Correct bias setting for mosfet amp w/out scope? - diyAudio
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Old 13th June 2011, 11:38 PM   #1
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Default Correct bias setting for mosfet amp w/out scope?

I'm trying to verify the "correct" bias setting for a BK ST-140. It's the first model (Iron core tranny, (+/-) 49V rails, single pair mosfet outputs/Ch).

I read (on another forum) it is 200mA (or 100mA / output). However, I also read (in a stereophile review) that the first St-140 used 160mA bias and that the second model used less (140mA) due to a significant increase in rail Voltage (+/- 60V) when they were comparing the different ST-140 models.

I tried contacting BK but then remembered that they (shut down) not too long ago. Does ANYONE happen to know the CORRECT bias setting?

PS: I don't own a scope; (so I can't apply a sine wave and increase bias until crossover distortion disappears)!

Much appreciated!
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Old 14th June 2011, 06:22 AM   #2
lineup is offline lineup  Sweden
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For MOSFET
100-200mA per transistor
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Old 14th June 2011, 07:25 AM   #3
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Do you have any more info about the amp ? Is it Class A A/B ? How many devices are in the output stage ?

Crossover distortion is normally pretty horrible to the ears.

If its Class A/B use the least amount of bias that produces a distortion free output.

If its Class A you need sufficient bias to ensure that the devices are conducting all the time.

Try to keep the output devices cooler than 65 Degrees for reliability.
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Old 15th June 2011, 11:41 PM   #4
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Quote:
Originally Posted by Andy5112405 View Post
Do you have any more info about the amp ? Is it Class A A/B ? How many devices are in the output stage ?

Crossover distortion is normally pretty horrible to the ears.

If its Class A/B use the least amount of bias that produces a distortion free output.

If its Class A you need sufficient bias to ensure that the devices are conducting all the time.

Try to keep the output devices cooler than 65 Degrees for reliability.
Yeah, I included all of that info in my initial post.... (SINGLE PAIR Outputs/ch). So, for 2 ch (stereo) that would be 2pair or 4 total!

I'm pretty sure it's A/B. It's definitely not A! Tranny & heatsink is too small!

I started the bias @ 150ma, didn't like the sound, so bumped it up to 200mA. To me, it still sounds somewhat "distorted" or veiled, but then again I've yet to "hear" a MOSFET amp that I thought sounded crisp/clean.

It's not running HOT but I don't wanna ramp up the bias too much and stress the outputs (there are on 2 (1pair) per ch).



Quote:
Originally Posted by lineup View Post
For MOSFET
100-200mA per transistor
Again.... This was included in my initial post! One of the recommendations was 200mA (or 100/output). However, the difference in (say 200ma from 100mA) is essentially double the current. That's a pretty broad range.




I was hoping someone here either owned or serviced/repaired this amp at some time (BK ST-140) and happened to know the correct bias.



Thanks;


B
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Old 16th June 2011, 12:43 AM   #5
bichi is offline bichi  United States
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a bit of additional BK ST-140 bias setting stuff:

Trouble with the bias B&K ST140
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Old 16th June 2011, 10:35 AM   #6
AndrewT is offline AndrewT  Scotland
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the schematic in the post5 link has an error.
It shows complementary output stage but the lower device is upside down.
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Old 16th June 2011, 07:29 PM   #7
amptech is offline amptech  Scotland
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I would set bias for 100ma for a signal pair of power fet's...and see have you get on.
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