6SN7 SRPP - oscillating dc offset

lordearl · 27th March 2012, 08:34 AM

Just finished a build of this circuit/pcb:

6SN7 SRPP Preamplifier PCB (Stereo) - Analog Metric - DIY Audio Kit

at the output there is between, say, -50mV and +50mV of dc present at any given time. what could be causing this?

Doz · 27th March 2012, 08:38 AM

There shouldn't be , there's no DC path. Check C12 isn't leaky. If you're measuring this with a DVM, it could be misleading you.

lordearl · 27th March 2012, 09:37 AM

Yes, appears very strange. C12 seems fine. Why would the dmm be playing tricks?

DF96 · 27th March 2012, 10:12 AM

You may be seeing supply rail variations due to your mains voltage varying. Your PSU is a low pass filter. The output cap and ground resistor is a high pass filter. In the subsonic overlap region where neither is attenuating too much you can get mains variations coming through.

This assumes that what you are seeing on your DMM is a varying 'DC' voltage, not a constant DC which happens to be different from time to time.

lordearl · 27th March 2012, 11:15 AM

OK excellent, thanks for the info

This may, or may not be, related, but there is a background humming sound which is present when the pot is at minimum, then disappears as the pot gets to 9 o'clock, then comes back and gets louder as the pot is turned up. The humming also disappears when I have my fingers on the pot itself - is this some kind of field interference or grounding issue?

DF96 · 27th March 2012, 11:41 AM

That sounds like it could be RF oscillation. Does the DC current drawn by the circuit vary with pot position?

Alternatively, you may have two sources of hum which happen to cancel when the pot is in mid position.

avp1 · 27th March 2012, 01:03 PM

Quote:

Originally Posted by lordearl

OK excellent, thanks for the info

This may, or may not be, related, but there is a background humming sound which is present when the pot is at minimum, then disappears as the pot gets to 9 o'clock, then comes back and gets louder as the pot is turned up. The humming also disappears when I have my fingers on the pot itself - is this some kind of field interference or grounding issue?

Get yourself an oscilloscope. Rigol 1052E DSO is below $400, old analog Tektronix'es are in that range too. You can't effectively diagnose circuit problems without it.

Palustris · 27th March 2012, 03:53 PM

"is this some kind of field interference or grounding issue?"

Make sure that neither RCA plug is grounded to the chassis where it is mounted. They should be grounded to signal ground and signal ground connected to the chassis where power supply ground is connected to the chassis.

kevinkr · 27th March 2012, 04:30 PM

DF96 is spot on about the variations in mains voltage causing this apparent wondering DC bipolar offset, you can also look at it as ridiculously low frequency AC.

Should be only one connection point between the electronics and the chassis. RCA jacks should be isolated and run back to this point as pointed out in another post.

A 1K grid stopper right at the input grid of the 6SN7 should help with RF susceptibility and a tendency to oscillate.

A scope is highly recommended - it will make diagnosing these sorts of issues much easier.

lordearl · 27th March 2012, 06:37 PM

I have not connected the signal ground to the chassis ground - will do this and see if any improvement is made.

Re the 1k grid stopper - I assume we are talking about R2/R10? I've used 250R (closest to the circuit specified 220R which I had to hand). Will try increasing this to 1k.

27th March 2012, 08:34 AM	#1
lordearl diyAudio Member Join Date: Jun 2007	6SN7 SRPP - oscillating dc offset Just finished a build of this circuit/pcb: 6SN7 SRPP Preamplifier PCB (Stereo) - Analog Metric - DIY Audio Kit at the output there is between, say, -50mV and +50mV of dc present at any given time. what could be causing this?

27th March 2012, 08:38 AM	#2
Doz diyAudio Member Join Date: Jul 2010 Location: Sat Down	There shouldn't be , there's no DC path. Check C12 isn't leaky. If you're measuring this with a DVM, it could be misleading you. __________________ "To err is human.. to make a real balls-up requires a computer"

27th March 2012, 04:30 PM	#9
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	DF96 is spot on about the variations in mains voltage causing this apparent wondering DC bipolar offset, you can also look at it as ridiculously low frequency AC. Should be only one connection point between the electronics and the chassis. RCA jacks should be isolated and run back to this point as pointed out in another post. A 1K grid stopper right at the input grid of the 6SN7 should help with RF susceptibility and a tendency to oscillate. A scope is highly recommended - it will make diagnosing these sorts of issues much easier. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan

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27th March 2012, 09:37 AM	#3
lordearl diyAudio Member Join Date: Jun 2007	Yes, appears very strange. C12 seems fine. Why would the dmm be playing tricks?

27th March 2012, 10:12 AM	#4
DF96 diyAudio Member Join Date: May 2007	You may be seeing supply rail variations due to your mains voltage varying. Your PSU is a low pass filter. The output cap and ground resistor is a high pass filter. In the subsonic overlap region where neither is attenuating too much you can get mains variations coming through. This assumes that what you are seeing on your DMM is a varying 'DC' voltage, not a constant DC which happens to be different from time to time.

27th March 2012, 11:15 AM	#5
lordearl diyAudio Member Join Date: Jun 2007	OK excellent, thanks for the info This may, or may not be, related, but there is a background humming sound which is present when the pot is at minimum, then disappears as the pot gets to 9 o'clock, then comes back and gets louder as the pot is turned up. The humming also disappears when I have my fingers on the pot itself - is this some kind of field interference or grounding issue?

27th March 2012, 11:41 AM	#6
DF96 diyAudio Member Join Date: May 2007	That sounds like it could be RF oscillation. Does the DC current drawn by the circuit vary with pot position? Alternatively, you may have two sources of hum which happen to cancel when the pot is in mid position.

27th March 2012, 03:53 PM	#8
Palustris diyAudio Member Join Date: Aug 2006 Location: Cape Cod	"is this some kind of field interference or grounding issue?" Make sure that neither RCA plug is grounded to the chassis where it is mounted. They should be grounded to signal ground and signal ground connected to the chassis where power supply ground is connected to the chassis.

27th March 2012, 06:37 PM	#10
lordearl diyAudio Member Join Date: Jun 2007	I have not connected the signal ground to the chassis ground - will do this and see if any improvement is made. Re the 1k grid stopper - I assume we are talking about R2/R10? I've used 250R (closest to the circuit specified 220R which I had to hand). Will try increasing this to 1k.

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