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Old 5th February 2018, 05:09 PM   #41
EUVL is offline EUVL  Europe
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I think his idea is that the Tx will drive the gates via C12, C17, loaded by R6 and R24, for AC signals.
And then by disconnecting C12 from R23 at the positive pole, the opamp can then drive the DC offset via the 4 resistors.
But it is isolated from the Tx at DC by C12 & C17. The Tx is low impedance to Gnd at DC.

Might also work.
But the "small cap" after R42 is additional load for the Tx in the audio band.


Patrick
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Old 5th February 2018, 05:45 PM   #42
RobLK is offline RobLK  United States
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Yes I know, I kept c2 in the circuit for the same reason. My circuit was basically the same as yours. I started over and now both ways work fine. They both have exactly the same distortion. The only difference is one doesn't have c2 in the circuit. I don't know why the circuit that looked like yours didn't work the first time.
Yes I know, a small cap after r42 in your circuit above is a bad idea. In the other circuit without c2 and that trace gone it made no difference with it in or out so I left it out.
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Old 5th February 2018, 07:25 PM   #43
EUVL is offline EUVL  Europe
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Can you post your .asc file ?


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Old 5th February 2018, 09:27 PM   #44
RobLK is offline RobLK  United States
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Well I had thought I would only try to explain it since it was just a very small change to what you had already posted. I was a bit clumsy at explaining it. Still except for maybe Zen people will figure it out. Basically I wanted to use a dc servo but without the need for c2.
Also some of the models I have such as the jfets used in the circuit aren't match well so I don't like to post with them. Do you know how to modify models to match components?
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Old 7th February 2018, 10:19 AM   #45
EUVL is offline EUVL  Europe
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The distortion in Spice, even with a perfect transformer, is not exceptional.
But I am getting a bit curious, so I might end up building one, just to find out for myself.


Stay tuned,
Patrick
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Old 8th February 2018, 10:49 AM   #46
EUVL is offline EUVL  Europe
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> If use DC servo, is the opto mandatory?
Yes.

> I thought that is for DC offset control,
No.
It is part of the auto bias control circuit.
See : Auto-Biasing Circuits for Complementary Followers

Only the DC servo is for DC control.

Sorry that I missed your post earlier.


Patrick
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Old 9th February 2018, 07:26 PM   #47
blackdod is offline blackdod  Europe
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Sorry boys.

I had a terrible week on my work and I didnīt nothing on the amp.

On the PSU, I removed Q1~4 and I bridged it, removed ledīs and I changed LM337/317 for other too but I have the same problem, voltaje drop to 14,80volt and -14,40volt. So I guess that I have a problem with the msr860 or maybe on the 5K trimpots that we have on R2.
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File Type: png 317_337 Cap Multiplier.png (37.6 KB, 119 views)
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Old 9th February 2018, 07:41 PM   #48
EUVL is offline EUVL  Europe
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This latest schematics you posted is not the same as in post#1 (Nazar shunt).
This is just a normal LM317/337 followed by a BJT cap multiplier, as used in the F5-HA.
The cap multiplier works in open loop and has some finite output impedance.
So some voltage drop is normal.

This has been proven to work in multiple examples of F5-HA using our PCBs.
So I cannot understand what your problem is.
If the output voltage is stable you can just trim out the DC offset as mentioned before.
Or just use the DC servo.


Patrick
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Old 9th February 2018, 07:56 PM   #49
blackdod is offline blackdod  Europe
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Thanks Patrick

Quote:
Originally Posted by EUVL View Post
This latest schematics you posted is not the same as in post#1 (Nazar shunt).
I confused the schemes, the last one that I have set is the correct one.

Quote:
Originally Posted by EUVL View Post
This is just a normal LM317/337 followed by a BJT cap multiplier, as used in the F5-HA.
The cap multiplier works in open loop and has some finite output impedance.
So some voltage drop is normal.

This has been proven to work in multiple examples of F5-HA using our PCBs.
I didnīt know that the voltage drop was normal . I thought this was a problem with my PSU. I can compensate the output voltage of each rail with the trimpot but I never get to have 15 volts in the output. The maximum voltage I get is 14'80volt in the negative rail and about 15volt in the positive.

When both rails were decompensated, the offset in the amplifier was terrible

Quote:
Originally Posted by EUVL View Post
If the output voltage is stable you can just trim out the DC offset as mentioned before.
Or just use the DC servo.


Patrick
I will try to use the amplifier with B + -14,5 volt and then I will eliminate the C2.
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Old 10th February 2018, 01:37 AM   #50
EUVL is offline EUVL  Europe
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You can also adjust the output voltage under load by adjusting the values of R2 and R7 in the power supply schematics.

Connect the PSU to the amp (without headphones).
Use a 5k trimmer, preset to ~2.7k in place of R2 and R7.
Adjust the trimmers until you get exactly 15.0V at the regulator output.
Then measure the values and replace with fixed resistors.

When this is done, then measure the output voltage of the amp with the input shorted to Gnd and C2 bypassed.
Adjust the trimmer as you have before.
Let it run for an hour and see if it is stable over time.

Easiest is still to use a DC servo.


Patrick
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