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Old 29th April 2014, 05:35 AM   #1
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Richard Murdey
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Default RJM Audio R40 (make fun of my amp)

I've never touched audio power amp design before, but I've studied the schematics for some time and thought I'd try my hand at it.

The schematic below is very loosely based on classic Accuphase amps, the A100 in particular, but I've dumbed it down and generally scaled it back. As is immediately obvious, I've left out many things like limiting and protection modules, and reduced other circuit elements to ideal voltage and current sources. The transistors are placeholders, being convenient choices in the LTSpice library.

However I was quite happy when the LTSPice sim actually started to work, and I found myself with an actual non-oscillating, tolerably-low-distortion simulation that could put out 40 W into 4 ohms!

In the tradition of this forum I present my circuit for your inspection and general ridicule...
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Last edited by rjm; 29th April 2014 at 05:43 AM.
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Old 29th April 2014, 06:35 AM   #2
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Hi - It looks neat and obviously works, so there is no need for ridicule - as far as the basic electrical simulation in isolation permits.

Though you may only be simulating ideal operating conditions, I see some practical problems:
First, there is no thermal compensation shown for the bias.
Second, there is no DC blocking capacitor at R15 or servo for negative feedback.
Third, the gain of 11 is too low for a real world amplifier having such a large output stage.
Fourth, there are no gate isolating resistors for the highly capacitive output devices.

Have a look at these topics and keep up the experimention - It's good to see established
designs like this one reconsidered occasionally.
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Last edited by Ian Finch; 29th April 2014 at 06:53 AM.
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Old 29th April 2014, 01:17 PM   #3
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Richard Murdey
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Thanks! Based on reading similar threads I was anticipating some of your points, but not all...

Quote:
Originally Posted by Ian Finch View Post
First, there is no thermal compensation shown for the bias.
]

There's no bias circuit yet, so that isn't too shocking I hope! The eventual circuit will have full thermal and OC protection.

Quote:
Second, there is no DC blocking capacitor at R15 or servo for negative feedback.
Good point. Servo, probably. Accuphase have always good that route.

Quote:
Third, the gain of 11 is too low for a real world amplifier having such a large output stage.
Good point, though the actual gain will have to be chosen less on the size of the output stage and more the overall gain/efficiency of the system in which it might typically be used. The 20 dB selected seemed about right since I normally use 30 dB integrated, and propose to pair with a 10 dB preamp.

Quote:
Fourth, there are no gate isolating resistors for the highly capacitive output devices.
It's not an oversight, just simplification for the purposes of simulation here as adding them makes no difference whatsoever to the LTSpice result.

All the details of the real-world extras and protection will come later, it this point I'm more worried that the basic layout is unsuitable or incorrect.

/R
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Old 29th April 2014, 01:50 PM   #4
Bigun is offline Bigun  Canada
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I have found that I like the sound of these double-LTP front-ends. My attempt is here: TGM7 - an amplifier based on Greg Ball SKA

You may want to add some emitter degeneration to your LTPs.
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Old 29th April 2014, 01:52 PM   #5
mcd99uk is offline mcd99uk  United Kingdom
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A few comments... (No ridicule)

1) Can't see any compensation (eg miller) on these schematics.

2) What happens with a capacitive load?

3) No output filter (L//R).

4) No output zobel.

5) No MOSFET Gate zobels.

6) Need to simulate amplifier stability (Phase margin / Gain Margin).

7) May need additional power rail filtering between MOSFETs and rest of amp.
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Old 30th April 2014, 01:58 AM   #6
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@mcd99uk & Bigun

Thanks for the input.

1) Can't see any compensation (eg miller) on these schematics.
Excuse the idiot question: is that the capacitor in parallel with the feedback resistor? If so I was considering adding it in later when I could get a handle of what it would do to the circuit performance.

2) What happens with a capacitive load?
Bad things probably, the simulation seems barely stable even with a resistive load. I wanted to get that issue sorted first.

3) No output filter (L//R).
Will not add until I move away from the pure resistive load shown in the schematic.

4) No output zobel.
see 3)

5) No MOSFET Gate zobels.
Will add later.

6) Need to simulate amplifier stability (Phase margin / Gain Margin).
Indeed. I need to learn to plot and analyse that information...

7) May need additional power rail filtering between MOSFETs and rest of amp.
Yes, actually the Accuphase circuits do this, and in many cases run the output devices off completely separate supplies. The latter option seems very sensible.
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