Abomination! or Painting a Mustache on the Mona Lisa meets the Island of Dr. Moreau - diyAudio
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Old 11th December 2008, 04:34 AM   #1
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Default Abomination! or Painting a Mustache on the Mona Lisa meets the Island of Dr. Moreau

...and I get a tip (from member Megajocke) that Arrow is selling off the thermal track transistors dirt cheap, so I order some. Now, what to do with these? I think to change an existing design for these to work in but then I remember seeing...somewhere...an amp that is made for these. It looks complicated and it has some "features" that I don't want. I figure I can make a few changes so I go to the trouble of capturing the schematic in Multisim to try and get an impression of the performance. Very nice this amp looks in simulation and my changing the input from jfet to bjt didn't seem to make that big of a...

Enough of that. First, I'm not looking for any input here and I fully expect this thread to be ignored by many BUT I have something too good not to share. Admittedly, it is not original - I ripped off another DIY design for this (apologies have been made). I made changes. I corrupted the design and produced a truly offensive board layout. I made a prototype and populated it with unholy parts. I ran it and low, I saw that is was good- sing praise.

In all seriousness, the schematic. Yes, it looks familiar.
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Old 11th December 2008, 05:41 AM   #2
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A very good amp, modelled on Roender's excellent amp.

Gudonya, John, Alleluyah.....

Hugh
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Old 12th December 2008, 12:00 AM   #3
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Is that a CFP driver setup ?? how would the EF oriented
T-track bias track tempco??
Is that why I noticed jumpered diodes in the Abomination
pix you posted??, to compensate.
OS
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Old 12th December 2008, 01:05 AM   #4
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Quote:
Originally posted by AKSA
A very good amp, modelled on Roender's excellent amp.

Gudonya, John, Alleluyah.....

Hugh
Thanks Hugh,
Yes, I twisted his into a new animal. It has very similar performance across the board. I'll be posting results as I go.


Quote:
Originally posted by ostripper
Is that a CFP driver setup ?? how would the EF oriented
T-track bias track tempco??
Is that why I noticed jumpered diodes in the Abomination
pix you posted??, to compensate.
OS

CFP like the original with the drivers running hot. They are all on the main heatsink along with the outputs. The TT diodes will track the temp. I have 5 of the 6 diodes in circuit but shorted one to allow for a lower idle current (50-60mA) to start. To use a higher idle current (140mA or more!) use 5 of the diodes. The schematic is not correct in this detail, I forgot to put the 5th one back in.

Here is the Fourier analysis of both simulated amps. This is not to show one being better than the other, but just to demonstrate how similar they are.
The simmed RMI first:

Click the image to open in full size.

And the Abomination!:

Click the image to open in full size.

These were done at the same output power, idle current and load.
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Old 12th December 2008, 01:18 AM   #5
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very good,mjl .I like that strange LTP/vas .. will "steal" it for sims.
But I thought you said LEDs were GAUDY... (I happen
to like the idea of a Xmas tree amp).

BtW, Just for reference , is the general topology of this one a "balanced cascode"? I never followed roenders thread (too
many trannies)
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Old 12th December 2008, 01:46 AM   #6
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Quote:
Originally posted by ostripper
very good,mjl .I like that strange LTP/vas .. will "steal" it for sims.
But I thought you said LEDs were GAUDY... (I happen
to like the idea of a Xmas tree amp).

BtW, Just for reference , is the general topology of this one a "balanced cascode"? I never followed roenders thread (too
many trannies)

I used all green LEDs that I'll paint black

Click the image to open in full size.

That's it operating at the full +/-70 rails.

It's a folded cascode. I'd explain but I'm not sure I know how. Maybe Hugh could pop in again.

Here's 20kHz clipping performance from the actual amp. This is at full power:

Click the image to open in full size.

Mild clipping shows no sticking. This is about 5Vpp over the amps input sensitivity.
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Old 12th December 2008, 02:06 AM   #7
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Quote:
I used all green LEDs that I'll paint black
DRab black ..I'll resim it so I can use red,blue ,green..
I like LED's.

I,m jealous of your nice boards , maybe i'll take take you up on your offer.. my layout is almost done..
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Old 12th December 2008, 02:18 AM   #8
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A folded cascode is pretty much any common emitter amp which operates directly into a common base circuit.

That is, a LTP collector drives the emitter of the voltage amp, with the base kept at a constant voltage (just like any other cascode).

Chief disadvantages are:

More noise
Poor offset control
Reduced output swing


All you need to know about them is

HERE.

I still like 'em though, because they are fast. Could you replace the VAS current mirror with simple resistors, do you think? That would improve offset control, and remove an area of non-linearity, though at the expense of OLG.... (but you could pick that up again if the output from the LTP was buffered through an EF).

Hugh
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Old 12th December 2008, 03:00 AM   #9
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Quote:
Originally posted by AKSA

Chief disadvantages are:

More noise
Poor offset control
Reduced output swing

Thanks again,
I can't comment on noise (yet) but the offset is as low as I've ever seen - less than 1mV. Output swing is not an issue in my case as I'm not utilizing the full voltage potential of the power supply anyway.
It is supremely stable with next to no compensation and very well behaved.

20kHz squarewave:

Click the image to open in full size.

I have tried a few different things, including resistor loads for the VAS, but all had a negative effect on the distortion spectrum - increases in the odd order harmonics, especially the 3rd.
It is the best amp that I have simmed (so far).
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Old 12th December 2008, 03:07 AM   #10
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Quote:
Originally posted by ostripper

I,m jealous of your nice boards , maybe i'll take take you up on your offer.. my layout is almost done..

Anytime, no problem.

"I see a red LED and I want to paint it black...No colors anymore I want them to turn black...
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