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Messages - shasam

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Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 23, 2020, 12:43:56 AM »
Thanks!
I have already built little amps, chipamp and class B amp drived by an ampli op (but simplier than this TD 742).
My real goal is to learn how to design a discret one, not just having a new amp.
I know I could buy PCB with TDAxxxx, or used ss guitar amp  for cheap. I could etch Randy Slone PCB too, and follow his schematic.
I just want to learn how to design it myself.  ;)

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Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 21, 2020, 06:13:15 AM »
Hi Phatt!

Likely over my head but I'll ask.
                              What is it you are trying to design?

I am trying to design a discret class B power amp, with CFP output. Something in the 15 -20 Watts range for beginning.


Re the last part of the link to SOA by Rod Elliot, (Section5  Protection Schemes)
To my mind that is how you cover the extremes.

So, I should place the number of output devices to endure most real life situations, and put I-V limiter for protection against anything worse?

I doubt any circuit is totally bullet proof but if it's just a 100Watt guitar amp then considering how many millions of 100watt amps get flogged and survive for years ,,, maybe you are over thinking the design. :-\
Or have I completely missed the point? xP

I am probably over thinking it  ;)

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Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 16, 2020, 08:41:51 AM »
Hi Edvard! Thanks!

What annoy me is, with real world measurments, some real speakers could be worse than the ones I have under my hand now, and input signal could came in wicker shapes that the ones I generate for testing. We can simulate the worse theorical case but this could be really worse than real practical situation so being useless. So I don't know what to really use...


"Tonal differences between speakers are often down to the material and construction of the speaker cone itself; the engineering of the power amp is only concerned with the magnet/coil part."

Could you please elaborate a little more on this? A bass reflex cab would generate other phase shift, why shouldnt we care about this?


I have seen on some websites that a phase shift of 45° it the worse case for output stage (ignoring 90°), as this exemple:
"As discussed in the Audioholics article, the key phase angle (denoted as Greek phi Φ) to keep in mind is +/-45°. This is the point where the amplifier must dissipate double the power compared to just a purely resistive load while the speaker receives only half the power. On either side of that key phase angle, the amplifier demand is reduced. Knowing this allows the amplifier designers to ensure that they can deliver adequate power and maintain the temperature of the devices at safe levels. I've seen my own speakers vary between +/-70° at the extremes with +/-60° being more common."

Douglas Self stated : "Average device dissipation also increases as the load angle increases. A 45 load increases average dissipation by 1.4 times, and a 60 load by 1.8 times.
Peak device dissipation increases more rapidly than average dissipation as the load angle increases.
A 45 load increases power peaks by 2.7 times, and a 60 load by 3.4 times."

From my simulation too (screenshot in attachement), 60° is worse than 45°. Is the first quote false, or am I missing the point?

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Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 10, 2020, 05:30:09 AM »
Thanks again Loudthud!

"To really study this subject, you would need a lab full of equipment. You would need to simultaneously monitor Voltage and current in a transistor, then multiply the two to get power. In a practical sense you would need to look at the instantaneous power over a time scale and compare that to the SOA limits from a transistor data sheet. I say practical because you don't want a 100 W power amp with 20 output transistors."

I was trying to have a theorical approch to avoid this, but as you say, this lead to lot of output transistors.
Do you think Spice simulations would differ a lot from reality?
What I find annoying, is what is a practical signal for testing a guitar amp? An hifi amp won't have to produce continuous square waves. But for a guitar amp, for exemple, a larsen with a super fuzz type pedal is a possible situation, and look really more painfull for the amp.
Should I think of it as "How could I suppidly distort the signal before the VI limiter began to conduct?"
Furthermore, what would you use for practical load? Results would vary a lot in fonction of the load used for measurments, with different impedance dips and phase shifts between real speakers.


"Some have a lower limit, a few higher up to 80V. Let me know if you find any bypolar transistors that are higher than 80V."

On SemiNJW21194G / NJW21193G seem to have 100V / 110V limit. (SOA graph in attachment)
https://docs.rs-online.com/420b/0900766b8126cc12.pdf

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Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 09, 2020, 05:25:37 AM »
Thanks Loudthud!

It is nice to see real measurement of what append with a scope!  :) Is there something else than phase shift between voltage and current due to speaker reactance that I should see here?

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Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 07, 2020, 11:09:56 AM »
Hi Loudthud!

Thanks for your answer!

I have join MEF, but I am having hard time to fine the thread you are talking about. Have you an idea of what keyword should I search, please?

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Amplifier Discussion / Peak dissipation in class B output stage
« on: September 04, 2020, 06:27:15 AM »
Hi!


I'm trying to understand how to design a class B power amp, and I am a little confused with the method to define the peak of power dissipated by the output transistors.
All the mathematical methods I have seen are based on sinewaves.
But it look to me that with an assymetrical heavilly clipped signal, like you could have from a fuzz, things could be really worse. With a big phase shift, you could have all the voltage (Vcc - Vee) accross a transistor in the same it would pass full curent. So Pd(peak) could be I(peak) * (Vcc - Vee). Is this really pessimistic?


Here, https://sound-au.com/soa.htm, the result look the same, but the method look strange to me :

"Having discounted the idea of any 'rules-of-thumb', I'm going to give you one anyway . Let's assume that you want to deliver 100W into 8 ohms, so you need a power supply with ±42V rails (I'm going to ignore losses here). The amp must also be able to drive nominal 4 ohm loads, so expect the minimum impedance to be 3 ohms. Worst case (resistive load) dissipation is therefore ...
I = V / 2 / R = 21 / 3 = 7 Amps
P = V / 2 * I = 21 * 8 = 168 Watts (peak)
This accounts for the resistive part of the load, and as we saw above, the reactive part of the load causes dissipation to double. Just like second breakdown, we aren't interested in the average dissipation - this influences the size of heatsink needed, but not the transistor's safe area. Therefore, Ppeak will be ...
Ppeak = P * 2 = 168 * 2 = 336 Watts"

How could the transistor pass full current with the output being at Vcc/2 with a resistive load?


What would you assumed to be the whorse impedance and phase shift from a 8ohms guitar speaker please?
I am right assuming at the lower impedance, the load would look only resistive, so there won't be phase shift, so the the worse cases couldn't come hand in hand? I have not found anything about this yet.


Thanks a lot for your help!
Please apologise my poor English.

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