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Old 21st March 2005, 08:15 PM   #1
wdekler is offline wdekler  Netherlands
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Default Newbie: how to calculate pot for volume control

Hello,


I'm planning on comparing a CD Player and the Hoerwege DAC on my Rotel RHB10 amp. The CD player has volume control but the DAC hasn't.

So I went to a electronics store and asked what value the pot should be and I've ended up with the wrong value: 20kohms (the volume is too loud).

Can someone please help me calculate which value shoud be correct in my case? 50k 100k?

I've been searching on the forums and I know that the value should be related to the voltage/impedance of the amp but I don't know how make the necessary calculations...

The Amp's manual says: 0.775V / 30K ohms

any help is much appreciated, thanks!


Wanne.
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Old 21st March 2005, 08:35 PM   #2
K-amps is offline K-amps  United States
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Looks like you have wired the pot in series, if thats the case maybe even 1 meg might be your ticket... Not that I'd go that way.

What you want to do is use the pot to "Short" the output signal, This will give you a dead quiet zero level. However shorting the output has consequences, so you may want to have a resistance, probably 10-56k in series with the output of the DAC and the pot so even with the pot shorted to ground, the DAC still see's 10-56k and will not misbehave.
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Old 22nd March 2005, 02:38 AM   #3
anatech is offline anatech  Canada
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Normally the fixed element in the pot goes from signal to ground. In this case a 30K resistance. The signal is taken from the wiper which can be varied from ground (0 signal) to full signal. Generally speaking you may want to buffer the output so you can drive the cable and input circuit of the next piece.
If you get the fixed part wired backwards the volume will increase as you turn it down, the taper will be wrong too. Don't worry, just reverse the connections to the outside terminals.
-Chris
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Old 22nd March 2005, 06:21 AM   #4
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there is some info on this site that might help you:

http://sound.westhost.com/pots.htm
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Old 22nd March 2005, 06:58 AM   #5
wdekler is offline wdekler  Netherlands
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Thank you for answering K-amps!

If I understand you correctly I directly connect the four wires and I also connect the pot to the the signal to pull it down to zero. And to be on the safe side I'll add a restistor so that the signal is never shortened. And even with this resistor, you'll still reach a dead quiet zero level.

I'm not sure though why that 20K pot should still be the right choice? Why not 10K or 50K?

Also, I know that the hoer-wege DAC uses relay's to "short" the output signal to ground when there's no data, but I'll be on the safe side and still use those resistors.


thanks!


Wanne.
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Old 22nd March 2005, 07:50 AM   #6
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Shorting the output as k-amps suggests achieves exactly the same as what the others have said about just connecting the pot properly, but uses an additional resistor. Just wire the pot across the output and tap off the wiper and ground like everybody else has suggested.
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Old 22nd March 2005, 01:40 PM   #7
K-amps is offline K-amps  United States
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Yes, If you only use the pot in that manner, you want to make sure it is connected properly so that the DAC sees a 20kohm load not zero ohms. Another reason why I have gotten into the habit of using a series resistor (perhaps not relevant in this case) is that some pots dont like to sink the entire load, the series resistor helps over loading them...
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Old 22nd March 2005, 08:40 PM   #8
wdekler is offline wdekler  Netherlands
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Everything is working fine now, I'm using the signal from the wiper as suggested. The website mentioned in the link was also very interesting...

(my second posting was actually after K-amps reply but it came a bit later due to moderation)


Now I'm considering some powersupply modifications for this new DAC but that's something for the digital forum


Wanne.
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