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Old 26th January 2012, 11:38 AM   #1
UV101 is offline UV101  England
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Default Amplifier Bias adjustment???

Hi,

Does anyone have any views on increasing the bias in my Restek Fantasy amplifier. I've asked the question on the thread I started replating to the mods I've applied to the amp but I think the original title means I'm not getting the potential exposure i'd hoped for. I hope no one minds, but I'll repeat the question here.

The standard setting of 10mV seems a little on the low side to me after reading the TNT article on bias adjustment???

For reference, the schematic is here
Restek Fantasy Schematic

I've taken my measurement accross each of the output resistors which are 0.15r. (R5L1 to R5L4)

Based on this, I've started to increment the bias.

This table is based on 0.15r
1.5mV = 10mA
4 mV = 26.7mA
8 mV = 53.3mA
10mV = 66.7mA
15mV = 100mA

I got as far as 10mV but felt that the amp was running too hot under extended heavy load so I've backed it down to 7.5mV 50mA.

Is there any adverse effect to increasing the the bias other than heat which will ultimately lead to component failure in the output stage? I have to say that with the setting at 66mA, it was like listening to a completely different amp!!!!

Incidentally, one channel is constant accross all 4 resistors, and one channel varies but up to about 1.5mV. I'm guessing thats where the FETs in one channel are not well matched. How much of a difference will this make replacing with a matched set?

Thanks, Ian

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Old 26th January 2012, 11:52 AM   #2
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If you call bias adjustment to preset wired to the base of the Vgamma multiplier at the base of the power stage, it only sets the quiescent current in them, the output offset voltage must be set in the first stage, normally a preset between the emitters of a differential pair. Keep in mind that a lower bias current will cause distortion at zero crossing signals, and too much will cause the output transistor to overheating because they go nearer class A.

Good luck!!!
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Old 26th January 2012, 12:00 PM   #3
UV101 is offline UV101  England
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Quote:
Originally Posted by Osvaldo de Banfield View Post
If you call bias adjustment to preset wired to the base of the Vgamma multiplier at the base of the power stage, it only sets the quiescent current in them, the output offset voltage must be set in the first stage, normally a preset between the emitters of a differential pair. Keep in mind that a lower bias current will cause distortion at zero crossing signals, and too much will cause the output transistor to overheating because they go nearer class A.

Good luck!!!
Hi, yes that is exactly what I mean. So the only thing to be aware of is heat dissipation then (and obviously staying on the good side of an output stage disaster)?
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Old 26th January 2012, 12:44 PM   #4
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Normally, it is said that an output voltage offset under 30mV is well, without load, which is your voltage there?
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Old 26th January 2012, 01:26 PM   #5
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Too much quiescent current can cause 'gm-doubling' distortion. Essentially, you move the crossover region away from zero crossing to higher voltage levels. This is theoretically wrong, but many people seem to prefer it - perhaps because the resultant distortion does not occur for very small signals.
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Old 26th January 2012, 01:26 PM   #6
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This is the DC offset?

less than 3mV
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Old 26th January 2012, 02:21 PM   #7
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It is well under 30mV, so, why to readjust it??
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Old 26th January 2012, 02:54 PM   #8
UV101 is offline UV101  England
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Eh? Sorry, I'm not following.

I've not adjusted the DC offset. Only the Biasing current to the base of the output transistors. The DC offset has always been virtually nothing. I think there may be a servo becuase I cant see another adjustment and no matter what, DC on the output is virtually nothing.
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Old 27th January 2012, 11:23 AM   #9
UV101 is offline UV101  England
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Quote:
Originally Posted by DF96 View Post
Too much quiescent current can cause 'gm-doubling' distortion. Essentially, you move the crossover region away from zero crossing to higher voltage levels. This is theoretically wrong, but many people seem to prefer it - perhaps because the resultant distortion does not occur for very small signals.
Can you explain a little more please. In what way will this change the sound?
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Old 27th January 2012, 11:39 AM   #10
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Isn't there adjustment for the bias in the output stage thru the 1 k pot? Did I download the wrong schematic?

If there is bias adjustment then shouldn't the bias be adjusted for the lowest distortion?

Dc offset is more a product of how well the front end /diff is matched isn't it?
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