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 27th July 2008, 06:26 PM #11 diyAudio Member     Join Date: Feb 2007 Location: Δραμα - North Greece Bias network? Saki I have in my documents a similar circuit. In the drive and in the output stage it is almost similar with your circuit, except the values of resistors etc. The main difference is an active bias network between the bases of drivers, which is absent from your circuit. In the past as i remember i had builded some amplifiers of this kind with success but only with +40/-40 Volts supply. In another two docs i have the plans of two big power amplifiers with an IC in first stage but there are also seperate VAS stages after the IC and before the drivers. I'll try to send you the schematics because i must take photos from the plans because these are from three well known comercial P.A. amplifiers and i can't find them in the web. Fotis
 27th July 2008, 06:34 PM #12 diyAudio Member     Join Date: Aug 2005 Location: K-town Hi Sakis To me the bias doesn't seem right. Removing the op-amp from the circuit, for DC bias 79.5/(15K + 130R)=5.25mA. 5.25mA X 130R=0.68V. 79.5/(15K + 220R)=5.22mA. 5.22mA X 220R=1.15V. 1.15V + 0.68V=1.83V. This is the base spread voltage for Q1 & 2, no? Subtracting Vbe for each one 1.83V - 1.2V=0.63V. 1K||22R= 21.5R. 0.63V/21.5R=29mA. Assuming a beta of 100 for Q1 & 2, Ib=290uA. 28.7 X 51R=1.46V across R22/23. Subtract Vbe of the output transistor and that leaves 0.864 across each 0.22 Ohm emitter resistor which is 3.93A for each output. A little high I think for class AB bias. Also 28.7mA X ~78V= 2.24W on each driver, Q1 & 2. For the CFP circuit, these drivers must be thermally compensated, not the output transistors or thermal run away will take place. This is a little bit different than I have seen. I remember something similar except there was a feedback loop setting the gain of the voltage amplifier (output stage). A voltage divider from the output to the emitters of Q1 & 2 (node of R16 & 17) and the op-amp feedback was still from the output node. You cannot set a gain of 1 with the CFP and expect a stable result, in my experience anyway. __________________ All the trouble I've ever been in started out as fun......
 27th July 2008, 07:17 PM #13 diyAudio Member     Join Date: Dec 2005 Location: the north Good post CBS240. Think you have done our math here, and found out the bad bias level. And like you, I very much doubt that such an critical output arrangement can be used without stability troubles. It is way trickier to use Power transistors with output from Collectors. = Have voltage gain in final output stage. Power followers are quite much more predictable. They most often work very well. __________________ lineup
diyAudio Member

Join Date: Feb 2007
Location: Δραμα - North Greece
Quote:
 Originally posted by lineup Good post CBS240. Think you have done our math here, and found out the bad bias level. And like you, I very much doubt that such an critical output arrangement can be used without stability troubles. It is way trickier to use Power transistors with output from Collectors. = Have voltage gain in final output stage. Power followers are quite much more predictable. They most often work very well.
I agree absolutelly with you my good friend Lineup.
I have a career of 25 years builded mainly in P.A. audio. During this they have passed many comercial amplifiers of enormous power from my hands. I have many service manuals from these devices. My expertise builded in these mainly and not in books or articles published on the web.
Sakis, as i believe, is trying to build a simple as far as possible and powerfull P.A. amplifier. In such type devices, the designer primarilly looks at the stability and rugedness. Therefore the CFP topology it is absolutelly rejected. Also the symetrical input stage - which offers as i know very well a better slew rate from the conventional simple LTP input stage - it is rejected in the 90% of P.A. designs as i have seen in a lot of plans.
I know that you are a lover of the symetrical LTP input stage And not unjustly.

Regards
Fotios

 27th July 2008, 11:46 PM #15 diyAudio Member     Join Date: Dec 2005 Location: the north Yes. for PA ( Public Announcement ) amp it is good to use a conventional LTP pair input. With power power followers output. I never build PA amplifiers. And not very often Power amplifiers. I very often design, MultiSim & publish Headphone & Line PreAmplifiers. Because this I can do. Well And when using lower Voltage = 2 x15, 2x24 and lower powers we can make very fast, rapido amplifiers, with complementary design and even output from DRAIN or COLLECTORS. And use some small capacitors to make stable. You have got my amplifier desire, philosophy right, fotios. I like you very much. But I do not know you much. I know you, precise like me .. you are a man making many thoughts other things than audio. You are Too, one Philosopher. Like your country man Sokrates was .. long time before. Regards Lineup ---------- Audio is not life. Is only a LITTLE part of life. Good Friends is life. They are BIG part of life.
 28th July 2008, 01:47 AM #16 diyAudio Member     Join Date: Aug 2005 Location: K-town Hi oopps, I did make a mistake, that 0.63V would be across 2 X 21.5R or 43R. This leaves 14.5mA in the drivers. 14.3mA X 51R=0.73V, so each output would have ~660mA. Still a little high and 14.3mA X 78V=~1.1W for the drivers so they will need compensated. I suspect instability, both oscillations and thermal imbalance. __________________ All the trouble I've ever been in started out as fun......
 28th July 2008, 03:44 AM #17 diyAudio Member     Join Date: Dec 2005 Location: the north Yeah. Oscillations can produce a lot of heat ... quickly! __________________ lineup
 28th July 2008, 08:17 AM #18 diyAudio Member     Join Date: Jan 2006 Location: Athens GREECE thanks guys ..... i read and understand all the posts and will take them under serious considaration but as usual let us not forget that this is a comercial working amplifier from hill audio that has been on production some time in the past .... for sure i cannot tell if it was good or stable the only changes i made was the input ic and the output transistors .... will go for 5534 this afternoon and see what changes __________________ SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
 28th July 2008, 02:11 PM #19 diyAudio Member     Join Date: Jan 2006 Location: Athens GREECE replaced TL071 with ne 5534 absolutely no effect ..... i start to belive that something might be wrong with the schematic __________________ SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
 28th July 2008, 02:26 PM #20 diyAudio Member   Join Date: Oct 2007 Yes, there is a lot worng with the schematic. First off, R6 and R7 need to be some sort of thermistor or diode stack with that can be adjusted for proper bias. If you overbias it'll burn out or run away on you. As-is, the output will be limited to +/-13V. The outputs are configured as followers the way you have them drawn. To make this simple circuit work, you need to turn the main power supply inside out. GROUND the collectors of the output transistors and take the speaker out from the center tap of the +/-79V supply. Then reverse the input phase (swap the + and - of the op amp). Do not use a TL071, it cannot provide enough drive current - the 5534 can. And the +/- supply for the op amp should remain isolated from the main supply (don't just use regulators, use a separate supply). You can boot strap it off the main supply, but that gets a little tricky if you've never done it before.

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