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Old 28th April 2009, 11:58 PM   #1
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Default VBE multiplier

I am trying to get the Luxman M-117 amp to bias properly. Attached to this post is the amp schematic. When I try to bias the amp higher to reduce cross over distortion I can't get it to thermally compenstate correctly. When monitoring the heat sink termperature and the emitter resistors the voltage stays constant but there is a very slow creep up in termperature. I think this means that the VBE multiplier is undercompensating at higher temperatures. I am trying to keep the amp at 45C at idle so that it won't overheat when driven. I am told it is better to slighly overcompensate. Right now I think it is slighly undercompensated at idle, which I don't think is safe.

I have mounted the hear sensor directly on top of the middle plastic sanken 2sc2922 output transsitor to reduce thermal lag. This is still a plastic to plastic interface so I don't know how effective it is. The original design simply mounted the sensor in the middle of the heat sink but the response was even slower.

I have read Self's book but he only talks about increased tempco vbe multipliers for CFP output stages not EF, like this is.

I tried the 2 transsitor Szikial pari ala Self which is supposed to have increased tempco over the 1 transistor version. It is stable and seems to have slighly higher tempco but the termperature still creeeps about .1 degree centrigrade per 30 minutes. After 2 hours it is up .5--.7 C. It does not seem to stop.

Some of you said on other threads that you could vary the tempco of the Szikail par by changing the base emitter resistor of the 2nd transistor. But I don't think this is true as the tempco does not vary much when I vary the value of this resistor.

Now I think I am left with 3 options:

1.Do a better job encasing the sensor in insulating material to avoid the slow heat loss?

2. Try using transistors in the Sziakel pari VBE with lower voltage (faster) or higher hfe to increase compensation? Will this work?

3. Try to come up with practical value of Self's increased tempco vbe multiplier based on the amp schematic I have attached. I am not smart enough to do this but I was thinking of using the one with a diode voltage reference? Can anyone suggest practical value to start.

4. I tried creating a 4 diode string a la Leach but it undercompensated. Someone suggested using Shotkcy diodes due to the lower Vf will have a higher compensation factor. Is this a worthwhile approach?


Bear in mind that the Vq of the emiiter resistor is held fairly firm. It seems to heat up, then the Vq goes slightly down to compenstate and then it heats up again, and then the Vq starts to go down again, which is all well and good. But eventually this pattern of going slighly down and then back up starts to drift upward. It takes at least a half hour to an hour before the slow upward drift starts.

I have spent almost 6 months trying to figure this out so I throw myself at the mercy of the forum.
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Old 29th April 2009, 03:11 AM   #2
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Sorry. The schematic is attached here.
Attached Files
File Type: pdf 20090428173727252.pdf (57.7 KB, 189 views)
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Old 29th April 2009, 03:11 AM   #3
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The schematic is upside down. I hope this one is clearer.
Attached Files
File Type: pdf 20090428184721105.pdf (97.6 KB, 180 views)
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Old 29th April 2009, 03:50 AM   #4
wg_ski is offline wg_ski  United States
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If the Vq (therefore Iq) is held constant, then it's properly compensated. If the temp drifts up with a constant current, that means the heat sink is too small.
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Old 29th April 2009, 04:37 AM   #5
kaos is offline kaos  United States
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If I understand your post correctly youíre worried about thermal runaway with extended operation (instead of reaching some stable working point the output bias current slowly creeps up over time and the amp temperature steadily rises accordingly)?

That schematicís far too hosed up and incomplete to be of much help. One thing that you can do is give the 2nd stage current source a negative temperature coefficient. To do this mount the diodes that are used for reference in the VAS source (just above and to the left of the Vbe multiplier in your diagram, if Iím seeing it properly) on the heat sink with some epoxy. As the temperature of the heat sink goes up the voltage across the diodes will go down and the current through the Vbe multiplier will go down accordingly. This will cause a drop in voltage across the Vbe multiplier thatís added to itís own TC. Seems to work fairly well, drawbacks are a decline in second stage slew rate as temperature goes up, and an increase in offset if a current mirror isnít used to interface the differential to the 2nd stage. The slew rate issue can be addressed simply by starting out with ample 2nd stage current to begin with. Offset probably wonít be a serious issue if the DC gain is unity (a capacitor is used in series with the gain setting resistor, or an effective servo is in use).

I know, probably not the answer you were looking for. Discrete Vbe multipliers are limited in what they can do though.
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Old 29th April 2009, 04:55 AM   #6
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I would try doubling the output transistor emitter resistors to
0.47 ohm * 6 after setting the bias current for low distortion with the
.22 ohm resistors in place. This keeps the current down while maintaining more class A drive on the output. It also reduces the
effects of heating on VBE as part of the drop in now across the output
resistors.
speaker output will not be affected much as .47 * 3 = .153 ohms
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Old 29th April 2009, 04:56 AM   #7
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That makes perfect sense.

The original amp was biased at 8mv across the emitter resistors where Self recomends 46.2 mv for .22 emitters.

So I tried to get it up to about 25mv, which is still suboptimal.

So I either need to get a bigger heatsink-- not a viable option

Fan cooling makes no sense if it needs a fan even when idling.

Or I could eliminate one pair of the output transistors.


Now there are more facts I have not disclosed because I could not put it all in one e-mail.

The amp was a stereo amp I converted into a monblock because the thermal protection circuity kept kicking in. So now instead of six 17 amp sankens (2sc2922) on one heatsink there are now 12 sankens on 2 heatsinks. If I eliminate one pair from each heatsink I would go down to 8 transistors on two heatsinks instead of 6 tranistors on one heatsink. I think this would permit me to bias the transistors more optimally without thermal runaway.

It drives a 2.5 ohm woofer tower. So now I am concerned about using only 8 transistors because of beta droop.

It did 200 watts into 8 ohms but only 250 into 4. But with the transformer now only supplying one channel and the filter capacitance going from two 15,000 uf (30,000 uf total) to 60,000 uf total and with plus/minus 80 vots on the rails, the amp may try to sink alot more current into 2.5 ohms.

With the low impedance of the woofer tower I am not sure whether it is better to bias more optimally with 8 transistors or suboptimally with 12.

By the way the amp sounds fanstastic.

Any thoughts?
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Old 29th April 2009, 05:26 AM   #8
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I would not do that as it will load the remaining transistors with the full
amount of power dissipation heating them further, possibly to their SOA limit. 2.5 ohms is an already low impedance load.
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Old 29th April 2009, 11:40 AM   #9
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Hi,
what does the schematic show immediately before the output device bases? It has three rectangles with a letter inside each.
It appears to connect the Vbe multiplier to the supply rail and to the output base resistor. Is there a resistor in the connection to the supply rail?
It also looks like there is a pair of resistors to the output feedback tapping. Can you confirm?
__________________
regards Andrew T.
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Old 29th April 2009, 04:03 PM   #10
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I cannot digest all of your great suggestions.


WGski: I am trying to observe carefully the long term temperature creep. This is moving up only .1c per half hour. The Vq drops as temperature goes up but then it can't fully compenstate and goes back up to approximately the same value. It would appear that to keep the temperature from creeping up the VBE would have to respond faster to the increase so that the temperature does not cumulate. This is a very long term phenomena but it would appear that vq would have to dcrease more or faster to prevent the slow thermal run away. 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?

Kaos, you are like a graduate school professor talking to a student in college. Which LEDs do you suggest I attach to the heatsink? There are 2 attached to the bottom of the resistor chain and one on the top. All 3 of them are Led LTZ MR15 (RED). Do I try mounting them all on the heat sink. I could put them in the same holes I made in the heat sink for the leach diode string for the VBE that I never implemented which are centrally located on the heatsink. Does anyone else agree with this approach because it is very creative? Does this make sense with an EF output stage?

Stocktrader. YOur idea is over my head. I know if I change the emitter resistor values it will change the current limited protection circuit. It will also improve current sharing which helps with 12 output transistors mounted on 2 heatsinks. I like the fact that it will lower the heat sink temperature because more current will be dissipated by the resistors and less by the transistors but I am afraid of this significant of a change. Does anyone else agree with this approach.

I need more help but thanks so much for all of your wonderful ideas.
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