Hello all,
My diamond output buffer, see PDF, eats output transistors, being Q1,Q4,Q27 and Q30.
I know it has to do with thermal issues. So i tried the following :
Place Q1,Q4,Q27 and Q30 together on the heatsink.
And couple Q6 with Q2 and Q28 with a piece of aluminium.
And the same with Q8, Q3 and Q29.
According to the logical rules this should be a perfect temeparture compensation , cause when Q2 and Q28 heat (and also Q3 and Q29) the current through Q6 and Q8 reduces and so the BE voltage over Q2,Q28, Q3 and Q29 decreases. Fine you should say. Well, when powering-up this immediately pops the power trannies.
So i also tried ; place Q1,Q4,Q27, Q30, Q2, Q28, Q3 and Q29 together on the heatsink. This seems to work.
But i see the quiescent current rise slowly as the heatsink warms-up. So that is not correct either.
And next to that.....
I used series resistors (13 ohm) in the powerline to protect for over-current.
When i remove these i get fireworks also. (i either set-up)
Somebody got some ideas ?
grtz
Simon
My diamond output buffer, see PDF, eats output transistors, being Q1,Q4,Q27 and Q30.
I know it has to do with thermal issues. So i tried the following :
Place Q1,Q4,Q27 and Q30 together on the heatsink.
And couple Q6 with Q2 and Q28 with a piece of aluminium.
And the same with Q8, Q3 and Q29.
According to the logical rules this should be a perfect temeparture compensation , cause when Q2 and Q28 heat (and also Q3 and Q29) the current through Q6 and Q8 reduces and so the BE voltage over Q2,Q28, Q3 and Q29 decreases. Fine you should say. Well, when powering-up this immediately pops the power trannies.
So i also tried ; place Q1,Q4,Q27, Q30, Q2, Q28, Q3 and Q29 together on the heatsink. This seems to work.
But i see the quiescent current rise slowly as the heatsink warms-up. So that is not correct either.
And next to that.....
I used series resistors (13 ohm) in the powerline to protect for over-current.
When i remove these i get fireworks also. (i either set-up)
Somebody got some ideas ?
grtz
Simon
blu_line said:
My recommendation:
Use 0.33ohm instead of 0.1ohm. I think, that it will solve Your problem, without any changes of the heatsink combination.
I think that Your current mirrors are over complicated, and the current depended by the supply voltage. I believe, that very simple current regulator do the job as well!
I think that You have to put all the BD139/140 on the same heatsink. This will give some thermal compensation. Or put all the power transistors to one big heatsink. Maybe it will work even with the 0.1ohm.
sajti
1) Q2, Q28, Q6, Q8, Q3, Q29 must be on the same heatsink not in contact with the main heatsink.
2) R2, 29, 48, 34 need to be higher, you only have <17.5mV under quiescent conditions which will not adequatly compensate 2mv/K from the transistors, and will not adequatly current share, as well as potentially be thermally unstable.
3) R9 and R10 set the quiescent current to ~~175mA per output pair (which seems high!) while the current sources only source 1.7mA, this is not enough for your output configuration.
4) The buffer has VERY close to Av=1, which means any feedback from output to input is an invitation to oscillation - this may be the probelm with your transistors instantly popping - given what you have written about thermal coupling of transistors that you tried, one would expect thermal runaway to be much slower. Id the input grounded when you try powering it up?
5) I see the reason for the current mirror current sources, as these can operate very close to Vcc, but you would probably do just fine with the common two transistor CCS. You may also want to divide R27 and R26 into two series resistors and put a LED or zener to the corresponding Vcc as otherwise you get bias current dependant on power supply.
2) R2, 29, 48, 34 need to be higher, you only have <17.5mV under quiescent conditions which will not adequatly compensate 2mv/K from the transistors, and will not adequatly current share, as well as potentially be thermally unstable.
3) R9 and R10 set the quiescent current to ~~175mA per output pair (which seems high!) while the current sources only source 1.7mA, this is not enough for your output configuration.
4) The buffer has VERY close to Av=1, which means any feedback from output to input is an invitation to oscillation - this may be the probelm with your transistors instantly popping - given what you have written about thermal coupling of transistors that you tried, one would expect thermal runaway to be much slower. Id the input grounded when you try powering it up?
5) I see the reason for the current mirror current sources, as these can operate very close to Vcc, but you would probably do just fine with the common two transistor CCS. You may also want to divide R27 and R26 into two series resistors and put a LED or zener to the corresponding Vcc as otherwise you get bias current dependant on power supply.
Hi Simon,
I estimate the current from the Wilson current source is about
3.5mA, 3.5mA*10/0.1=350mA is not very large even to BD139.
And I think the first ( couple Q6 with Q2 and Q28 with a piece of
aluminium) is more reasonable than the second one, but if the
temperature of Q2.Q3 Q28.Q29 rise too quickly, the Vbe will
lower than the Q6.Q8, the quiescent current of Q1.Q4...will add
about 200mA per 10 Centigrade each. Because Q1's Vbe will lower
2mV per 1 Centigrade .
I guess this is the major reason why power transistor will blowout
quickly then the second one (all on the same "large"? heatsink) .
M.T.
p.s. u can see my sziklai-DB power stage PDF,
I use another method to provide the quiescent current =Vbe/2*Re .
I estimate the current from the Wilson current source is about
3.5mA, 3.5mA*10/0.1=350mA is not very large even to BD139.
And I think the first ( couple Q6 with Q2 and Q28 with a piece of
aluminium) is more reasonable than the second one, but if the
temperature of Q2.Q3 Q28.Q29 rise too quickly, the Vbe will
lower than the Q6.Q8, the quiescent current of Q1.Q4...will add
about 200mA per 10 Centigrade each. Because Q1's Vbe will lower
2mV per 1 Centigrade .
I guess this is the major reason why power transistor will blowout
quickly then the second one (all on the same "large"? heatsink) .
M.T.
p.s. u can see my sziklai-DB power stage PDF,
I use another method to provide the quiescent current =Vbe/2*Re .
@chalky
Why are the resistor values different for the top and bottom sections of the circuit?
[ST : You are right those 47 ohm in the bases of Q3 and Q29 should be 10 as well . In the schematic they are wrong , on the bench they are OK.
@ilimzn
2) R2, 29, 48, 34 need to be higher, you only have <17.5mV under quiescent conditions which will not adequatly compensate 2mv/K from the transistors, and will not adequatly current share, as well as potentially be thermally unstable.
[ST : with an Iq of 1 amp per transistor this should be 100mV !
Thanks for the comments, i'll try your suggestions.
(won't be tonight i'm afraid)
grtz
Simon
Why are the resistor values different for the top and bottom sections of the circuit?
[ST : You are right those 47 ohm in the bases of Q3 and Q29 should be 10 as well . In the schematic they are wrong , on the bench they are OK.
@ilimzn
2) R2, 29, 48, 34 need to be higher, you only have <17.5mV under quiescent conditions which will not adequatly compensate 2mv/K from the transistors, and will not adequatly current share, as well as potentially be thermally unstable.
[ST : with an Iq of 1 amp per transistor this should be 100mV !
Thanks for the comments, i'll try your suggestions.
(won't be tonight i'm afraid)
grtz
Simon
> those 47 ohm in the bases of Q3 and Q29 should be 10 as well
I see 47 and 100 on the schematic. You type 10 in your message. This is a critical point, so I don't know which of the several numbers to use in guess-work.
What are the base resistors there for? These can really screw-up the thermal compensation.
That CE-CE output stage is prone to fatal oscillation. For initial fire-tests, I would put 0.01uFd from collector to base of every output transistor. That will be too slow for good audio, but you can't put good audio through burnt transistors.
I'd make the output emitter resistors MUCH higher than 0.1, at least for initial testing. Work your way down to a reasonable value. I don't see why 4-8 ohm loads need 0.05 ohm resistance.
I'd also increase the emitter resistors on the inputs. 50mV drop seems a bit large for theory, but (with appropriate output resistor scaling) a lot less likely to thermal runaway.
What ARE you trying to drive? The 1.8mA(?) current sources seem lightweight for many-Amp loads, even though these transistors have high Beta.
I see 47 and 100 on the schematic. You type 10 in your message. This is a critical point, so I don't know which of the several numbers to use in guess-work.
What are the base resistors there for? These can really screw-up the thermal compensation.
That CE-CE output stage is prone to fatal oscillation. For initial fire-tests, I would put 0.01uFd from collector to base of every output transistor. That will be too slow for good audio, but you can't put good audio through burnt transistors.
I'd make the output emitter resistors MUCH higher than 0.1, at least for initial testing. Work your way down to a reasonable value. I don't see why 4-8 ohm loads need 0.05 ohm resistance.
I'd also increase the emitter resistors on the inputs. 50mV drop seems a bit large for theory, but (with appropriate output resistor scaling) a lot less likely to thermal runaway.
What ARE you trying to drive? The 1.8mA(?) current sources seem lightweight for many-Amp loads, even though these transistors have high Beta.
This is a theory but not reality I'm afraid. I became very awaremtlin12 said:
of this when I developed my diamond buffer. Why? Vbe is certainly not a constant! and very different for different types of transistors. I'll guess here is a case when Bob Pease is right! You can't solve everything with simulations.
peranders said:
I don't know how to run Pspice or any other simulation software,
so I just use caculator only.
I've design a sziklai input diamond buffer last year for DIYZONE,
PCB picture ,
it is also 6-transistor design ,but only small ZTX450/451 and
ZTX 550/551, (or MPS8099/8599 ) it works and sounds very well,
according to DZ's boss Mr. Wu, but occaionally it will oscillate
when the input signal is too large .
I think maybe the power transistors pop also by some kind of ocsillation.
I know that Vbe of different transistor is not easy to control, so I
use another method to control the quiescent current at my ULTIMA-SDDB power Amp.
There are two reasons I found for that to occur. You need to maintain loading at high frequencies (add a zobel network to the output). The current in Q1 & Q3 can get too low, did you try it without Q2 & Q4? Low current in Q1 & Q3 will also reduce the slew rate and increase distortion. You could also add a resistor b-e on Q2 & Q4 that would increase the current in the first transistors.
In the end, I would eliminate Q2 & Q4 I think.
-Chris
In the end, I would eliminate Q2 & Q4 I think.
-Chris
I have not tried any thermal experiments as of today.
What i tried is to remove the Q1,Q4,Q27 and Q30 and have it powered on for a while.
At first the 47 ohm resistors, R1, R4, R28, R32 smoked.
That clearly meant the Q2,Q3, Q28,Q29 where fully open.
So i reduced it to 0.
For what other reason are R9 and R10, besides VBE and emitter degeneration, in anyway ?
I use BD139/BD140's. SInce these are more happy at 100mA, i'll try to stick in 2n5401 and 2n5551's saturday.
grtz
Simon
What i tried is to remove the Q1,Q4,Q27 and Q30 and have it powered on for a while.
At first the 47 ohm resistors, R1, R4, R28, R32 smoked.
That clearly meant the Q2,Q3, Q28,Q29 where fully open.
So i reduced it to 0.
For what other reason are R9 and R10, besides VBE and emitter degeneration, in anyway ?
I use BD139/BD140's. SInce these are more happy at 100mA, i'll try to stick in 2n5401 and 2n5551's saturday.
grtz
Simon
I was wondering....
I want to set the amp in class A. 1 Amp each transistor
That means, w.r.t. the 10/0.1 formula, i just need 10 mA.
While the BD139/140 is most happy at around 100mA, i could reduce these resistors to 1 vs 0.1. (or less)
Will i need a lot of current to steer it or not ?
grtz
Simon
I want to set the amp in class A. 1 Amp each transistor
That means, w.r.t. the 10/0.1 formula, i just need 10 mA.
While the BD139/140 is most happy at around 100mA, i could reduce these resistors to 1 vs 0.1. (or less)
Will i need a lot of current to steer it or not ?
grtz
Simon
blu_line said:
Hi, Simon
I don't quite understand your meaning, but I think you should try
1. use 0.33 ohm not 1 nor 0.1 ohm at first.
2. use power supply with adjustable current limiter.
3. use 2 bi-direction zeners between input & output for protection.
4. do not flow your input add a resistor say 10K to ground.
I've test on my sziklai-output stage headphone amp, the power
supply will act the same wave as the output, I think that may pop
your power trannies. My headphone amp use 10 ohm as Re.
See BUF634 page 7 Figure 1
BUF634 use 7 diodes each direction for protection.
and http://focus.ti.com/lit/ds/symlink/opa627.pdf page 1
use 2 diodes between input and output.
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