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Old 22nd December 2009, 06:19 PM   #1
gl is offline gl  United States
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Location: Sierra Foothills - California
Default F4 DC offset drift performance question.

Could any F4 builders please comment on the DC offset drift performance of this circuit.

I am building an F4 and I would like to be able to set the DC offset to the lowest possible value and have the drift be as close to nil as possible.

Thank you.

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Old 22nd December 2009, 09:36 PM   #2
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Depends on the devices used and the stability of the DC
supply, but I recall being able to keep it around 10 mV drift
or so.

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Old 22nd December 2009, 10:13 PM   #3
gl is offline gl  United States
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Join Date: Dec 2003
Location: Sierra Foothills - California
Hi Nelson,

Thank you for the reply.

I'm using 5 each IRF610's and 9610's running 10 ma per with an unregulated supply sort of like a scaled down F4. Power output is about 2W I figure, and it's driving a Jensen 1:1 line output transformer (JT-11-DMPC). I don't want to use a huge electrolytic coupling cap if I can avoid with it.

I'm also trying to avoid regulated supplies for several reasons. Do you think they would reduce DC offset drift?

Anyway the project is a super upscale D.I. box for bass guitar. It needs to drive 300 feet of Belden 9451. It'll be finished in a week or so.

I'm just trying to anticipate issues so I threw out this question.


Last edited by gl; 22nd December 2009 at 10:18 PM.
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Old 5th January 2010, 04:44 PM   #4
gl is offline gl  United States
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Join Date: Dec 2003
Location: Sierra Foothills - California
Well it works and sounds crystal clear playing the bass guitar in one end while bypassing the output transformer and listening with headphones.

Using unregulated supplies there's no hum, and no noise.

The DC offset drifts over a range of 1.5mv. But I've only observed it for a couple of hours. Setting the offset was kind of twitchy so I changed R22 to 3.01K and P2 to 500 ohms (F4 Rev.1). This made it quicker and easier to get the offset to less than 1mv. It helped too that I had matched Vgs on the output devices to within .01V. Plus the cross match was pretty close. It was cool - I don't think I had to pull more then 10 devices from each tube to get this tight matching.

I didn't use a coupling cap between the output and the transformer. Jensen recommend keeping the DC across the primary to less than 100uV. I am not in that range. However, nothing is burning and the sound is great so I think I'll just carry on until I find a reason to change things.

Thursday night I'll take the new box out on the town for a trial run in the real world - huge PA - line arrays, subs under the stage, 95-100dB. We'll see how it does.

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