I don't really understand why mirrors....
hi guys,
I don't really understand why current mirrors in the LTP
affect or degrade sound quality to some extent.
actually im using here at home an amp based from some Self, Slone,
and other known guys.
Self says, putting current mirrors on the LTP's forces the two
BJT's to share almost equal currents and at the time lowers down
the overall THD of the system, Slone seconded this one, therefore
persuaded me to do the same on my amp. Actually I used this amp
for my two subwoofers at 4ohm each and the output was
tight and dynamic.
But as what the thread says, that mirrors also affect
sound quality, the question is, how is this so ?
<graham maynard> says as follows:
There is nothing to worry about when mirroring a differential stage - if - the three transistor arrangement is used. A perfect dc balance can be set up where the VAS bias current is matched by the third transistor (base plus base-emitter resistor) current. The resistor is likely to be between 4k7 and 10k in value, and the arrangement minimises distortion, offset error also output impedance. As attached;->
I dont understand those lines, attaching some images may clarify them.
.
regards,
hienrich
__________________________
"to learn is to be ignorant......"
hi guys,
I don't really understand why current mirrors in the LTP
affect or degrade sound quality to some extent.
actually im using here at home an amp based from some Self, Slone,
and other known guys.
Self says, putting current mirrors on the LTP's forces the two
BJT's to share almost equal currents and at the time lowers down
the overall THD of the system, Slone seconded this one, therefore
persuaded me to do the same on my amp. Actually I used this amp
for my two subwoofers at 4ohm each and the output was
tight and dynamic.
But as what the thread says, that mirrors also affectsound quality, the question is, how is this so ?
<graham maynard> says as follows:
There is nothing to worry about when mirroring a differential stage - if - the three transistor arrangement is used. A perfect dc balance can be set up where the VAS bias current is matched by the third transistor (base plus base-emitter resistor) current. The resistor is likely to be between 4k7 and 10k in value, and the arrangement minimises distortion, offset error also output impedance. As attached;->
I dont understand those lines, attaching some images may clarify them.
.
regards,
hienrich
__________________________
"to learn is to be ignorant......"
I have a small collection of power amps: 4 class ABs using bipolars, 1 class A with valves, 1 SE class A with FETs.
The SE class A FET amp ranks bottom for sound quality.
The class A PP valve amp and one of the bipolar class AB amps rank = 1st
My conclusion is that design matters much more than class of operation.
The SE class A FET amp ranks bottom for sound quality.
The class A PP valve amp and one of the bipolar class AB amps rank = 1st
My conclusion is that design matters much more than class of operation.
"Class A" has come to have great value as marketing hype. A lot people buying high-end audio gear have little or no idea what it means. They think it is some kind of consumer rating like "Grade A eggs". No one design feature trumps the others; you could have in input section with all the bells and whistles giving you truly unmeasurable, inaudible noise and, distortion and blowing it all away in the output section. Or the other way around -- which is what I think some "marketeer's Class A" designs have done.
Same with current mirrors I've listened to amps with and without that sounded the same.
The heat isn't really an issue for a reasonable sized stereo pair of 100 watts per channel. Even somebody that really likes their music loud would be happy with that.
A well executed AB design sounds just as good as a well executed A design. Class A just doesn't sound better, I'm not saying it sound worse, but it doesn't sound worse.
A well executed AB design sounds just as good as a well executed A design. Class A just doesn't sound better, I'm not saying it sound worse, but it doesn't sound worse.
Hi Heinrich.
When I wrote the words you have quoted, they were about the three transistor mirror. They related to an earlier comment that a two transistor mirror can be imbalanced by VAS stage base current.
A 2 or 3 or 4 transistor mirror introduces a change in HF propagation phase angle. Whether this has any impact upon performance depends upon the rest of the circuit; i.e. the number of active stages, including CCSs, also stabilisation arrangements.
It is very easy for some folk to associate their experience of poor sound with a particular circuit arrangement, and introducing a mirror without watching for internal phase changes - where there had not been a mirror before - could actually introduce problems.
My view is that a mirrored differential input stage can be better than a plain un-mirrored differential, but it is not automatically so.
Not all class-A amps automatically sound good either.
As you say Sam ....... they all need to be *well executed*.
Cheers ........... Graham.
When I wrote the words you have quoted, they were about the three transistor mirror. They related to an earlier comment that a two transistor mirror can be imbalanced by VAS stage base current.
A 2 or 3 or 4 transistor mirror introduces a change in HF propagation phase angle. Whether this has any impact upon performance depends upon the rest of the circuit; i.e. the number of active stages, including CCSs, also stabilisation arrangements.
It is very easy for some folk to associate their experience of poor sound with a particular circuit arrangement, and introducing a mirror without watching for internal phase changes - where there had not been a mirror before - could actually introduce problems.
My view is that a mirrored differential input stage can be better than a plain un-mirrored differential, but it is not automatically so.
Not all class-A amps automatically sound good either.
As you say Sam ....... they all need to be *well executed*.
Cheers ........... Graham.
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