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Old 23rd September 2012, 08:49 AM   #1031
AndrewT is online now AndrewT  Scotland
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Kean,
how do we tell where the decimal places are in all those values?
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Old 23rd September 2012, 10:31 AM   #1032
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Originally Posted by keantoken View Post
Your output frequency response still indicates that your circuit is oscillating (upturned phase). In a real amp it may ork because the output stage has a high source impedance, but it ill still have the resonance and positive feedback problems hich ill limit it's stability.

This is another example of one of those circuits people tend to dismiss as impossible because they don't kno of any ay to stabilize them... An RC snubber in just the right place can do amazing things. Try this Dadod.

Also notice I rearranged the circuit a bit. It provides the same benefits, but this time the RF current loops are smaller and less convoluted. I like this arrangement better.
I've had to increas output transistors base stoppers higher then your values to get stable simulation. Attached is the zip LTspice file with two different OPS, my first suggestion and your last one.
I am not satisfied with nether of them, but if you can make anything of it, please show it.
dado
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File Type: zip amp-newOPS.zip (23.1 KB, 40 views)
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Old 23rd September 2012, 08:28 PM   #1033
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Your first circuit ithout the BC5xx collectors bootstrapped may need different compensation than the RC.

If you consider each difference beteen this output stage and a traditional EF, you'll see that it's essentially optimized for speed. As for distortion, there are a lot of intervening factors, such as the quality of the 100mA CCS's, and quasi-saturation of the drivers. In the end the circuit is biased at the margins. So I agree that this arrangement leaves us anting more.

Again, the MJE1503x are overkill for the CCS's, MJE253/243 are perfectly safe here and probably ill perform better. But the BC5xx are not prepared for 80V Vce at max volume. You could try a simple capacitor bootstrap (attached). It is pretty much a free mod, since they only have 2mA Ic. Interestingly the RC's are still needed here.

Furthermore, putting the CCS's after the diode in the C-multiplier may be a bad idea. For one thing, you limit voltage range again and it is the voltage range of the CCS hich limits this output stage. Secondly, look at the diode as a filter device. At 10mA it is about 2.5R. By moving the CCS here you increase it's bias to 100mA hich decreases it's resistance to .25R, reducing its filtering ability by a decade. No it is basically doing nothing, and actually reduces the benefit that the proceeding circuitry gets. I think it is better to hang the CCS off the main rails.

Also, look at the Ib of Q23 and Q25. If the Hfe of these transistors is say 100, then Ib ill be 1mA. R44 conducts about 1.6mA. The 1mA ill be gobbled up by Q23/25 and ill leave only .6mA for Q24/26. It could be orse; if R44 as 100k there ould be a measely 100uA left for Q24/26 and this ould cause them to saturate hile driving the bases of Q23/25 and cause glitches and high order distortion. Furthermore, consider the logarithmic behavior of the Vbe of Q24/26. ith 1mA Ic, they ill have a transresistance of about 33R. This is seen in parallel ith R38 and R41. 33/5.6=5.9. So any current variations through R44 ill be magnified in the CCS output current by about 6 times! So I ould decrease R44 to 27k or so, though it may dissipate more.
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Old 24th September 2012, 03:31 AM   #1034
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My most interesting prototype yet is on the bench. It has 3 20-turn trimmers and 2 1-turn trimmers. It uses a clever self-feedback output stage that eliminates current AND voltage distortion from the output stage, loosening requirements for the frontend. The output stage itself performs ell enough to be used ithout a frontend (belo .01% THD at 5A into 6R ith one pair of outputs) depending on your personal standards of course. The output stage has just 6 transistors. The frontend is a buffered single-ended non-LTP thingy hich presents a sort of fussy challenge. The VAS is almost, but not entirely, unremarkable. I have to mess ith time constants to get the hole circuit to start up properly.

It is also my most compact point-to-point iring job. All 15 transistors, 30 resistors, and 18 capacitors fit very tightly in 4 cubic inches, ith most of the mass in the middle. I am considering submitting it to a local art (sculpture?) sho to see just ho far this circuit can take me. I had to sandich some aluminum foil in some masking tape and edge it in the thing to shield more sensitive traces from the output and VAS. I should probably be a bit more careful of layout.

I just anted to update for anyone ho might have an interest in my affairs. I no yield to the topic flo...
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Old 24th September 2012, 07:44 AM   #1035
dadod is online now dadod  Croatia
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Quote:
Originally Posted by keantoken View Post
Your first circuit ithout the BC5xx collectors bootstrapped may need different compensation than the RC.

If you consider each difference beteen this output stage and a traditional EF, you'll see that it's essentially optimized for speed. As for distortion, there are a lot of intervening factors, such as the quality of the 100mA CCS's, and quasi-saturation of the drivers. In the end the circuit is biased at the margins. So I agree that this arrangement leaves us anting more.

Again, the MJE1503x are overkill for the CCS's, MJE253/243 are perfectly safe here and probably ill perform better. But the BC5xx are not prepared for 80V Vce at max volume. You could try a simple capacitor bootstrap (attached). It is pretty much a free mod, since they only have 2mA Ic. Interestingly the RC's are still needed here.

Furthermore, putting the CCS's after the diode in the C-multiplier may be a bad idea. For one thing, you limit voltage range again and it is the voltage range of the CCS hich limits this output stage. Secondly, look at the diode as a filter device. At 10mA it is about 2.5R. By moving the CCS here you increase it's bias to 100mA hich decreases it's resistance to .25R, reducing its filtering ability by a decade. No it is basically doing nothing, and actually reduces the benefit that the proceeding circuitry gets. I think it is better to hang the CCS off the main rails.

Also, look at the Ib of Q23 and Q25. If the Hfe of these transistors is say 100, then Ib ill be 1mA. R44 conducts about 1.6mA. The 1mA ill be gobbled up by Q23/25 and ill leave only .6mA for Q24/26. It could be orse; if R44 as 100k there ould be a measely 100uA left for Q24/26 and this ould cause them to saturate hile driving the bases of Q23/25 and cause glitches and high order distortion. Furthermore, consider the logarithmic behavior of the Vbe of Q24/26. ith 1mA Ic, they ill have a transresistance of about 33R. This is seen in parallel ith R38 and R41. 33/5.6=5.9. So any current variations through R44 ill be magnified in the CCS output current by about 6 times! So I ould decrease R44 to 27k or so, though it may dissipate more.
I wanted to simulate this OPS behavior i GNFB. I just used my old amp's input stage and the diodes in the power lines are left over from it, but it does not influance main goal, to show how stable is the OPS in GNFB amp.
You are correct about R44, it was way to high, now it is 10k(dissipation isnot a problem) and CCS works better, but the main problem is high peak at 9MHz. It could be tamed the with the RC at the output transistors bases.
Can you show the schematic of that yours amp prototype, or OPS only?
dado
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Old 24th September 2012, 03:15 PM   #1036
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Changing Q23 and Q25 to MJE253/243 ill partly help the 10MHz spike. You may also try collector resistors on these transistors as a clever ay of inserting damping. You can see it this ay: the VAS output impedance has more than 90 degree phase shift. The OPS input impedance also has more than 90 degrees phase shift. This is because in both cases, capacitors are bootstrapped. Much of the amp's stability characteristics are determined by this junction and if the combination of the source and input phase gets too high, you get a local oscillator or at least a troublesome resonator. This is a big Caveat Emptor hen it comes to using bootstrapping and cascoding to eliminate parasitic capacitances; you only eliminate them at audio frequencies - at RF they become sources of phase dismay.

Other than that I ended up increasing the RC values to 68n/.33R to try and get rid of the 10MHz spike. This asn't very satisfying.

I ould like to share my OPS but I'm still trying to ork out the thermal behavior. I have shared it ith one person ho as going to build it but hasn't gotten around to it yet. It is thermally stable at 20V but hen the supply increases to 30 or 40 volts it starts doing the 2-steps-forard, 1-step back-thing and it makes me anxious. There must be such a thing as thermal oscillation.
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Old 25th September 2012, 07:12 AM   #1037
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Quote:
Originally Posted by keantoken View Post
Changing Q23 and Q25 to MJE253/243 ill partly help the 10MHz spike. You may also try collector resistors on these transistors as a clever ay of inserting damping. You can see it this ay: the VAS output impedance has more than 90 degree phase shift. The OPS input impedance also has more than 90 degrees phase shift. This is because in both cases, capacitors are bootstrapped. Much of the amp's stability characteristics are determined by this junction and if the combination of the source and input phase gets too high, you get a local oscillator or at least a troublesome resonator. This is a big Caveat Emptor hen it comes to using bootstrapping and cascoding to eliminate parasitic capacitances; you only eliminate them at audio frequencies - at RF they become sources of phase dismay.

Other than that I ended up increasing the RC values to 68n/.33R to try and get rid of the 10MHz spike. This asn't very satisfying.

I ould like to share my OPS but I'm still trying to ork out the thermal behavior. I have shared it ith one person ho as going to build it but hasn't gotten around to it yet. It is thermally stable at 20V but hen the supply increases to 30 or 40 volts it starts doing the 2-steps-forard, 1-step back-thing and it makes me anxious. There must be such a thing as thermal oscillation.
Here is my last incarnation of the amp with new OPS. THD20k is 0.81ppm at 70 W on 8 ohm.
I am not completely satisfied with the Gain Margin and how the phase twists around 15 MHz.
I simulated the CCS with MJE253/243 instead MJE15032/15033 with no differences at all, so I keep the more powerful pair here as the power dissipation go up to 9 W on the CCS. I tried MOSFET CCS with DN2540 with similar result.
Big question here is how to go for more output pairs??
I did not scratch thermal stability yet.
dado
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File Type: jpg amp-newOPS-e-graf.jpg (263.4 KB, 187 views)
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File Type: zip amp-newOPS-e.zip (18.5 KB, 17 views)
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Old 25th September 2012, 07:10 PM   #1038
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100mA max base drive for the outputs, means 10A max output for an Hfe of about 100. So an extra pair can still make use of the measely max 100mA drive since the outputs lose Hfe after 6A. But after 3 pairs or so you ill run out of drive at max volume; then you ill need to increase CCS current or add an extra pair of drivers.
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Old 27th September 2012, 02:42 PM   #1039
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As for capacitor resonance, pleas look here for some measurements.
Courtesy of Glen Kleinschmidt.

Cheers,
E.
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Old 28th September 2012, 11:13 AM   #1040
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Default Bypassing electrolytic caps.

Glen has updated his website with measurement on larger (electrolytic) caps.
See also: Bypassing large electro's with smaller caps - Page 1 for additional info.

Thx Glen!

Cheers,
E.
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