Cascode Output Stages

[unclejed613]

if the CCSs were thermally coupled, they might not be so problematic.

[ppl]

Hi
Thanks for the Zero tempo operating tip. I arrived at the same conclusion, however for the Complete Amplifier once the DC offset is set to 0 the Amp Uncased sitting on the Bench will drift about -1 to +3 mV after 1 Hour of operation. It will stay below 5 mV after 8 Hours of operation however if large air currents are continuously circulating then DC can drift up to +25 mV. I think that once cased up this should be again under 5 mV over time and thus not a Problem. While the Output stage is inside the overall feedback loop there is only about 12 dB of Neg feedback at 1KHz. The feedback Loop contains a Cap and pot to add Bass Boost. When the Bass Boost is Full on the Amount of feed back is reduced to about 2 dB at DC. it is in this position that the thermals were an issue.

I have this working good now with only one or two minor complaints one i believe may be an issue with the Vbe Multiplier i use to set the Icq. if the Bias is set to more than 50 mV across the 1.1 Ohm output transistor Emitter resistors instability starts and is excited by probing the Base of the pre driver Transistors< however if set for under 40 mV the instability is completely Gone and probing in this area causes no instability, However i am not comfortable with an amp that can be adjusted into instability so im thinking different pot value or Divider values on the Vbe Multiplier so as to prevent adjustment greater than 40 mV however this all could then require pre-screening of transistors and such.

As Peter noted this type of Topology is hard to get stable and requires more work that one would initially suspect to obtain this, However once working the performance and sound quality are superb so i think this is a topology, That with further refinement could be the Output stage of one's Dreams

[unclejed613]

i gather you replaced the diode stack with a Vbe multiplier..... did you put at least a 1uf cap across it? the oscillation could be from the bias voltage collapsing when the transistor begins conducting, and since the base bias for the Vbe multiplier transistpr collapses, the transistor reduces conduction and the voltage goes back up and the cycle begins again.

[ppl]

" i gather you replaced the diode stack with a Vbe multiplier..... did you put at least a 1uf cap across it? the oscillation could be from the bias voltage collapsing when the transistor begins conducting, and since the base bias for the Vbe multiplier transistpr collapses, the transistor reduces conduction and the voltage goes back up and the cycle begins again."

Thanks
I never used a diode stack in my version always a Vbe Multiplier. Yes a cap is placed from C to E on the Vbe however it is 0.1uF not 1uF.

[Kostya-M]

I can suppose variant of cascode replacement of pair npn transistors. Using of MOS power transistors is preferred because its SOA is larger and output impedance much higher. First MOS transistor must be ordinary. Second - logic level.

[ppl]

" I can suppose variant of cascode replacement of pair npn transistors. Using of MOS power transistors is preferred because its SOA is larger and output impedance much higher. First MOS transistor must be ordinary. Second - logic level. "

Replacing Mosfets ith BJT's is not my Cup of Tea. I am one of them that actually Dislike the sound of Mosfets, Moreover this has already Been done but with only one stage as opposed to Three stages we are talking about See http://www.amb.org/audio/beta22/

[Kostya-M]

Replacing Mosfets ith BJT's is not my Cup of Tea. I am one of them that actually Dislike the sound of Mosfets, Moreover this has already Been done but with only one stage as opposed to Three stages we are talking about See http://www.amb.org/audio/beta22/ [/B]

I prefer bjt in output too. But I think, that primary bjt transistor witll determine sound. And upper mosfet has minor impact on sound, but can do good work.

[pietjers]

i gather you replaced the diode stack with a Vbe multiplier..... did you put at least a 1uf cap across it? the oscillation could be from the bias voltage collapsing when the transistor begins conducting, and since the base bias for the Vbe multiplier transistpr collapses, the transistor reduces conduction and the voltage goes back up and the cycle begins again.

There is a 47uF capacitor across the diodes.

I did some research to get standing current as constant as possible. Especially because the triple is know for its thermal runaway. Tried Vbe multipliers and diode solutions. But by far the best way is to use stacked diodes (5 in my case) mounted on top of one of the power transistors. The picture shows how this can be made with a crimp terminal ring, 1N4148 diodes and some two part epoxy resin adhesive. The results are great! After power-up, within 20 seconds the final standing current is reached, and it remains stable within 10% during operation.

Peter

[unclejed613]

diode stacks work. what's the capacitance between the diodes and the lug? or do you possibly have a short between one of the diodes and the lug? i've seen oscillation caused by a bias transistor and a heat sink clip shorting together. this happened regularly on an amplifier production line, so i saw it often enough for it to stick in my mind....