see frequency response variations when the pot is adjusted. A friend has a Welborn labs thing so I might be able to just try it and see.

[paulb]

No, the DAC would see the parallel combination of the lower part of the pot (depends on the setting) and the upper part of the pot in series with the source. At full volume, this would be about the same as the source. At midpoint, it would be about 2.5K. If the input capacitor is too low a value this could affect low frequency response, but I doubt this would be the case.
If it's a concern you could use a 5K pot instead...

[Tenson]

Hi,

How do I work out the load that a voltage divider presents? I know how to calculate the attenuation but not the impedance.

2.5K is too low a load for my DAC, it won't like driving that at all.

Thanks

[Pano]

I don't know how you've set up your transformer output - but I've "transformerfied" many a DAC. (My DCX2496 is next).

Voltage output DACs like the AKM in the Behringer like a low load - 1K is often good. As you know, the DAC will see the load reflected back thru the tranfo. The reflected load will depend on the load on the output of the transformer x the winding ratio.

So if you have a 10K load on the secondary side of the transformer with a 1:1 ratio, then the DAC will see 10K. Higher than you want. But if your DAC load resistor is ~ 1.2K, then the DAC will see a total load of 1K. Perfect!

I've used a 10K TKD pot with very, very good results. It means a lower output resistance than a 50K pot.
If you had a six gang 2K pot, you'd be golden.

Hope that makes sense. Basically, use a low value pot and do quick math to be sure you don't load down the DAC too much. No series resistance needed or wanted.

Please ask if this does not make sense.

[Tenson]

Hi thanks for the response.

Interesting that you say it likes a low load. On the spec sheet it says to load 600R or greater. I assumed 600 was an absolute minimum and it should be loaded with more.

If the DAC will always see the reflected load, what is the effect of the impedance of the windings themselves? For example how would a 150R : 600R setup differ from 600R : 2K4 ? Is it only of consequence when there is no load connected to the secondary so 600R : 2K4 would better protect the DAC in this case?

I have the transformer in a 1:1 ratio at the moment, and it seems perfectly happy driving a 20K load, and a 10K load.

I just tried a voltage divider of 3k3 (Z1) and 6k6 (Z2) it behaves just fine. I then tried it reversed with 6k6 (Z1) and 3k3 (Z2) and it works fine like that too. This is driving the 10K load of my soundcard input. Great

So how do I calculate the load seen by the DAC, and the source impedance seen by the amp with a voltage divider like this?




In this case would the DAC see Z1 + Z2 as the load? And the amp would see Z2 as the source?

Thanks.

[Pano]

When I've talked to the chip designers they usaully say that they spec them at a certain load, e.g. 1K, and don't know how they perform at other loads. There seems to be a wide operating margin, tho.

Don't know why you need a voltage divider, is the output from the DAC too high? I thought the AKM chip in the Behringer output about 1.7V. Is it more?

Or is your voltage divider shown actually the volume pot?

[Tenson]

Hi,

I didn't have a suitable pot, so to test if it would work well at all, I used a voltage divider from fixed resistors.

So I'm not on the look out for a suitable 6 channel pot. I'd prefer a switched ladder network but I think it will be hard to find one with 6 channels, that would need 12 layers!

Its a shame the Welborn Labs unit is a shunt design as that would be perfect otherwise. You can just stack more boards on it, and its remote control with an LED readout.

[anatech]

Hi Tenson,
There is more to consider than simply how well an input stage is driven. The impedance of your line (interconnect) and source also has a lot to do with how much noise you pick up. On top of that, if the input is a little non-linear, any resistance in the source will determine how much distortion is created.

I'll be honest about my stance on "passive" preamplifiers. I don't like them. What you are talking about is essentially the same thing here. I feel some people ignore the entire picture and focus on perceived weaknesses. Often, a good buffer is far superior to "no active components in the signal path". Of course, poor buffer designs or poorly executed designs will degrade the signal every time.

I know that an absence of active components in the signal path sounds romantic, but that's all. There is little truth to the statement that the less components the signal goes through, the more "pristine" it is. What is more important is that you account for every problem in a system and deal with it.

Now, the only thing a transformer will buy you is galvanic isolation. This really shouldn't be a problem in a well designed system using intelligently designed devices. The other "feature" that comes along for the ride with a transformer is distortion and reduced bandwidth. This depends on both the quality of the transformer and how well it has been loaded. Transformers need to see the proper impedances on the primary and secondary.

So, which of the two evils would be the better choice?

-Chris