SOZ DC Offset question.

moe29 · 7th April 2003, 04:19 AM

I recently finished one channel of my SOZ.

Rails = 16V Vgs reading of 4.58 and 4.55 on the FETs

DC Offset after 1 hour of warm up .588 Volts !!

To lower the offset i put an 8ohm resistor in parallel with R5, like
Mr. Pass mentions in the SOZ article. This brings the offset down
to 38mV.

My question is, is it normal to see that much offset with Vgs
readings like i got above? Is there any penalty in fidelity from
paralleling resistors to achieve minimum offset? Should I try
harder to find two better matching MOSFETS?

By the way, i was listening to the one channel through a
Paradigm bookshelf speaker and it sounded fine... at what point
does DC Offset start to get dangersous/sound degrading?

The BOSOZ/SOZ combination was deadly silent - free of hum and
hiss - and this was as set up on the test bench! I feel i'm going
to be very happy with this combo

Then when i get the bug
i'll try to X them.

Thanks for the help

moe29 (pics when i get them boxed up)

jh6you · 7th April 2003, 06:28 AM

In my opinion, the tolerances of resistor values also affect the differential offset. The parallel resistors would play roles to compensate the tolerances, without payment to the amp’s performance.

The differential offset is known in general as acceptable up to 100mV. This small offset would not degrade the amp’s output character. It merely creates negligible offset of the speaker’s piston motion center.

JH

Nelson Pass · 8th April 2003, 07:54 PM

No problem with trimming resistors to get low offset. Most of
the time it's the result of matching, but whatever it takes to
get less than 50 mV or so is just fine.

gnomus · 12th August 2003, 05:14 PM

I have 40mv dc offset in my SOZ. The mosfets are mounted on separate heat sinks. Would mounting them on the same heat sink(of the right size) lower dc offset too?

Thanks in advance.

moe29 · 12th August 2003, 05:28 PM

i don't think thermal considerations have much to do with
DC offset in the SOZ. Matching the FETS is what gets you the
lowest offset.... 40mv isn't bad at all.

7th April 2003, 04:19 AM	#1
moe29 diyAudio Member Join Date: Dec 2002 Location: Florida	SOZ DC Offset question. I recently finished one channel of my SOZ. Rails = 16V Vgs reading of 4.58 and 4.55 on the FETs DC Offset after 1 hour of warm up .588 Volts !! To lower the offset i put an 8ohm resistor in parallel with R5, like Mr. Pass mentions in the SOZ article. This brings the offset down to 38mV. My question is, is it normal to see that much offset with Vgs readings like i got above? Is there any penalty in fidelity from paralleling resistors to achieve minimum offset? Should I try harder to find two better matching MOSFETS? By the way, i was listening to the one channel through a Paradigm bookshelf speaker and it sounded fine... at what point does DC Offset start to get dangersous/sound degrading? The BOSOZ/SOZ combination was deadly silent - free of hum and hiss - and this was as set up on the test bench! I feel i'm going to be very happy with this combo Then when i get the bug i'll try to X them. Thanks for the help moe29 (pics when i get them boxed up)

12th August 2003, 05:14 PM	#4
gnomus diyAudio Member Join Date: Jan 2002 Location: New York City	I have 40mv dc offset in my SOZ. The mosfets are mounted on separate heat sinks. Would mounting them on the same heat sink(of the right size) lower dc offset too? Thanks in advance. __________________ Paul

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7th April 2003, 06:28 AM	#2
jh6you Account Disabled Join Date: Aug 2002 Location: Near to the Pacific Ocean	In my opinion, the tolerances of resistor values also affect the differential offset. The parallel resistors would play roles to compensate the tolerances, without payment to the amp’s performance. The differential offset is known in general as acceptable up to 100mV. This small offset would not degrade the amp’s output character. It merely creates negligible offset of the speaker’s piston motion center. JH

8th April 2003, 07:54 PM	#3
Nelson Pass The one and only Join Date: Mar 2001	No problem with trimming resistors to get low offset. Most of the time it's the result of matching, but whatever it takes to get less than 50 mV or so is just fine.

12th August 2003, 05:28 PM	#5
moe29 diyAudio Member Join Date: Dec 2002 Location: Florida	i don't think thermal considerations have much to do with DC offset in the SOZ. Matching the FETS is what gets you the lowest offset.... 40mv isn't bad at all.

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