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Old 25th April 2014, 01:26 PM   #581
wg_ski is offline wg_ski  United States
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The test load was two pairs of dual Delta10A cabs with Selenium HF. They're just top boxes (relatively high tuning), but I was running them full range. On this low a power it's pretty safe.

The other amp is not 3055-based, it is TIP35/36. It's actually 2 pairs - one active and one for the commutators. The 3055's don't have enough Hfe to do that.

"Double Barreled" 3055 is how I would have done it from the beginning, instead of grounded bridge. Voltage stresses and dissipation are EXACTLY the same, as long as you're not in slew limit. I have a pair of double-barreled (cascode) amps that will do about 1200 watts in 2 ohms -and melt the plugs off the extension cord doing it. Those were the last big class AB amps I did.

Last edited by wg_ski; 25th April 2014 at 01:27 PM. Reason: typo
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Old 25th April 2014, 01:47 PM   #582
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Quote:
Originally Posted by wg_ski View Post
The other amp is not 3055-based, it is TIP35/36. It's actually 2 pairs - one active and one for the commutators. The 3055's don't have enough Hfe to do that.
Interesting project too.

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Originally Posted by wg_ski View Post
"Double Barreled" 3055 is how I would have done it from the beginning, instead of grounded bridge.
Well, the most interesting part of it is the grounded bridge topo. This will be very cool when we get one running, and then later, perhaps with a 4 pairs version, when we bridge a bridge. I simulated john ellis' bridge in the bridged bridge situation and that works fine. We'll have cool amps done soon.

I think revisiting the leach double barreled on 3055 will be a good future project as well. This one can get quite a bang and also can be bridged.

One 3055 project that I really want to build as well is the one I posted about much earlier, with a quad comp output stage based on bryston's topo. My simulations on that one show superior performance for 3055s.

But one thing at a time, let's get a grounded bridge working first.

One thing that I'm very curious about, is to find out how well that performs when actually measured, compared to the simulations.
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Old 26th April 2014, 05:16 PM   #583
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Here is a state of where we are on that fig 41 limiter.

I switched the protection transistors to the BC547/57C for higher gain, which improved the action of the limiter, reduced their base resistors from 10k to 4k7 and that too improved it a little more, then switched also the vas protection transistors to the same higher gain and increased the 12ohms resistor for the sensing to 27ohms, so that protection is at the threshold of action and that limiter clamps the vas max current to about 22mA, which also improves the action significantly, but we're still not close enough to a full fold back action from the limiter and I am afraid this doesn't keep the dissipation enough within the soa as set by the locus.

The values for the fig 41 are now as follows:

R1 = 12k
R2 = 430
R3 = 147
R4 = 14k
R5 = 4k7
R6 = 1k5
3 zeners: 12V + 12V + 10V = 34V to set the Vref at 1V

The plotted locus does slightly encroach on a part of the critical second breakdown area, but that is on the 50C de-rated DC SOA curve, so it's not too bad. This is the theory/calculations and the reality may be a little different with the real parts. Plus the screened and tested 3055s for higher Vce0 should help this a bit too.

I ran this sim with the output shorted with 1mohms, so that's a frank short! at a drive level on the input of 1.27Vpk, which gives nearly 100W on 4ohms when not shorted, so it's driven almost to clipping.

Attached is a plot of the dissipation only for T21 and I plotted its soa curve/load line, which looks very funky in a big loop, and I drew up a part of the de-rated @ 50 DC soa curve on top of it, which does show how much above the soa it still is. The average power in T21 shows ~115W already, and we can see peaks at about 410W.

The limiter is fully activated, as it should be, according to the values in place.

Please someone verify my values, just in case. But with almost full drive and output shorted, the limiter is definitely active but not effective enough to clamp the dissipation within the soa as it's supposed to.

I didn't have the flyback diodes before for all the other sims, but this time since I'm testing the limiter, I added them to see if they help, and I guess they would probably help on a reactive load and not a short.

I played a bit with the time constant R5/C1 (R68/C13). When I had R5 (R68) at 10k and C1 (C13) at 22n, for a 200us constant, which is below the max of 500us as prescribed by Cordell, the limiter's action showed low levels of oscillations.

The dissipation increases with frequency and a longer time constant dampens oscillations of the limiter, however it also increases the dissipation a lot, so I tried finding a compromise that works across the whole range of 20-20k, and I ended up using a 22n for C1 (C13) (~103us constant), which keeps the oscillations to a bearable minimum while acting towards limiting the dissipation. However as this dissipation info shows, it's too much above soa, and worse at 20k with ~138W average power dissipated in T21 alone.

I thought less limiter action was needed at higher frequencies and expected lower dissipation, but it's the opposite.

We can see peaks of dissipation and oscillations over pretty much the whole range. The shorter time constant helps bring down average dissipation, at the expense of higher peaks and more oscillations.

My question for now is: Do we look only at the averaged out power dissipation or should we consider the peaks?

Also how much oscillation should we tolerate. More oscillations cause more issues and more high frequency dissipation.
Attached Images
File Type: png test output shorted 10khz.png (775.9 KB, 226 views)
File Type: png dissip T21 output shorted prot active 10khz.png (352.0 KB, 202 views)
File Type: png SOA-T21-output-shorted-prot-active.png (202.0 KB, 179 views)
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Old 26th April 2014, 05:37 PM   #584
sbrads is offline sbrads  United Kingdom
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The way you're going, you might as well have relays shorting out T13 and T14 b/e along with what you've got now. That'll do it. Now I think about it, it'll bang on and off as quick as it's capable of - perhaps that's what's sort of happening with your present circuit with its oscillations, so I suppose all that can be striven for is a low duty cycle repetition rate to keep dissipation down.
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Old 26th April 2014, 05:57 PM   #585
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Originally Posted by sbrads View Post
The way you're going, you might as well have relays shorting out T13 and T14 b/e along with what you've got now. That'll do it. Now I think about it, it'll bang on and off as quick as it's capable of - perhaps that's what's sort of happening with your present circuit with its oscillations, so I suppose all that can be striven for is a low duty cycle repetition rate to keep dissipation down.
Oscillations are "normal" and unavoidable on such limiters, it's their nature of operation. We use a time constant there to dampen those oscillations but we can't eliminate them entirely, or the circuit would no longer work at all.

It's a matter of compromise between what amount of oscillations we keep possible and the dissipation.

On 2EF or 3EF stages, this limiter works a lot more efficiently due to the larger gap to squeeze when acting, but here we only have one junction's worth of gap and the protection transistors have a harder time closing that gap well enough.

When I saw that a higher gain type was more able to squeeze this gap, I thought about using darlingtons there, but then the draw back is that they need twice the Vbe to act, so we'd be back to square one and require a higher value for Re.

If we can't make this limiter act hard enough to keep the dissipation within the soa, then other protection means will be required regardless.

The main issue is with this topology with one junction only. That was this same issue that was giving me a problem making a better bias spreader.

The original elektor schematic with the 2 transistors used as diodes as a bias spreader, with a very low value only resistor in between, was really not so practical.

Using TO92s to couple them thermally wasn't too hard to do, but the bias trimmer resistor needed to be only a few ohms and it was difficult to bring the bias down low enough.

Actually when I used cordell's bd139/140 models, their Vbe being around 400mV only, the 2 bc639/40 were not able to keep the bias current down enough even with ony 1 ohms between them.

I have been thinking about the latch found on crimson's amps, but since we may have some action sometimes on legit signals if speaker impedance drops too low with an overly high reactive load, then we'd have an amp locking up all the time when driven a bit too hard.

I think with 3 pairs of 3055s that making a bridged bridge would require keeping the drive well below max power. I plotted a 4 pairs soa and that looks much better for driving 2ohms to the max. That will be an other incarnation of this amp, but for now let's get a workable 3 pairs model.

The main issue with the detection of overload is with the output shorted, because when the limiter isn't acting on legit signals at max power, the dissipation stays reasonable. It's the limiter's action that causes more dissipation.
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Old 29th April 2014, 10:43 PM   #586
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For the purpose of testing the vi limiter's action on various complex loads and checking how stable the amp is on those types of loads, I picked up a few models of configurations.

The most complex, I got from Rod Elliot's site, for 2 ways with a passive filter. 2 variations of that. Plus an old thread on the forum had what is to be the worst case possibilities for inductive and capacitive, both with 60 degrees phase shift, which I haven't verified, although I changed the 4ohms series resistors for 2ohms, because I want to simulate as close as possible to 4ohms real world load.

And finally I don't remember where that came from, but I also used a single speaker model that is supposed to simulate the real complex load that it presents, with all the caps, resistors and coils to emulate it.

I'm posting what I used for reference.

ESP's 2 examples of 2 ways complex load seem to me to be for 8ohms nominal, and despite their apparent complexity, they don't pose a problem to the amp at all (elektor's quasi), and the amp doesn't break a sweat. Nothing bad going on with phase shift, thd stays very close to the resistive load and there is nowhere near enough phase shift to cause any significant extra dissipation in the outputs, their soa usage stay very well within their limits and the limiter doesn't activate.

Then I tried both supposedly worst case loads with 60 degrees capacitive and inductive, and those don't pose any problem either, the dissipation in the outputs is no threat at all.

I can see there is a phase shift, which for the capacitive load is hardly noticeable at the lowest frequencies and it increases with frequency, while the amp's output current and power transistor's dissipation is also very low at the lowest end and much higher at the high end (20k) although less than 15W worth of dissipation in T21...

The phase margin on 4 ohms resistive is at almost 70 degrees and the gain margin a little too low at a bit over 1.54db.

The capacitive load decreases the phase margin down to a bit over 61.8 degrees and it increases the gain margin to a little more than 2db. There are no oscillations at all on that load and the thd goes down as well, actually cut almost in half at 20khz.

Things don't go quite that way on the inductive load, as the phase margin increases a bunch to nearly 79 degrees, while the gain margin drops to negative at some -1.33db, and there are some very high frequency oscillations superimposed on the sine signal, and curiously, the thd also drops some 200ppm lower than on the 4ohms resistive load, as with the capacitive load.

I am thinking perhaps those oscillations happen because there is no more gain margin, being negative and that's enough to allow them. So I will have to tweak some more the compensation scheme for more gain margin, hopefully to stabilize this better.

I haven't yet been able to do simulations on lower impedance loads, as those are as they are and I don't know what I should tweak to change realistically their impedance, perhaps down to some 3 or 2 ohms. This will be needed to test the limiter.

All the tests done so far show me that on a full short, the limiter doesn't suffice and it should be supplemented by something else more efficient and perhaps as radical as causing something to shut down somewhat quickly, to prevent damage, as the dissipation in the ouputs while the limiter is active on a short is much too high and quite above the soa.

Although I haven't yet tested the limiter on complex low impedance loads, I think it probably will be sufficient and should keep the outputs in their soa. I will have to confirm that.

What I'd like to do now is tweak the complex loads to make them 4ohms nominal instead of 8, and somewhat badly behaved with impedance dropping to say 2 or 3 ohms, which should at least some of the time activate the limiter, which is the while point.

Then perhaps try to have those load also present a 2ohms nominal, with their bad behavior accordingly, which should trigger the limiter pretty much all the time at full power.

When I put in place the new calculated values for the limiter (M.K.'s fig 41), I found although it wasn't really activating, that it was a tiny bit turned on anyway, from having a threshold close above the full power (see soa and locus), and doing so, it was adding a little too much extra thd, which I cured by increasing the time constant from about 103us to 470us. The lowish values calculated for the sensing resistors (R2=430 & R3=147) also caused unwelcomed loading on the pre-drivers and caused significant extra thd (more than 200ppm more), so I went back to the calculations and managed to make that higher (R2=953 & R3=310), with a different breakpoint threshold as well, slightly negative this time at -3V.
Then the thd increase from the addition of the limiter was minimized.

It's funny that the more complex and reactive loads are actually making the thd go down...

I'd like some pointers on tweaking the speaker simulated loads for the lower nominal impedance, keeping them as realistic as possible. And I'm going to attempt getting more gain margin, hopefully to cure the hf oscillations on the inductive loads.

Last edited by spookydd; 29th April 2014 at 10:48 PM. Reason: the attachments aren't working, posting them next
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Old 6th May 2014, 09:43 PM   #587
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I have tested the fig 41 limiter on both the elektor quasi and the john ellis complementary version of the bridge.

It works roughly the same and won't be much help in case of a short, however on overly difficult reactive loads, it does ok and as long as the impedance doesn't drop too low, that is close to a full short, the limiting action should suffice.

If there are shorts, they may not be near enough zero ohms, and they may not last very long, so this may be fine. If we want a real short protection, then something beyond the limiter would be required.

I've been running a lot of sims with various loads, and the most difficult ones aren't even the highly reactive, it's the short and with a short, there isn't much reactance left, mostly the zobel and output network, so nothing much to cause phase shift.

Now I've tried again to make the elektor based one to work in the bridged configuration, and I think something isn't right. It won't work that way and I think despite all our discussions to arrive at a bridging somewhat like crown's, we're not quite doing it right.

I tried keeping each side separate and test them side by side. With a feedback on the low side like on the high side, both side work just fine, but not when bridged.

I thought perhaps the TMC compensation could be an issue, not being certain where the tap from the output should really be coming from, so after comparing all the compensation methods and seeing how TPC performs quite well in this amp, I used that in the bridge configuration, because it refers neither to ground nor the output, only the rails, which seems ok to me in a bridge with flying rails.

With the TPC compensation, the thd went down to somewhere a bit above 200ppm, which is far better than plain miller when we started this.

What I observe when trying to give the low side the types of signals that I think it should see in the bridge, is an open loop behavior. We can tap the ground for feedback for the low side, so it should take that from the high side, but it's an opposite phase signal, so not a negative feedback. And this may be where there is a need for more reflections.

It's only a matter of getting the bridge to work now, as I think I have the limiter working properly (mostly. not on shorts)

Until we get this bridge working right, I will further the work on john ellis' bridge, as that one works and the limiter is the same on it.
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Old 10th July 2014, 05:28 PM   #588
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please offer me the best amp with 2N3055 transistors.
thanks
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Old 10th July 2014, 06:24 PM   #589
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Originally Posted by Hadighorbani View Post
please offer me the best amp with 2N3055 transistors.
thanks
That is the goal here. We're working on it.
Although I have not been able to work on it for some time, I will get back into it soon.
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Old 11th July 2014, 08:14 PM   #590
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i was find original 2N3055 and want creat best amp with them with best sound.
thank you
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