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Old 9th September 2005, 10:09 PM   #1
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Default Preamp volume control??

Hi,

I was wondering how to use a volume control on my preamp, (Aikido using 12AX7's).
It makes sense to use it at the start of the amp before the first tubes grid, as putting it on the output would waste the gain and would run the amp at a consistently higher distortion level.
But i dont want to load my CD player at lower volumes with the POT on the input.
What is the way to get around this and maintain a reasonable Zin?

Cheers
Craig
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Old 10th September 2005, 12:36 AM   #2
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Quote:
But i dont want to load my CD player at lower volumes with the POT on the input.
Just out of curiosity. Why you don't want this?

Erik
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Old 10th September 2005, 06:01 AM   #3
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I have the same question as Craig.

Eric, if the CD player has an output coupling cap, installing the pot at the input of a preamp will cut off the low frequencies too early when the volume is set to low. Viewing from the preamp's main circuit, the output coupling cap from the CD player and the pot at the start of the preamp form a high pass filter with a -3dB point at 1 / (2 x Pi x R x C). R is very small at low volume which means the low frequency will roll off very early.

I am thinking about modifying my CD player and remove the output coupling cap. In that case low frequency is not a problem. However, depending on the DC offset of the CD player the pot may not like it. This can be a problem.

Regards,
Bill
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Old 10th September 2005, 06:05 AM   #4
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Opp! After reading Craig's question more carefully I now understand that Craig and I were talking about different things and Craig meant that he does not want to load the CD player excessively.

I would still like to get an answer for my question from some experts in the forum.

Regards,
Bill
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Old 10th September 2005, 06:07 AM   #5
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Hi,

Connect the input to one end of the pot and the other end to ground and then connect the wiper to the input of the amplifier eventually with a series resistor of say 10kohm as some amplifiers get unstable if the source resistance is too low.

In this way the signal source be it a CD player or anything else will see a contant load with same value as the pot independant of the volume setting, this is how it is usually done.

Regards Hans
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Old 10th September 2005, 07:04 AM   #6
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Hans,

Thanks. That answered Craig's question perfectly.

What about low frequency rolloff then?

Regards,
Bill
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Old 10th September 2005, 07:27 AM   #7
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Quote:
What about low frequency rolloff then?
If you use an input cap the low frequency roll off will be the same for any setting of the pot, note that an input cap is not needed with a tube amplifier as the grid normally is on ground potential.

There is one possible problem with this connection of a pot and that is if the pot is of very high value and the tube has high input capacitance, in that case you can get high frequency roll of for lower volume settings when there is a high resitance in series with the tube input, however as long as you use reasonable values of pots it s not a big problem. Old tube amplifiers could have pots with values of 1Mohm and in that case the HF rolloff is quite noticeable but more modern equipment normally use 100kohm or even less.

Regards Hans
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Old 10th September 2005, 10:32 AM   #8
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Hans, lol thanks i cant believe i didnt think of that i had it fixed in my head that i would have to put the CD player output on the wiper.

My POT is 100K so i should be fine.

Cheers
Craig
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