Altec 9440A DC Offset Instability - diyAudio
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Old 2nd June 2010, 04:32 AM   #1
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Default Altec 9440A DC Offset Instability

My 9440A has a curious problem I haven't come across with any other amp tested. If you run 10 k sine wave into it into clipping, the dc offset bounces around with approx. 2 volt swing. All parts test good , new power caps, etc.
All offset and bias checked, perfect. Signals below 10 k dont cause this, but high level ultrasonic signals cause a wild dc offset fluctuation. Any ideas?
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Old 2nd June 2010, 06:53 AM   #2
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Don't run 10Khz through it, it's not intended to run that way. The outputs are quite slow.
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Old 2nd June 2010, 07:15 AM   #3
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I understand. However, if used as a tweeter amp, direct coupled, I'd be concerned with that offset causing trouble. If this behavior is normal, I'll just use it for subs.
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Old 2nd June 2010, 03:28 PM   #4
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It has the same MJ15024 used in many older pro amps.I doubt it's the nature of the device,this is the only amp I've seen do this.
Have run this issue by several people over the years...no one can figure it out.
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Old 3rd June 2010, 08:19 AM   #5
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"I understand. However, if used as a tweeter amp, direct coupled, I'd be concerned with that offset causing trouble."

So, what tweeter can handle 280W/8R ? Why don't you want to run a protection cap?

I have used the 9440 to drive compression drivers at 500hz in a two-way system, and driven it into clipping. Everything was fine. I was smart enough to use a protection cap.

"It has the same MJ15024 used in many older pro amps.I doubt it's the nature of the device,this is the only amp I've seen do this."

Only if it has been repaired by someone who doesn't have service data. The Altec 9440 uses the 2N6259 which has 250W SOA at 100V, the MJ15024 only has 100W at 100V. The 2N6259 is only 800Khz.
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Last edited by djk; 3rd June 2010 at 08:27 AM.
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Old 3rd June 2010, 04:04 PM   #6
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Quote:
Originally Posted by djk View Post
"I understand. However, if used as a tweeter amp, direct coupled, I'd be concerned with that offset causing trouble."

So, what tweeter can handle 280W/8R ? Why don't you want to run a protection cap?

I have used the 9440 to drive compression drivers at 500hz in a two-way system, and driven it into clipping. Everything was fine. I was smart enough to use a protection cap.

"It has the same MJ15024 used in many older pro amps.I doubt it's the nature of the device,this is the only amp I've seen do this."

Only if it has been repaired by someone who doesn't have service data. The Altec 9440 uses the 2N6259 which has 250W SOA at 100V, the MJ15024 only has 100W at 100V. The 2N6259 is only 800Khz.
Multiple tweeter arrays.

The point is moot, however, since, as you pointed out, the amp appears to have the incorrect set of outputs.

Looks like the originals were discontinued back when they quit the 554/424 Toshibas...not a pretty picture.!
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