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Old 14th July 2004, 04:43 PM   #1
Rudy is offline Rudy  Belgium
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Default Any comments/changes on this schematic ?

I am about to finisch my amp, have already completed the 2 channels, all of it Amp,supply and servo is on 1 pcb/channel.

Wat worries me is that the hights or not as clear as i suspected, the rest off the amp sound very naturally and overall great.
( without the servo it sounds better but thats not an option )

or there any things i could try to impove it before i start building it into its enclosure ( witch arives in 4 weeks ).

Click the image to open in full size.

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What i did not like about it in the first place was the DC offset without the servo, is there any way to to something about this, becouse of this its needs about 10sec to stabilize the DC offset to les then 100mV. ( i allready matched the diff pairs and the resistors, this helped but not that mutch as i hoped )

Rudy
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Old 14th July 2004, 05:45 PM   #2
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Only two comments : Too high current on VAS and too high voltage on zeners in gates ( give there 15 V, it's enought ). Very clear circuit .
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Old 14th July 2004, 06:34 PM   #3
MikeB is offline MikeB  Germany
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I would say, that C8 with 1n is quite big, i prefer 100p.
The same might be with C10.
You might work without the servo, if you put a big cap in series
with r25 (220-470uf ?)
But if the DC is that big, you should try to adjust the DC somewhere
else, maybe by adjusting r13 (with a pot ?)

What i don't really understand is the biasvoltage for T8,
Its not clear to me, what the voltage is. If it's 20Volt, this would give
~1 Amp through T8.
For good DC, its important that T7/T8 gets the same "biasvoltage".
This means if current through R9 is for example 2ma, you get a
bias of ~0.94 Volt, which results in a current through T7 of ~18ma.
The same settings should be for T8.
But for a current that big in the VAS you should add a buffer to T7/T8.

Mike
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Old 14th July 2004, 07:05 PM   #4
Rudy is offline Rudy  Belgium
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@Upupa Epops
Any suggestions on the vas current, any other disadvantages accept for the currentdraw from the diff pair.

@MikeB
Servo is not an option, as this is my third amp, and the other 2 or without servo, it was time i started experimenting the servo.
"If" i want to insert a pot, it defenetly would not be there, as with that resister the diff pair current is set.
The current in T8 is not set by that zenerdiode, but with the CCS of the diff pair. ( VAS current is 30mA ) ( total diff pair current is 5,5mA )

could the DC problem be coused by the huge difference from R5 and R25 ?
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Old 14th July 2004, 07:16 PM   #5
dimitri is offline dimitri  United States
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Mike, don't be that fast > what the voltage is. If it's 20Volt
There is T6

Rudy,
R18 drop is 540mV,
approx. the same drop will be on R13,
as R18=R17, your input differential pair will work asymetrically,
change R9=R10 approx 2*R13

frequency compensation - don't like it and leave it for your simulation

servo - you have three poles servo! R40C7, R39C17, R38C16. Have you simulate group delay/phase shift?
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Old 14th July 2004, 07:39 PM   #6
Rudy is offline Rudy  Belgium
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"approx. the same drop will be on R13"
not really, as the drop on T8be is bigger then T5be
You may also not forget the current for the VAS pair drawn out off/into the diff pair.

"frequency compensation - don't like it and leave it for your simulation
servo - you have three poles servo! R40C7, R39C17, R38C16. Have you simulate group delay/phase shift?"

I actually dont do simmulations, values or measured from the amp itself.

I could try to find a match on the diffpaircurrents.
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Old 14th July 2004, 07:56 PM   #7
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Try putting a big capacitor in series with R25 to allow 100% feedback for DC.
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Old 15th July 2004, 03:37 PM   #8
Rudy is offline Rudy  Belgium
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Changed C10 to 100p
R17 and R18 changed to 56E for a VAS current of 11mA
but i have had not the time to listen to it yet ( with this current )

at this point the diff pair is perfektly balanced ( 1mV difference R9//R10 )yet i have a dc offset off -6V without the servo ( but i am leaving this like it is )

@dimitri R40C7 is not a pole, R4 prevents this for being one.

/edit .. also changed the zeners

Rudy
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Old 15th July 2004, 03:50 PM   #9
dimitri is offline dimitri  United States
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Quote:
@dimitri R40C7 is not a pole, R4 prevents this for being one.
Rudy, this is most common mistake, as R4 is shunted by the output resistance of the preamp!
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Old 15th July 2004, 03:57 PM   #10
Rudy is offline Rudy  Belgium
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Quote:
Originally posted by dimitri


Rudy, this is most common mistake, as R4 is shunted by the output resistance of the preamp!
would you be so kind to explain this
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