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Old 29th April 2009, 04:03 PM   #11
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Andrew, at least I can answer this question.

the rectangular box with the BCE designations refers down to the bottom edge of the schematic where you see the 4 small square boxes each of which are darlingtgon pairs.

There is one for the positive and negative drive circuit.

I think Self would refer to this as an output triple or beta enhancer , since it has a predriver and driver transistor in each box.

There are two 270 ohm resistors (540 ohm total) between the plus minus predrivers

Self uses one 330 ohm resistor for faster switch off. Thought about listening and changing this value to 330 ohms, make sense????

The drivers have two 68 ohm resistors for 144 total, which is very close to Self's circuit with 150 ohm so I was going to leave it alone. But I did add the suck out capacitor across the resistors

I think this is a EF type II output stage according to Self.
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Old 29th April 2009, 04:54 PM   #12
wg_ski is online now wg_ski  United States
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Quote:
Originally posted by fglabach
I am not sure whether this is a heat sink deficiency or a delayed response that causes cumulative error. Does it make sense that this problem would get worse when I try to bias at a higher vq?

It is hard to seperate the two phenomena. In a deayed response situation, the bias may increase temporarily (or decrease) and again settle out after a change in dissipation. It's only really "thermal runaway" when the steady-state bias current creeps up as the temp increases. If the bias current is stable, the temp will eventually stop increasing - but it may be at a temp that is too high for comfort. The only remedy in that case is either reducing the bias (dissipation) or getting a fan. But if you overcompensate such that the bias goes down with temp, it will no longer be optimally biased per Self as the temp goes up. In situations where the heat sink isn't really big enough, it is the only acceptable answer. The optimal bias is quite a bit higher than is typically designed for when manufacturers select the heat sink.
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Old 29th April 2009, 04:56 PM   #13
kaos is offline kaos  United States
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The diode that Iím referring to is circled in green. If this were mounted to the heat sink to sense temperature while the associated transistor is allowed to remain at ambient, then the current source would develop a negative TC related to the heat sink temperature. The diode would need to be a red LED or a combination of series diodes (not a blue type). If you do choose to try this just be careful that you donít accidentally hook the diode up backwards or damage it with ESD. Whether a single red LED will be enough to give you the TC that you want will remain to be seen.

Click the image to open in full size.
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Old 29th April 2009, 05:26 PM   #14
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Kaos:

Thank you for the helpful diagram.

Yes, that is a red LEd for the current source.

It should be easy to move that off the board to the main heatsink

I will finish this project during the weekend. Sorry for the delaly and then I will find out whether there is enough additional negative co-efficient from the one red LED.

Some more information. The temperature rise is very slow. This is a very slow long term thermal delay. The VBE multipier actually lowers the vq in response to higher termperature.

What I don't understand is what is the difference between increasoing the tempco of the VBE multiplier as opposed to your idea of adding negative tempco from the VAS which may current limit the driver stage. Why have you shifted your attention to the VAs? This stuff is over my head but I still will pursue your idea.

By the way I live in LA as well.
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Old 29th April 2009, 06:26 PM   #15
kaos is offline kaos  United States
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A single Vbe multiplier can only give so much in terms of a practical temperature slope. In a discrete design it will always have a lower temperature rise than the outputs in an EF stage due to the mechanical nature of the situation. After that one has to start looking elsewhere for options to tweak the temperature curve. In your case it sounds like there may be a general heating of the circuit over time causing drift. The VAS current is a simple option to explore to counter this longer term effect. The red LED can be replaced with several discrete diodes to increase the TC if needed, though the value of the associated resistor in the current source would need to be adjusted to maintain proper VAS current if that were done.
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Old 29th April 2009, 06:45 PM   #16
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wg ski:


Here are my readings starting one hour after warm up:

41.7 C 21.3mv at 9:30 a.m.
42C 21.41mv at 9:45 a.m.

42.5C 21.35mv at 10:00 a.m.

42.6C 21.6mv at 10:05 a.m.

42.7C 21.7mv at 10:10 a.m.

42.8C 22nv at 10:12

42.9C 21.54 mv at 10:13

42.7 21.26 at 10:14 am

42.8 21.23 at 10:14

42.8 21.17 at 10.14 a.m.

42.9 21.76 at 10:15

43.5 21.85 at 10:30

43.4 21.5 at 10:38

43.5 21.6 at 10:45.


Is this thermal run away? Insufficient negative tempco? or inadequate heat sink?
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Old 29th April 2009, 06:47 PM   #17
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Kaos: Thanks for the explanation. I will pursue your option. If negative tempco insufficient will add another diode.
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Old 29th April 2009, 07:07 PM   #18
kaos is offline kaos  United States
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That doesnít look too bad, though itís difficult to tell from just a few degrees change. Ideally a slight negative TC is better than a slight positive one, but itís hard to get a discrete design to be right on the mark. Looks like the current will slowly rise with temperature, but not destructively so. I guess you have to ask yourself ďwhatís my goal?Ē If you want the amp just to not blowup youíre probably there (though cooler running is better than hot, especially with a heavy load like you have). If you want a closer tracking or a negative TC then you still have some work to do.
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Old 29th April 2009, 07:15 PM   #19
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Kaos

Since the amp is driving a woofer tower I think my goal has to be to get a slight negative tempco.

Do you think there would be any benefit from putting it in a lower voltage faster VBE multiplier such as a BD139.

By the way the readings I provided are with a Szikai two transistor vbe multiplier that I installed which I believe has slightly higher negative tempco then the one transistor used in the circuit.

If your idea works I may go back to the one transistor VBE because it is less prone to problems.

Thanks again.
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Old 29th April 2009, 09:21 PM   #20
wg_ski is online now wg_ski  United States
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Doesn't look like it's going anywhere. Those mV fluctuations could even be due to the line voltage varying. To be sure, either run it for a week straight and monitor it, or crank the music up for half an hour, or whatever it takes to get the heatsink up to about 60-70C. Then back off the music and take readings. It should settle out (with the HS temp still elevated) within a few minutes if it's stable. If it's still in the 22mV range I'd say it's pretty good.

If you're running over 40C just idling, the heat sink really isn't big enough for a class AB/B amp.
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