DC offset blues

grapplesaw

Veteran and General Yakker
Joined
Aug 13, 2014
Messages
1,838
Location
Vancouver
Tagline
---
#1
I was going to run my Bridged 700 ( in stereo) at a higher Bias setting to see the sound and heat differences. I always set at .350v and am now going to try .380 v then .390v.

as I warm up the unit to set Bias I have a strange DC offset reading in the Right channel. I tried two different meters and same thing. What is happening is higher than wanted offset that is rapidly changing from 18 mv to 21mv and sometimes as low as 16 and high of 27mv

Normally I see some change in DC offset but slow changes only. The other channel is stable at .49mv which is excellent.

I guess I should change out the Q105 and q101 outputs.

still strange Any other thoughts welcomed
 

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
9,400
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#2
Appears you might have a leaker on the back wall. If you take the bias down to 0.250, what does the offset go to?
 

grapplesaw

Veteran and General Yakker
Joined
Aug 13, 2014
Messages
1,838
Location
Vancouver
Tagline
---
#4
Interesting results Joe

I turned down to 250mv and offset dropped to 10 mv

then turned all way down bias to .5mv. As I brought back to .250 mv the offset remained low and back at .400v it’s now .47mv offset And at .380 it’s .39mv offset. Continues to drop as it warms up

could rail capacitors effect that. I have two 1.5uf and one .15uf on each rail ?
 

Attachments

grapplesaw

Veteran and General Yakker
Joined
Aug 13, 2014
Messages
1,838
Location
Vancouver
Tagline
---
#5
Powered off and repowered
Dc offset at start up was 10-12 mv both channels and instantly started dropping. 5 minutes of warning and now less that 1 mv both channels
Bias set at .380v and mains is 121volts.
 

grapplesaw

Veteran and General Yakker
Joined
Aug 13, 2014
Messages
1,838
Location
Vancouver
Tagline
---
#6
I am going to add .046uf cap and two 1n4148 diodes to the 101 and 201 cap. It passed good signal with low distortion even when it had the high offset
 

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
9,400
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#7
What is a 101 and 201 cap Glen?

Earlier you reported:
"What is happening is higher than wanted offset that is rapidly changing from 18 mv to 21mv and sometimes as low as 16 and high of 27mv"

Has that condition changed?
 

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
9,400
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#8
Interesting results Joe

I turned down to 250mv and offset dropped to 10 mv

then turned all way down bias to .5mv. As I brought back to .250 mv the offset remained low and back at .400v it’s now .47mv offset And at .380 it’s .39mv offset. Continues to drop as it warms up

could rail capacitors effect that. I have two 1.5uf and one .15uf on each rail ?
What control board are you using Glen?
 

grapplesaw

Veteran and General Yakker
Joined
Aug 13, 2014
Messages
1,838
Location
Vancouver
Tagline
---
#9
What is a 101 and 201 cap Glen?

Earlier you reported:
"What is happening is higher than wanted offset that is rapidly changing from 18 mv to 21mv and sometimes as low as 16 and high of 27mv"

Has that condition changed?
Hi Joe
Yes the DC offset is once again normal, less than 1 mv. I just powered it on again after working outside for a while. All good now. Very puzzling.
Sorry Joe but I got this board years ago and
is a board Nick Soundas (THD+N in our forum) made which has a dual op amp so we can invert one channel to make a mono (bridged) output. I did a thread on the build a while back. You can use in bridged mode when connecting only the two hot outputs to a single speaker . It also has stereo output in normal operation ( but one channel is inverted so you have to flip one set of speaker wires on the binding post to keep them in phase.

The cap I am referring to is doing the job of your C4 on your g board to which I will add the components.
This board does not have the c5 or d5,d6 diodes you use. I thought I might add them to prevent any latch up in the future.
 

Attachments

Last edited:

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
9,400
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#10
Hi Joe
Yes the DC offset is once again normal, less than 1 mv. I just powered it on again after working outside for a while. All good now. Very puzzling.
Sorry Joe but I got this board years ago and
is a board Nick Soundas (THD+N in our forum) made which has a dual op amp so we can invert one channel to make a mono (bridged) output. I did a thread on the build a while back. You can use in bridged mode when connecting only the two hot outputs to a single speaker . It also has stereo output in normal operation ( but one channel is inverted so you have to flip one set of speaker wires on the binding post to keep them in phase.

The cap I am referring to is doing the job of your C4 on your g board to which I will add the components.
This board does not have the c5 or d5,d6 diodes you use. I thought I might add them to prevent any latch up in the future.
Lots of ECOs there Glen...
 

grapplesaw

Veteran and General Yakker
Joined
Aug 13, 2014
Messages
1,838
Location
Vancouver
Tagline
---
#12
Amp is working again. I must have done something with the probe on the first test. No issues. No real difference in heatsink temps at the .380v bias.
Sound seams a little sharper. There is no real amount of cross over distortion at the .350v setting so it’s all in my head and I like it
 

mlucitt

Veteran and General Yakker
Joined
Jun 24, 2011
Messages
2,444
Location
Jacksonville, FL
#17
Before you can have an Engineering Change Order, you need to have:
ECP - Engineering Change Proposal
ECN - Engineering Change Notice
ECC - Engineering Change Committee
ECE - Engineering Change Estimate
ECM - Engineering Change Model
ECS - Engineering Change Sequence
ECC - Engineering Change Change (corrections)
ECP - Engineering Change Parameters
ECQA - Engineering Change Quality Assurance
ECT2P - Engineering Change Theory To Practice
ECR - Engineering Change Review
ECU - Engineering Change Update

How else are you going to keep all the engineers busy?
 
Top