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Old 20th March 2015, 11:49 PM   #1
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Default Potential problem of 3EF dimond in simulation

I am doing stability simulation of my 3EF diamond OPS and I saw a potential problem. Attached is the asc file. The circuit on the right side is diamond configuration where the collector of Q14 and Q18 pre-drivers are connected in diamond configuration. The circuit on the left is exactly the same EXCEPT the collector of the two pre-drivers ( Q49 and Q60) is connected to rail instead of diamond. both are adjusted to conduct 200mA per stage.

Also attached is the frequency sweep plot from 1K to 100MHz. Notice the peak around 10MHz of the diamond configuration? This is BAD. Look at the phase shift also.

From experimenting with simulation, the output inductor is the key to stabilize the amp. Both will peak without the inductor.

Anyone have experience with this? Looks like I'll be doing some cutting and jumping before I even receive the boards.

BTW, do you guys simulate beyond 5MHz? If not, do so!!! I always ran to 5MHz. Just last night when I want to make sure about the stability and ran to 10MHz, I notice the gain curved up, so I ran all the way to 100MHz. I am glad I did that. This can be a very serious problem. Adjusting TMC will not help at all. I tried RC lead lag network on different part of the 3EF, not very effective until the R is in 50 ohm range. That's not good as it will distort the signal without even have to do any simulations.
Attached Files
File Type: pdf 3EF diamond stability.pdf (72.1 KB, 59 views)
File Type: asc 3EF diamond amp 200mA alone.asc (29.3 KB, 27 views)

Last edited by Alan0354; 21st March 2015 at 12:00 AM.
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Old 21st March 2015, 02:00 AM   #2
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I isolated to only the 3EF portion, no more IPS and VAS, no more GNFB, just imposingly unit gain. Attached are the asc and the output gain phase plot. Look at the phase of the the diamond circuit.
Attached Files
File Type: asc 3EF diamond amp stability.asc (11.7 KB, 18 views)
File Type: pdf 3EF only gain phase.pdf (65.5 KB, 16 views)
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Old 21st March 2015, 09:53 AM   #3
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Hi Alan

So no problem provided the output inductor is fitted? I've not had any dramas with this topology in practice (with output inductor fitted, obviously). You won't see the inductor on my schematics/boards because I mount the output inductor near the speaker terminal, with a second zobel on the speaker side.

Why are you using a low Hfe TO-126 pre-driver device? One of the advantages of this topology is that you can use fast, high Hfe predrivers since it is only exposed to the Vbias.

EDIT: I just realised the music I'm listening to as I type is playing through one of my amps with this output stage (blameless front-end). So it definitely works!
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Old 21st March 2015, 04:16 PM   #4
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Quote:
Originally Posted by Ranchu32 View Post
Hi Alan

So no problem provided the output inductor is fitted? I've not had any dramas with this topology in practice (with output inductor fitted, obviously). You won't see the inductor on my schematics/boards because I mount the output inductor near the speaker terminal, with a second zobel on the speaker side.

Why are you using a low Hfe TO-126 pre-driver device? One of the advantages of this topology is that you can use fast, high Hfe predrivers since it is only exposed to the Vbias.
Hi

I since did a lot of simulations. I found the the fT of the driver transistor ( second EF stage) is of utmost importance. My original simulation used MJE15032 and 15033. They are only 30MHz. That gave the peak and had to stay away from diamond.

But in this simulation, I replace it with 2SC4793 and 2SA1930. The peak is GONE!!! I remove the inductor and all. I tried 2SC5171, the peak is much lower, well below the -3dB, so it does not matter, the phase at -3dB is below 180 deg by quite a bit. Attached are the graph and the asc file. There is NO difference between diamond and normal configuration. I am happy

About KSC and KSA, the important thing is fT. For 3EF, I don't think beta is important as long as it's over 50. The reason is for high frequency performance and more importantly, the stability as shown. I tried the higher fT transistor because I kind of suspect it. I trace the output waveform to show how each transistor affect the former stage. As frequency goes up, the beta drop. High beta at low frequency does not mean anything, it ONLY means the frequency roll off of beta starts early. The beta at high frequency depends ONLY on the fT, not the low frequency beta.

For example, if fT of MJE15032 is 30MHz. At 3MHz, beta is 10. Where as another transistor has fT of 60MHz, the beta at 3MHz is 20 no matter what the low frequency beta is. I am convinced that I got to the bottom of this peaking stuff. I don't like to use Zobo, inductor etc. to bandage the problem, I like to get to the bottom. Of cause I am going to put the output inductor and Zobo at the output for insurance, but I want the design to be sounded without.
Attached Files
File Type: asc 3EF diamond amp 200mA alone.asc (29.9 KB, 6 views)
File Type: pdf 3EF diamond gain phase faster drivers.pdf (71.9 KB, 12 views)

Last edited by Alan0354; 21st March 2015 at 04:42 PM.
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Old 21st March 2015, 10:26 PM   #5
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Interesting; I use 175MHz drivers, so that's probably another reason why I haven't seen this problem.

Assuming the amp is sensibily band passed and the ULGF is not too high (1 - 1.5MHz seems to be about the upper limit with modern devices in EF3) then is it realistic to expect stability problems in the ~10MHz region?
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Old 21st March 2015, 10:50 PM   #6
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Quote:
Originally Posted by Ranchu32 View Post
Interesting; I use 175MHz drivers, so that's probably another reason why I haven't seen this problem.

Assuming the amp is sensibily band passed and the ULGF is not too high (1 - 1.5MHz seems to be about the upper limit with modern devices in EF3) then is it realistic to expect stability problems in the ~10MHz region?
The problem is not the pass band frequency. I believe is the slower transistor go into negative impedance or something. For example, I probed two sides of the 10 ohm resistor from the pre-driver stage to the base of the driver stage. The side on the emitter of the pre-driver stage has very little change in amplitude and phase, but on the other side of the resistor that connects to the base of the slow driver, you see big change in amplitude and phase.

Change the value of the 10 ohm resistor does not help. Adding a small resistor at the collector of the driver stage does not help.

I tried removing 4 out of 5 stages of the output transistors, does not help much, maybe just slightly. I looked at the base and emitter of the power transistor, they track good. Changing the 2.2 ohm base resistor does not help either. Bottom line, the problem is at the 2 stage ( the driver stage).

What driver are you using. It is hard to find one with fT of 175MHz. I am using 60MHz TO-3P drivers, I yet to find the model for them. I might have to go back to TO-220 driver to get higher fT.
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Old 22nd March 2015, 07:16 AM   #7
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KSA1220A and complement, which are only TO-126 parts, but since I'm only using them to drive a single pair of outputs that's ok. Not stout enough for your output stage - you would need to put several in parallel which gets silly. The 60-70MHz Sanken's would be perfect for your usage I think.
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Old 22nd March 2015, 07:49 AM   #8
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Quote:
Originally Posted by Ranchu32 View Post
KSA1220A and complement, which are only TO-126 parts, but since I'm only using them to drive a single pair of outputs that's ok. Not stout enough for your output stage - you would need to put several in parallel which gets silly. The 60-70MHz Sanken's would be perfect for your usage I think.
Yes, that's the ones I use now: 2SC1186 and 2SA2837. I was hoping there are faster ones but no luck. I posted to ask for the LTSpice model for those two but apparently nobody has them.
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Old 22nd March 2015, 08:01 AM   #9
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In your simulation I don't see any zobels around the driver base-collector pins or the output base-collector pins. Nor do I see any rail decoupling between the driver and output collectors. Does your pcb include these provisions?
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Old 22nd March 2015, 09:56 AM   #10
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Quote:
Originally Posted by Ranchu32 View Post
In your simulation I don't see any zobels around the driver base-collector pins or the output base-collector pins. Nor do I see any rail decoupling between the driver and output collectors. Does your pcb include these provisions?
Yes, I have 0.1u and 10u on the collector of every single transistor point to point bypassing to ground. I have zobel on the output on the real board. I did not put into the simulation because it does not make any difference. I did put bypass caps in the simulation, no difference. Simulation assume perfect ground and power. Power is just as ground. Bypass does not make a difference in simulation. It does make a day and night difference in real circuits.

That's the reason I insist in using power and ground plane to make it as ideal as possible. You see I have components at an angle, all those just to make sure I fit it tight AND avoid cutting up ground and power plane. Layout is everything. The battle is won or lost in the layout.
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