Passive Volume Control for DACs/Sources

Also not trying to derail the topic, but, do you mind sharing a few impressions of the Zkit60? I've been thinking about a "summer amp" lately, and, I think it's the Van Alstine SET 120 that's based off the same design, which I had been considering. And that got me thinking, maybe I should tackle this amp. Easy to build? Sounds good?

And I love the idea to put the Slagleformers in the amp - might have to steal that idea if I go for this build. One less thing to throw off some heat!

 

I really like it, and it wasn't bad at all to put together. Decware's forum at their site has a section devoted to it, I don't think there's many people that have built it. Send me a PM if you have any questions or want more info.

https://www.decware.com/cgi-bin/yabb22/YaBB.pl?board=ZKIT60

 

Thanks! Still in the "kicking the tires" phase, but if I get more serious or buy a kit, I might take you up on it.

 

So, I recently purchased the Khozmo DIY source selector and shunt kit, with Z-foil 50K loading resistors and AMRT resistors.


https://khozmo.com/product/shunt-type-64-steps-attenuatorwith-6x-input-selector/

I am still waiting on a custom case for everything, however I did manage to put it in a small crap case and just hook it up with some short 1ft XLR cables and one RCA out just to see how it would work and how it would sound. I got rid of my Freya for this because I was only really using the Freya 1x mode and something that was crazy is that my unbalanced outpt from my FM receiver was always louder and cleaner when going passive on the Freya than going through the 1x buffer, especially on unbalanced output, and I can't figure out why. But that is a different story.

All I can say for a few days using this, I never thought I needed a passive and thought it was always defeating the point of a good RK27, and I think that still holds true, because you ARE reducing voltage of the incoming signal and then having to send that through cables to your amp or whatever. We are talking way low mV here. Micro volts even. But the signal is clean, has good transients and top end and air.

The relay noise is much quieter than the Freya and also does not cut volume as you go up and down - it is pretty smooth. I also found that 63 steps is way more than enough for me. Maybe because I also went with 50K. I still find that for normal listening I am in the high 20's or low 30's.

This particular DIY setup does some pretty cool things, like customize the input names, adjust sensitivity of the encoders, and pushing the selector/volume is customizable as well to things like mute and standby. So my volume is set to mute when pushed in and the selector is set to go to standby.

I think the limitations of a passive is obviously going to be: length of cable run of output, quality of cable for output, and the quality of the resistors (if ladder or shunt) or the quality of the pot. I just really like the options and price of this Khomzo stuff. Of course you can go with their pre-built stuff or the more high end designed Hattor stuff. All are really good options.

 

I was told at DIY audio that I can convert my DIY Slagle autoformer from RCA to XLR an unexpectedly simple way:
1. pin 1 ground is simply connected bypassing the autoformer
2. pin 3 cold is a former RCA ground, connected to common autoformer pin and also directly to pin 3 output
3. pin 2 hot is a former signal in RCA, split by autoformer, the moving contact goes to output pin 2

Can you confirm it will work? I should have it done in a few days, I'll need few days of listening then.

 

This is no different than using an RCA->XLR adapter. It works, but it doesn't make your signal "balanced".

 

AI says it may add even order distortion:
" Symmetrical Processing (Ideal Balanced): When both the positive and negative halves of the signal are amplified or attenuated by the same amount, any non-linearities in the system tend to generate predominantly odd-order harmonics (3rd, 5th, 7th, etc.). These distortions create a "harsher" or "brighter" sound and are often less musically related to the original signal.
Asymmetrical Processing (Your Proposed Attenuation): When the positive and negative halves of the signal undergo different amounts of attenuation (or other processing), the resulting non-linearities introduce more even-order harmonics (2nd, 4th, 6th, etc.). Even-order harmonics are octaves of the original frequencies, which tend to sound more "musical" or "warm" because they are harmonically related to the fundamental tone. However, they are still a form of distortion, an alteration of the original signal."

I wonder if this is correct because it doesn't explain. Yes this doesn't make the signal balanced, but imagine if I attenuate pin 2 halfway it will be 1/2 balanced vs RCA unbalanced.

Also the part I don't understand is if suppose the attenuation is off completely, does it mean the 4V XLR signal is twice louder than 2V RCA signal using the RCA->XLR adapter, or the amp takes care of this and the volume is comparable? Is there any preference for the amp to have both legs active vs pins 1 and 3 shorted?

 

Don't get scared off just because of the possibility of noise boogeymen. If your environment is relatively free of interfering signals and you're not criss-crossing a ton of signal wires and power wires around each other, you're probably fine. If there weren't any noise gremlins to begin with (which I would think so if you were already using RCA connectors before), then trying to bootstrap XLR to get rid of noise that doesn't exist isn't going to make any difference.

Also XLR is not magically louder than RCA. Either connector can handle huge magnitudes. Hell, I've run speaker level signals through both XLR and RCA before. The whole 2V vs 4V is simply a general standard that the output stages general follow.

if you use an adapter to go XLR->RCA by shorting pins 1 and 3 (which I never recommend under normal circumstances), then you lose half the signal. So yes, it goes from a 4V down to a 2V signal. Half the voltage typically translates to a quarter the power, thus -6 dB. The amp side *usually* doesn't care (unless it has some kind of smarts built it to do level matching between sources).

As a general rule, don't short pins if you can avoid it. Not as crucial on an amp receiving signal, but potentially very bad on source components. It's in the very word that we use. If you short an electrical circuit, you get a ton of current and hopefully pop the breaker because it knows something is wrong. On a dac/source/whatever, shorting output pins means you're draining a relatively high amount of current which can fry your outputs. Pro gear usually has protection circuits built in or current limiters. Hifi may or may not.

 

I forgot to explain what I am trying to achieve - I was told that the XLR connector should sound better than RCA, unrelated to noise (I don't have RCA noise). I have no idea by myself, I never tried anything XLR for the only reason that my autoformer volume control is RCA by design. So there is a certain ephemeral sound quality that statistically speaking people suggest - some people say for them XLR sounds the same, some people say better, nobody ever said it's worse.
So I would say if it's for sure not worse the way I want to do it, it's worth doing. So I'm really just interested to be sure I'm not doing something likely to be wrong or sounding bad, "identical" outcome is acceptable. Even is the solution is identical for all technical reasons, XLR connectors are low mass by design, so I better invest into good XLR connectors and get better outcome just because of the connectors, than waste money on upgrading RCA Amphenol connectors I have.

Update: no I don't use XLR->RCA now, Bifrost has an RCA output. I suppose the cold pin is simply not connected to anything. On the receiving side, Purify has an RCA input as well, which is simply shorting 1 and 3 internally with a switch. And BTW it sounds scary because this switch can short DAC output if I was using XLR output. I hope the DAC has protection.

Update2: if there is any difference besides noise, this is what I think could be different: the DAC has 2 16 bit (?) DACs per channel inside, they convert digital for pin 2 and 3 separately. Could be that the DAC resolution is higher when both pins are in use?

The amp input buffer is using 2 channel opamps. It means it amplifies the pins 2 and 3 separately _before_ applying the differential circuitry to merge them. It means the current taken from opamps is 2X, and the amplifier likes to have high current input for better dynamics.

And finally, if pins 2 and 3 are separated before the DAC, effectively the whole analog pipeline is separate up to the final amplification. Any irregularities in the signal randomly inserted by the analog pipeline (unrelated to the cable noise, that part is no issue) will have only 1/2 effect if the other pin has no identical irregularity.

 

Done, sounds amazing! This very unusual XLR wiring approach is #605. Soundstage noticeably deeper (it was wide enough before). Bass tighter and 5 Hz deeper - from 43 to 38 Hz, on a desktop nearfield. Shivers factor up. A/B with Schiit Sys RCA pot - not even close. Completely different no Shiit level.

Wired with a simpler transformer wire, Neutrik connecters, low capacitance winding around teflon tubes. Noise is slightly up, because I went from shielded RCA to unshielded - will fix it eventually, nothing urgent.

Strongly recommended, if you already have an RCA autoformer volume control rewire it to XLR as above.

 

I've been looking into passive volume control Slagle Autoformers lately & found this on eBay
https://www.ebay.com/itm/205479968396?

At under $800 is this the cheapest pre-built Autoformer available?

I also caught this in one of the recommendation in that page
https://www.ebay.com/itm/135677100612?

Would that be considered a passive volume control Slagle Autoformer as well, or is it different?

 

A Slagle is not active, so yes , passive. Just a different type of passive.

 

Well you need someone to have them and test them. In contrast, you can easily know that Slagle is kicking the ass because I have and my ass is kicked. Slagle balanced vs Schiit Sys RCA is like a major upgrade, imagine replacing your amp with 2x of it.