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Old 31st January 2013, 10:47 PM   #33941
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Back to basic line amps:
If we have to select an input pot or resistive attenuator, it is always a tradeoff. Sometimes it is mostly limited to what is available. For example, a P&G pot might have only a 10K max resistance rating, so for tubes, usually we have to add a series resistor on ONE position of the selector switch for a vacuum tube product to be connected.
Another problem is balanced input. It is next to impossible to have perfectly tracking multiple pots, but you might need a 2 gang or 4 gang pot, in order to make a convenient balanced input. You never get perfect common mode cancellation, BUT you get enough common mode reduction to be advantageous.
Potentiometer quality varies with 'sound quality', distortion, tracking, noise, inductance, etc, so many really high quality attenuators are made of discrete precision resistors, so that control of every aspect can be maintained.
In other words, a perfect potentiometer is both very expensive and virtually non-existent, but we muddle through.
Of course, another approach, often taken by mid-fi, and others, is a digital volume control that gives you remote control, good tracking, and perhaps a low impedance drive for the following line amp. This is one way that PMA could use his AD797 line amp and still have it quiet. It is difficult to find or make a PERFECT digital control, and I am still looking hard, because I need one for future designs, myself. Any suggestions anybody?
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Old 31st January 2013, 10:53 PM   #33942
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Quote:
Originally Posted by john curl View Post
It is difficult to find or make a PERFECT digital control, and I am still looking hard, because I need one for future designs, myself. Any suggestions anybody?
PGA's are NOT midfi.
Just look at Ebay to find very cheap PGAs based digital volume controls and play with it (the command is done, and there is very little to do to make them near perfect on the audio side.
You can start to listen in less than one hour.
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Old 31st January 2013, 11:01 PM   #33943
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That easy, huh? Oh good.
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Old 31st January 2013, 11:23 PM   #33944
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Originally Posted by RNMarsh View Post
With the capability to measure in everyones hands - we ought to start measuring this more often in respect to tube pre/amps (transformers) as well as termination of speaker cable lines at a given loudspeaker and the like... Microphones, too. Sware and HWare that run by PC can do this easily now.
How does terminating a cable at the speaker terminals improve Group delay? What would it take? Thx-RNMarsh
I'll await your results. In the meantime, I'll just enjoy some good music.

se
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Old 31st January 2013, 11:30 PM   #33945
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Quote:
Originally Posted by john curl View Post
Another problem is balanced input. It is next to impossible to have perfectly tracking multiple pots, but you might need a 2 gang or 4 gang pot, in order to make a convenient balanced input. You never get perfect common mode cancellation, BUT you get enough common mode reduction to be advantageous.
Can just use a good input transformer for your balanced input/common-mode rejection/ground isolation, and the attenuator on the secondary can be unbalanced.

Quote:
Potentiometer quality varies with 'sound quality', distortion, tracking, noise, inductance, etc, so many really high quality attenuators are made of discrete precision resistors, so that control of every aspect can be maintained.
In other words, a perfect potentiometer is both very expensive and virtually non-existent, but we muddle through.
No need to muddle.

Click the image to open in full size.

se
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Old 31st January 2013, 11:59 PM   #33946
mlloyd1 is offline mlloyd1  United States
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john:

not perfect, but have you ever investigated the wolfson digital volume controls? no op amps included; just the resistor networks and logic to control them.

they are very nice.

nelson and wayne know about another device from japan, but i have not played with that one at all.

mlloyd1


Quote:
Originally Posted by john curl View Post
Back to basic line amps:
If we have to select an input pot or resistive attenuator, it is always a tradeoff. ... It is difficult to find or make a PERFECT digital control, and I am still looking hard, because I need one for future designs, myself. Any suggestions anybody?
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Old 1st February 2013, 12:29 AM   #33947
fas42 is offline fas42  Australia
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Just to add my voice ... digitally controlled volume controls, properly integrated, are no worse, or better, than a high quality, audio friendly, opamp. They certainly don't add the pernicious distortion that taints a very high percentage of expensive hifi ...

Edit: I suspect why analogue controls are still so beloved, is that they add precisely the right type, and quantity, of distortion to make, generally accepted as "bad", recordings sound really vile, and, add just enough interest to "audiophile" recordings so they don't sound completely mundane ...

Frank

Last edited by fas42; 1st February 2013 at 12:35 AM.
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Old 1st February 2013, 12:38 AM   #33948
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Originally Posted by mlloyd1 View Post
not perfect, but have you ever investigated the wolfson digital volume controls? no op amps included; just the resistor networks and logic to control them.
mlloyd1
Same with the old DS1666

http://datasheets.maximintegrated.com/en/ds/DS1666.pdf

Slap a 170FET buffer on it! Sweet!

Another thing about these and stepped attenuators is repeatable exact volume setting. A slight increase/decrease can make such a difference to the sound.

Last edited by AudioLapDance; 1st February 2013 at 12:43 AM.
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Old 1st February 2013, 12:49 AM   #33949
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And what devices are used to switch between ladder or R-2R resistors inside all those chips? Mosfets! And that's another can of worms, so to speak.
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Old 1st February 2013, 12:57 AM   #33950
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Originally Posted by elektroj View Post
And what devices are used to switch between ladder or R-2R resistors inside all those chips? Mosfets! And that's another can of worms, so to speak.
PGA2311 distortion ?
0.0002% at 2Vrms on 600ohms load, and < 0.001% at 20KHz.
Unity−Gain Bandwidth, Small Signal 10MHz
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Last edited by Esperado; 1st February 2013 at 01:00 AM.
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