Balanced Volume Controller / Line Stage

[alexcp]

Originally Posted by zgtc

Would it be possible to get the schematics in the .sch type, not an image?

Just sent you PM.

Originally Posted by zgtc

Also, where do you buy the WM8816 chips? I think they're not available anywhere

I have a small stash. Otherwise, eBay is (usually) your friend for things that are not available elsewhere.

[alexcp]

The schematics posted above raise questions on why so many opamps for a simple volume control, and why just not limit yourself to one, or better no opamps at all?

The question is valid, and of course, alternatives exist - see, for example, post #27 above.

In the attached schematic, however, each opamp has a role:

• U3 together with R5,R6,R7 provides high (2.2Meg) common mode input impedance and reasonably high (~100k) differential mode impedance. Besides lighter load for the signal source, this helps to mitigate the imbalance in the source's output impedances on the "cold" and "hot" wires.
• U4 compensates for the DC component of the input signal, as well as for the DC offset that is created by U3's input currents flowing though R5.
• U5A is the difference amplifier, which calculates the input signal as the difference between the "cold" and the "hot" inputs.
• U6 is the volume control proper; its gain is controlled by the WM8816.
• U5B is the ground sense output stage that allows the following stage (say the power amp) to be referenced to its local ground (that is, to have a single ended input) without creating e.g. ground loops.

[zgtc]

Hello again,

Thank you so much for all the details. I really need to digest all the info, too complex for me at the moment. Just a pair of questions though.

1. Looks like providing balanced in for each channel is compulsory as it is explained above, right?

2. If that is not the case, if we can input unbalanced V+ for each channel, and if (big if) we already have passed the signal through an active filter (opamp based), impedances would already be OK, so wouldn't it be possible to start at U5 position?

Sorry again, I know these are very newbie and maybe stupid questions, but I'd like to learn a bit more.

Also, again, not trying to criticise the design at all.

[alexcp]

The article that started this whole project can be found at edn.com. In it, Bruno Putzeys comprehensively and thoroughly explores the balanced/unbalanced/grounding topic and, I believe, answers your questions better than I can. Spoiler: he suggests that "the RCA connector and all it stands for should be banned by law".

[zgtc]

Originally Posted by alexcp

The article that started this whole project can be found at edn.com. In it, Bruno Putzeys comprehensively and thoroughly explores the balanced/unbalanced/grounding topic and, I believe, answers your questions better than I can. Spoiler: he suggests that "the RCA connector and all it stands for should be banned by law".

Yeah I know about the excellent article. I think there’s a misunderstanding tho, in question 1 I was referring to the balanced signal before the volume controller, not after. I will create the balanced out at the exit of the preamp, not in the DAC, as it seems to be the case here, if I understood correctly.

[alexcp]

Originally Posted by zgtc

Yeah I know about the excellent article. I think there’s a misunderstanding tho, in question 1 I was referring to the balanced signal before the volume controller, not after. I will create the balanced out at the exit of the preamp, not in the DAC, as it seems to be the case here, if I understood correctly.

Sorry I did not understand your context. If you want a volume control to be connected to the output of the active filter following your DAC and follow the logic of Bruno Putzey's article, then:

• You can probably omit U3 and U4 and the associates circuitry if your active filter can drive 1kohm load directly.
• If you can have your active filter use the volume control's local "ground" (the point where R19 and R21 connect) as the active filter's output reference point, the way U5A uses it as its output reference, then the active filter can replace U5A and everything before it.
• If you active filter uses some other reference point for its output, you'd need to keep U5A.
• You can omit U5B if you do not need the ground sensing feature.
• When you take stages out, keep in mind that the absolute phase of the output signal should be preserved.

It would be helpful if you posted the schematic of your DAC's analog output.

[zgtc]

OK I think it all makes more sense to me now. still need to digest all your recommendations tho. What’s the “ground sensing feature”?

[zgtc]

Originally Posted by zgtc

OK I think it all makes more sense to me now. still need to digest all your recommendations tho. What’s the “ground sensing feature”?

OK just read about it in first page of the thread, all clear now. Thank you again