Nelson Pass said:A simple Vbe multiplier like you have without R23 and D1
should be fine.
Does the Vas have enough current? Depends. I see that you
have very high rails, so are you planning on delivering +/- 20 amps
into the load?
That sounds about right for a 3ohm load! So the answer is yes!
What does the crazy 8 ball show?
Thanks, DonS
You didn't offend me - I transported myself to Inverness, where
I am sitting on the beach eating pistachios.
Yes, I think you can do 20 amps. For a thumbnail analysis, let's
take the gain of the outputs at 50, so the drivers have to
come up with about 400 mA, and if their gain is about 100,
then the front end will have to source 4 mA, which it can do.
You don't have a lot of feedback, so don't expect particularly
low distortion, and I wouldn't try driving a short if I were you.
munch munch....
I am sitting on the beach eating pistachios.
Yes, I think you can do 20 amps. For a thumbnail analysis, let's
take the gain of the outputs at 50, so the drivers have to
come up with about 400 mA, and if their gain is about 100,
then the front end will have to source 4 mA, which it can do.
You don't have a lot of feedback, so don't expect particularly
low distortion, and I wouldn't try driving a short if I were you.
munch munch....
Mr. Pass, can I bribe you with pistachios? Maybe?
I have an oscillation at about 11MHz in the output stage. It doesn't matter if the drive inputs are open or shorted to ground. The level is very slightly lower when they are shorted.
I tied the frontend to the output drivers with no change. I even increased the FB with no help.
I also seem to have a problem with the Vbe. I have about 2.5V across it. The bias levels seem off the charts!! Does that voltage appear correct for a 160mA output bias?
I may be wrong, but if I correct the first problem we may not have a problem with the second.
JJ, I was out buying pistachios, here is the update!
Thanks, DonS
I have an oscillation at about 11MHz in the output stage. It doesn't matter if the drive inputs are open or shorted to ground. The level is very slightly lower when they are shorted.
I tied the frontend to the output drivers with no change. I even increased the FB with no help.
I also seem to have a problem with the Vbe. I have about 2.5V across it. The bias levels seem off the charts!! Does that voltage appear correct for a 160mA output bias?
I may be wrong, but if I correct the first problem we may not have a problem with the second.
JJ, I was out buying pistachios, here is the update!
Thanks, DonS
Oscillation Update!
Mr. Pass, after further investigation I believe I found the source of the oscillation. The frontend board is still hooked up to the same power supply. If I touch the drains of Q1, Q4, or Q6 the oscillation stops. I can see this oscillation at different points on the ground rails and the power supply rails. It does not appear at the output of the frontend though.
Depending on where the test point is, the oscillation varies from 20mV to 100mV P2P.
What gives?
Thanks, DonS
Mr. Pass, after further investigation I believe I found the source of the oscillation. The frontend board is still hooked up to the same power supply. If I touch the drains of Q1, Q4, or Q6 the oscillation stops. I can see this oscillation at different points on the ground rails and the power supply rails. It does not appear at the output of the frontend though.
Depending on where the test point is, the oscillation varies from 20mV to 100mV P2P.
What gives?
Thanks, DonS
The resistances have been checked about 6 times, along with the voltages. Funny it always comes out the same. Checked pinouts ect.
Unless I calculated wrong the beta of the Vbe multiplier is ~54 in this circuit.
I am glad to hear from you Mr. Pass. Were you in the Scottish Highlands by the Beauty Firth again eating pistachios?
If I lower R17 and R18 to about 1K0 will this help?
Is the high value of C4 a problem here?
Also any idea where the oscillation is coming from?
Thanks, DonS
Unless I calculated wrong the beta of the Vbe multiplier is ~54 in this circuit.
I am glad to hear from you Mr. Pass. Were you in the Scottish Highlands by the Beauty Firth again eating pistachios?
If I lower R17 and R18 to about 1K0 will this help?
Is the high value of C4 a problem here?
Also any idea where the oscillation is coming from?
Thanks, DonS
That problem reminds me of the regulator I made for my aleph mini. It put out real low voltage--like 5 volts below the unregulated voltage. I guess the gate, er, base was sucking down so much current the voltage on the base was dropping.
An interesting lesson in the differences between mosfets and bipolar transistors. My guess is that your problem is something similar to that and, yes, lower value resistors will help if that is the problem. (it did for the regulator, which I scrapped anyway),
JJ
An interesting lesson in the differences between mosfets and bipolar transistors. My guess is that your problem is something similar to that and, yes, lower value resistors will help if that is the problem. (it did for the regulator, which I scrapped anyway),
JJ
OK, no mystery here:
R18 has .108 mA going through it (P2 at min)
R17 has .465 mA going through it
Therefore the Base is taking the difference,
which is .357 mA.
20 ma / .357 mA indicates a beta of 56, pretty
close to what you thought.
Probably you want to lower your impedances or
get a higher beta device.
R18 has .108 mA going through it (P2 at min)
R17 has .465 mA going through it
Therefore the Base is taking the difference,
which is .357 mA.
20 ma / .357 mA indicates a beta of 56, pretty
close to what you thought.
Probably you want to lower your impedances or
get a higher beta device.
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