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Old 29th October 2008, 06:26 PM   #1
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Default Hafler 9180 DC Offset

I'm looking for a fix that will help me eliminate (or reduce) the 50mv DC ofset (in one channel) of my Hafler 9180 series amp. The DIY mavens were very helpful in curing the DC offset problem in my DH200.
see DIY Thread for that background: Hafler DH200 DC Offset

ANYWHO...My need is to direct-drive a Morel EM1308 from this amp. I'll need as little DC offset as I can get so the dome remains centered in the magnetic gap. (5 mV max would be nice)
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Old 29th October 2008, 10:53 PM   #2
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Even if the dc offset is low (5mV or whatever) the amp will still spike to 100mV-300mV during power up and shut down. You will need to use a large capacitor (20uf or more I'd say) in series with your driver.

Also, doesn't the 9180 have a dc offset adjustment potentiometer? Been awhile since I've looked at the schematic so maybe I'm mistaken.
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Old 29th October 2008, 11:50 PM   #3
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Default 9180 features

The 9180 has a soft start and shut-down to eliminate spuriious noises during power supply stabilization and shut-down. (nice).

Bias adjust - YES. There is not an offset adjust in this amp.

Looks like q1,2 q3, 4 are selected/graded. If I can get a matched set that may be the trick. 2SK163 and 2SJ74. Anyone know where to get those?
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Old 1st November 2008, 07:21 PM   #4
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Default Wasted Time & Parts

I decided to try the same tweak which brought the DC offset in my Hafler DH200 down from 300mv to 4mv. In the DH200 they are Q1,2,5,6. In the Hafler 9180 they are Q5,6,7,8. The devices I used are Matched / Paired 2n5551 and 2n5401. Unfortunately in the 9180 NO CHANGE! Still 50mV on the left channel and 25mV on the right. This wouldn't be so bad except I need to directly power a low excursion midrange directly from the amp. Does anyone have a suggestion to get the offset down to below 5mV?
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Old 1st November 2008, 11:12 PM   #5
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Like DickW said in the other thread, Q5-Q8 are just the current sources for the Jfet's, Q1-Q4. You need to get a matched set of the Jfet's, or buy a lot of them and match them yourself, to get the offset down any lower in this amp.
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Old 1st November 2008, 11:28 PM   #6
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Good evening !

If the offset is consistent and does not fluctuate with time/temp,
then you can eliminate it with the opposite amount of offset injected
to the input.
Attached Images
File Type: jpg offset adj.jpg (10.0 KB, 94 views)
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Old 1st November 2008, 11:47 PM   #7
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Default Could it be that simple?

Looks like a simple trim pot straddling the PS rails.
What are the critical components to include here P1 ( a 1meg trimmer) ; R8, R7. Anything else needed?
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Old 1st November 2008, 11:53 PM   #8
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Default Replacing Matched Fets

I couldn't find anyone to supply those replacement FETs. I can't do the matching (no facility, no equipment and limited skills).

2SK163 and 2SJ74 Not easy to locate......so far impossible to buy matched pairs.
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Old 2nd November 2008, 01:45 AM   #9
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I am sorry you've not yet found the FETs needed. Now you know why I said on the other thread that I was avoiding this amp. FETs are often difficult to find a few years later.

On the other thread I told you how I fixed the DC offset in my DH-200. It is essentially the same circuit as Lord Winter just described. His parts values are quite high so you will only get a trickle of either + or - voltage, depending on the setting of the pot's wiper. Reread my message on the other thread on how to do this.

Good luck.
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Old 2nd November 2008, 04:29 PM   #10
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Default FETS from Europe & China

Thanks Dick:
You have been helpful and I may have a little luck finding those FETS.

WHAT Idss do I need to specify?
Any other parametric matching required?

FYI:

ERNO BORBLEY writes to me:

Hi Ron, thanks for your email. I have plenty of K170 and J74 matched, but I need to know what Idss they should have.
Do you have the manual of the 9180? Can you email the schematic and the parts list? With those I should be able to
suggest the Idss range.
BTW, min. order is 50 Euro. Pleae fax your VISA/MC details, incl. sec. code when you order.
Thanks, Erno.

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There is also some on E-BAY a guy in China claims to have sets available too. Claims to have matched +/- 3% IDSS matched devices.

WHO TO TRUST ?

If I had to buy a min $$ amount, I'm sure there are other DIY folks who would like to buy some of the "excess" ones I might have on hand after this deal?

As I say....never confuse motion with progress. I'm getting a little motion sickness but I'm encouraged.
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