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Old 26th March 2012, 06:01 PM   #2201
mikelm is offline mikelm  England
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Thanks Jay, that explains why the conversation seemed to be going on ad infinitum.

It's funny though because I'm just about coming around to the idea that valves might be able to give more clarity & life than solid state - at least a valve front end.

Perhaps it's the caps & iron that can end up sounding mellow ? ?

mike
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Old 26th March 2012, 09:36 PM   #2202
hahfran is offline hahfran  Germany
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Originally Posted by hahfran View Post
Fear not! I am almost through with an all BJT version of a non switching class B
I give up...I cannot get the three requirements
current driven all over
non switching power section
thermal compensation with thermaltrak BJTs

under one umbrella.
not enough spare time for that.
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Old 26th March 2012, 09:48 PM   #2203
hahfran is offline hahfran  Germany
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Originally Posted by mikelm View Post
Thanks Jay, that explains why the conversation seemed to be going on ad infinitum.

It's funny though because I'm just about coming around to the idea that valves might be able to give more clarity & life than solid state - at least a valve front end.

Perhaps it's the caps & iron that can end up sounding mellow ? ?

mike
tube amps really excel in mini dynamics reproduction i.e. resolution.
That is why these give more clarity . While there are some tube MOSFET hybrids no one has tried a tube BJT hybrid ..that could be interesting
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Old 26th March 2012, 10:17 PM   #2204
AKSA is offline AKSA  Australia
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no one has tried a tube BJT hybrid
Wouldn't be so sure about that...... I'm sure they are around, they just don't seem to meet the commercial standard for some reason.

BTW, I agree with you about the non-switching Class AB. I have tried this one too, over some years, but never found the benefits I expected sonically. I really don't think that the switching of the output stage is such a big problem as long as it's clean and well controlled.

A good compromise is to ensure that the drivers never switch off.

Hugh
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Old 26th March 2012, 11:00 PM   #2205
Sheldon is offline Sheldon  United States
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With K30ATM and BF245A had tested only. But mine's LAT and low driver bias suffices. Check if your input BJTs still run about the same Ie, and if the amp's output tracks sine wave well symmetrically when cranking it up on the scope. Say at 30V pk-pk. About HF noise, see if its acting up so to revisit comp caps or input filtering 'cause the CCS boosts its speed. Will some NTC find enough temp to react when bundled with two small BJTs running 30mW each? We must get to know which exact type works for LC first. Still the CCS is fixing the drivers bias stability not the input drift. But in a bridge everything interacts non the less.
Upon further review (tried a DN2540), the CCS prevents bias of the outputs from drifting too high. That part is fine. But the problem is that the voltage drop across the CCS is small until it begins limiting current, but then it continues to rise. This eats up the rail voltage on one side. I don't think this will work. I await the NTC recipe.

Sheldon
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Old 26th March 2012, 11:15 PM   #2206
Salas is offline Salas  Greece
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I used low pinch off Jfets for that reason. Up to what level did the DN2540 hold before starting to chew up?
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Old 27th March 2012, 02:09 AM   #2207
Sheldon is offline Sheldon  United States
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I used low pinch off Jfets for that reason. Up to what level did the DN2540 hold before starting to chew up?
The DN2540 holds up fine, it's a 400 volt device. It keeps the current through the bias diodes from drifting higher. The problem is that the driver transistor keeps trying to turn on harder, increasing the voltage at the top of the CCS. That effectively reduces the rail voltage on the rail driving the CCS. Clipping is asymmetrical, of course.

Sheldon
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Old 27th March 2012, 06:04 AM   #2208
Salas is offline Salas  Greece
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Was mild on ~zero gate current Latfet. Not for BJT output then.
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Old 27th March 2012, 06:54 AM   #2209
hahfran is offline hahfran  Germany
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Originally Posted by Lazy Cat View Post
VAS cascodes has 10 mA collector bias current, I don't think they ever go to off state ...

P.S. Vbe multiplier voltage is always present and quite constant, so no sign of FET crossover distortion. FET bias is independant from output stage and can be several hundred mA if you like. Voltage part of the circuit also has +/-10 V more PSU potential than the output stage.
Ah
how would you bias the FETs for class AB?
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Old 27th March 2012, 07:03 AM   #2210
hahfran is offline hahfran  Germany
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Wouldn't be so sure about that...... I'm sure they are around, they just don't seem to meet the commercial standard for some reason.

BTW, I agree with you about the non-switching Class AB. I have tried this one too, over some years, but never found the benefits I expected sonically. I really don't think that the switching of the output stage is such a big problem as long as it's clean and well controlled.

A good compromise is to ensure that the drivers never switch off.

Hugh
It could well be that the idea of non switching class AB does not yield any sonic superiority, agreed. But I think the idea of only current amplification all over should have audible benefits . ( And hence a tube BJT hybrid !)
if it were not about those tubes made in China and Russia having unknown quality I had possibly thought deeper on a kind of "current dumping" tube BJT hybrid where the tube supplies the voltage and the BJT supplies the current.
BJTs are unbeatable if they do what they can do best, current amplification.
Same stuff with speakers, speakers are current controlled not voltage controlled.
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