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Old 29th April 2014, 07:05 AM   #21
AKSA is offline AKSA  Australia
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RC,

Yes, I have used the DMM5401 but be aware that they are VERY TINY. At my age, I have a problem seeing the buggers..... I moved to singleton input stages (a la the JLH) some time back and so the thermal connection is not required. I was very pleased to give it away, they are, to use a description of our good friends the Yankees, 'pesky an' ornery' to fit.

I have just looked over your quasi circuit. I would foster your thoughts on this. Quasis are asymmetrical output stages, with two n devices in totem across the rails and they create a lot of H2 and H4. A lot of people actually love quasis.......

You have the emitter of Q4 on the output rail. You might put a 4004, a 22nF filmcap and a 100R resistor, all in parallel with each other, in between Q4 emitter and the output rail. This makes it very easy to set the bias current in the output stage and softens the switching transistors between the output devices under load.

Cheers,

Hugh
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Last edited by AKSA; 29th April 2014 at 07:16 AM.
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Old 29th April 2014, 07:09 AM   #22
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by Ranchu32 View Post
Very generous of you to take the time to simulate this. Any chance you would send me the files to play with?
Yes of course but they are on a different PC so I'll do it shortly
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Old 29th April 2014, 08:11 AM   #23
Mooly is online now Mooly  United Kingdom
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Here you go. I've added a simple bias network. The file will run straight off in LTSpice using the inbuilt models.
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File Type: zip Simple Class B Amp FET.zip (1.3 KB, 80 views)
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Old 29th April 2014, 10:30 AM   #24
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Quote:
Originally Posted by AKSA View Post
RC,

Yes, I have used the DMM5401 but be aware that they are VERY TINY. At my age, I have a problem seeing the buggers..... I moved to singleton input stages (a la the JLH) some time back and so the thermal connection is not required. I was very pleased to give it away, they are, to use a description of our good friends the Yankees, 'pesky an' ornery' to fit.

I have just looked over your quasi circuit. I would foster your thoughts on this. Quasis are asymmetrical output stages, with two n devices in totem across the rails and they create a lot of H2 and H4. A lot of people actually love quasis.......

You have the emitter of Q4 on the output rail. You might put a 4004, a 22nF filmcap and a 100R resistor, all in parallel with each other, in between Q4 emitter and the output rail. This makes it very easy to set the bias current in the output stage and softens the switching transistors between the output devices under load.

Cheers,

Hugh
Thanks for dropping in Hugh. I've seen this resistor in this configuration in Quasi's NMOS project and wondered about its function; however, his design doesn't have the diode or bypass cap.

I'll include these in the redraw I'm about to post shortly.
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Old 29th April 2014, 10:39 AM   #25
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Latest redraw incorporating Mooly's revised R values for the tail resistor and feedback network, tail resistor and output gate stoppers, as well as Hugh's suggested changes.

I've also tidied up the front end, band-passing the signal (added 330p ceramic), adding a 100k resistor to prevent any big bang in the speaker if the input is accidentally connected while powered on, and adding inverse parallel diodes around the ground lift to pass fault current.
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File Type: png CSH2.png (14.5 KB, 439 views)
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Old 29th April 2014, 11:06 AM   #26
The Dastardly Dad of Three
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Didn't peter baxandall have some diode trick that improves linearity ? Maybe worth a google. Or maybe Hugh's post covers that already.

Edit : worth checking if the trick is for BJTs so maybe needs more diodes for a fet ops ? Blind speculation omp

Last edited by kasey197; 29th April 2014 at 11:10 AM.
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Old 29th April 2014, 11:10 AM   #27
Mooly is online now Mooly  United Kingdom
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The Baxandall diode is more applicable to bjt output stages where there is significant driver current flow. I put the latest version into simulation, I'll post the result in a minute.
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Old 29th April 2014, 11:13 AM   #28
Mooly is online now Mooly  United Kingdom
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Here we go. Output set at 2.83 vrms approximately.
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File Type: jpg Circuit.JPG (135.1 KB, 384 views)
File Type: jpg Distortion.JPG (113.5 KB, 367 views)
File Type: jpg FFT.JPG (137.8 KB, 340 views)
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Old 29th April 2014, 11:31 AM   #29
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Oops... forgot the RC filter on the LTP tail.
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Old 29th April 2014, 11:36 AM   #30
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Mooly, I've never used Spice, so I don't know how to interpret the result. Is it good or bad?
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