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Simple Symetrical Amplifier
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Old 15th September 2011, 02:51 PM   #801
Esperado is offline Esperado  France
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Quote:
Originally Posted by shaan View Post
Offset +-5mV(varies all the time).
May-be you can add a 4700F electrolic cap between feedback resistances R17, R28 and ground to increase DC feedback ? (joke)
5mv is a real nothing....

Last edited by Esperado; 15th September 2011 at 03:05 PM.
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Old 15th September 2011, 11:37 PM   #802
shaan is offline shaan  India
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Originally Posted by Esperado View Post
May-be you can add a 4700F electrolic cap between feedback resistances R17, R28 and ground to increase DC feedback ? (joke)
5mv is a real nothing....
I know esperado. But that is not really the problem, the offset is too much sensitive to slider movement, still.

However, I have reached a solution to this problem while using only one pot for offset, still at the input, decreasing slider sensitivity, decreasing parts count and increasing input impedance and power bandwidth. Current through the input BJTs are 5.5mA and 5.8mA. I am attaching the input stage. Please comment.

What I found is that the offset is best controlled at a specific ratio of the resistors R3 and R4. For my instance it is 3:2 and I'm going to test it right away with a 10K pot in place of R5.
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Last edited by shaan; 15th September 2011 at 11:40 PM.
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Old 16th September 2011, 01:35 AM   #803
Esperado is offline Esperado  France
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Originally Posted by shaan View Post
...
Then R3= R4=10k and between us a 4.7k pot with the cursor at ground ? It will go for an other pair or an other amp ?
And why to keep D2 and D3 ? If you want to keep them to filter, they would be better between R2 and C1 & R5, C2

Last edited by Esperado; 16th September 2011 at 01:43 AM.
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Old 16th September 2011, 03:46 AM   #804
shaan is offline shaan  India
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Finally, got what I needed! Fine tuning of the offset.

The idea was really simple, I wonder why I didn't think of it first!

VR1 and VR2 is both 10K.

VR2 has about +-500mV offset control over its 10K range, i.e. 1Volt.

Started with both pots at 50%, set VR1 first to about 1volt, then VR2.

That's all and it worked!

However, offset is still dancing between +-5mV.

Simulating SSA now.
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File Type: png SymTree.png (10.9 KB, 747 views)
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Old 16th September 2011, 04:07 AM   #805
Esperado is offline Esperado  France
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...
C5 would be better in the bases of Q1 & Q2. You can make a nice lowpass at 100kz, and it will filter RFI from power supply in the same time (you can use SMPS)
If you are bored by this offset problem, you can, with a simple OP amp, build a DC servo, and get rid of offset as well as tuning-it. I like very much this feature, specialy when your design allow no cap in the input.

Last edited by Esperado; 16th September 2011 at 04:10 AM.
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Old 16th September 2011, 04:19 AM   #806
shaan is offline shaan  India
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Yes I have already played a lot with servos. They are touchstones capable of making any naughty amp behave. I use them in my DoZ amp which are direct coupled and fed from dual supply and are assymetric, so servos were essential.

I am not bored! Come on, did I ever look so? After this much of mess? Don't worry.
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Old 16th September 2011, 05:20 AM   #807
Esperado is offline Esperado  France
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Lazy cat, one idea (sorry, not so easy to read ).
Your input architechture + error correction power stage from Bob Cordell.
i'm sure it would done something very special fast as light and smooth sounding with very low distortion and huge damping with less global feedback.
The best of the two best sounding world ?
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Old 16th September 2011, 06:41 AM   #808
Lazy Cat is offline Lazy Cat  Slovenia
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Quote:
Originally Posted by shaan View Post
However, offset is still dancing between +-5mV.

Simulating SSA now.
+/-5mV is really nothing, what matters is to look at this in freq domain. Look what I posted to you a while ago:

Quote:
Originally Posted by Lazy Cat View Post
So for the begining I suggest you to place 15V zeners in parallel to C1, C2, replace VR1 with two 1N4148 + trimmer in series and certainly "must" is thermall coupled input NPN/PNP pair. That would help DC bias conditions to be much more stable.
For a sensitive/high impedance input DC bias control arrangement you need a very stable DC supply rails, so two 10-15V zeners are essential.

DC servo, forget about it.
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Old 16th September 2011, 07:35 AM   #809
Lazy Cat is offline Lazy Cat  Slovenia
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Quote:
Originally Posted by shaan View Post
However, I have reached a solution to this problem while using only one pot for offset, still at the input, decreasing slider sensitivity, decreasing parts count and increasing input impedance and power bandwidth. Current through the input BJTs are 5.5mA and 5.8mA. I am attaching the input stage. Please comment.
Please try something like this for the input arrangement.
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Old 16th September 2011, 08:00 AM   #810
Lazy Cat is offline Lazy Cat  Slovenia
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Exclamation SSA Full Balanced Input

Quote:
Originally Posted by Esperado View Post
See attachment, that's what you means ? I just wonder why you want me to change the emitter values from 150 ohms to 10, it increase current and open loop gain, right ? it need 4watts for the 330 ohms then, and, if i'm correct, the gain will be 33 if 330ohms are set to 660 now ?.
Where did you see this 100F, where i have put the 1000 ?
Schematic will explain everything.

What we got here is full balanced input with symmetrical input impedances for positive and negative input and at top of it in current feedback loop arrangement, with minimal parts count.
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