Thanks for all replies. I have been thinking some more about this
and I am somewhat less confused about the design now. On
the other hand, I think I am quite confused about what exactly
I am/was confused about. 🙂
I guess the phenomenae that confused me are more common
than I realized, they just became more obvious to me in this
particular design. I realize now that it does not matter if we
have a symmetrical push-pull VAS or a non-symmetrical one
loaded with a CCS. I didn't think of that before. The issue then
is basically whether we have a purely capacite load or a
mainly resistive load. Perhpas it doesn't matter to have a
purely capacitive load, as long as we don't get current
starving, but one wonders how/if it affects the sound, which
Nelson also remarked on. Of course, we will usually get
a dominant capacitive load above some frequency also when
we load the VAS with resistors or at least BJTs, and if this
does not happen above the audible band then the same
concerns apply here, I suppose. This latter case I agree appears
also in most op amps, but usually not the purely capacitive load
case. Well, back to the books and the drawnig board now, I
guess, with a somewhat better understanding of this.
and I am somewhat less confused about the design now. On
the other hand, I think I am quite confused about what exactly
I am/was confused about. 🙂
I guess the phenomenae that confused me are more common
than I realized, they just became more obvious to me in this
particular design. I realize now that it does not matter if we
have a symmetrical push-pull VAS or a non-symmetrical one
loaded with a CCS. I didn't think of that before. The issue then
is basically whether we have a purely capacite load or a
mainly resistive load. Perhpas it doesn't matter to have a
purely capacitive load, as long as we don't get current
starving, but one wonders how/if it affects the sound, which
Nelson also remarked on. Of course, we will usually get
a dominant capacitive load above some frequency also when
we load the VAS with resistors or at least BJTs, and if this
does not happen above the audible band then the same
concerns apply here, I suppose. This latter case I agree appears
also in most op amps, but usually not the purely capacitive load
case. Well, back to the books and the drawnig board now, I
guess, with a somewhat better understanding of this.
Christer:
Did you ever look at the Leach Low TIM Power Amp Proejct web site
It may help you gain some addtional understanding.
mlloyd1
Did you ever look at the Leach Low TIM Power Amp Proejct web site
It may help you gain some addtional understanding.
mlloyd1
mlloyd1 said:
Good idea! I have downloaded the circuit diagram before but
I don't think I ever read the comments on the design. It is not
immediately obvious how related it is to my specific "problem"
since the Leach amp uses BJTs, but on the other hand, current-
starving is one of the interesting issues here, so Leach should
have something to say on that I guess. I'll have a look at it.
Thanks.
Christer:
I was interested in using Opti-MOS to drive an ESL, so I asked Randy about that, and he said:
By the way, his previous book explains the thoughts behind his design in much more detail than his later design. I'd recommend you buy the book(s) if you are at all interested in his design, even if you don't want to build it as is
Take that for what it's worth.
I'm currently building six (yes, six) for a home theater setup. I have most of the boards soldered up but am scrambling to find misc parts (now I need some TO-3p silipad insulators ...)
I'll let everyone know how it goes, once it goes ...
I was interested in using Opti-MOS to drive an ESL, so I asked Randy about that, and he said:
By the way, his previous book explains the thoughts behind his design in much more detail than his later design. I'd recommend you buy the book(s) if you are at all interested in his design, even if you don't want to build it as is
Take that for what it's worth.
I'm currently building six (yes, six) for a home theater setup. I have most of the boards soldered up but am scrambling to find misc parts (now I need some TO-3p silipad insulators ...)
I'll let everyone know how it goes, once it goes ...
"Now to answer your questions. John, the OPTI-MOS amplifiers will drive electrostatic speakers better than any amplifier that I am aware of. They have a phase margin of only 120-degrees, and I have tested these amps with only 1/2 of the specified compensation values with 2.2-uF aluminized-poly caps in direct parallel to the typical audio loads (i.e. 8-ohm and 4-ohm) without a hint of instability. They also remain stable with any load impedance down to a virtual short. The majority of audio power amplifiers, to be quite frank, will not even survive such testing."
I am of the belief that the majority of the amplifiers out there
will meet this criteria.
I am of the belief that the majority of the amplifiers out there
will meet this criteria.
I don't disagree, I think anything a vendor says has to be taken with a grain of salt 😀
I have heard however, that some ESL's will definitly smoke some amps, so although most of them probalby work, there's some anecdotal evidence that some, definitly, will not, although I don't have personal experience with this.
Ironically, I heard one of your amps last week drive a set of Martin Logans. Nice! 🙂
I have heard however, that some ESL's will definitly smoke some amps, so although most of them probalby work, there's some anecdotal evidence that some, definitly, will not, although I don't have personal experience with this.
Ironically, I heard one of your amps last week drive a set of Martin Logans. Nice! 🙂
Prof Leach writes:
"the bias current in each output transistor is in the 40 to 45 mA range. This is enough current to cause all crossover distortion on the output voltage waveform with an 8 ohm load to essentially disappear. This was determined by using an oscilloscope to observe the output waveform from a distortion analyzer connected to the amplifier."
Does anyone know what this guy is a Professor of?
"the bias current in each output transistor is in the 40 to 45 mA range. This is enough current to cause all crossover distortion on the output voltage waveform with an 8 ohm load to essentially disappear. This was determined by using an oscilloscope to observe the output waveform from a distortion analyzer connected to the amplifier."
Does anyone know what this guy is a Professor of?
I have no reason to disbelieve that statement.
His project description is very analytical but I am not sure that the measurement methodologies he chooses correlate that well to sound quality. Nor does he discuss this. Perhaps that isn't the primary goal of his project, and that is fine.
His project description is very analytical but I am not sure that the measurement methodologies he chooses correlate that well to sound quality. Nor does he discuss this. Perhaps that isn't the primary goal of his project, and that is fine.
traderbam said:
I must comment this title thing. Professor means different in different countries. In USA (correct me if I'm wrong) is a professor a teacher but in Sweden a professor is a chief of a institution at an university. In Sweden doesn't a professor title automaticly mean = highly skilled....but some professors think that about themsleves....😉
It's OK, I was kidding. 🙂
If you check his web site, he's published and taught some classes on audio design, and low noise design - interesting, as most EE work now days is not in discrete design, it's in VLSI or analog integration.
If you check his web site, he's published and taught some classes on audio design, and low noise design - interesting, as most EE work now days is not in discrete design, it's in VLSI or analog integration.
It seems that every university's EE department has some
old fogey who remembers the good old days of tubes, and
they let him teach the one analog course.
😉
old fogey who remembers the good old days of tubes, and
they let him teach the one analog course.
😉
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