+/- 36 Volt Voltageamp with TMC error correction - diyAudio
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Old 9th November 2012, 07:07 PM   #1
lineup is offline lineup  Sweden
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Default +/- 36 Volt Voltageamp with TMC error correction

This is a very precise Voltage amplifier.
It goes well together with my other project - The Power Buffer.
Because of TMC error correction the performance is intact throughout the whole range.
The THD distortion is never more than 0.00006%.
The sweet spot of TMC is set at +/- 20 Volt.
At this level it shows only THD 0.00004%

The input uses transistors in Sziklai config. This makes high input impedance.
There is a mirror on top.
The VAS is a darlington pair and the output is standard in Class A.

Further improvement is hard to make with discretes.
But who knows what you guys can come up with ......
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Old 9th November 2012, 07:39 PM   #2
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Perhaps you should drop this into the Discrete Op Amp Open Design thread?

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Old 9th November 2012, 07:43 PM   #3
lineup is offline lineup  Sweden
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I do not know about it, Bear.
But I will have a look around
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Old 9th November 2012, 11:08 PM   #4
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it is here:Discrete Opamp Open Design

But don't let that stop the discussion here... I'm really interested in these sorts of designs, especially if they do well as the freq goes up...

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Old 9th November 2012, 11:47 PM   #5
jcx is offline jcx  United States
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with my suggestion for the 2-pole bootstrapped compensation mod to Scott's circuit I found in the sim that the R had to come from the drive Q, not the output for there to be much advantage
presumably the "VAS" output node is so high Z that the nonlinearity of the output Q emitters being coupled back was bigger than the positve feedback gain boost

so I 'd guess that a double follower output stage would still be an improvement in Lineup's circuit
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Old 10th November 2012, 01:33 AM   #6
Struth is offline Struth  Canada
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Hi Guys

I found that the Darlington VAS will readily blow up its input BJT when driven to clipping - at least in a power amp circuit. Current limiting is needed to protect it.

Along the same line, you might want to add some clamp diodes between the output node and the output BJT bases - cathodes up. This will speed recovery from clipping.

jcx's idea of adding a driver stage to the outputs would allow the center-tap of two Rs between the emitters to be the feedback point for the internal compensation. Since reading Cordell's and Leach's articles back so many moons, I have never built an amp since with C-dom returning to the VAS input.

Another Lineup triumph!

But...

Can you dial the gain down to unity and have it be stable?

Have fun
Kevin O'Connor
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Old 10th November 2012, 09:30 AM   #7
lineup is offline lineup  Sweden
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Quote:
Originally Posted by bear View Post
But don't let that stop the discussion here... I'm really interested in these sorts of designs, especially if they do well as the freq goes up...
_-_-bear
I will do some 20 kHz SPICE as well.
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Old 10th November 2012, 09:34 AM   #8
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Quote:
Originally Posted by jcx View Post
with my suggestion for the 2-pole bootstrapped compensation mod to Scott's circuit I found in the sim that the R had to come from the drive Q, not the output for there to be much advantage
presumably the "VAS" output node is so high Z that the nonlinearity of the output Q emitters being coupled back was bigger than the positve feedback gain boost

so I 'd guess that a double follower output stage would still be an improvement in Lineup's circuit
Interesting. So you think the Resistor in my circuit shoud come from a driver.
Certainly a double floower output can be a real improvement.
I might put 2SC2240 and 2SA970 to drive the BD139 and BD140 output.
Thanks jcx
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Old 10th November 2012, 09:43 AM   #9
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Quote:
Originally Posted by Struth View Post
Hi Guys

I found that the Darlington VAS will readily blow up its input BJT when driven to clipping - at least in a power amp circuit. Current limiting is needed to protect it.

Along the same line, you might want to add some clamp diodes between the output node and the output BJT bases - cathodes up. This will speed recovery from clipping.

jcx's idea of adding a driver stage to the outputs would allow the center-tap of two Rs between the emitters to be the feedback point for the internal compensation. Since reading Cordell's and Leach's articles back so many moons, I have never built an amp since with C-dom returning to the VAS input.

Another Lineup triumph!

But...

Can you dial the gain down to unity and have it be stable?

Have fun
Kevin O'Connor
As far as I have tested in SIM the circuit is well enough compensated with 100//680pF.
I always do AC Analysis to see how it behave.
And this circuit as is shows no tendence to oscillation.

Now at UNITY GAIN is another thing.
This would probably make to have another compensation.
Now, the circuit is not built to be stable at all levels of GAIN.
It was built for gain like 20.
To feed a power Buffer with gain=1

I did not build this like Universal Op Amp.
The voltage gain should be like I show in my circuit.


But if I should follow jcx advice to add a driver to the output pair it might need more compensation. And I have to redesign the TMC network.
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Old 10th November 2012, 10:08 AM   #10
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Quote:
Originally Posted by bear View Post
Perhaps you should drop this into the Discrete Op Amp Open Design thread?

_-_-bear
Rather not, this is a used to death outdated design, the only new ( now rather old) development is TMC but weve already seen how it works in a couple of other designs including the amp PCB sold by DIYAUDIO. Rather keep that thread for innovative designs.
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