35 years old and still going strong

[latala]

I have this week been looking at a power amplifier that I made for a friend about 35 years ago !
It was rated at 30 watts rms per channel into 8 ohms
I made this for Steve when I was about 22 years old using what was at the time the latest thinking re power amplifiers
Input is via a low pass filter 2.2k and 220 pf to avoid T.I.D. distortion This was then fed into a npn diff pair Bc 182 these being fed via a constant current source this being a ring of 2 Bc182 output from the diif pair being fed into a Bc212 vas that had an unbypassed 100 ohm emitter resistor collector load for the vas was another Bc182.
Bias for the output pair was via a vbe multiplier yet another Bc182
The output pairs were inverted darlingtons using base stopper resistors as experiments had shown that using this system I could acheive a very stable set of outputs
I did not use a capacitor between collector and base on the vas but between the collector of the vas and base of diff pair
After extended testing i.e.35 years I have measured both dc offset worse channel being 15 mv and bias being 12 mA not bad for a design thats pre Self and blameless
This was all done using a progressive method of listening and simple instruments an Eagle signal generator and a low cost Heathkit scope I also measured distortion under 0.1% @ 1 khz at any power below clipping.
I have made a drawing re the schematic and taken photos would any one like to see them?
How does it sound ? fine mighty fine !
Regards Trev

[Cal Weldon]

Of course we would. Quit teasing us.

[destroyer X]

please..continue..i am excited!

regards,

Carlos

[chris661]

This sounds oddly familiar. My guitar amplifier is of a similar age. I find it amazing how stuff just keeps going. They don't build them like that any more.

[latala]

I hope that these pictures attache ok
The amplifier now appears a little rough but its had a long working life !
I can even remember marking and cutting the boards by hand
This was way before the days of PC and cad software
I have also noticed that their is no dc blocking capacitor on the input to the diff pair but as the pre-amp was capcitor coupled it did not matter ( what a risk taker)
I shall post the schematic tommorow with a few more comments
This is only 1 of many hundreds of amplifiers I have made and designed over the last 40 years or so
But as they say we can all learn something new!
Ps the only failure in all this time has been the left hand led power indicator ( I knew these new fangled Leds were crap)
regards Trev

[destroyer X]

without loose it's focus... we could see these lovely construction in more details with the macro feature.

Some cameras does not have macro function, to make focus when the lenses are 10 to 20 centimeters from the target to be captured in photography.

Thank you Latala..... was good to see..from time to time real diyers appear.... a rarity those days.

regards,

Carlos

[latala]

Please find hand drawn schematic of the power amplifier
I have used this scheme many times with great success However my designs that I use today have evolved from this design
To me the main advantage of this circuit is the thermal stability due to the use of the inverted darlingtons in the output stage.
I made many tests at the time that this circuit was created relating to thermal stability and can say that this was in the order of 10 times better re bias drift related to output transistor temperature when compared with both Quasi and true Darlington output stages. You will note from the earlier pictures that the bias monitor transistor is not even connected to the heatsink
I would say that this is a good design for any one looking for this power level
In this day when we have a very large selection of high performance devices it must be remembered that the devices we could obtain where very limited
especialy re operating voltage etc
regards trev

[latala]

I forgot to mention that The amplifier is effected by the capacitor that is connected between the Vas collector and the input to the diff pair it is 100 pf here however when I was testing this I found that distortion could be made as low as 0.02% if this was reduced to 22pf,however stability was affected I decided by listening tests that the value of 100 pf gave me the best sound and a reasonable measured performance!
To me one of the most desirable properties on any power amplifier is stability
I should imagine that this amplifier would be very good with modern devices
Like Carlos I listen to my hifi! I can measure it but in the end I decide with my ears
Regards Trevor

[lanchile]

Originally Posted by latala

I forgot to mention that The amplifier is effected by the capacitor that is connected between the Vas collector and the input to the diff pair it is 100 pf here however when I was testing this I found that distortion could be made as low as 0.02% if this was reduced to 22pf,however stability was affected I decided by listening tests that the value of 100 pf gave me the best sound and a reasonable measured performance!
To me one of the most desirable properties on any power amplifier is stability
I should imagine that this amplifier would be very good with modern devices
Like Carlos I listen to my hifi! I can measure it but in the end I decide with my ears
Regards Trevor

Wow, after 35 years playing, that amp is still kicking some butt, I bet you if you change those electrolytic caps for better ones, that amp will live for 35+ years more. I guess it was well made.
I think today some companies can still build very good and reliable amps. Bryston is one of them.

[Miles Prower]

A basic design, done well can perform magnificently. There is no need to go uber fancy. This design is about as basic as you can get. The Sziklai pairs on the back end also sound very good, if you can keep 'em stable. That, and the QC topology, definitely sound better to me.