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?
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?
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.
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.
No need to muddle.
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se
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
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
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
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
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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.
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PGA2311 distortion ?
0.0002% at 2Vrms on 600ohms load, and < 0.001% at 20KHz.
Unity−Gain Bandwidth, Small Signal 10MHz
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Are we back to qouting THD+N numbers? I thought the progress have been made in the past 30 or so years.
If it works for you, Esperado, that's fine. Me? No thanks!
Sandman's article has a Fig. 2 which the old guys might recognize as the
basis for Mike Wright's ill-fated XEC1000 power amplifier. The idea was to
use a smaller amplifier correcting the larger one. The problem of course is
that the smaller one had to deal with the large load current....
It isnt a problem if you design properly for it. His problems doesnt have to be others problem unless it is an unsolvable issue. Two power amps as a single mono-block? Then see how it does in thd and sound. The thd at highest freq would be interesting. Maybe a flat thd vs freq without the usual rising thd as freq increases.
I can see other apps also; A S.E. input to balanced output circuit that has low thd for line levels.
Either topology might be used with IC Opamps of lower cost to get very low thd.
Easy way to have super low thd in oscillator circuits. Instead of just higher gain/fb approach.
No good reasons yet to not try it and see the thd curves resulting.... what about those abnotious upper middle/high freq everyone talks about -- gone?
Has anyone done much on these circuit approaches -- got articles?
Thx-RNMarsh
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Mike Wright went further in the last generation of XG - 10 mk III electrostatic included the transformer of the electrostatic speaker in the feedback loop and the user needed a second amplifier for the error correction signal.
XG 10 Mk III error_correcting-ESL
XG 10 Mk III error_correcting-ESL
If one wishes to control volume "digitally", I think the only decent solution is a bunch of digitally controlled relays that switch resistor networks. Life cycle of the relays is a problem, but everything else - specialized ICs, logdacs aroung ICs, A/D-DSP-D/A and so on is too much of a compromise. YMMV
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