New 200 watt Kit amp features ultra low distortion and Thermaltrak devices

[Nordic]

If I had to guess almost none of the high volume production amps incorporate matching, just due to the extra time it takes.... and yet we don't have blocks of flats going up in flames due to failure.

[snoopy]

Originally posted by Nordic
If I had to guess almost none of the high volume production amps incorporate matching, just due to the extra time it takes.... and yet we don't have blocks of flats going up in flames due to failure.

I think you will find that the Japanese amplifiers used matched devices. They had access to matched devices long before anyone else did

[snoopy]

Originally posted by john_ellis
Hi Andrew T

Try simulating the effect of a 47 uF /12k input resistor on a 20 Hz square wave, and compare with 4.7 uF.

It is surprising how big the input capacitor needs to be to support good LF performance. I tend to use DC coupling, normally, but where offset voltages are of a concern, at least 22 uF!

I have noticed that MJL's need higher current to reduce crossover distortion in simulations, (but not measured significant differences using higher bias currents). This seems to be related to the drop of fT at low currents. gm doubling isn't an issue if a current source drives the VAS. The Miller cap of course reduces the impedance (increasing the problem of gm doubling) at higher frequencies.

I'm surprised that this Self clone amp still uses such a large miller capacitor with output transistors 10 times faster than the MJ802 etc. which Self wrote about in Electronics World. One would have thought that other improvements might have been done ...

cheers
John

The Douglas Self circuit has a potentially fatal flaw in it. Because there is no current limiting on the driver or VAS transistor it is possible to destroy it if it goes into saturation and the emitter follower transistor before it feeds excessive current into the base. I have been told that this is indeed what happens to practical implementations of this circuit. Normally the base current is limited by the diff pair but as soon as an emitter follower is used then the base current can be much larger and potentially destructive to the VAS transistor.

Simple fix is to use an emitter resistor of say 10 ohms for the VAS transistor and a current shunt transistor to the base of the emitter follower In actual fact if you look on page 134 figure 5.24 of his 3rd edition this other circuit does actually use this protection

[col]

I was wondering if anyone has actually tried one of these amps yet? Jaycar has the kits on their "new in 2008" flyer priced at $89.95 for the amp and $54.95 for the power supply. Which seems to me quite a reasonable price if they are any good.

col.

[ostripper]

Wow, I recently read D. Selfs book and am in the process of
building/designing a 4 OP device main amp based on "blameless".

To my astonishment the amp in this thread is almost the
"clone" of my endeavors except for the thermaltraks
and undercompensated vbe
that Roender commented on.

I am using the same drivers (mje15030/1) ,almost the same outputs,
(mjl3281/1302) and to get the amp thermally stable I had
to mount a real Vbe,(bd139) with a thermal insulator on one of the outputs
(2 mica pads) to get a +- 20mv bias tracking between 25-60C.

Bias wise, I noticed switching distortion below 15ma so
I settled for 35Ma per device.
Otherwise, my amp is the same except for no VAS buffer
and a LED CCS.

Since they are selling this as a kit it must
not have any real safety issues and it's nice
to see my device choices commercialized.

I do have a question about a EF amp with this topology.

Which driver arrangement is better:

A.Drivers with emitter resistors tied to output(like this amp),or

B.Drivers with "floating" single resistor between the emitters.

Why do different EF designs use both topologies and what
are the advantages and/or disadvantages of them.
(I left both possibilities open on my PCB's)
Any comment on this would be appreciated,
Thanks ,OS.

BTW , Its nearly impossible to get thermaltrak's here...,
but even if I could I'd opt for the more "traditional"
output choice.

[roender]

Originally posted by ostripper
Wow, I recently read D. Selfs book and am in the process of
building/designing a 4 OP device main amp based on "blameless".
...

Which driver arrangement is better:

A.Drivers with emitter resistors tied to output(like this amp),or

B.Drivers with "floating" single resistor between the emitters.

Why do different EF designs use both topologies and what
are the advantages and/or disadvantages of them.
(I left both possibilities open on my PCB's)
Any comment on this would be appreciated,
Thanks ,OS.

Read again D Selfs book.
All the answers are there

[JSW]

Be very careful about fake mje15030/1 , I suspect the ones I got with the modules are just that , changed them with new ones and low and behold the bias voltage is exactly what they said it would be with a 68 ohm resistor in series with the bf470, must now do some testing before it will be used