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PROS? High Voltage VS Low Voltage Amp
PROS? High Voltage VS Low Voltage Amp
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Old 23rd September 2017, 05:01 PM   #101
anatech is offline anatech  Canada
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PROS? High Voltage VS Low Voltage Amp
Hi SMY14INCH,
In my 40 or so years in audio service, the link between temperature and defects has been demonstrated practically time and time again. As well, 5 years is nothing. There has been high numbers of computers break down do to temperatures being too high. I recently had a modified Hafler DH-110 come in. The signal transistors ran far too hot. Possibly like your own preamp does. It needs a set of transistors as they are all suspect now with a few failing intermittently. High temperatures are the enemy to trouble-free performance in electronics. 3 months is nothing. I'm also working on 40 year old products. That is reliability.
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For my 3 pairs class ab vs 7 pairs output class ab. Both situations idle current was kept excatly the same.
Per transistor, or total?
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Can hear a big diffrence when going from 5to 6 pairs,
Of course! This has zero to do with temperature and everything to do with making the change of current with the signal in each transistor a lower percentage. You also gain a lower output impedance doing this. These things are not affected by how hot the transistor is unless it dies. Try not to confuse current flow with temperature.

-Chris
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Old 24th September 2017, 06:46 AM   #102
HI-FI PRO is offline HI-FI PRO  China
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Total bias current dosen't change from 3 pairs upgraded to 7 pairs.
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Old 28th September 2017, 10:53 AM   #103
jan.didden is offline jan.didden  Europe
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Just by coincidence (I believe) in EDN:

A negative feedback model for transistors with emitter/source degeneration | EDN

Jan
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Old 28th September 2017, 11:35 AM   #104
DF96 is offline DF96  England
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Originally Posted by traderbam
I have never before heard anyone describe a resistor as a NFB device.
Who else believes this? Because it has to be if an emitter follower is too.
So who is bold enough to confess and justify why?
As I said, I was merely showing where your logic leads. All circuits can be analysed using circuit theory alone. Many circuits (almost all circuits, as I have shown!) can also be analysed using feedback theory. Your argument was that as an emitter follower (at low frequencies - although you didn't say that) can be analysed using circuit theory then it cannot use feedback. This is logically false, because it assumes that any circuit can have one and only one analysis. I merely demonstrated that a circuit using just resistors can be analysed using feedback theory, thus (according to your logic) it must be analysed using feedback theory alone and not circuit theory.

The advantage of using feedback theory (where it is appropriate) is that it gives useful insights into circuit operation (e.g. stability) which would be harder to determine (although still possible) using pure circuit theory. Feedback theory is certainly applicable to an emitter follower (as I have demonstrated); it is appropriate to an emitter follower because it gives insights about stability and input impedance and distortion which might be harder to obtain from the naive 'potential divider' model. In any case, it is blindingly obvious (to almost everyone) that an EF subtracts the whole output signal from the input and therefore has 100% feedback.

What is slightly more subtle, and so catches out more people, is that emitter degeneration (with the output taken from the collector) is also feedback.
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Old 28th September 2017, 11:45 AM   #105
jan.didden is offline jan.didden  Europe
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Originally Posted by DF96 View Post
What is slightly more subtle, and so catches out more people, is that emitter degeneration (with the output taken from the collector) is also feedback.
Actually I find that rather obvious, if you realize that the power supply rail is AC ground; connecting the top of Rc with the bottom of Re and voilá - a regular feedback divider ;-). It even follows the familar gain equation!

Only shows that any attempt to fool mother nature is doomed at its outset!

Jan
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Old 4th October 2017, 09:40 PM   #106
Mrcloc is offline Mrcloc  South Africa
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Originally Posted by HI-FI PRO View Post
amps I design are able to get a little bit more perfomance when having absolutley fixed vcc to keep precise amp biasing precise.
Interesting. My design goals are that varying supply doesn't affect biasing, and this is precisely to get the best performance possible by rejecting power supply changes. In fact, neither driver transistors nor VAS transistors need to be the same, let alone matched (although the same transistors should be used).

My latest design can be used with 12-0-12 transformer up to 18-0-18 without changing a single component. Biasing should remain constant. The only thing that changes is output power.

But then again, class A is a different art.
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Old 5th October 2017, 04:31 AM   #107
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Originally Posted by HI-FI PRO View Post
amps I design are able to get a little bit more perfomance when having absolutley fixed vcc to keep precise amp biasing precise.
Just so you guys do not get confused I am talking about having fixed vcc so voltage gain stage transitor bias can stay in best optimum operating point for extra sound.
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Old 5th October 2017, 08:05 AM   #108
Mrcloc is offline Mrcloc  South Africa
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Just so you guys do not get confused I am talking about having fixed vcc so voltage gain stage transitor bias can stay in best optimum operating point for extra sound.
Yes, but why? Isn't your VAS controlled by a current source? I know you can use bootstrapping and other resistive current sources, but a current source in the VAS is the absolute bees knees.
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Old 5th October 2017, 08:09 AM   #109
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Yes, but why?
This is one factor out of many that will make my amp diffrent and sound better than others.
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Old 5th October 2017, 08:13 AM   #110
Mrcloc is offline Mrcloc  South Africa
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Originally Posted by HI-FI PRO View Post
This is one factor out of many that will make my amp diffrent and sound better than others.
No, I mean why does Vcc affect bias? Bias should be completely independent from Vcc for this optimal quality.

Either way, how do you determine that bias point?

ETA: Vcc dependent bias point is (IMO) a recipe for oscillation.
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