Son of Dork: Attenuator

[dorkus]

The Son of Dork (SOD) project is the result of the diyAudio.com multi-channel preamp initiative (The diyAudio.com preamp project!). The project has been split into threads dedicated to discussion of the preamp's different functional units:

- General Discussion (project status, functional requirements)
- Active Circuitry
- Attenuator
- Power Supply
- Connections and Layout (including input switching)
- Chassis and Construction
- Hardware Logic, Controls, Display
- Microprocessor (programming, implementation)

I will be managing the project as a whole, as well as initially overseeing individual threads. For a background on the project, please see the original thread referenced above. A website dedicated to the project will be located at http://dorkus.org/diy/sod.

Let the fun begin!

[dorkus]

the WM8816... i know some don't like it because it requires an opamp, but was wondering how it sounds in combination with a "good" IC, e.g. AD825 or what not... if the chip is capable i may try it with a discrete opamp design.

thanks,
marc

[sonnya]

I guess i have to etch some more boards next week!? Actually i have five pieces laying around. They are stereo devices so i could mail two of them too you!? Should be enough.

Sonny

[pmkap]

the WM8816... i know some don't like it because it requires an opamp, but was wondering how it sounds in combination with a "good" IC, e.g. AD825 or what not... if the chip is capable i may try it with a discrete opamp design.

Absoluted no need for an op amp whatsoever, discrete or otherwise. All that is needed is a inverting amp, you could do it with a plate output triode if you so desired, a single transistor...

Sonny,

Have you tried the ad8610 with the wolfson yet?

[sonnya]

No Paul ... i have not been able to find time for this right now... But in this summer i will have 4 weeks vacation so there should be plenty of time...

For the inverting and no feedback...

The performance of the WM8816 is based on a feedback design... But if the output of the WM8816 is held onto virtuel ground then this should be possible!?

Nice idear Paul ... Worth a try! ... With the right circuit the output will also be in phase with the input.

Drawback ... The resistor network inside the WM8816 will change a lot from unit to unit .. ~30% So you have to compensate for this gain variation

Sonny

[dorkus]

i'm running out of attenuator ideas...

other than the wolfson, i have maybe one other scheme. we could use a dual-stage circuit, as i saw posted on some german website... there would be a 10-position attenuator at the input of the preamp (0dB to -9dB in 1dB), followed by the gain stage, then followed by a 8-position attenuator (0db to -70dB in 10dB steps), then followed by a buffer or another gain stage. this would effectively give us 0dB to -79dB attenuation in 1dB steps. suggested gain stages are Borbely balanced SE JFET, Borberly "super buffer", maybe even a simple single-ended FET buffer for the 2nd gain stage. the actual attenuators would be fixed-series, variable shunt using solid state switches to select the shunt resistor (e.g. AD SSM2404). since one end of the switch will be connected to ground, an FET switch should be fairly linear here, with low on-resistance.

thoughts?

[mlloyd1]

dorkus:

sounds good.
now get busy, I want to see some results!
mlloyd1

[AuroraB]

Anyone got some real down to earth thoughts on the placement of a single attenuator...input or output..???
Seems like putting it in the output leg makes you more dependent on a higher load impedance..??
Any other pros and cons....??

[mlloyd1]

The Dallas/Maxim DS1808 attenuator is also an "op amp less" part. It might be worth investigating. I haven't been successful yet at getting any samples though. So, I wonder if it's a real part or vaporware.
mlloyd1

Originally posted by dorkus
the WM8816... i know some don't like it because it requires an opamp, ....

[tvi]

There are Japanese manufacturers of Volume Control ICs, alot include bass, treble and loudness functions, the following don't:
<a href="http://www.semicon.toshiba.co.jp/eng/prd/mpsig/ft_mpsig.html">Tohiba Electronic Volume Control ICs</a>
<a href="http://www.semicon.toshiba.co.jp/en/bucat_2/bucat_3/bucat_14/td_7/TD.pdf">TC4959</a> is 91 step 0 to -89dB
<a href="http://www.semicon.toshiba.co.jp/en/bucat_2/bucat_3/bucat_14/td_8/TD.pdf">TC9482</a> is 97 Steps 0 to -95dB, 6 channels in one chip.

<a href="http://service.semic.sanyo.co.jp/semi/ds_pdf_e/LC7536M.pdf">Sanyo LC7536 </a></body> is 81 step 0 to -79dB

As to sound ????


James