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VSSA Lateral MosFet Amplifier

This what I've got with Bigun file.This is at quite low level, at higher level it is much worse.
dado

For such a simple amp it is still all very good, at your first sim at 1 kHz, H2 still -100dB.

Normal home usage with moderate to high efficiency speakers will do fine.

VSSA is ideal for an active speaker (2-3 channels per speaker), low parts count, direct connection to drivers, active crossover, etc.

If you would like to get lower THD add appropriate driver, plus two output pairs more ...
 
I setup the circuit from your diagram.
The DC-offset is very small, 12 mV.

The first thing I noticed is that circuit needs some compensation to be stable.
This was fixed with one 22pF from base to collector of KSA1381 and likewise 22pF around KSC3503.

The THD distortion at 1 Watt is remarkably low for such a simple amplifier:
THD 0.0002% at 1 Watt RMS.

Very nice amplifier :)

Yes, thanks lineup. It would be a shame not to share this amp among others, especially for young DIY-ers to start with some discrete amp built.

Agree, 22pF base collector compensation and 100pF parallel with input 10k also recommended.
 
For such a simple amp it is still all very good, at your first sim at 1 kHz, H2 still -100dB.

Normal home usage with moderate to high efficiency speakers will do fine.

VSSA is ideal for an active speaker (2-3 channels per speaker), low parts count, direct connection to drivers, active crossover, etc.

If you would like to get lower THD add appropriate driver, plus two output pairs more ...

This is THD20k before clipping(this amp clips at 28 V p with +-35 V of power supply)).
dado
 

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Yes it's a really little nice circuit and I think to publish it in a German forum. Especial younger people are searching often good and simple circuits. I will of course mention the author ;)

No problem, some resistors values still can be calibrated to optimal values, like OLG (470ohm, 10 ohm) vs. bandwidth (feedback resistors ratio), input/VAS bias etc, but in general as it is now, VSSA will work. ;)

Also please invite German fellows to DIY Audio too. :cheers:
 
With only six transistors you can get genuine current feedback, high bandwidth amplifier, capable of driving 4 ohm speakers. PCB can be very simple and small too. ;)

With additional pair of transistor for current source (and paralleled output), it can deliver up to 180W with better performance. And the best stability I have seen from all amplifiers I have simulated.
 
All of them ever? :eek:

I mean capacitive load. Hehe :D And low impedance load too.

ADD: Simulated with 2 Ohm load and/or paralleled with 56uF, no oscillation even with 2V input (and no thermal runaway). Not sure with real measurement but never seen something like this before but the amp sounds good. I learned a couple of things from Marantz CFB amplifier.

My latest play is with your simplest BJT SSA. So only 4 transistors. But I used darlington output. Really "simple". And can be biased very low with acceptable performance.
 
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With the bias resistor of 1 kohm amp is in A class(more then 4A of bias). For B class it should be between 160 and 200 ohm depends of used bjts.
dado

Actually, in my sims, only needs 120 Ohm for around 120mA of bias, which is about as low as you want to go without temperature compensation. But even at this low current, the distortion is still excellent.
 

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Few more words about VSSA, tips for a potential builder to follow.

Input BC550C/560C pair has to be thermally coupled simply by gluing flats of their cases together or by greasing the flats and than firmly tied by plastic cable tie.

Each of VAS gain transistors should have small heatsink to dissipate its 0,5W thermal power. It would be also fine to have only one heatsink for both of them, in this case mandatorily connected to GND potential. 2SA1381/2SC3503 must be electrically isolated from the heatsink by mica insulators if they're not already in isolating cases.

10uF poliester film capacitors and 2,2mF/10V elcos should be quality performers here since they're in direct AC feedback to GND path.

Output transistors of course have to be mounted to the main heatsink (0,5°K/W), I suppose the best version would be that they're completely positioned underneath the PCB serving as spacers-holders for PCB.

There are three adjustments for VSSA to set it to normal working conditions.

Two 15k input DC current injection resistors should be trimmed to get 150mV on each 10 ohm VAS emitter resistors and at the same time to get 0mV DC offset at the output.

1k multiturn trimmer serves to adjust proper output mosfet's bias, something around 150mA is pretty optimal value here. After one hour when the amp is warmed up, readjust trimmer to set exact 150mA bias current.

At the end short explanation why so low distortions we get in so simple amp as it is.
Well beside high slew-rate there's also large bandwidth at 80dB of OLG, which is very high concerning this is CFB design, so 55dB of feedback correcting the output is fast and accurate to uV levels.

Another very important feature of VSSA is correct subtraction between input and output signal due to constant quiescent current provided simply by resistor, not by using some complex constant current source. How is this possible? Well 15k resistors are located between two almost constant DC points with no AC voltages present there, so constant DC and constant resistance gives constant current simply by using plain resistor. Even better performance results would be achieved by regulating and filtering rails DC to a mV level. ;)