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Old 28th June 2013, 07:01 PM   #131
mcd99uk is offline mcd99uk  United Kingdom
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Its a good thing in a way that you didn't make the connection. It made me think. Best way to learn. It all makes sense now and knowledge has been gained about LTspice.

It all came down to following your suggestions earlier. I now know that the output stage is benign. The exicon models are ok and that the bootstraps are fine. Thank you!

Bootstrapped drivers work a treat now . This is the driver topology sorted then.

Adding voltage sources is a good way. I was thinking of adding one source to remove the offset but keeping the caps.

Shall now analyse the attachment.
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Old 1st July 2013, 10:37 PM   #132
mcd99uk is offline mcd99uk  United Kingdom
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Default Convergence at last

Here is what I hope to be close to the final schematic. Done a lot of experimenting and there doesn't seem to be any real improvements that can be made without significantly changing topology. Anyhow, feel I need to move on to the next stage. Which I think you can guess what that is and its not a prototype

The power rail snubber positions are representative. Added VAS current limiting as I noticed during clipping the VAS was running way too much current. Lots of extreme tests have been simulated and the amp appears to be well behaved. Decided to keep the current source caps. Without them to diamond buffer current sources had a strange dip in output. Also, they don't seem to affect recovery as I think other mechanism kick in before they saturate completely.

PM = 83 degrees / GM = 12dB.

Current activity is re-laying the PCBs out. Almost complete redesign.
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Old 2nd July 2013, 10:06 AM   #133
AndrewT is offline AndrewT  Scotland
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Is this a current feedback amplifier?

Congratulations !
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regards Andrew T.
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Old 2nd July 2013, 10:28 AM   #134
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Is this a current feedback amplifier?

Congratulations !
Without going into the arguments about what a CFB amplifier really is, yes.

Please hold off any congratulations until I present a scope picture... The journey still has much distance to go. But progress has been made.
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Old 3rd July 2013, 11:53 AM   #135
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Originally Posted by mcd99uk View Post

PM = 83 degrees / GM = 12dB.
Much better, this is around the min a allround high performance amp should have. You can use any speakers or cables without any worries.

The RC network on the bases of the vas is near always necessary to smooth a slight step that is caused by EF augmented vas in CFB. You can increase the gain margin by increasing the resistor in either the RC on the NFB compensation or the compensation of the vas but at the cost of PM. Yours is just dandy as is.

Its a nice high performance design, I would just cascode the input diamond so that I could use low noise RF transistors but your way with error correction to improve performance is a good alternate way.
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Old 3rd July 2013, 12:21 PM   #136
mcd99uk is offline mcd99uk  United Kingdom
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Originally Posted by manso View Post
Much better, this is around the min a allround high performance amp should have. You can use any speakers or cables without any worries.

The RC network on the bases of the vas is near always necessary to smooth a slight step that is caused by EF augmented vas in CFB. You can increase the gain margin by increasing the resistor in either the RC on the NFB compensation or the compensation of the vas but at the cost of PM. Yours is just dandy as is.

Its a nice high performance design, I would just cascode the input diamond so that I could use low noise RF transistors but your way with error correction to improve performance is a good alternate way.
Thank you. Hopefully, it'll sound good. It's up against the Leach, Sansui AU517 and Cyrus2 trio of amps.

I'm steering clear of cascodes in this version. I'm worried about cascode oscillation.

In the future there will be mk2 which I plan to try and push performance further.
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Old 13th July 2013, 03:28 PM   #137
mcd99uk is offline mcd99uk  United Kingdom
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Agghhhh.......The perils of parasitic simulations....

This has been a lot of work.

Have now got the all parasistics simulated down to 5n and some even below that.

THD has gone up massively and LTspice is having problems. It just spends ages source stepping.

The good news is that the circuit is stable and still has much the same spec other than thd.

If only I could get LTspice to simulate without stepping so much then I could make some progress with the THD.

Frustrating.
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Old 13th July 2013, 03:38 PM   #138
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I hear you! That can be one of the most challenging things.

I have found on many occasions that if I leave a capacitor "dangling" with one side not connected to anything, it will stop convergence. LTSpice doesn't seem to recognize open-ended caps as singular matrixes and I'm not sure why. Deleting these caps or terminating their loose ends somehow often unfreezes the simulation. You might try Rclick->Rpar=100meg.

Also, try running the amp without solving the DC operating point (sim cmd->skip op point). This will allow you to see start-up behavior. Sometimes the sim can't find the operating point simply because the circuit in it's present state wants to latch on startup, or is unstable during startup.
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Old 13th July 2013, 03:46 PM   #139
mcd99uk is offline mcd99uk  United Kingdom
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"This is war".

At least its not just me. I have noticed that reducing the mosfet bias improves things. Could this be a clue?

Just tried skipping operating point and it is unstable. There's some oscillation at the beginning that dies down. Then the simulation settles down and there is no output from the amp.
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Old 13th July 2013, 06:16 PM   #140
mcd99uk is offline mcd99uk  United Kingdom
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Very basic question. This is very new to me.

What do you set Rpar on the trace inductors and capacitors to? Do you set to something stupidly high?

I have formulated a plan of attack. Unfortunately could be very time consuming.
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