Explendid amplifier designed by Michael Bittner, our MikeB

[Sheldon]

Thanks,

With that info, should be a breeze. I'll try it. May also try the GEM amp. Dang, I never shoulda looked at the SS forum, but Carlos's enthusiam sucked me in. That, and waiting for speaker parts has got me making amps in multiple numbers.

Sheldon

[sajti]

Mike,

yesterday evening I replanned Your amplifier, little bit. The input stage use PNP devices, with 2mA bias. The VAS use BD139/140 transistors, with 8mA bias. I keep the 68ohms as common emitter resistor, but I applied 82ohm local feedback in the VAS. I changed the bias network, to get better thermal compensation. So, now I working on the PCB design...

sajti

[MikeB]

Hi Sajti !
So, a complete redesign ? Can you show/send me schematic ?
With that much changes it will be necessary to readjust
feedbackcompensation. The thermal compensation worked "perfect",
for doubledarlington a simple vbe-multiplier does a very good job.

Just a hint, high current in the inputstage is not necessarily a good
thing, a lower current here in combination with lowgain-bjts in
the vas can improve quality as you get more a currenttransfer
from 1st stage to 2nd stage instead of voltagetransfer. (In theory)

Don't want to give a chance to my version ?

Mike

[destroyer X]

Symassym
Aksa 55 with nirvana upgrade
GEM Diamond...(the last published)
GEM 25W
JLH 1969 and DOZ

I construct 3.7K amplifiers...when i stop to count them... observe that no one in the list i wrote above are industrial amplifiers, as those i only apreciate an old enormous tube Mac Intosh amplifier.

Those ones i wrote above I like, all them, very much, and they were not written in my preference sequence.

I did not answered you Sheldon, because i left space for Michael enter, as he is tracking the thread, in the reality, i introduced, but the thread belong to him, he is the designer.

Satji:

I really made Michael amplifier with BD139, but was one of his first designs, and did not worked that first one, and i am sure was my construction failures, as Michael constructed and the unit worked....but was not aproved by him, he perceived harshing distortions in the treble range.

As a result, i avoid to use those BD139...but i jumped to his brother, the BC639 and BC640, and those ones have the same silicon die inside... but beeing a very small transistor, can only dissipate 1 watt, and cannot accept more than 1 ampere.

But those ones, alike the BD139, have not wonderfull gain, have to select, and was hard to find units with gain around 150.

As Michael said, i really constructed with BD139...as BC639 is the same transistor...but i had problems inverting leads....this transistor have strange pins.... watching the label, with the leads pointing to the floor, from left to rigth, Base, Colector and Emitter.... the same as BD139...but, with the label facing you.... if you read BD139 with the label facing you, will result wrong, will be emitter, colector and base....and this confuses me.... i am having some agging effects, as i could perceive.

I have constructed 4 Symassym, and more 2 boards are ready to construct it very small, 2 inches by 2 inches...but i am very lazy to start....but when started i will never stop untill the unit is ready to run or to receive a 12 gauge shot!



To Sheldon or Sunrise, as someone asked some about that.

Thank you Sheldon, because of the enthusiasm you said that i have.

- The gain i use to say, and i imagine the same Mike are talking about is the DC gain. I use my digital multimeters to measure this...but you can assemble something to measure too.... with batteries and resistors you calculate 1 miliampere flowing into the base to emitter current, and them measure the current that will flow into the colector to emitter circuit.... divide the second by the first and you will have the DC gain.

- To big transistors, use more current to base...let's say... 20 miliamps...and do the same measurement.

- I think Rodd Elliot, in ESP pages, explain how to do that better than i can explain.

- This amplifier, Symassym4, had 3 milivolts off set measured.... was hard to measure, it is a hell stable and reproduce with a very impressive dinamics.

regards,

Carlos

[MikeB]

Hi !

There is one detail about the symasym i forgot to tell...
The gain for this amp is set to a level, so that it can be directly
driven by linelevel from CD-player for example.
When connected to a preamp, the gain might be too high,
a solution might be using a voltagedivider at input.

Mike

[sajti]

Originally posted by MikeB
Hi Sajti !
So, a complete redesign ? Can you show/send me schematic ?
With that much changes it will be necessary to readjust
feedbackcompensation. The thermal compensation worked "perfect",
for doubledarlington a simple vbe-multiplier does a very good job.

Just a hint, high current in the inputstage is not necessarily a good
thing, a lower current here in combination with lowgain-bjts in
the vas can improve quality as you get more a currenttransfer
from 1st stage to 2nd stage instead of voltagetransfer. (In theory)

Don't want to give a chance to my version ?

Mike

Hi Mike,

I never make copy from any amplifier. I like to change them to my taste. I use more local feedbacks, intstead of the large open loop gain, and overall feedback. This method means less compensation, even it works without any compensation, if the overall feedback is low.
I will plan some different compensation possibility to the PCB, to optimize the final result.
I don't really understand Your comment regarding the current/voltagetransfer.
I use smaller emitter resistors for the output transistors. (0.1R) I need more temperature compensation with them...

Anyother issue: I planned two pairs of output devices, to handle more output current, or even class A working.

I will post my schematic, after I count all the values!

sajti

[sajti]

One more thing: I use 33uF WIMA capacitor for the feedback, and I not use input coupling capacitor.

sajti

[MikeB]

Hi sajti, my early prototype also used 0.1ohm, it showed the same
thermal stability. This voltage/current-transfer is something i am
still rasearching, i have not really understood it...
But this is something that has big impact on sounding.
These caps in vas to gnd in my schematic, the do not only serve
as feedbackcompensation, they have 2 additional functions:
- add a constant phaseshift to reduce dynamic phasehifts,
that might be the reason for the great soundstage.
- lower zout of vas for higher freqs, in sims these caps lower
thd at 20khz.

Sajti, should a bypassed electrolyt not be sufficient ?
A asymetrical inputstage without inputcoupling, is this a good idea ?

Mike

[sajti]

Carlos,

thanks for the comments! I used BD139/140 many times in my amplifiers, and I never had any problem with them. The 8mA/40V is bit too high dissipation for BC639/640. Maybe they can handle it, but I'm not 100% sure, so I don't want to take the risk to smoke my loudspeakers
For BD139/140: I want to screw them together, to keep the temperature, as close as possible. It's much easier, than with BCs...
But of course the listening test will do the final decision.

Thanks for the notes regarding the pins!

sajti

[sajti]

Originally posted by MikeB
Hi sajti, my early prototype also used 0.1ohm, it showed the same
thermal stability. This voltage/current-transfer is something i am
still rasearching, i have not really understood it...
But this is something that has big impact on sounding.
These caps in vas to gnd in my schematic, the do not only serve
as feedbackcompensation, they have 2 additional functions:
- add a constant phaseshift to reduce dynamic phasehifts,
that might be the reason for the great soundstage.
- lower zout of vas for higher freqs, in sims these caps lower
thd at 20khz.

Sajti, should a bypassed electrolyt not be sufficient ?
A asymetrical inputstage without inputcoupling, is this a good idea ?

Mike

Thanks for the comments! I just add one more transistor to the bias network. It it too much, I just apply jumper on the PCB to forget it.
Interesting, what You told about the I/V transfer. Maybe I will try to make some test with the sound of the different bias on the input stage. I use 2mA long time ago, and it has good sound to me. I use some more series collector resistors as well. It results lower dissipation for the input devices, and gives better sound, but I don't know how.
I saw Your compensations, and I understand the 330pF capacitors halps to reduce the output impedance. I will use smaller resistors over there. And due I not use any simulation, I will set this calacitors with measuring the real amplifier.

sajti