You said that the 1/8th watt base resistor would not allow you to listen to music.
We have shown that a 1/8th watt 2r2 resistor is rated to pass 238mA.
That 238mA into the base of the output transistor results in an output current of roughly 6A.
i.e. a 1/8thW 2r2 base resistor will allow an output from each output transistor of roughly 6A.
Your amplifier has three output transistors (I think, but have not checked).
Your amplifier will be able to pass three times that ~ 6A, i.e. ~ 18A to the load when fitted with 1/8thW 2r2 base resistors.
The heating of the base resistors takes time. The shorter the time that you expose the base resistors to current, the more current they can pass. The peak current of an AC sinewave is sqrt(2) times the rms current.
We showed a calculation earlier that the approximate output current when allowing for that sqrt(2) factor was about 30Apk. That is an almighty big output current for the loudest of music.
Shorter transients heat the base resistor even less.
For a very short transient I would expect a metal film smd 2r2 1/8thW resistor to pass double the current i.e. 2times 238mA or about 476mApk.
That would allow the 3pair output stage to pass nearly 50Apk to the load.
In normal operation a 2r2 1/8th base stopper on a 3pair output stage will allow massive music currents to pass to the speaker/s.
We have shown that a 1/8th watt 2r2 resistor is rated to pass 238mA.
That 238mA into the base of the output transistor results in an output current of roughly 6A.
i.e. a 1/8thW 2r2 base resistor will allow an output from each output transistor of roughly 6A.
Your amplifier has three output transistors (I think, but have not checked).
Your amplifier will be able to pass three times that ~ 6A, i.e. ~ 18A to the load when fitted with 1/8thW 2r2 base resistors.
The heating of the base resistors takes time. The shorter the time that you expose the base resistors to current, the more current they can pass. The peak current of an AC sinewave is sqrt(2) times the rms current.
We showed a calculation earlier that the approximate output current when allowing for that sqrt(2) factor was about 30Apk. That is an almighty big output current for the loudest of music.
Shorter transients heat the base resistor even less.
For a very short transient I would expect a metal film smd 2r2 1/8thW resistor to pass double the current i.e. 2times 238mA or about 476mApk.
That would allow the 3pair output stage to pass nearly 50Apk to the load.
In normal operation a 2r2 1/8th base stopper on a 3pair output stage will allow massive music currents to pass to the speaker/s.
Last edited:
I did blow 2r2 600mW base stoppers in my Krell Klone.
Not sure what test I carried out at the time, but it was oscillating if I added 100nF to the speaker out. That may be what caused the blow as well as taking out the rail fuses.
The same amp in dummy load testing delivered >400W without blowing the base stoppers.
Not all my amps have base stoppers but that one occasion is the only time I have blown them.
I count that as abuse.
Not sure what test I carried out at the time, but it was oscillating if I added 100nF to the speaker out. That may be what caused the blow as well as taking out the rail fuses.
The same amp in dummy load testing delivered >400W without blowing the base stoppers.
Not all my amps have base stoppers but that one occasion is the only time I have blown them.
I count that as abuse.
Hi Guys,
I tried to tackle this one more time. I used the board on which I cut in the base stoppers. I'm getting oscillation. When I start it up cold, the offset will be fairly large like over 10V for a few seconds but then it gradually drops to about 6mv after maybe 6-10 seconds. It will sit there like that until I try to take any measurements. As soon as I touch the probe to any of the transistor legs in the front end, the light bulb will burn brightly and the bias will shoot up. I have and asc made with the same components as I used. I am attaching it hoping one of you can take a look and see where I might need to change or add capacitance to get rid of the oscillation.
Thanks Terry
I tried to tackle this one more time. I used the board on which I cut in the base stoppers. I'm getting oscillation. When I start it up cold, the offset will be fairly large like over 10V for a few seconds but then it gradually drops to about 6mv after maybe 6-10 seconds. It will sit there like that until I try to take any measurements. As soon as I touch the probe to any of the transistor legs in the front end, the light bulb will burn brightly and the bias will shoot up. I have and asc made with the same components as I used. I am attaching it hoping one of you can take a look and see where I might need to change or add capacitance to get rid of the oscillation.
Thanks Terry
Your .asc was good.
First look at the "peaking" at output in the bode plot (below 1).
This is a unstable amp.
Next , we find out why ??? (below 2 ) The design has MIC comp , lead comp.
and TMC all at the same time. 3.3mhz peak (oscillation ?)
Ahhh , lets fix it !
Eliminate MIC/lead , 470p-100p /1.5 K "badger" TMC = (below 3)
76 degree margin / 636khz unity gain = close to the "badger" (stable).
This amp should now rock !
All those extra caps so it could oscillate ???
PS - I changed gain to 33X (more reasonable)...
OS
First look at the "peaking" at output in the bode plot (below 1).
This is a unstable amp.
Next , we find out why ??? (below 2 ) The design has MIC comp , lead comp.
and TMC all at the same time. 3.3mhz peak (oscillation ?)
Ahhh , lets fix it !
Eliminate MIC/lead , 470p-100p /1.5 K "badger" TMC = (below 3)
76 degree margin / 636khz unity gain = close to the "badger" (stable).
This amp should now rock !
All those extra caps so it could oscillate ???
PS - I changed gain to 33X (more reasonable)...
OS
Attachments
Last edited:
R4 / R5 = 68R , this increases unity gain point to 636K and closed loop gain
to almost 100db (lower THD20K).
edit - and C12/ c8 in a 1:4.7 ratio (100p/470p) - this could also be 68p/390p or 47p/270P (higher UG). these reduced values might require
R4/5 to be 100R for stability. optimum would be 68P/390P with R4/5 at 82R. 10ppm THD20K would be achieved (close to the wolverine).
OS
to almost 100db (lower THD20K).
edit - and C12/ c8 in a 1:4.7 ratio (100p/470p) - this could also be 68p/390p or 47p/270P (higher UG). these reduced values might require
R4/5 to be 100R for stability. optimum would be 68P/390P with R4/5 at 82R. 10ppm THD20K would be achieved (close to the wolverine).
OS
Last edited:
OK I got the chance to make the changes. The closest I could get for the caps was 100p/480p. I installed the 68R for R4/R5. R16=33K, R57=1K5
Offset went up to 240mV. Oscillation is way better but set with the bias pot at 1K it will actually play music but I can see oscillation on the signal. If I play a sine wave I see oscillation at about 1.2Vac output. The - side of the VAS is not turning on all the way. I'm attaching a schematic with the voltages I am getting. Now if I turn the bias pot to raise the bias as soon as the bias starts to rise, it starts oscillating and the bias shoots up hard. Getting closer, just not quite there. Waiting for some more suggestions.
Thanks, Terry
Offset went up to 240mV. Oscillation is way better but set with the bias pot at 1K it will actually play music but I can see oscillation on the signal. If I play a sine wave I see oscillation at about 1.2Vac output. The - side of the VAS is not turning on all the way. I'm attaching a schematic with the voltages I am getting. Now if I turn the bias pot to raise the bias as soon as the bias starts to rise, it starts oscillating and the bias shoots up hard. Getting closer, just not quite there. Waiting for some more suggestions.
Thanks, Terry
Attachments
I'm still learning simulation tricks. I can finally get that nice flat 'floor' on the FFT graph. I played with the simulation of this one some time ago and was never really satisfied with it. I will have to fire it up again sometime soon and see what else catches the eye, keeping OS's suggestions in mind too.
Terry, you are using the lower ft MJL2119x devices on this one if I'm remembering correctly. Seems to me triples in general work better with faster devices.
As for protection, I tried the simulation with all protection related components removed and it wasn't any better. Sketchy at best. I'm not thinking the protection is likely at fault, but almost anything is worth investigating.
Funny (actually not) that the designer, Struth, never came back with ANY support on this even after my sending him a set of boards. The same faulty board appears to be for sale on his website too.
Terry, you are using the lower ft MJL2119x devices on this one if I'm remembering correctly. Seems to me triples in general work better with faster devices.
As for protection, I tried the simulation with all protection related components removed and it wasn't any better. Sketchy at best. I'm not thinking the protection is likely at fault, but almost anything is worth investigating.
Funny (actually not) that the designer, Struth, never came back with ANY support on this even after my sending him a set of boards. The same faulty board appears to be for sale on his website too
.- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.