Another amp

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The schematic looks sound on quick glance. Fairly straightforward design with classic topology. since this seems to be a compilation of ideas from different sources I might consider adding a current mirror to the LTP. If you use a ccs in the LTP and VAS, might as well.

Put a resistor in series with the base of the current source for the second voltage amp, 470R would be a good place to start. Improves recovery from clipping. The 1N4007 in the limiter circuit should be fast diodes, try the 1N4935. Flyback diodes across the outputs are cheap insurance, 1N4935. A pair of power supply bypass caps in the range of 22µF~47µF at 63V would make the bass seem more solid. Input cap to be a film type. Feedback cap to be non-polar or back-to-back electrolytics with a small film bypass. The diode across this cap should be increased to two in series, and then two more in opposite polarity. The poles are staggered wrong, supply is 1hz (4R), feedback is 9.6hz, input is 3.3hz. Change feedback cap to 470µF, 4.5hz. Change input cap to 1µF, 7.2hz. Input pole must predominate, feedback pole must be between input and power supply poles. If not staggered properly amplifier will ring at infrasonic frequencies when clipped, very hard on woofers and very poor sound quality. I would also consider doubling the current in the second voltage amp, you could be running out of drive current at low impedance. Looks very nice!
Hi all,

I was just notified that my amp (called M250)
was being discussed here. Thanks to Djk for the detailed comments.
If you would like to try my design
I think that you should take djk's suggestions
into consideration. I will try these changes myself as soon as I find the time. I have
added FR diodes at the output in my amp, but
regrettably I have had too little time
recently to update the page.

Best Wishes,

how about decoupling the output section from the driver and ltp section via 10 ohm resistor and 470uf cap
helps in preventing any noise from the output section entering the cruical low noise areas(ltp)

for your emitter resistor, i feel u utilize slightly big value here
i used 0.1 ohm normally. it is enough to force sharing
bigger value means higher cost.

i see u include the miller dominating cap here, i guess the output transistors can be swap for sc3281,sa1302. Because motorola transistor hfe drop quite abit when current is higher while the toshiba as a relatively constant hfe throughout the current values

how come u utilize mje350 for the ccs? any particular reason
u could use the bs556 back rite?
i feel the design is solid and good. i guess no harm in tweaking it
who knows u could sell it as a kit :)

The elektor amp is from april 1982. I have a papercopy of the article, but it is in dutch (elektor is dutch). It is specked at being 100W into 4 ohm! I have build the amp for PA use long time ago. So 500VA transformer and 2 * 20000uF elko. Also using a fan with matching heatsink. It does not get hot, not even warm..
And a 19i case you can stand/dance on :->

In this setup it is fine for PA use, it was the only one to drive the FANE bas speakers we had in those days.. Fun. But i dont think it is serious hifi. Elektor published more amps after this one which are more serious hifi.

One think it lacked: a shortcircuit on the output terminals killed it.
Not really much PA there..... Case of replacing a BDX and back in business (i have a DC protection circuit installed, so only the amp breaks.).


Re: THE M250

Drafance007 said:
I also need some answers about the M250 amplifier...
What is with the diode on the input....what does the resistor 15 ohm does on the base of pre output ??? Please Whod do i Ask 4 Help ?
The LED diode is used like a volt reference,
in the constant current source.
The resistor 15 ohm is used, when the current limiting transistors, T6/T7 gets active, to limit the current flow through the transistors.
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