Erno 320 jfet phono DC offset drifting between +0.2V and -0.18V

Sunsun22 · 29th September 2010, 06:28 PM

I am using Erno's 320 jfet phono with his 255 super shunt regulator for a while and it is super beautiful until one day........ I discovered that one channel's voltage drifts up and down. Since the 255 regulator is a CCS, I think the 320 is drawing too much current therefore I made some fine tune on the 320 and then problems - I measured one channel with 1mV DC offset while the other is drifting between +0.2V and -0.18V even with the DC servo IC plugged in. I did nothing to the 320 except I took this out from the chassis and fine tune it. I believe such a high DC offset is a concern. Am I right?

320 uses 4 pairs of N P input jfets with cascodes. I measured the junction between the four 2SJ74 input jfets and cascodes, they are 2.023V, 2.257V, 1.744V and 2.015V on the problem channel while the normal channel is having readings close to 2V on all of them. Could such a variation cause the DC offset to such a high degree?

Erno is too busy for his last batch of orders and is not able to help. Can anyone give me some clues?

Leon08 · 29th September 2010, 07:05 PM

This is called drift and is normal.

Sunsun22 · 30th September 2010, 01:54 AM

Does this mean such a DC drifting will not harm? Why does this happen since the problem channel was also having 1mV DC offset before?

ticknpop · 30th September 2010, 02:37 AM

You need to reduce airflow around the front jfets . When there is consider
breeze ( like open chassis without top cover ) the jets will drift - you need stable temperature enviornment to get offset drift down.

lineup · 30th September 2010, 03:53 AM

RIAA Phono amplifiers have very high gain.
So even a small dc-offset somewhere can be amplified
and reach several Volts at output.

But in your case I think there is an issue.
If one channel is very good, shows 1 mV,
the other should not show +200 to -180 mV variations.
This is not what you want.
But I do not think it is harmful to anything. You do not have to be afraid, as long it is below 0.2V.

Hope someone can figure this out.
If not, you have to wait until Borbely can help you.

29th September 2010, 06:28 PM	#1
Sunsun22 diyAudio Member Join Date: Jul 2004 Location: Hong Kong	Erno 320 jfet phono DC offset drifting between +0.2V and -0.18V I am using Erno's 320 jfet phono with his 255 super shunt regulator for a while and it is super beautiful until one day........ I discovered that one channel's voltage drifts up and down. Since the 255 regulator is a CCS, I think the 320 is drawing too much current therefore I made some fine tune on the 320 and then problems - I measured one channel with 1mV DC offset while the other is drifting between +0.2V and -0.18V even with the DC servo IC plugged in. I did nothing to the 320 except I took this out from the chassis and fine tune it. I believe such a high DC offset is a concern. Am I right? 320 uses 4 pairs of N P input jfets with cascodes. I measured the junction between the four 2SJ74 input jfets and cascodes, they are 2.023V, 2.257V, 1.744V and 2.015V on the problem channel while the normal channel is having readings close to 2V on all of them. Could such a variation cause the DC offset to such a high degree? Erno is too busy for his last batch of orders and is not able to help. Can anyone give me some clues?

30th September 2010, 03:53 AM	#5
lineup diyAudio Member Join Date: Dec 2005 Location: the north	RIAA Phono amplifiers have very high gain. So even a small dc-offset somewhere can be amplified and reach several Volts at output. But in your case I think there is an issue. If one channel is very good, shows 1 mV, the other should not show +200 to -180 mV variations. This is not what you want. But I do not think it is harmful to anything. You do not have to be afraid, as long it is below 0.2V. Hope someone can figure this out. If not, you have to wait until Borbely can help you. __________________ lineup

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29th September 2010, 07:05 PM	#2
Leon08 Banned Join Date: Sep 2010 Location: at Home	This is called drift and is normal.

30th September 2010, 01:54 AM	#3
Sunsun22 diyAudio Member Join Date: Jul 2004 Location: Hong Kong	Does this mean such a DC drifting will not harm? Why does this happen since the problem channel was also having 1mV DC offset before?

30th September 2010, 02:37 AM	#4
ticknpop diyAudio Member Join Date: May 2005 Location: toronto	You need to reduce airflow around the front jfets . When there is consider breeze ( like open chassis without top cover ) the jets will drift - you need stable temperature enviornment to get offset drift down.

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