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Old 13th May 2005, 12:29 PM   #1
kmtang is offline kmtang  Canada
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Default Matisse Line Amp

Refering to the attached schematic, I am wondering if there's a fraud with this circuit.

The value of R14 is 47K in the above schematic. I just feel that it is rather weired having such low impedance at the 12AT7 grid. It reflects the same impedance to 12AX7's plate load. I am wondering if it was a mistake in the design or not. The plate load resistance of 12AX7 is about 70K. The plate load resistor should be 3 times of the plate resistance which is approx 210K. If we load it with 47K, surely it would have higher distortion. May be that's what the designer wants. Please correct me if I am wrong.


I have changed the R14 to 500K and C51 to 0.1uF. It improves the quality and dynamic of the sound dramatically.


I wish I could have the Tube CAD software to check this out.
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Old 22nd May 2005, 04:10 AM   #2
kmtang is offline kmtang  Canada
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Overloading of the first stage 12AX7 would cause severe distortion. However, the heavy feedback could reduce the amount of distortion.

May be the designer want to make it this way to create some magic. I still think this is not logical at all. I remember my friend said the guitar amps were designed with lots of distortion. Possibly, the designer of Matissee intended to apply the same magic to this line amp.
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Old 22nd May 2005, 09:02 AM   #3
PRR is offline PRR  United States
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> I am wondering if there's a fraud with this circuit.

Fraud is a very strong accusation.

> I am wondering if it was a mistake in the design or not.

47K is not the value commonly used, but there are no "right answers" and this is sure not wrong.

> surely it would have higher distortion.

What distortion? We are asking for 1V-2V signal from 400V power supply. Distortion is going to be very low even if we "screw-up" the "right answer for lowest distortion".

Also the 12AT7 has significant gain, which means the 12AX7 works at much lower signal level (about 50mV output) than the 12AT7 (1V output), and the 12AT7's distortion will dominate.

Also the two distortions are in opposite directions, and can cancel. I don't believe it gives large cancellation, but it might.

> I wish I could have the Tube CAD software to check this out.

There was a time when we did not have TubeCAD, SPICE, computers, or even LED calculators. Somehow we managed, scribbling on napkins.

A resistance loaded triode will approach 5%THD when peak signal is 20% of supply voltage. So the 12AT7 will run 5%THD at about 80V peak, 50V RMS. THD does not go down exactly with signal voltage, but that's an easy approximation. At 1V RMS output, the 12AT7's THD will be around ~0.1%.

With hi-Z loading, the 12AX7 working at 0.05V signal level will be down around ~~0.005%. With 47K loading, signal current is high, and Gm/Ik nonlinearity will raise THD around 10 times, to say ~~0.05%. Indeed it may semi-cancel the 12AT7 distortion.

Working hi-Z and bypassed, a 12AX7-12AT7 pair can have forward gain of around 2,000. We often want closed-loop gain around 10, and that gives a LOT of negative feedback. No matter how you feel about high-NFB audio, 200:1 feedback can be hard to stabilize.

Even with the heavy 47K load, and unbypassed cathodes, this pair can have forward gain around 200-400. Fed-back to closed loop gain of 10, this is 20:1-40:1 of feedback. If we assume that distortion does not cancel between stages, and is around 0.1% open-loop, it will be below 0.01% closed-loop.

> my friend said the guitar amps were designed with lots of distortion.

The classic (not fuzz-box) amps were originally designed without regard for distortion. They use hi-Z loading for high gain, and tend to run very high signal levels, without any feedback except a trice around the speaker-jack for slight damping.
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Old 22nd May 2005, 02:50 PM   #4
kmtang is offline kmtang  Canada
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Thanks PRR for you detailed comments. I sincerely thank you sharing your knowledge and experience with me.
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Old 24th May 2005, 04:53 PM   #5
fhwong is offline fhwong  Malaysia
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This???
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Old 24th May 2005, 05:02 PM   #6
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by PRR
There was a time when we did not have TubeCAD, SPICE, computers, or even LED calculators. Somehow we managed, scribbling on napkins.
Don't forget slide rules! I once had a book of 7 figure log tables for accurate calculations...
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Old 24th May 2005, 11:09 PM   #7
PRR is offline PRR  United States
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> Don't forget slide rules!

Unretouched this-moment snapshot of what is in front of me:
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Old 25th May 2005, 08:28 AM   #8
EC8010 is offline EC8010  United Kingdom
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I'm envious - that's a much nicer slide rule than mine. And truth be told, it's ages since I've used mine - calculators that didn't eat batteries were a marvellous invention.
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Old 25th May 2005, 09:14 AM   #9
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Quote:
Originally posted by PRR
Unretouched this-moment snapshot of what is in front of me:
Nice set of persuaders (vice grips) there too... i've a couple slide rules kicking around too... haven't used one much since i got the HP25C in 1976... and now it is either the RPN calc on my 'puter or Trapeze for the more complicated tasks. Would be nice if TubeCAD got ported to OS X, i'd buy it in a flash.

dave
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Old 25th May 2005, 01:53 PM   #10
kmtang is offline kmtang  Canada
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Hi FH Wong,

Yes, it is the one as your picture shown.

I am using Jensen copper paper-in-oil capacitor between the 12AX7 and 12AT7.

Johnny
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