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Old 12th August 2011, 01:08 PM   #11
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Using a VBE multilpier does not neccesarily make it class AB.
If you ditch the current source then you will need to bootstrap or lose power.
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Old 12th August 2011, 01:29 PM   #12
Bigun is offline Bigun  Canada
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bootstrap is a good idea, more headroom to +ve rail for sure, maybe it's possible to bootstrap for the -ve rail too ?

yes, without Vbe multiplier thermal runaway is a problem, I'd overlooked that. I wonder what value of emitter resistors at the output would be needed to stabilize without Vbe (distortion could be higher) ?
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Old 12th August 2011, 01:41 PM   #13
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Would a standard CE or CFP output stage give better performance? Lateral fets even?
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Last edited by Stormrider; 12th August 2011 at 01:44 PM.
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Old 12th August 2011, 02:40 PM   #14
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Originally Posted by Stormrider View Post
Would a standard CE or CFP output stage give better performance? Lateral fets even?
A standard CE or CFP output stage will give better performance, especially in frequency response and ergo slew rate. The TIP142 and TIP147 does not seem to be the fastest trannies in the spares drawer.

MOSFETS is a possibility. They don't need need emitter resistors like BJTs do. You don't even need a Vbe multiplier. Here is a variation on the theme, with a high-voltage op-amp. Not quite simplified yet and I haven't tried it at a lower supply voltage, but you get the idea.

I will ditch the Vbe multiplier and use the current source from the previous circuit and report back. The feedback around the op-amp can be left out at a small cost in phase margin as well.
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Last edited by ingenieus; 12th August 2011 at 02:43 PM.
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Old 12th August 2011, 02:52 PM   #15
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Originally Posted by Bigun View Post
I wonder what value of emitter resistors at the output would be needed to stabilize without Vbe (distortion could be higher) ?
That's certainly worth a look.

Another option would be to use the Thermaltrak transistors from ON Semi. They have built-in diodes for thermal tracking.

Here is a full-house circuit for the Thermaltraks from an application note. Combined with a HV op-amp, the parts count might be just right. They will need drivers, but losing the Vbe multiplier with its need for adjustment would make it worthwhile. There seem to be a market/need for DIY amps that do not need any adjustment between build and use.
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Old 12th August 2011, 05:01 PM   #16
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Originally Posted by ingenieus View Post
Here is my suggestion for an amplifier that is as simple as possible, but no more than that. The main design objective was to get a low-priced amplifier with as few components as possible, while still getting good performance.

To this end, I went for integrated circuits and Darlington transistors. I even went as far as using the LM334 current source. A conventional current source has at least five components, maybe more. The design might not be the last word in state of the art and high performance, but it is as simple (and cheap) as I can make it.

LTspice has this amp giving at 1kHz:
  • 16W into 4 ohms with a THD of 0.045%
  • 32W into 8 ohms with a THD of 0.027%
The limiting factor is the power supply voltage of the NE5534 op-amp, which is + 22V. If more power is required two of these can be bridged. The generously dimensioned Darlington output transistors will handle the additional current required, but only into an 8 ohm load.

A 4 ohm load might be okay in a bridged configuration but each output transistor then has to source 7A of current. They have an absolute maximum rating of 10A, so we are sailing a little close to the wind here.

At these power levels, we might be able to do away with the Zobel network, or maybe just bits of it. Any thoughts on this would be appreciated.

There are various ways of getting more power out of this design, but then the cost and/or number of components goes up. The first way is to use a higher supply voltage and to drop the supply to the op-amp with Zener diodes. You then need an output stage with voltage gain. This can be achieved with complimentary feedback pairs and local feedback, However, all of the simplicity is gone.

Another way is use a high voltage opamp like the OPA551, which can handle a supply of + 40V. This will give about 70W of output power. However, these opamps are fairly expensive, which defeats the design objective. And for just a little more money you can use the LME49830 input stage from National Semiconductor.

Using an op-amp in a power amplifier seems to be a bit of a technological dead end, for the reasons mentioned above. But if you don't want a lot of power, it serves its purpose.
I intend to realize such an amp for checking various older and currently available OP-Amps in the DIP-8 outline and the SO-8 outline.
An interesting fact is the reaction of the error of the output buffer, which is still present in the not "Class-A" mode (distortion with complex character through idle current only between 20 and 50 mA).
Because this buffer stage is inside of the NFB loop, the necessary standarts of the OP-AMP (e. g. GBP = product of gain and bandwidth) must be much more higher than by normal line stages (and even than by RIAA stages with high voltage gain factor).
Thus the differences in sonic character between the various OP-AMP types must be actually also much more larger (and from other kind) by use in such an amp topology than by use in usually line stages.
How is here the audible perception of the differences between various op amp IC's from Analog devices (AD797), Burr-Brown OPA627, OPA827) and National Semiconductor (LME series) ?? This would be very interesting to know.
The maximum achievable output power by +/-15V is enough for checking this.

I will replace the darlingtons with discrete components, because here I can use various values and topologies for the emitter resistors of the first darlington stages (driver transistors). This isn't possible by darlingtons, because this are actually integrated circuits with two transistors and two resistors.

BTW - where goes the current from the emitter of transistor U6 (VBE Multiplier transistor from attached schematic post #1) ?
And what is the advantage for use of the integrated CS unit instead two resistors from the voltage rails to C and to E from U6?

Last edited by tiefbassuebertr; 12th August 2011 at 05:19 PM.
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Old 12th August 2011, 08:35 PM   #17
Boscoe is offline Boscoe  United Kingdom
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Could separate supply rails be used to obtain higher rail voltages for the output stage?
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Old 13th August 2011, 12:50 AM   #18
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For more power out of an opamp frontend the QSC topology is better. They have schematics on their website.

Here is a simple one:

http://qscaudio.com/support/library/...20One/1080.pdf
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Old 13th August 2011, 05:01 AM   #19
gannaji is offline gannaji  India
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Originally Posted by Stormrider View Post
For more power out of an opamp frontend the QSC topology is better. They have schematics on their website.

Here is a simple one:

http://qscaudio.com/support/library/...20One/1080.pdf
Very interesting output stage topology! Am I right in assuming that the collectors of both output transisters are at ground potential and hence, they can be mounted directly on heatsinks without insulators? Any other adverse effects or points to be taken care of?
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Old 13th August 2011, 09:59 AM   #20
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Originally Posted by tiefbassuebertr View Post
BTW - where goes the current from the emitter of transistor U6 (VBE Multiplier transistor from attached schematic post #1) ?
I got the original idea for the circuit from a sub-woofer amp circuit in the book "Build your own high-end audio equipment" published by Elektor. I wondered about the same thing myself, but it works. It seems that the op-amp sinks it. I confirmed this in LTspice, but can that be the right way of doing things.
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