Coupling caps on long wires

somejohndoe · 13th November 2011, 11:12 PM

I am looking at an amp's input circuit that has coupling caps going from the input op amp to the wire that leads to the level pot, which I understand, as well as one on the return wire coming from the pot going to the next stage. Why would it need the second cap? Surely the first cap would eliminate any DC coming from the output of the op amp. The pot is connected via 18" of unshielded 22AWG twisted pair with no bypass caps on either end.

gootee · 14th November 2011, 01:03 AM

Sorry. You need to post a diagram, or photo, or describe it more clearly.

somejohndoe · 14th November 2011, 01:28 AM

cap.png

Its the second cap that I am interested in. The pot sits off the circuit board connected by an 18" twisted trio of wires.

nigelwright7557 · 14th November 2011, 01:32 AM

Put a voltmeter across the caps and see if there is a dc voltage there.

somejohndoe · 14th November 2011, 01:35 AM

Quote:

Originally Posted by nigelwright7557

Put a voltmeter across the caps and see if there is a dc voltage there.

I figured that already. There is none on the second. The first has a few mV.

nigelwright7557 · 14th November 2011, 01:39 AM

Quote:

Originally Posted by somejohndoe

I figured that already. There is none on the second. The first has a few mV.

In that case they probably arent required.

Jeff R · 14th November 2011, 01:44 AM

What does the rest of the circuit to the right look like? There may be something feeding dc back to the volume control that would upset things.

crispycircuit · 14th November 2011, 02:14 AM

Even a tiny amount of DC on the pot would make the volume/level control very nosiy when adjusting... So it must block a small amount to DC....

syklab · 14th November 2011, 02:29 AM

Quote:

Originally Posted by crispycircuit

Even a tiny amount of DC on the pot would make the volume/level control very nosiy when adjusting... So it must block a small amount to DC....

Hi,
Would that imply the cap on the right hand side will be needed to have clean signal going into the volume?

somejohndoe · 14th November 2011, 02:34 AM

From the second cap, a voltage divider leads to the +'ve input of another op amp. No measured DC but that might not always be the case I guess. I could see it if there was a feedback loop attached to it but there is not.

13th November 2011, 11:12 PM	#1
somejohndoe diyAudio Member Join Date: Jul 2010	Coupling caps on long wires I am looking at an amp's input circuit that has coupling caps going from the input op amp to the wire that leads to the level pot, which I understand, as well as one on the return wire coming from the pot going to the next stage. Why would it need the second cap? Surely the first cap would eliminate any DC coming from the output of the op amp. The pot is connected via 18" of unshielded 22AWG twisted pair with no bypass caps on either end.

14th November 2011, 01:32 AM	#4
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	Put a voltmeter across the caps and see if there is a dc voltage there. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

14th November 2011, 02:14 AM	#8
crispycircuit diyAudio Member Join Date: Feb 2007 Location: Near Buffalo NY USA	Even a tiny amount of DC on the pot would make the volume/level control very nosiy when adjusting... So it must block a small amount to DC.... __________________ The stuff you don't know always humbles us.........

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14th November 2011, 01:03 AM	#2
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	Sorry. You need to post a diagram, or photo, or describe it more clearly.

14th November 2011, 01:28 AM	#3
somejohndoe diyAudio Member Join Date: Jul 2010	cap.png Its the second cap that I am interested in. The pot sits off the circuit board connected by an 18" twisted trio of wires.

14th November 2011, 01:44 AM	#7
Jeff R diyAudio Member Join Date: Nov 2002 Location: Florida	What does the rest of the circuit to the right look like? There may be something feeding dc back to the volume control that would upset things.

14th November 2011, 02:34 AM	#10
somejohndoe diyAudio Member Join Date: Jul 2010	From the second cap, a voltage divider leads to the +'ve input of another op amp. No measured DC but that might not always be the case I guess. I could see it if there was a feedback loop attached to it but there is not.

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