Bob Cordell Interview: BJT vs. MOSFET - Page 55 - diyAudio
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Old 9th January 2007, 11:08 PM   #541
andy_c is offline andy_c  United States
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Quote:
Originally posted by JPV
My question is about your graph: why is the trend at high current not R=0,22 because hib ( 1/gm) is then very low?
Looks like it's because I did the sim with two devices in parallel. So it should approach 0.11 Ohm at high currents. I didn't go to quite high enough of a current to get there though.
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Old 9th January 2007, 11:28 PM   #542
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Quote:
Originally posted by Nico Ras


For all those fans out there building clones, you could save a small fortune not having a 1 KVA transformer to heat up the transistors as well as massive heatsinks to prevent them from setting the furniture alight.
Heh, heh ... But one day you'd want to throw a party with several pairs of relatively inefficient, current hungry speakers (like ATC SCM100s) in parallel. I assure you that big heatsinks and massive toroids have their place under the sun
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Old 9th January 2007, 11:30 PM   #543
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Quote:
Originally posted by Nelson Pass


I will spare you my further involvement.


Hi Nelson,

I did not think that your comments were the least bit off-topic or in the nature of trying to drag the thread in another direction. I, and I am sure the great majority of the other readers, welcome and enjoy your involvement. I am personally honored that you participate in this thread.

Bob
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Old 9th January 2007, 11:40 PM   #544
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Quote:
Originally posted by JPV
Hi andy_c

I looked at your thread and have seen your graphs.
This is great. It seems that a simulation ( yours), a calculation ( Oliver) and tests ( Self) are showing the same trend.

My question is about your graph: why is the trend at high current not R=0,22 because hib ( 1/gm) is then very low?
Seeing this in linear scale is also surprising. What I said on narrow window doesn't make sense.

Mr Cordell

I understand of course the thermal dynamic problem. I though I showed my concern. A question is : is the thermal lag effect creating a lot of "transient distortion" ? What is the weigth of this effect in the overall aural perception?


Mr Pass

I have VERY good burgundy in my cellar in Brussels. If you ever PASS by you are welcome

JPV

You have posed a very, very good question. We should always connect all the dots and close the loop on all of our well-intetioned hypotheses, yet unfortunately we often don't or can't. Although I have measured the dynamic thermal effect on bias, I have not done a transient THD or transient twin-tone distortion test to be able to put a number on the objective result. I don't think its an easy thing to do, but maybe someone here has done it. Similarly, while I and others hypothesize that a temporary under-bias situation may cause pretty serious crossover distortion, we really don't know (at least I really don't know) that the effect is all that audible.

Bob
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Old 9th January 2007, 11:44 PM   #545
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Quote:
Originally posted by andy_c
Nelson, I apologize for my comment. I was out of line.
OK. Now you will have to put up with me.

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Old 10th January 2007, 12:19 AM   #546
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Quote:
Originally posted by Bratislav


Heh, heh ... But one day you'd want to throw a party with several pairs of relatively inefficient, current hungry speakers (like ATC SCM100s) in parallel. I assure you that big heatsinks and massive toroids have their place under the sun

Not if you are using four 15 Hz Horns, 15 watt will blow you away, but still I use 4 down biased KSA50s running only hot when I irritate the people in the next village.

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Old 10th January 2007, 12:38 AM   #547
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Quote:
Originally posted by Nico Ras



Not if you are using four 15 Hz Horns
My room is big, but not that big
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Old 10th January 2007, 07:30 AM   #548
JPV is offline JPV  Belgium
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Andy-c

Can you do a run where you change Io the bias current like you did but you change also the Rc ( emitter resistor) in order to keep Rc x Io = 26mV.
Thanks

M; Cordell

This transient thermal effect can perhaps be done rather easy.

If you use thermaltrack transistors mounted in push pull with a normal vbe multipler but not using the diodes on chip.
Then, you can apply a stable signal to the amplifier but pulse short high power bursts in the diodes. This will heat the chip in a transient way with respect to the main signal. By using a low duty cycle it must be possible to see the distortion induced if you have good instruments.

By using the push pull stage (perhaps with a driver in the loop) with the LM4562 (low distortion) you can include or remove a resistor R between the + and - of the LM4562 and decrease or increase the loop transmission so the distortion without changing the gain. I would do it with an inverting configuration to avoid common mode.
The loop transmission is a(s) (R //R1)/(R2+R//R1) and gain remains R2/R1 whatever R not to low
By coupling this with pulses on the diodes and listening to it it is perhaps possible to hear the effect of crossover distortion and dynamics. I dont'have yet the set up to do this but I will in the futur and will do it

What do you think?

JPV
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Old 10th January 2007, 09:01 AM   #549
AndrewT is offline AndrewT  Scotland
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Hi,
it appears we are worried more about under-biasing than over biasing in our push pull arrangements.

The window of adjustment seems to be 15 to 25mV across the individual Re.

What if one were to set the "normal" bias to near 25mV and accept that when cold it will be slightly overbiased, but more importantly when the output die is cooling after a high power burst the Re bias voltage is allowed to drop to near 15mV, then we stay in the near optimum bias window.

Is it possible to scale the sinks and Re and number of output devices to achieve something like this set-up?

Then, there would be no argument about whether post burst under-biasing is audible or not (hopefully there would be no under-biasing).
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Old 10th January 2007, 02:06 PM   #550
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Quote:
Originally posted by JPV
Andy-c

Can you do a run where you change Io the bias current like you did but you change also the Rc ( emitter resistor) in order to keep Rc x Io = 26mV.
Thanks

M; Cordell

This transient thermal effect can perhaps be done rather easy.

If you use thermaltrack transistors mounted in push pull with a normal vbe multipler but not using the diodes on chip.
Then, you can apply a stable signal to the amplifier but pulse short high power bursts in the diodes. This will heat the chip in a transient way with respect to the main signal. By using a low duty cycle it must be possible to see the distortion induced if you have good instruments.

By using the push pull stage (perhaps with a driver in the loop) with the LM4562 (low distortion) you can include or remove a resistor R between the + and - of the LM4562 and decrease or increase the loop transmission so the distortion without changing the gain. I would do it with an inverting configuration to avoid common mode.
The loop transmission is a(s) (R //R1)/(R2+R//R1) and gain remains R2/R1 whatever R not to low
By coupling this with pulses on the diodes and listening to it it is perhaps possible to hear the effect of crossover distortion and dynamics. I dont'have yet the set up to do this but I will in the futur and will do it

What do you think?

JPV
This is an interesting suggestion, but I may be mis-understanding a part of it. It sounds like you want to heat the power transistor die on a controlled transient basis by running current through the ThermalTrak diode to cause heating via power dissipation in the diode. Is that correct? If so, can the ThermalTrak diode handle the needed amount of current and dissipation?

Bob
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