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Old 6th March 2017, 07:51 PM   #1
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Default Super Leach amp simulation woes

I know, a bit late, but how did this clean up job go?
Any more photos?
Did the amp work again? Better?

I'm curious about oscillations, since I never could get any superamp to simulate properly without oscillations, was this amp built exactly as described by leach?
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Old 6th March 2017, 09:14 PM   #2
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since i became very busy with tube amps, this project was put in the back burner,
wrt oscilations, i used gate stoppers on the output trannies to take care of that...
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Old 6th March 2017, 09:32 PM   #3
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You must mean "base stoppers" (this is BJT).

What values?

The original superamp didn't have them but his latest version added them, at 10ohms. But I can't get it to stop oscillating in simulations, and base stoppers don't stop anything.
I just made up a sim of just the OPS, without the resistor dividers setting the cascode bias, which I replaced with voltage sources.
I got it biased and working, and at input voltages less than about 35V it works about ok, but above that, it goes haywire, loads of distortion, and oscillations.
Trying to stop that and make it work right, but something is wrong.
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Old 6th March 2017, 09:35 PM   #4
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i used 4.7 ohms.....
funny but in all my super leach amp builds i never had any
problem about oscillations at all...
maybe because i used those amps for music listening and not
for mobile disco....
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Old 7th March 2017, 07:52 AM   #5
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Well, oscillations would have nothing to do with how the amp is being used.

4.7 is what I've been using in sims lately, after starting out with the 10ohms that leach has in his latest superamp version.

But still, it just won't work without oscillations, and no amount of compensation can help or stop those.

What I can't understand is why an amp that works well in reality, just can't work right in simulations.

Makes no sense. Unless there is something specific about the cascode topo that calls upon some aspects of the models that cause this. It's true that when switching to different models, the oscillations change a lot, but never go away completely.

We just can't count on compensation and loop gain to correct the defects caused by oscillations. The loop gain can only do so much to compensate, and can't eliminate all distortion. Those pesky oscillations are causing a lot of extra distortion, but how could simple base stoppers in a real amp make it work right, and not in a sim? Beats me!
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Old 7th March 2017, 09:10 AM   #6
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i do not sim so i am unable to help you...
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Old 7th March 2017, 09:38 AM   #7
AndrewT is offline AndrewT  Scotland
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Have you tried using transistor models from R.Cordell, A.Grey, etc?
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Old 7th March 2017, 11:37 AM   #8
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Quote:
Originally Posted by AndrewT View Post
Have you tried using transistor models from R.Cordell, A.Grey, etc?
I didn't know about A.Grey, where can we get those?

But yes, I've been trying this out with many models from various sources, including Cordell of course, as well as fairchild, onsemi (modepex. yuk?!?), and others...

The thing is, the simulations works differently with different models, but keeps giving bad results not matter what. Although those same models work just fine in other sims, giving expected "normal" results, in the leach design, they don't give those same results.

Since those amps have been built over and over, for real, by so many people, using many different parts, it MUST be working, so why not in sims?

So it must be something with the models, that have some aspects that aren't bothering in usual circuits, but do in this cascoded topo.

What I'd like to do is use the major advantage of the cascoding to allow using 3055/2955 in a superamp with 60V rails, which would not push the devices beyond the specified limit for Vce0.

I haven't seen any such builds out there, and I'd like to see this done.

One thing I found out when I tried simulating only the output stage from the superamp with the 3055/2955 models at hand, is that they are likely representing the earlier vintage of devices, with a rather low Ft, possibly even the overly low Ft from those early devices at some 800khz.
One thing that hinted at this is the comparison with the MJ15003/4, which when I switched back and forth, showed a huge difference in bandwidth, with a comfortable one for the MJs, even though by today's standard they would be deemed "slow" devices, the 3055s were a whole lot slower, with the rolloff starting a little before 10khz, while it was beyond 1mhz for the MJs.

It would be nice to have tweaked 3055/2955 models to make them correspond to what's made nowdays, which are nearly on par with the MJs.

I did even try out the MJ15015/16 models, but those are modpex from onsemi, and gave the same wild results.

A leach superamp with 3055s would be nice to see working. With 60V rails (under full power), that would be about 200W on 8ohms, and with 3 device sets, it should handle a 4ohms load (2ohms accounting for most reactive loads).

The key is getting those models to work right.
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Old 7th March 2017, 11:51 AM   #9
AndrewT is offline AndrewT  Scotland
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Andy Grey is/was a member that became disillusioned with the way he was being treated/disrespected. He left to start his own technical forum.

But did leave behind a lot of information and links to his Forii (plural of Forum?)
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Old 7th March 2017, 02:03 PM   #10
PB2 is offline PB2  United States
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Quote:
Originally Posted by spookydd View Post
I didn't know about A.Grey, where can we get those?

But yes, I've been trying this out with many models from various sources, including Cordell of course, as well as fairchild, onsemi (modepex. yuk?!?), and others...

The thing is, the simulations works differently with different models, but keeps giving bad results not matter what. Although those same models work just fine in other sims, giving expected "normal" results, in the leach design, they don't give those same results.

Since those amps have been built over and over, for real, by so many people, using many different parts, it MUST be working, so why not in sims?

So it must be something with the models, that have some aspects that aren't bothering in usual circuits, but do in this cascoded topo.

What I'd like to do is use the major advantage of the cascoding to allow using 3055/2955 in a superamp with 60V rails, which would not push the devices beyond the specified limit for Vce0.

I haven't seen any such builds out there, and I'd like to see this done.

One thing I found out when I tried simulating only the output stage from the superamp with the 3055/2955 models at hand, is that they are likely representing the earlier vintage of devices, with a rather low Ft, possibly even the overly low Ft from those early devices at some 800khz.
One thing that hinted at this is the comparison with the MJ15003/4, which when I switched back and forth, showed a huge difference in bandwidth, with a comfortable one for the MJs, even though by today's standard they would be deemed "slow" devices, the 3055s were a whole lot slower, with the rolloff starting a little before 10khz, while it was beyond 1mhz for the MJs.

It would be nice to have tweaked 3055/2955 models to make them correspond to what's made nowdays, which are nearly on par with the MJs.

I did even try out the MJ15015/16 models, but those are modpex from onsemi, and gave the same wild results.

A leach superamp with 3055s would be nice to see working. With 60V rails (under full power), that would be about 200W on 8ohms, and with 3 device sets, it should handle a 4ohms load (2ohms accounting for most reactive loads).

The key is getting those models to work right.
andy_c was the guy with the models, but he only did a few parts - a very sharp person:
MJL3281A and MJL1302A SPICE Models

Why do you want to use such old parts? You don't want to Cascode them rather you
would put them in series as was done in the Double Barrel amp. Or you could just use
about half the rail voltage and run very high current amps bridged. BUT look at the SOA
on those old devices, you'd probably need 2X as many as the best modern devices. Other
old amps with series outputs are the Ampzilla and Tigersaurus.

If you start a new thread about your sim of that amp, I'll try to help. I'm also seeing near
oscillation with the Krell KSA-50 in sim but obviously people have built them with no issues.
I think wiring inductance, especially to the base of the outputs acts as a base stopper, but
I don't think that is the whole story.
One tip is not to trust any of the models, I would use all models from Bob Cordell and just
sub similar parts to the small signal types. Note that you can use devices that don't meet
the breakdown voltage since that is not modeled in SPICE. BD139/140 is fine for med power
for example.

Last edited by PB2; 7th March 2017 at 02:10 PM.
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