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Old 18th December 2011, 08:21 PM   #1
sturmj is offline sturmj  United States
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Default Help!! with attenuation.

I have a diy(ish) preamp, that have padded inputs (360kohm, in series). I don't want to pad there any more (I think) that is as high as the kit supplier suggest (to avoid noise floor problems). The problem is that the volume at the first click on the pot (DACT 10k) is as loud as I would normally listen, meaning low volume listening is not possible. The designer suggests padding the amps input, which leaves me with two questions.

1. Would there be any difference between adding a series resistor to the output of the preamp, and adding one to the input at the amp? (interconnects are short, 1 meter). Adding to the preamp out is easier at this point.
2. How do I best determine which value resistor I should use? (avoiding willie-nillie attemps) and what effect will this have on the preamp? (impedance that is sees etc.) It sounds great! just not sensitive enough in terms of volume control.
I don't have a sound meter, but I suspect the first click an the DACT is close to 82dB I see is used to calculate such things (I saw this on some website).

The preamp is 12au7 based, I have measured the output voltage, It is very low in the current configuration (0.1Vac peaks and averaging about 0.05Vac measured at the rca out jack, inner-outer, which seems low to me (but what do I know?)
The amp is 300B SE and works fine with a commercial preamp (another option, but its not homemade).
Speakers are rated as 97dB sensitivity.
Any help you can give a newbie is appreciated!
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Old 18th December 2011, 09:20 PM   #2
DF96 is offline DF96  England
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If the first click on the volume pot is too loud then you probably don't need a preamp.

If you want to attenuate the signal then a potential divider may be better than a single resistor. A resistor at the input may add noise. A resistor at the output may interact with the capacitance of the cable. In either case a potential divider will have less problems.
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Old 18th December 2011, 11:05 PM   #3
sturmj is offline sturmj  United States
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With what I have on hand this is what I will try:
a 1k ohm resistor from the center of the output jack to the output lead, and a 330ohm resistor also from the center of the jack to the ground.
If my calculation-understanding id correct this would reduce the voltage by 1/5
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Old 19th December 2011, 11:57 AM   #4
DF96 is offline DF96  England
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If I understand you correctly you have it back to front. The 330 needs to go from output lead to ground, not jack centre to ground. This will reduce the voltage by 1/4 (=330/(1k+330)). However, this is likely to present too much load to the source so better to use 10k and 3k3 instead.
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Old 19th December 2011, 06:29 PM   #5
sturmj is offline sturmj  United States
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I tried my idea, It seems to work, I only had things running long enough to see how the volume worked. Volume control was much better. The way I have it now is, output wire is connected to the 1k the 330 connected at the opposite end of the 1k the junction of the two goes to center pin of rca the opposite end of the 330 to ground (L-pad).
I will try upping the values, but keep in mind this change is after the tube stages, will the source see the difference? If so, then I will definitely change to the higher values.
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Old 19th December 2011, 09:28 PM   #6
DF96 is offline DF96  England
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If the attenuator is after the valve stage, then that will be affected by the load - the main effects will be an increase in distortion and a reduction in low frequencies. Few valve line stages can drive a 1.3k load successfully.
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Old 20th December 2011, 05:28 AM   #7
sturmj is offline sturmj  United States
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Ok then higher value resistors it is.....Thank you very much!
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