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Old 2nd May 2009, 10:21 PM   #11
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If I understand what Douglas Self is promoting, to run the Sankens at 80ma you would increase the emitter resistors to .3 ohms. This would give you 24mv across Re at your 80ma bias point.
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Old 2nd May 2009, 10:34 PM   #12
Mr Evil is offline Mr Evil  United Kingdom
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Is the current from/to N_Out/P_Out constant? If so, then you could replace R12 with a constant current source to save having to adjust it.
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Old 2nd May 2009, 11:04 PM   #13
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Quote:
Originally posted by Mr Evil
Is the current from/to N_Out/P_Out constant? If so, then you could replace R12 with a constant current source to save having to adjust it.

That is a good idea. But then in fact you do the same as what the LME49810 already have inside.

With best regards,
Bas
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Old 2nd May 2009, 11:07 PM   #14
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Quote:
Originally posted by Gary P
If I understand what Douglas Self is promoting, to run the Sankens at 80ma you would increase the emitter resistors to .3 ohms. This would give you 24mv across Re at your 80ma bias point.

Dear Gary,

Thank you for your response. I stubbornly choose for 0,1 ohm Re's because I prefer the bass response with this lower output impedance with a single pair of transistors.

With kind regards,
Bas
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Old 2nd May 2009, 11:13 PM   #15
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Quote:
Originally posted by jackinnj
yes, Jan -- I've been running them "hot" with no issues -- but I haven't been able to actually listen to the things.

It would be nice to have a better idea of the configuration of the VAS stage -- as it is now you're dealing with a bit of a black box.

Dear jackinnj,

Either I would try to built a truth (cascode?) current source to bias the STD's (like mr. Evil suggested) Or give it a try with the LME49810 which allow you to set a perfect 2,5mA pre bias for the STD03 diodes.

Ps: If you go for a current source for bias you really should take a look to Jan Didden's Elector article of his PAX amplifier design, where he use current sources as well to bias the Diode string.


With best regards,
Bas
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Old 3rd May 2009, 11:41 AM   #16
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Sebastiaan
I stubbornly choose for 0,1 ohm Re's because I prefer the bass response with this lower output impedance with a single pair of transistors.
the thermal stability of the output stage is determined by the thermal resistance of the output devices, the supply voltages, the emitter resistors and the transistor gain.
Cordell gave a formula to check this.

High gain transistors and low value emitter resistors can become thermally unstable.

You must raise the value of Re.

Then you can apply the correct Vre.

BTW,
the correct (total) Vre is 26mV at room temperature. This includes the voltage drop across the internal and emitter referenced base resistors as well.
The external Vre will be less than 26mV. It could be considerably less.

I'd suggest 20mV across 0r22 emitter resistors as a starting point.
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Old 3rd May 2009, 12:16 PM   #17
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Quote:
Originally posted by Mr Evil
Is the current from/to N_Out/P_Out constant? If so, then you could replace R12 with a constant current source to save having to adjust it.

Yeah, it's raining here in the PRNJ so instead of putting in lillies and hosta I wll be putting in a CCS.

Andew, I apologize publicly for my prior incendiary remark. I am just really t-ee-d off at Nat Semi's management for canning Troy, Mark and Bob and didn't think clearly before I wrote.
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Old 3rd May 2009, 04:05 PM   #18
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Quote:
Originally posted by Sebastiaan



Dear Gary,

Thank you for your response. I stubbornly choose for 0,1 ohm Re's because I prefer the bass response with this lower output impedance with a single pair of transistors.

With kind regards,
Bas

... but the bass response is not determined by the Re, only very remotely so.
I don't think you can measure any difference in Zout between 0.1 or 0.22 ohms, if you figure in all speaker cabling, the output zoble etc. IF there is a difference it will only appear at high frequencies anyway.

Jan Didden
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Old 3rd May 2009, 05:57 PM   #19
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Quote:
Originally posted by AndrewT
the thermal stability of the output stage is determined by the thermal resistance of the output devices, the supply voltages, the emitter resistors and the transistor gain.
Cordell gave a formula to check this.

High gain transistors and low value emitter resistors can become thermally unstable.

You must raise the value of Re.

Then you can apply the correct Vre.

BTW,
the correct (total) Vre is 26mV at room temperature. This includes the voltage drop across the internal and emitter referenced base resistors as well.
The external Vre will be less than 26mV. It could be considerably less.

I'd suggest 20mV across 0r22 emitter resistors as a starting point.

Quote:
Originally posted by janneman



... but the bass response is not determined by the Re, only very remotely so.
I don't think you can measure any difference in Zout between 0.1 or 0.22 ohms, if you figure in all speaker cabling, the output zoble etc. IF there is a difference it will only appear at high frequencies anyway.

Jan Didden
Dear Jan and Andrew,

I know both of you are theoretical right about the higher Re.

In my case the amplifier is for an active speaker. And I tried different Re's from 0,1 ohm till 0,5 ohm. in listening sessions. And 3 other people with me prefer the sound of 0,1 Re and believe me, the difference in sound is very noticeable in terms of "speed" and bass tightness in listening experience.

However the system is thermal stabile with 8mV across both Re's, have never thermal runaway or instabilities. Even after listening session on full power for while the bias get stable fast already and back to it's 8mV. point after a couple of minutes of cooldown.

With best regards,
Bas
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Old 3rd May 2009, 06:23 PM   #20
AndrewT is offline AndrewT  Scotland
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Yes, reduce the emitter current in the output device and you reduce it's gain. That reduced gain has the consequence of improved thermal stability.

But reducing the emitter current to below optimum for an EF output stage reduces the sound quality of the amplifier (increased crossover distortion).
I do not know what the optimum value of emitter resistor voltage is, but 8mV sounds too low to me.

If the only way to get optimum bias current is to increase the value of the emitter resistor, then that is the solution you should be pursuing.
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