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Messages - shasam

#1
Hi! Some news about the amplifier :
I have greatly improve the THD... by disconnecting the multimeter who was tracking the bias on the emitters resistors... I should have think about it before.  :-[
I have made some measurments of how the resistor on the base of the transistor CCS voltage reference influ THD. I have use 1k. With the reference connected to the VAS CCS, the THD without it is 0.08%, with the resistor it is 0.008%, 10 time improvement!  :tu:

With the reference connected to LTP CCS, the THD without it is 0.1%, and with it, again 0.008%. I think the major distortion now come from elsewhere in the circuit, so the difference in the transistor position is not measurable.

I have not improve the thermal tracking for now, so the results are approximate.
#2
Hi!
I have find some informations about this today  :)

First quote from ESP, that suggest to use the input stage source as reference :
"A special note for the unwary - If one is to use a single current control transistor for both the LTP and Class-A driver, do not use the Class-A (aka VAS - voltage amplifier stage) current as the reference, but rather the LTP.  If not, the varying current in the Class-A circuit will cause modulation of the LTP emitter current, with results that are sure to be as unwelcome as they are unpredictable [ 4 ].  Where the current source reference is based on the VAS (Class-A driver), it's advisable to decouple the voltage reference for the LTP source to minimise interactions."
(https://sound-au.com/amp_design.htm)

And a second one, from Douglas Self, that suggest to use the VAS current source as reference, and give an explanation about the base resistor :
"Bias isolation resistors are not unique to the amplifier of Figure 15.1; they are very commonly used. For an example taken at random, see Meyer.7 My own purpose in adding R23 was not to isolate the two current sources from each other at AC (something it utterly fails to do) but to aid fault-finding. Without this resistor, if the current in either source drops to zero (e.g., if TR1 fails open-circuit), then the reference voltage collapses, turning off both sources, and it can be time-consuming to determine which has died and which has merely come out in sympathy. [...]
The original amplifier used an active tail-source, with feedback control by TR14; this was a mere whim, and a pair of diodes gave identical THD results. It seems likely that reconfiguring the two current-sources so that the VAS source is the active one would make it more resistant to feedthrough, as the current-control loop is now around TR5 rather than TR1, with feedback applied directly to the quantity showing unwanted variations"
The amplifier of figure 15.1 is in attachement.
#3
Hi!
I have done some measurements this morning.
With the circuit on the right on my first picture (base connected to VAS current source emitter), I have a THD between 0.04% and 0.09%. (The thermal tracking is not great, and the bias shift, I think this is why the THD varies like this.)

With the one on the left, that you suggested, the THD goes up to 0.6%, 0.7%!   :(

For the moment I have not use any base resistor.

Have a good week end!
#4
Thanks a lot Loudthud!
I will look deeper into this next week end, when I get more free time.
#5
Quote from: Loudthud on November 28, 2020, 08:35:59 PM
Use the circuit on the left. Problems occur when the VAS clips. The VAS transistor turns off and the VAS current source has no where for the collector current to go, so it sucks a big base current until it's Voltage reference gives up. R7, limits the base current and prevents the input stage current source from going crazy.

Thanks Loudthud! So I should use a circuit like the one in attachement, with no other base resistor?
In a lot of schematics in Douglas Self's book, he use the other circuit (base of the reference transistor connected on the VAS current source emiter). Do you know why, please?


Quote from: Loudthud on November 28, 2020, 08:35:59 PM
Another subtle problem. C1 is intended to filter the reference current for both current sources. But, the way you have it grounded, it doesn't do it's job. It needs to be connected between the plus rail and the junction of R3 and R13 so R3 doesn't see the ripple on the plus rail. Be sure to connect the + side to the plus rail.

Sorry, my drawing was not clear, but this is how the capacitor is connected. :)


Quote from: Loudthud on November 28, 2020, 08:35:59 PM
This creates another problem, it limits how fast the LTP current source turns on when the amp powers up. That creates a big pulse on the output. If you can make the current source turn on fast, the output will be almost silent when the amp  turns on and off.

Thanks! I will look for that. Do you have a suggestion for a faster turn on please?
#6
Hi!

I would like to use current source for the input stage and for the VAS of my amplifier, and I see different implementations of these, but I don't realy understand what it does change...

When using current sources for the input stage and for the VAS, with a transistor used for voltage reference, sometimes the base of this transistor is connected on the emiter of the LTP curent source transistor, sometimes it is on the VAS one. Could someone explain me what difference does it make please?

Also, resistors are sometime used on the base of one or more of those three transistors. Randy Slone write 1k is a good value, but I haven't find any further information about this. What is their purpose please?

Thanks a lot for your help!
Please apologise my poor English.
#7
Hi! I have been away from the workbench for the last two weeks, sorry for my late answer.

I have finally run some tests  :cheesy:

Just before clipping level, with a 8 ohm load (resistive),  the power supply sag to +/- 19.8V, with 35V ptp output (approximately, I have only measured it with the scope).
#8
Thanks!

You are right, those transistors are oversized.
I have redone the calculations for BD243 / BD244.
(datasheet here : https://www.onsemi.com/pub/Collateral/BD243B-D.PDF)

I got this :

Derating = 0,52W/°C

Max dissip 25°C = 65W

Maximum_case_temperature = (Pmax_at_25° - Ppeak_reactive) / derating + 25° = (65-23,17) / 0,52 +25 = 105,45°C

Pdiss_worst_case =  Vps² / (19.75 * RL) = 38,5² / (19,75 * 8 ) =  9,38...W

Heatsink_max_thermal_resistance = (Maximum_case_temperature - Ambiant_temperature) / Pdiss_worst_case = (105,5 - 50) / 9.5 =  5,85 °C/W

Quote from: phatt on September 29, 2020, 09:38:05 AM
Others here will correct me if I'm wrong but all that maths assumes the Transformer is an infinite current source and the tX is the size of a car battery.  :lmao:
Meantime back in the real world your 30vct Tx supply will dramatically sag long before the circuit can pull that much current.

Maybe there is something that I dont understand, but this voltage should be under load.
With no load, I have 44.9V (mesured) from Vcc to Vee.
It is true that I have negligected ripple, that would reduce the usable voltage.
#9
I am going to use a 2*15V transformer, and NJW0281G (NPN) / NJW0302G (PNP) output transistors, in CFP configuration, for 8Ω output.
Datasheet can be found here : https://docs.rs-online.com/41ca/0900766b813e9eb0.pdf


For now, my calculations are :

Total DC power supply  =  2 * (Veff *√2 - 2Vdiode) = 2 * (15 * √2 – 2) = 38,5V

Output power =  (power supply - loss)² / (8*Rload) = (38,5 – 5)² / (8*8 ) = 17W (I found similar result in LTSpice)

Output peak current = (power supply - loss) / (2*Rload) = (38,5 – 5) / (2*8 ) = 2,1A

Output peak power  = (power supply - loss) * Output peak current =  (38,5 – 5) * 2,1 = 70,35W


With Rod Eliott rules-of-thumb previously quoted :
Quote
"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"
(from https://sound-au.com/soa.htm)

That give me : I = (Vrail / 2) / R = (19.25 / 2) / 8 = 1.203125A
Ppeak_resistive = Vrail / 2 * I = (19.25 / 2) * 1.203125 = 11.58...W(peak)
Ppeak_reactive = Ppeak_resistive * 2 = 23.16...W(peak)


Max dissipation for NJW0281G / NJW0302G is 150W at 25°, 0W at 150°
Derating = 150W / (150° - 25°) = 1.2W/°C

Maximum_case_temperature = (Pmax_at_25° - Ppeak_reactive) / derating + 25°
= (150 - 23.17) / 1.2 +25 = 130°C


I have use the formula given here for average worse dissipation : https://www.updatemydynaco.com/documents/Class_B_Amplifier_Dissipation_Calculations.pdf
Pdiss_worst_case =  Vps² / (19.75 * RL) = 38,5² / (19,75 * 8 ) =  9,38...W
I don't really understand the calculation here. I have use it because it give me the worse result of all the calculation I have read online. What do you think about it?

Heatsink_max_thermal_resistance = (Maximum_case_temperature - Ambiant_temperature) / Pdiss_worst_case = (130 - 50) / 9.5 = 8.42... °C/W
I don't really know what to use for ambiant temperature, I have use 50°C here. I will use bigger heatsink to limit the max temperature lower than 130°C too.


I tried to make it as clear as possible. What do you think about it, please? Could you spot some mistakes? Should I take bigger margin?

Thanks a lot!
#10
Thanks a lot Phatt!

I try to une rules of thumb too, but different ones look to give really different results sometimes (for heatsinks calculation for exemple, I find results from simple to double with different rules of thumb I have found).

I won't try current feedback for this one. Like you said I try to keep it simple (and I still find it not so simple...).
Maybe for a next project.

I am not familiar with polyfuse, I need to learn more about them, but this look really interesting!  :cheesy:

I have noticed you use different heatsinks for the driver transistors. Do you find the thermal tracking to be better like this?
#11
Thanks Phatt!

How would you limit the bandwidth please? With bigger Miller capacitor? Or in feedback network? I was thinking to limit it in the preamp section.

Quote from: phatt on September 25, 2020, 04:04:47 AM
Some of the greatest guitar sounds ever heard were produced by what would now be considered a very bad design. :lmao:

I am aware of that, but without going to digital, i prefer take a modern approach, with tone and distorsion independant of volume control (OK, speakers and ears are still going to react differently with volume change ;) )


Quote from: phatt on September 25, 2020, 04:04:47 AM
As for design help if you can find a copy of "Art of Electronics" it's well written for those of us who are less gifted at complex maths. :duh
Thanks! I have something similar, in french (my native langage), but this one look more complete.

Quote from: phatt on September 25, 2020, 04:04:47 AM
As noted in the ESP page you quoted, anything running below +/-35VDC is not likely to give much trouble and that covers a basic amp of 40~50Watts so 15 ~20 Watts is fairly safe.
That's the reason why I begin with this, I already have collect bigger transformers for future projects :)


Quote from: phatt on September 25, 2020, 04:04:47 AM
I'll try and dig up some old pictures of my testing circuits to give you ideas.
I would really appreciate it, thanks a lot!
#12
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.  ;)
#13
Hi Phatt!

Quote from: phatt on September 18, 2020, 10:19:23 AM
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.


Quote from: phatt on September 18, 2020, 10:19:23 AM
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?

Quote from: phatt on September 18, 2020, 10:19:23 AM
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  ;)
#14
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?
#15
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