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Passive volume control and resistance matching with digital sources

thaimusic

New member
Elements of Alan's fantastic Shock! and Horror! thread caused me to think more seriously about passive volume control and source switching for a primarily digital set-up. These guys can get you going for $49.00. Whether their unit or someone else's, I learned that there is one variable that may need consideration: resistance offered by the volume (variable attenuation) control. That unit's is 10k ohms output impedance. But other manufactures/builders spec them with up to 100k ohms. Some build-to-order shops request customers choose. So is this something that should be matched to the following amplifier’s input impedance?

Looking up the specs on a few of the solid state power amps often popping up here, I’ve found input impedances of 15k, 33k, 50k and 100k ohms.

Given: a) that most dacs seem to present 50-100 ohms to whatever’s next in line, and b) recognizing HUG wisdom that any needed attenuation of the source voltage has occurred to better match the power amp’s input sensativity, is the power amplifiers's input impedance the major variable (if there is one) to address this question. Are there trade offs or does it really matter?
 

thaimusic

New member
More on matching

More on matching

Searching a bit more on line revealed what appears some guidance. Evidently I used the wrong term in the title. What we’re talking about here is not matching but bridging. You can read about it here. Now this is from a specialized passive pre amp builder, so not sure if what’s offered is generally accepted practice of not, but hopefully others much more in the know can comment.

Generally it states that one should attempt to achieve an impedance bridging ratio between the source dac's impedance, and that of the combined impedance of the passive pre amp and amp, of 1:10, and preferably higher. The following equation is offered to calculate the total impedance of amp and passive pre: Rt = (Ramp x Rppre)/(Ramp + Rppre).

Trying this out using an amp with a 33k ohm input impedance and 10k ohm passive pre amp yields 7.7k ohms presenting itself to the source dac. That represents a 1:77 impedance bridging ratio for a dac with a 100ohm output impedance and a 1:154 ratio for a dac with a 50ohm output impedance. Assuming the validity of this rational, would be useful to know at what point this impedance bridging ratio is considered too low? A 100k ohm amp and 100k ohm passive pre amp pushes this ratio to 1:500 for the 100ohm dac and 1:1000 for a 50ohm dac.
 

willem

Well-known member
Or a DAC with volume control

Or a DAC with volume control

The impedance issue is beyond me. However, you are absolutely right that in a digital world there is far less need for a separate pre-amplifier. Just connect any good power amplifier, and you are done, other than that you will have no tone and balance controls (but your source may provide those).

That Shiit passive pre amp is a steal at 49 dollar (and far cheaper than many exotic offererings) if you already have a good DAC with all the inputs that you need. However, if you still need to buy a DAC, you might as well buy a DAC with volume control and switching between multiple inputs.

A cheap version of the idea would be the new Aune X1s at just over 200 euro, a more expensive one the Teac UD-301 at just over 400 euro, and e.g. the Benchmark DAC2 if you want the very best and have deep pockets (though not really by nutty audiophile standards - this is pro audio gear). As an aside, it would be interesting to find out if there are any audible differences between these offerings when used in a domestic setting.
 

thaimusic

New member
The vcariables in level matching equipment

The vcariables in level matching equipment

Willem, thanks for the commentary. To me, the rational toward a simple passive switch and volume control includes:

a) A bit more user electronics’ control. The increasing Hug discussions on mismatched components seems to be pretty easily resolved for those using dacs. With a dacs output specs known as well as those of the following amp, spec the volume control accordingly and you’re done.

Somewhat related, I tried recently to obtain specs from an often mentioned integrated amp manufacturer here about the specifics of their internal dac’s output stage and following pre amp’s input stage, and all they would tell me is that they were matched. I wanted to know how much of the volume control I’d have access to during dac playback, but I could not get that from them. Was kind of keen on that all-in-one, but since I can’t actually demo it...

b) Digital (d)evolution. The digital format thing has yet to settle down. One may desire access to some of the higher res stuff, not necessarily for the enhanced sonics, but the quality of the overall recording process that may have gone into these new productions. So only need the new dac—or new internals therein. Less expensive than a whole new unit with pre amp.

As you pointed out, the Teac’s relatively inexpensive though. And I agree, I’ve longtime admired the Benchmark. I see they even have 10db and 20db attenuation jumpers for their balanced output stage. And for my situation, a remote is mandatory.

Options for tone controls in the digital domain are increasingly available, but absent a more traditional pre amp element the only thing I’ve found that will take care analog as well is DSpeaker’s Anti Mode 2.

Horses of courses, and I am just looking to better understand the variables one might consider if pursuing a passive volume control between their dac and power amp.
 

willem

Well-known member
DSpeaker Antimode Dual Core as pre amplifier.

DSpeaker Antimode Dual Core as pre amplifier.

Yes the DSpeaker Antimode Dual Core is another variant of the idea. It is a DAC (plus one analogue input) with volume control and can serve as a pre-amplifier (with remote control).

The difference with other DAC's with volume control is, of course, that it also does automatic room equalization of the lower frequencies and advanced manual tone control/equalization.

It could do with a few more digital inputs, however. Also, Alan experienced some as yet unexplained sonic issues with it to do with high background hiss levels when used between a pre and power amplifier.
 

acroyear

Active member
Loud enough to cover hiss?

Loud enough to cover hiss?

...

Horses of courses, and I am just looking to better understand the variables one might consider if pursuing a passive volume control between their dac and power amp.
I recently tried the emotiva 'control freak', that will work as an attenuator/volume control between a preamp/integrated or power amp, it is one input only so could work on the end of a multi input DAC but you do have a quite a lot of cord to contend with as you have to piggyback it to the interconnect going into your amp (a very short interconnect would be useful here).

There are RCA and XLR models, I do wonder though (as Alan has suggested) that with quieter recordings would the signal be loud enough being fed directly into a power amp (especially a pro unit) to get full power if needed.

Edit: on the topic of noise, I still feel it is worth listening to how hissy your amp is, a recently tried NAD solid state amp has pretty much no audible hiss, another (modern) amp ive tried recently had quite loud hiss and hum at the speakers, almost as noisy as a big pro amp with its pots wide open, beware.
 

Jeff_C

Member
Matching should be straightforward

Matching should be straightforward

...I do wonder though (as Alan has suggested) that with quieter recordings would the signal be loud enough being fed directly into a power amp (especially a pro unit) to get full power if needed.
This should be straightforward. Providing the equipment feeding the passive volume control is capable of outputting a higher voltage than is necessary to get full power out of the power amp, then the passive volume should give you adequate control over the signal. A typical DAC or CD output of 2V with a power amp with input sensitivity of say between 0.5V to 1.8V should be fine

In my opinion the only reason for having a very quiet passage on a recording, is so that later in the recording there is a better contrast with a much louder passage (good use of dynamic range). If the recording meanders through from the start of the CD to the very end quietly, with a maximum voltage (for a CD of say 0.2V) and the recording is a symphonic work, then that is just a poor recording and should not induce a reaction whereby we worry about whether our components are adequately matched.

Sometimes it is better or even imperative if the recording is quiet relative to something else. If I play a gentle piano recording where the keys are being tickled I expect it to be recorded quieter than a piece where the piano keys are being hammered. It is better to experience that loudness difference (even when swapping from one CD to another) without having to jump for the volume control to create the contrast for yourself.
 
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