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Old 26th September 2012, 07:24 AM   #3151
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Quote:
Originally Posted by shaan View Post
Anyways, I am currently simulating my SSA with a servo, in order to eliminate my headaches/confusions/doubts/caring about offset once and for all, concentrating solely on the bias of the input and VAS.
Maybe something in this way ...
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Old 26th September 2012, 08:35 AM   #3152
ersa191 is offline ersa191  Equatorial Guinea
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I have a little problem finding common part BF245B local stores have only A,C grade. is this OK or should I go for other jfet like 2N5459? The amp is TSSA BIGBT HP?
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Old 26th September 2012, 09:00 AM   #3153
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2N5459 is OK if its Id is more than 8 mA at Vgs=0 V, so it has current regulation margin more than 100% at 3 mA idle Id. Of course Rg has to be trimmed to get correct current value and than replaced with fixed resistor.
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Old 26th September 2012, 10:07 AM   #3154
shaan is offline shaan  India
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Originally Posted by Lazy Cat View Post
Maybe something in this way ...
I am simulating something in this way.

Red trace is amp freq response with 10uF input cap. Green trace is servo response. As simmed, the servo with the shown values would work at about 0.2Hz and quickly fall in response in both sides. I used this with my DOZ amps and it worked pretty well, the thing to keep in mind is not to provide voltage gain in the VLF zone where the servo will work.

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File Type: png SSAServoV1-graph.png (10.1 KB, 412 views)
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Old 26th September 2012, 10:36 AM   #3155
sajti is offline sajti  Hungary
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Originally Posted by Lazy Cat View Post
Maybe something in this way ...
You can avoid the 100ohm resistor, or change it to some 1-10k. 100ohms means hard load for the opamp, and no necessary.

Sajti
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Old 26th September 2012, 10:38 AM   #3156
sajti is offline sajti  Hungary
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Quote:
Originally Posted by shaan View Post
I am simulating something in this way.

Red trace is amp freq response with 10uF input cap. Green trace is servo response. As simmed, the servo with the shown values would work at about 0.2Hz and quickly fall in response in both sides. I used this with my DOZ amps and it worked pretty well, the thing to keep in mind is not to provide voltage gain in the VLF zone where the servo will work.


Is the 1M is not too high? How much input offset it can handle?

Sajti
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Old 26th September 2012, 11:22 AM   #3157
mikelm is offline mikelm  England
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When I tried a servo on my fetzilla the nice qualities were lost ( transparency, spaciousness & bass integrity ) unless I set the -3db point to an incredibly low subsonic frequency so I will be interested to hear your before & after impressions.

Good Luck !
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Old 26th September 2012, 11:29 AM   #3158
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Quote:
Originally Posted by sajti View Post
You can avoid the 100ohm resistor, or change it to some 1-10k. 100ohms means hard load for the opamp, and no necessary.

Sajti
Nope, there will be max +/-1 V on 100 ohm meaning max. 10 mA OP amp output current.
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Old 26th September 2012, 11:38 AM   #3159
FdW is online now FdW  Netherlands
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Quote:
Originally Posted by Lazy Cat View Post
Maybe something in this way ...
There is a DC-servo'ed version that I posted (some time ago) maybe it is helpfull Simple Symetrical Amplifier post #1063 (it includes a simulation file).
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Old 26th September 2012, 11:52 AM   #3160
shaan is offline shaan  India
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Originally Posted by sajti View Post
Is the 1M is not too high? How much input offset it can handle?

Sajti
Yes. Thanks for the tip. With 100K resistor from servo to input it can handle +-5V of output offset(will it ever happen? dunno).

If the servo is just removed then the default offset with the 10K from input to ground is about +900mV, with the currents in the upper and lower halves balanced.

Quote:
Originally Posted by mikelm View Post
When I tried a servo on my fetzilla the nice qualities were lost ( transparency, spaciousness & bass integrity ) unless I set the -3db point to an incredibly low subsonic frequency so I will be interested to hear your before & after impressions.

Good Luck !
Yes this happened in my listening tests with DOZ too and I had to increase the capacitor at the servo from 1uF to 4.7uF and decrease the feed resistor from 100K to 33K, with a -3dB of about 0.4Hz. Obviously the servo was adding its own voltage gain to the amplifiers LF response. There was funny cone excursion with the default values.

I will add the servo to my SSA and update ASAP.
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Last edited by shaan; 26th September 2012 at 11:59 AM. Reason: forgot pic
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