Leaving out the potentiometer?
May 2, 2012 at 4:26 PM Thread Starter Post #1 of 7

shake

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Hey guys, I'm planning to build a new desktop amp and I got two quick questions.
 
1. Since I can control the volume on my PC/phone, is there really any point in putting a potentiometer in the amp?  I know a big issue for amps is the quality of the potentiometer, so not having one would be best?
 
2. If I were to leave out the potentiometer, how would the wiring change?  Would I need to have a resistor in place of the potentiometer going to ground, the input capacitor, and the resistor going to ground after the input capacitor?
 
Thanks.
 
May 2, 2012 at 5:12 PM Post #2 of 7

johnwmclean

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A volume control is not mandatory, controlling digitally or with an analog pot both have pros and cons. Digital will have better channel and impedance matching, analog has better resolution.

Wiring is simple, there is no need for extra resistors or capacitors. Inputs would go straight to the amp board, outputs directly to the headphone jack.
 
May 2, 2012 at 5:16 PM Post #3 of 7

Yoga Flame

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Yes, leaving out the pot and controlling the volume from the source will avoid the small amounts of noise that pots introduce. No channel imbalance too. But it also depends on how volume control is implemented on your source. With CD quality music, you get 16 bits of dynamic range. As you lower the volume digitally, that range decreases. "Throwing bits away" as it is sometimes called.
 
On the other hand you can have quality volume control on a DAC itself, such as with the Buffalo DAC. These DACs process everything at 24/32 bits internally regardless of the source material. So even if some bits are discarded to decrease the volume, there is still plenty left to represent the full dynamic rage of human hearing.
 
Maybe some software audio players / PC sound drivers do the same? You can experiment by digitally lowering your volume as much as possible, then turning up the pot on your amp to compensate. If the music sounds worse, then that particular digital volume control implementation probably isn't that great.
 
 
If you leave the pot out, the replacement resistor should go in the same position as the pot would have. That is, it would be the equivalent of turning the pot all the way up.
 
May 2, 2012 at 5:24 PM Post #4 of 7

shake

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Ok, so not having a potentiometer may cause quality loss as well.  I'll try both methods, seeing as 3 extra resistors wouldn't cost more than $1.  Would 10K resistors be good?  It's a solid-state amp.
Thanks for the input guys.
 
May 2, 2012 at 7:40 PM Post #5 of 7

nikongod

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If you rely on a "digital" pot life REALLY sucks if it ever goes to max. Some DACs do this periodically. Its also a point of user error if you forget to turn the volume down after hooking things up differently. 
 
The noise argument is a double edged sword and not really all that simple. or even valid. There is some noise made by resistors, but the DAC produces noise too. 99.995% of the time the DAC makes more noise than the pot. When you look at what happens to SNR when you use digital attenuation (SNR is measured against maximum output level and drops like whoa with digital attenuation) noise is not a valid argument at all. 
 
There are only 2 and a half valid arguments for not including a potentiometer in an amp. 
cost
space constraints
channel matching
power amp hooked up to a dedicated preamp (does not really count here) 
 
If you have the room, just get a pot and put it in. If you want to do digital volume control turn the pot up to max and do it that way. 
 
You only need 1 extra resistor. Well, I guess 2 for stereo.
 
May 2, 2012 at 8:43 PM Post #6 of 7

BK_856er

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In the absence of an "attenuator" device and wiring directly to the heaphone jack, what happens to the output impedance, i.e., 50k, 10k, etc. and is it necessary to compensate?  Is that the "extra" resistor referred to above?  Been thinking lately about casing up my balanced b-22 without an attenuator since my Lavry DA-11 and Buffalo-II both offer digital control and current funds are limited.
 
BK
 
May 2, 2012 at 8:58 PM Post #7 of 7

shake

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Quote:
If you rely on a "digital" pot life REALLY sucks if it ever goes to max. Some DACs do this periodically. Its also a point of user error if you forget to turn the volume down after hooking things up differently. 
 
The noise argument is a double edged sword and not really all that simple. or even valid. There is some noise made by resistors, but the DAC produces noise too. 99.995% of the time the DAC makes more noise than the pot. When you look at what happens to SNR when you use digital attenuation (SNR is measured against maximum output level and drops like whoa with digital attenuation) noise is not a valid argument at all. 
 
There are only 2 and a half valid arguments for not including a potentiometer in an amp. 
cost
space constraints
channel matching
power amp hooked up to a dedicated preamp (does not really count here) 
 
If you have the room, just get a pot and put it in. If you want to do digital volume control turn the pot up to max and do it that way. 
 
You only need 1 extra resistor. Well, I guess 2 for stereo.

I'm not too worried about the volume being up too high.  If I'm not listening on my headphones, the sound it being output to my speakers, which I keep at ~60% volume, so I leave my computer volume ~5-20%, but you do make a good point.
The potentiometers I initially bought  to use are the ALPS rk27 (blue velvet), but then I came across a thread comparing this potentiometer to stepped attenuators and that made me think that the ALPS weren't sufficient.  Then I hypothesized that no potentiometer would be the best.

And yes, just 2, I forgot that the ground channel doesn't use one.
 
Quote:
In the absence of an "attenuator" device and wiring directly to the heaphone jack, what happens to the output impedance, i.e., 50k, 10k, etc. and is it necessary to compensate?  Is that the "extra" resistor referred to above?  Been thinking lately about casing up my balanced b-22 without an attenuator since my Lavry DA-11 and Buffalo-II both offer digital control and current funds are limited.
 
BK

Yes, that's what the extra resistors are for.
 

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