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dadod 20th October 2012 12:14 PM

Little GEM
3 Attachment(s)
This amp I called “Little GEM”, very simple circuit with TMC(with a twist) and very low THD. It uses one pair of the ThermalTrak ONSEMI output transistors.
Thermal compensation is divided in two part, output transistors with TT diodes and the drivers with Vbe multiplier mounted on separate small heat sink. I simulated thermal behavior and it looks promising, it will need find tuning during practical implementation.
I use similar circuit in one amp(normal output transistors) and it sounds exceptional good.
THD20k at 100W/8R is 0.000391% and at 4W/8R 0.000294%
THD1k at 100W/8R is 0.000047% and at 4W/8R 0.000029%
all simulated.
This amp it’s all you need for domestic listening, easy to build, standard easy obtainable components.

east electronics 20th October 2012 12:40 PM

well pair of outputs @ 45+45 rails is a bit on the risky side ...what will happen in 4 Ohms loads ? especially if this is working without a VI limiter

then again doubling the mistake adding darlingtons will make the soar even worst ....

Lets say that the rest of the circuit looks far more promising but still you should recalculate everything with rails 35-40 volt

Again your simulator results might be a good point to start but to be useful you need to have a working prototype to understand how pcb , arrangement,implementation of the circuit and power supply will effect the zeroes for the worst ...

I am not trying to be negative but i am getting a bit fed up with all this simulation around me that has no actual use in real life ....Many forget to add what you correctly did that the impressive zeroes we see are simulator's not even result ...its just an estimation .

Kind regards

dadod 20th October 2012 01:47 PM

OK, we can keep power voltage at +-40 V and that is good for 4 ohm load. There are different wattage for ONSEMI TT transistors, so it could be scaled accordingly.
I seams that you don't like spice simulation. I've got very good exeriance with LTspice and practical result were close to the simulated one. I can't say it for THD as I never measured it on real amp, but the sound was the one to say it is good.
And one more thing regarding this amp, it is not only simulation(by the way simulation shows how good it could be is all role, as layout p.e. are followed) but if you read carefuly my previous post, I am listening a very similar amp.
Don't be fed up with spice simulation, with it you can design better amps.

dadod 20th October 2012 04:50 PM

2 Attachment(s)
Close Loop Gain and FFT at 1kHz first watt.

dadod 22nd October 2012 02:05 PM

1 Attachment(s)
This one uses “normal” output transistors not ThermalTrak in OPS. It is the one I am listening now. The circuit diagram is very similar, technical specification the same as the one with TT output transistors, except how good and how fast it kips the bias current stable regarding temperature changes of the output transistors and in part the drivers.

dadod 23rd October 2012 07:52 AM

1 Attachment(s)
This amp surprises me every day, it do magic with the sound. For such simple circuit diagram it just should not did it, but it did. The bootstrapped VAS with beta enhancer and the TCM to do all the magic?? Moving C13(part of the TCM) from Q3 collector to the emitter halved THD20k. I tried that with my TT amp but the result was not so good. It could be for different reason, already very low distortion, JFET LTP, TRIPLE output stage, I am not sure, but here it did this magic.
Here is PCB layout for non TT version, I would like some comments on it as I tried to follow the role of minimal interference from the power supply. This amp simplicity is in pair with P3A. I have heard P3A some time ago and the sound was very good, but this one I like better, could be because it’s mine.

jaycee 23rd October 2012 08:28 AM

Can you post some more graphs about stability ? Have you tested with a 10KHz square wave to the performance on rising/falling edges ?

dadod 23rd October 2012 08:42 AM


Originally Posted by jaycee (
Can you post some more graphs about stability ? Have you tested with a 10KHz square wave to the performance on rising/falling edges ?

What graphs you want to see? I have tested this amp before but with two output pairs and standard TMC, look here I don't think that moving TMC capacitor can influence square wave performance.

dadod 23rd October 2012 08:47 AM

Sorry wrong link.

jaycee 23rd October 2012 08:54 AM

Ah, that jogged my memory :) I have read that thread before... good stuff !

I've been noodling with a similar amp myself, so this has me interested. The main difference is I have an output triple (similar to Leach) and a regular current sink for the VAS rather than a bootstrap. I have been playing with TPC and TMC compensation, ironing out the stability is the sticky point.

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