[Folsom]

Hi guys. I've been intending on making some PCB's for this, but that'll be a separate ad (and it'll have other features)

So here's the diagram and part numbers of what I mentioned in another thread. These attenuators despite their originals are flat out very good. And the ground isolation seems to do great things for classD amplifiers in particular. The enclosure MUST be metal.

*Please notice the RCA jacks are only grounded to chassis following the ground isolator and DACT. You can simply negative the isolation rings that come with them if you sand a little bit on the enclosure so it makes contact if it's not already bare metal. The ones prior must be isolated from enclosure.

*You're responsible for RCA jacks and enclosure.

Mouser Part Numbers:
2x SC-02-300JV
2x PTF563K3000BYEK

eBay DACT 10k

These are nice project boards that allow anchoring of the CMC's with solder. They're a bit heavy.





*Ground isolator credit to Keantoken on DIYa.

*I use 3.3kohm resistors as best fit. If music sounds too dark you can lower the resistance up until 2k. If it's too bright you can move up as far as 10kohm. But I'd try the 3.3kohm first. Adjusting your system this way might just be somewhat inappropriately trying to muddle with problems it has elsewhere.

[eis]

I'm going to build this project in a few weeks. One question - I know it's a noob question - what is the purpose of the choke?

[Folsom]

I'm going to build this project in a few weeks. One question - I know it's a noob question - what is the purpose of the choke?

It isolates the grounds. No ground loops can form, and it reduces a dozen possibilities (common ones) of ground noise. It's not required, but since attention to the scheme for wiring with the volume device is important, this is an apt configuration.

For my setup the TPA3116 only now shines as being good with the ground isolator.

*Order the eBay item soon, it takes a couple weeks.

[keith_correa]

I'm assuming that this would work well with non stepped Attenuator pots and with values of pots other than 10k too. Correct?

Also, can I request you to please point me to an equivalent Coilcraft choke. They're more easily available where I'm at.

[Folsom]

I'm assuming that this would work well with non stepped Attenuator pots and with values of pots other than 10k too. Correct?

Also, can I request you to please point me to an equivalent Coilcraft choke. They're more easily available where I'm at.

Yes but I wouldn't ever recommend anyone use a pot. They sound terrible by comparison. The Chinese DACT sound is better than something like Dale Vishay attenuators and such. The only bad thing about it is only having 24 positions.

Maybe the CMT4-32-4L is worth a shot.

 

[keith_correa]

Yes but I wouldn't ever recommend anyone use a pot. They sound terrible by comparison. The Chinese DACT sound is better than something like Dale Vishay attenuators and such. The only bad thing about it is only having 24 positions.

Maybe the CMT4-32-4L is worth a shot.

Thanks!

[Davey]

It isolates the grounds.

It does?

Dave.

[Speedskater]

It does?

Dave.

I don't think that this very strange circuit does anything good.
It would be interesting to see the frequency response and distortion measurements.
And O-scope photos of square waves.

[Folsom]

It does?

Dave.

Yes. And it has no perceivable distortion to worry about. I'm using one right now, and again, for the first time my TPA3116 sounds not just ok, but good.

Kevin you keep weighing in on this, but I've never seen you produce designs for buffers, capacitor multipliers, or new and improved LTspice modeling... anything really. Why should someone be inclined to jump on your nay-say wagon when you're not really demonstrating the capability of/near/over someone like Keantoken? I'm well aware that you know a lot, particularly when it comes to AC wiring, but the just walking in to piss all over isn't beneficial to the forum.

[kc8apf]

As your diagram shows, a common-mode choke is an inductor which is a piece of wire with frequency-dependent impedance.  Adding a resistor in parallel reduces the DC resistance component just like paralleling two resistors will.  That doesn't isolate the grounds as there is still a DC path.  It just has a bit of resistance in the way.  An ohmmeter between the input terminal ground and output terminal ground will still show continuity.  What you _have_ done is create a star-ground topology and added common-mode noise suppression.  Both are good things but they are not isolation.

[Davey]

Yes.

No.  It doesn't....at least not the way you have it drawn.  At least not galvanic isolation.

If I connect my ohmmeter between RCA outer connections I'm not going to measure infinite resistance....I'm going to measure zero ohms (300 milliohms.)  At high frequencies my meter will measure 3.3kohms.

I like the idea of galvanic isolation of input/output in certain situations.  But if you're going to do it, do it right.....with a proper transformer scheme.  Not exactly cheap but it is the best way.

If you say it works well, I believe you.  But let's not make technical claims that aren't true.....or at least use more precise language so the newbies aren't confused.

Dave.

[Folsom]

While there may not be a definition as such _______ ground isolation that would fit your criteria, it does perform isolation. It may not be 100%, but it does for nearly all intensive purposes isolate. There's a lot of devices and schemes for preventing ground noise/loops that get applied in equipment (and not at all in some) that aren't necessary with this.

It works so well that if you were to try and connect the enclosure to the input side grounds you may get audible noise. I've performed this feet myself when playing with it initially. (the enclosure and cables become an antenna together on the amp side)

It's funny not a soul on DIYaudio has mentioned the concerns you guys have, including some of it's most experienced and senior members (that have seen it, numerous times).  The question is the validity to its usefulness, to which I say, try it, even if you simply intend to measure it and show it's wrong.

[*Scotty*]

Jensen makes transformers for this application. I for one would not be inclined to put an inductor in the signal path at that point in my system. I have never been a big fan of hysteresis distortion.
You may indeed be unable to hear any negative consequences from inserting this circuit into your system.
Scotty

[Folsom]

Jensen makes transformers for this application. I for one would not be inclined to put an inductor in the signal path at that point in my system. I have never been a big fan of hysteresis distortion.
You may indeed be unable to hear any negative consequences from inserting this circuit into your system.
Scotty

They're also rather expensive.

At first I didn't think this was wise, either, but it's proven different.

[Davey]

If a person is interested in a nice volume control that actually DOES have isolation keep your eyes peeled for one of these:
You might find one cheap.

This is the NHT PVC (no longer available) that incorporates two Jensen JT-11P transformers and a very decent ALPS 5k dual control.  The transformers are on the input side and the 5k load is optimal to create a non-peaked response at the top of the audio band.  The transformers roll-off the top end above 20khz.  The controls on the two units I have exhibit excellent left/right channel matching.

Dave.

[Folsom]

Except it uses a lot...

[KR500]

It's funny not a soul on DIYaudio has mentioned the concerns you guys have, including some of it's most experienced and senior members (that have seen it, numerous times)

I tried to find any discussion on DIYaudio using the term volume Device+ Folsom but couldn't find any posts .
Is there a link or thread title anything may have appeared under ?

thanks

I wonder what is in the Emotiva or Schitt VCD other than a potentiometer ? Can't be much more at that price point

[srb]

I wonder what is in the Emotiva and Schitt audio volume Controls other than a potentiometer maybe ?

Just a potentiometer in the Schitt SYS.  I would imagine the same for the Emotiva Control Freak and the TC Electronic Level Pilot.

Steve

 

[KR500]

 Steve , I just checked the company site
Guess I needed the second cup of coffee this am to make that leap

[Davey]

The Schitt is a 10k control.  I have one of those too. 
It doesn't have the same excellent left/right channel matching of some others I've tested, but there may be some unit-to-unit variation.  As usual with these, if you get the rotation near 12 o'clock and higher the channel matching is much better.

A nice little box with two inputs for $50.  You really can't beat that.

Dave.