Digital/analog volume control experience

[janneman]

Hi all,

Yesterday, with a few friends (Leo, Walter, Guido (gydotron.com) we tried out the digital volume control system from Wolfsom Microelectronics, the WM8816. The WM8816 differs from other similar system like the PGA23xx series in that it comes without an opamp. The chip just contains the control logic, switches and attenuator ladders. Another unusual feature is the constant input impedance (around 7k). In most systems, the input imp. varies with the level setting.

I bought the WM8816 evaluation module shown here, which contains a two channel system with an MC33078 opamp. More on that opamp later. The module also has a subwoofer output, which we didn't use:

[janneman]

The control was done by the micro-controller board which also is available from Wolfson (I bought all from profusionplc in the UK, btw). This board has a simple interface, but can control up to 3 8816 boards for 5.1 plus subwoofer systems! The 8816 board is about 100 euro's, the controller board about 150.

We tested this system against Guido's passive level control (switched attenuator), using our own favorite music. No blind testing, I'm afraid...
Power amps were LC Audio class-T boards (Good job Lars, I was sceptical but these power amps are first-rate), on Guido's Apogee Scintilla's. This is the control board, set for -2dB attenuation (as is, the range is from +15.5 to -99.5dB). The board also has provisions for peak level detect, mute and balance:

[janneman]

After two hours of all kinds of music, some beers etc, we agreed that the WM8816 made the sound more rough, less smooth than the passive control. But now our problems only began. because, what is 'best' ? Some preferred the passive, because it was more smooth, some preferred the wolfson, because it showed more detail (or so we thought). So, although we DID agree on the difference, we DIDN'T agree on what was preferable. (Right, guys?) Oh, and yes, I caused Guido almost a heartattack when I remarked that on the best of passages, his system came really close to my car stereo. Lucky for me, his sense of humor is as big as his audio savvy.

Anyway, we found that we could comfortably live with either system.

This is Walter (left) and janneman trying to make sense of what they hear:

[janneman]

Forgot the pic, and apparently this cannot be edited in:

[janneman]

Leo and one of his friends are putting together a demo board with a PGA23xx. When that's ready, we'll A-B it against the Wolfson, see what that gives.

Anyway, we would be gratefull for any experiences with this stuff anyone had.

Oh, and yes, that MC33078. This was an unknown opamp for me, but apparently it is a specially designed audio opamp. What is interesting is that it has the usual lf rolloff from a few 10's of Hz like they all have, BUT the THD vs freq curve is rather flat until beyond 20kHz. That IS unusual. It looks like they have some clever feedforward stuff inside. Also, another unusual feature is the class A output stage. Anybody using this chip?


Jan Didden

[janneman]

Nobody ever used that MC33078? Really?

Jan Didden

[SY]

Originally posted by janneman
Nobody ever used that MC33078? Really?

Jan Didden

Really.

But it's on the list.

[JensRasmussen]

Hi Jan,

I have used the MC33078 from Onsemi - I think it's their version of the NE5532

I have some 8 pin SMD types if you need any I'll be happy to send them to you - no charge

\Jens

[A 8]

I found it in a soundcard from Creative.

[VT67]

Jeez, it took some digging to find this thread again. Not many people seems to care about a digital volume control.
I'm almost done with my version of Elektor's "Audio Link", a preamp/volume-control based on the PGA2311PA from TI/BB. This is controlled by a PIC18LF452-I/L from Microchip. Al the parts are soldered on the board except for the analog power supply. Don't like the 7805/7905 combo and some crappy cap's to get a deicent symmetric supply to feed the analog part of the IC. Also managed to program the µcontroller, this was the first time I did this so whe'l call it beginners luck!
So, Janneman in the near future we can have a "Battle of the Best" to see which one performs the better.

Happy listening

Walter