Peranders,
I'm intrigued as to why you would have the thought the AKSA might be a piece of crap. When I first read Hugh's website, I was sufficiently interested to investigate in some depth and eventually lashed up a version of his circuit. I was shocked how musical the amp sounded, I then bought the real thing and was even more impressed. I did manage to find a few ways to improve the sound further and I still have some concerns over minor aspects of the implementation, however it's the best sounding amp I have built so far. In the end what matters to me is good sound, good reliability and sensible price.
I have also built a P3A and although it's not quite as good sonically it is a bit cheaper to build and it easily meets the 3 requirements.
See:
http://diyaudio.com/forums/showthread.php?threadid=21305&perpage=15&pagenumber=3
Graham,
I can assure you that the P3A has no switch on thump and a barely detectable switch off thump (ear to the bass unit!!). If I could be bothered I would post a scope photo.
Could you explain the mechanism by which the bootstrap cap causes a thump.
In practice you will find that the diff pair keeps the output near zero volts as the PSU rails reduce, it's only when the diff amp loses control that the output can "thump". In the AKSA there is a bass "whoosh" at turn off because the diff pair is resistor fed and loses control of the output at a higher voltage than the cs fed front end in the P3A. If you calculate the power during the AKSA turn-off you will see that it is harmless (even to a directly connected tweeter).
By the way - has anyone posted any listening impressions for Rod's MOSFET amp versus the P3A or AKSA yet?
I'm intrigued as to why you would have the thought the AKSA might be a piece of crap. When I first read Hugh's website, I was sufficiently interested to investigate in some depth and eventually lashed up a version of his circuit. I was shocked how musical the amp sounded, I then bought the real thing and was even more impressed. I did manage to find a few ways to improve the sound further and I still have some concerns over minor aspects of the implementation, however it's the best sounding amp I have built so far. In the end what matters to me is good sound, good reliability and sensible price.
I have also built a P3A and although it's not quite as good sonically it is a bit cheaper to build and it easily meets the 3 requirements.
See:
http://diyaudio.com/forums/showthread.php?threadid=21305&perpage=15&pagenumber=3
Graham,
I can assure you that the P3A has no switch on thump and a barely detectable switch off thump (ear to the bass unit!!). If I could be bothered I would post a scope photo.
Could you explain the mechanism by which the bootstrap cap causes a thump.
In practice you will find that the diff pair keeps the output near zero volts as the PSU rails reduce, it's only when the diff amp loses control that the output can "thump". In the AKSA there is a bass "whoosh" at turn off because the diff pair is resistor fed and loses control of the output at a higher voltage than the cs fed front end in the P3A. If you calculate the power during the AKSA turn-off you will see that it is harmless (even to a directly connected tweeter).
By the way - has anyone posted any listening impressions for Rod's MOSFET amp versus the P3A or AKSA yet?
Listening Impressions at ESP
ESP - The Audio Pages
No summary as I haven't finished reading them myself.
ESP - The Audio Pages
No summary as I haven't finished reading them myself.
peranders said:
I think that (speaking of one's own experience) is very constructive.
My objection to GM is that more often or not, the guy gets out of his comfort zone and opines on things he obviously doesn't know, or argues against reality.
Had he just stated what he believes to be right and coveats that as his personal view and accepts the possibility that his view may / could be wrong, he would have gained a lot of respect and credibility.
And I will state steadfastly that none of the issues GM critized the P3A for was present in my build of the P3A. GM may be right that the P3A suffers from those issues but it is extremely unresponsible and unprofessional for him to say so without any back-up: building one himself would be a good starting point.
Opinions are like *********, everyone has one. Theories are nice to have but until and unless he has built that amp, I don't think he is qualified to judge its sound quality.
Ultrachrome - thanks for the link, I had not found this one.
On the P3A subject, even though there is no thump, I did find a couple of areas that the amp needed a bit of refinement. Here are some notes I took when I built it, of course my variations may have neccessitated the mods:
"The P3A is built on the standard pcb with some track modifications to allow dual mono construction. I used different power semiconductors – the VAS transistor (BD140) was replaced by 2SA1360, the drivers (BD139, BD140) were replaced by 2SA1306/2SC3298 and the output devices are 2SC2922/2SA1216. The Sanken output transistors were used to enable 4 Ohm loads to be driven with the 42V rails. The power supply for each channel consists of a 225VA 30-0-30VAC toroid with MUR820 diodes (one bridge per rail) and BHC ALS20A 10,000uF 63V capacitors (one per rail). I found it necessary to add a 10uF capacitor across the Vbe multiplier to provide sufficient drive to the negative side of the output stage when driving low impedance or capacitive loads. Using a bench power supply with 1ft long leads the on-board decoupling capacitors (0.1uF) were found to be inadequate, resulting in oscillation into capacitive loads. I replaced these with 56uF 63V Panasonic HFQ capacitors (in the chassis this may not be necessary due to the close proximity of the main reservoir capacitors, however I left them in place). I also added an output inductor and resistor to ensure stability into highly reactive loads, without this the amplifier could be provoked into oscillation."
On the P3A subject, even though there is no thump, I did find a couple of areas that the amp needed a bit of refinement. Here are some notes I took when I built it, of course my variations may have neccessitated the mods:
"The P3A is built on the standard pcb with some track modifications to allow dual mono construction. I used different power semiconductors – the VAS transistor (BD140) was replaced by 2SA1360, the drivers (BD139, BD140) were replaced by 2SA1306/2SC3298 and the output devices are 2SC2922/2SA1216. The Sanken output transistors were used to enable 4 Ohm loads to be driven with the 42V rails. The power supply for each channel consists of a 225VA 30-0-30VAC toroid with MUR820 diodes (one bridge per rail) and BHC ALS20A 10,000uF 63V capacitors (one per rail). I found it necessary to add a 10uF capacitor across the Vbe multiplier to provide sufficient drive to the negative side of the output stage when driving low impedance or capacitive loads. Using a bench power supply with 1ft long leads the on-board decoupling capacitors (0.1uF) were found to be inadequate, resulting in oscillation into capacitive loads. I replaced these with 56uF 63V Panasonic HFQ capacitors (in the chassis this may not be necessary due to the close proximity of the main reservoir capacitors, however I left them in place). I also added an output inductor and resistor to ensure stability into highly reactive loads, without this the amplifier could be provoked into oscillation."
Millwood, watch the language. I hate having to manually censor stuff.I've only one comment on the amplifier design which I'm surprised no-
one else has mentioned so far except for Rod-E in the original article.
To me the voltage inefficiency rankles and goes hand in hand
with limited performance into low (common nowadays) loads.
Rod-E thinks its an acceptable compromise, I don't, but JMO.
sreten.
one else has mentioned so far except for Rod-E in the original article.
To me the voltage inefficiency rankles and goes hand in hand
with limited performance into low (common nowadays) loads.
Rod-E thinks its an acceptable compromise, I don't, but JMO.
sreten.
Howdy Nelson...
I reckon a current compensated Vbe multiplier will more than compensate for such variation in practice.....
http://www.essex.ac.uk/ese/research...cs/J8 Optimization of the amplified diode.pdf
Nelson Pass said:
I reckon a current compensated Vbe multiplier will more than compensate for such variation in practice.....
http://www.essex.ac.uk/ese/research...cs/J8 Optimization of the amplified diode.pdf
Re bootstrapping, I've never used it but recalled Self had some comments about it's use. I checked and his motive seems to be that it "fools" the signal souce into "seeing" a suitably high input imedance while enabling one to use lower value resistors in the input and NFB return hence reducing Johnson noise. I don't know if this is Elliot's intension or not.
The description used above referrs to a bootstrapped input stage, which is a completely different usage of the technique. There are 2 forms of bootstrapping -
1) input stages, to obtain a high input impedance using low value resistors, and
2) VA Stages, where the bootstrap acts as a constant current source.
Obviously, it was not my intention in the MOSFET amp to boost the input impedance, and the technique is used to provide a constant current to the VAS.
A regulated supply is not necessary at all, and a great many of these amps have been built (and used) with no problems encountered (other than construction errors).
Two of the amps (supplied as fully built units) are installed in recording studios in Australia, with another about to follow suit. This has been after extensive listening and comparison with other 'high end' amps. The comments so far indicate that the amp has excellent detail and accuracy, two properties that are essential for studio monitoring applications. Obviously, the overall sound quality has been considered superior to other amps that have been auditioned.
As for 'low efficiency', this is not really true either. Admittedly, under heavy loading, the amp will not provide as much peak-to-peak swing as a bipolar amp (or a MOSFET amp with auxiliary supplies), but in terms of real losses, the difference only amounts to about 1dB. At (or near) speaker resonance (where the highest instantaneous swings occur) the difference is far less.
Naturally, all of the criticism that has been levelled at this amp is easily resolved by building and listening, but it is far easier to make negative comments based on what one has heard from someone else (who didn't build one either). Hardly a scientific approach
Cheers,
1) input stages, to obtain a high input impedance using low value resistors, and
2) VA Stages, where the bootstrap acts as a constant current source.
Obviously, it was not my intention in the MOSFET amp to boost the input impedance, and the technique is used to provide a constant current to the VAS.
A regulated supply is not necessary at all, and a great many of these amps have been built (and used) with no problems encountered (other than construction errors).
Two of the amps (supplied as fully built units) are installed in recording studios in Australia, with another about to follow suit. This has been after extensive listening and comparison with other 'high end' amps. The comments so far indicate that the amp has excellent detail and accuracy, two properties that are essential for studio monitoring applications. Obviously, the overall sound quality has been considered superior to other amps that have been auditioned.
As for 'low efficiency', this is not really true either. Admittedly, under heavy loading, the amp will not provide as much peak-to-peak swing as a bipolar amp (or a MOSFET amp with auxiliary supplies), but in terms of real losses, the difference only amounts to about 1dB. At (or near) speaker resonance (where the highest instantaneous swings occur) the difference is far less.
Naturally, all of the criticism that has been levelled at this amp is easily resolved by building and listening, but it is far easier to make negative comments based on what one has heard from someone else (who didn't build one either). Hardly a scientific approach
Cheers,
"The description used above referrs to a bootstrapped input stage, which is a completely different usage of "
<< That's suppossed to indicate embarrassment. That's what happens when I read posts on the fly (and try to comment) with a lot of local excitment and a lot of noise vs. signal around and behind me. Looking at it on a calm quiet morning, I find the correct D.Self reference which says the technique may look "old fashioned" but works as well more modern solutions. He makes the claim that very low speaker impedances can cause a problem, however, it ocurrs to me that this may be offset by the zobel.
In any case I think it is interesting to speculate on how the amp works, but rather silly to bad mouth it if you haven't built it. I don't want to be distracted from a couple of other things I'm struggling with currently or I would have my check in the mail to ESP for a rair of boards. I enjoyed the P3A and expect this one as well will perform better than seems reasonable from such a semi-minimalist design.
<< That's suppossed to indicate embarrassment. That's what happens when I read posts on the fly (and try to comment) with a lot of local excitment and a lot of noise vs. signal around and behind me. Looking at it on a calm quiet morning, I find the correct D.Self reference which says the technique may look "old fashioned" but works as well more modern solutions. He makes the claim that very low speaker impedances can cause a problem, however, it ocurrs to me that this may be offset by the zobel.In any case I think it is interesting to speculate on how the amp works, but rather silly to bad mouth it if you haven't built it. I don't want to be distracted from a couple of other things I'm struggling with currently or I would have my check in the mail to ESP for a rair of boards. I enjoyed the P3A and expect this one as well will perform better than seems reasonable from such a semi-minimalist design.
I built one and I posted some info here.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=33189&highlight=
I couldn't care less what technology is in the amp........ as long as it sounds great, I don't care and it does sound great.
I don't know how it compares to the P3A but there has been a bit of discussion on the ESP forum.
http://www.instantboard.com/users/rode/
http://www.diyaudio.com/forums/showthread.php?s=&threadid=33189&highlight=
I couldn't care less what technology is in the amp........ as long as it sounds great, I don't care and it does sound great.
I don't know how it compares to the P3A but there has been a bit of discussion on the ESP forum.
http://www.instantboard.com/users/rode/
Bootstrapped VAS
What I learn from AKSA (thanks Hugh), is that bootstrapped VAS produces nice to hear amp. This is due to harmonic produced, and behavior for low frequencies.
The designers using this, like AKSA or RodE, are very clever guy. They are not putting bootstrapped VAS because they dont know there is CCS VAS existed. They put it there on purpose.
But is this implementable to small-medium size power amp only? Why I never see bootstrapped VAS on high output power amp, like pro power amp? Is it when we are into the area of huge rating watt, CCS VAS is a must?
I also dont understand about TRANSCONDUCTANCE in mosfets. I can read the definition, but cannot imagine what is it. HFE is easy to understand, it is amplifying factor. What is TRANSCONDUCTANCE? What is the difference between one mosfet has low transconductance, and other mosfet have high transconductance? The transconductance figure is somehow inverted. One with 0.3 is lower transconductance than one with 0.03 transconductance? Is the gain factor of mosfet depends on load resistance (from transconductance formula)?
Other question (maybe someone can help). IF I wanted to put 5 pairs of K1058 for final output stage, with rail about +/-80V, what is the bias current needed in the VAS enough for driving all output stage?
What I learn from AKSA (thanks Hugh), is that bootstrapped VAS produces nice to hear amp. This is due to harmonic produced, and behavior for low frequencies.
The designers using this, like AKSA or RodE, are very clever guy. They are not putting bootstrapped VAS because they dont know there is CCS VAS existed. They put it there on purpose.
But is this implementable to small-medium size power amp only? Why I never see bootstrapped VAS on high output power amp, like pro power amp? Is it when we are into the area of huge rating watt, CCS VAS is a must?
I also dont understand about TRANSCONDUCTANCE in mosfets. I can read the definition, but cannot imagine what is it. HFE is easy to understand, it is amplifying factor. What is TRANSCONDUCTANCE? What is the difference between one mosfet has low transconductance, and other mosfet have high transconductance? The transconductance figure is somehow inverted. One with 0.3 is lower transconductance than one with 0.03 transconductance? Is the gain factor of mosfet depends on load resistance (from transconductance formula)?
Other question (maybe someone can help). IF I wanted to put 5 pairs of K1058 for final output stage, with rail about +/-80V, what is the bias current needed in the VAS enough for driving all output stage?
beta or hfe is the ratio of output current to input current.
transconductance is the ratio of output current to input voltage
transresistance is the ratio of output voltage to input current
voltage gain is the ratio of output voltage to input voltage
In the case of a mosfet the input voltage is the voltage between gate and source. The output current is the drain current. Because of biasing requirements the transconductance usually refers to the slope of the Id/Vgs curve at a particular Id.
Most cases of rail sticking I've seen are due to slow saturation
recovery of bipolar devices. This often appears in amplifiers
where the front end Vas has a separate (higher) supply.
Apart from avoiding saturation altogether, it's best to make the
fastest devices in the chain saturate first, avoiding saturation
in the slower devices. Alternatively, you look at the use of a
Baker clamp (look it up).
recovery of bipolar devices. This often appears in amplifiers
where the front end Vas has a separate (higher) supply.
Apart from avoiding saturation altogether, it's best to make the
fastest devices in the chain saturate first, avoiding saturation
in the slower devices. Alternatively, you look at the use of a
Baker clamp (look it up).
Nelson Pass said:
One would've thought the opposite is the case...!?
Nelson Pass said:
Apart from avoiding saturation altogether, it's best to make the
fastest devices in the chain saturate first, avoiding saturation
in the slower devices. Alternatively, you look at the use of a
Baker clamp (look it up).
Tried baker clamp....distortion unacceptable....
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