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Old 7th May 2009, 05:34 PM   #1
jetbat is offline jetbat  United States
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Default Impedance question

Hi. I'm having trouble finding information on impedance for the circuit in the schematic I have attached. I understand the low source/ high load impedance rule. But I am not sure the pre gain volume pot affects the output impedance from the op amp.

The output impedance from the op amp is 55ohms. By using a 500k pot, does that change the source impedance from a low to high? The cathode follower stage has a very high impedance load, but I don't know if this high enough if the pot raises the source impedance.

Should I use a 10k volume pot instead? That would be over 10x the source and less the 10x the load.

Thanks
Scott
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Old 7th May 2009, 05:42 PM   #2
Jeb-D. is offline Jeb-D.  United States
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Yes, the source impedance will change depending on volume setting.

I would use a pot in the 10k-100k range instead.
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Old 7th May 2009, 06:27 PM   #3
Yvesm is offline Yvesm  France
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Default Re: Impedance question

Quote:
Originally posted by jetbat

. . .
The cathode follower stage has a very high impedance load, but I don't know if this high enough if the pot raises the source impedance.
. . .
Scott
The input impedance of this CF is well over the value of its grid bias resistor because it is boostraped. Probably more than 10Mohms.

Thus, the link cap is over dimensioned

Yves.
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Old 7th May 2009, 07:12 PM   #4
jetbat is offline jetbat  United States
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I'll try a lower value pot. Does that cause the signal volume to drop?

And for the capacitor, does having a 0.1uF affect the sound or is it just larger then needed.

Thanks
Scott
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Old 8th May 2009, 06:35 PM   #5
jetbat is offline jetbat  United States
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Well I had a 10k pot on hand so I tried it and I like it. This is a guitar amp and the notes opened up with the 10k compared to the 500k. The fundamental notes seem the same but you can hear much more of the harmonics now. The sound seems more complex then before and the signal volume does not seem to have changed. Very nice.

Now I have to look over the rest of the schematic to see if I have any other questionable impedance areas.
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