You are right Upupa Epops, 99 out of 100 amps have higher or same voltage in VAS as output.Upupa Epops said:Pa with gain cca 45 dB is nonsens... Second nonsens is using of higher voltage at output stage then at VAS, mainly if output stage is made with lateral mosfets... I'm sorry, my friend....
But not this one.
If it is bad is when the output stage start to put out voltage near to VAS.
Gradually VAS can not drive output no more.
With this type you have to sure that you will not output any higher voltages.
Because MOSFET need maybe like 6-8 V G-S to drive, and the VAS transistors needs at least a couple of Volt, too.
In this case we shouldnt output more than like 50 Volt - 10 Volt = 40 Volt.
Seems like a waste of COOLING, because output current will create
an additional power across output MOSFETS
Current x (55V-40V) = Ix15.
Say peak current is (40V/4 Ohm) 10A.
Will be alot of waste of power to cool down.
But as long as we can put good cooling and do not put out more than 35-40V peak, nothing really bad will happen.
Personally, just like Upopa, I do not understand why VAS voltage supply should be lower.
Better would be if VAS was 5-10 volt HIGHER than output supply.
As to what I can see it.
Optimal value of voltage at VAS, in case when in OS are MOSFETs, is cca 7 - 8 V higher ( depend on concrete connection of VAS ). This voltage must be floating with voltage at OS, so you must to use power supplies connected in SERIES, not fixed regulated voltage for VAS ( in case, when voltage at OS isn't regulated ).
In the article JLH shows a justification of the +/-50V VAS supply - it's based around sufficient voltage to achieve 80W in 8R output. He does not mention why he chose 55V for the output stage, my guess is that he designed around the lowest dropout voltage he could reasonably get away with.
IMHO the o/p stage regulation is unneccessary (I also think his practice of deriving the positive reference off the negative rail and vice versa might lead to supply rail modulation, although I doubt this really matters in practice). JLH also did a 1989 acticle where he changed the regulator pass trs to FETs.
I did find it very beneficial to improve the VAS regulators though.
I got the chance to compare my version with the offical Hart one at Shaun's house. I thought mine was slightly better and Shaun thought his was slightly better (predictable?), the offical one was a bit more laid back sounding.
Oh and BTW, I built mine without the active balance control and with a gain of 50 IIRC.
Unfortunately I dissasembled mine to use the parts for other projects. It would be interesting to hear it again, in comparison with my later projects.
IMHO the o/p stage regulation is unneccessary (I also think his practice of deriving the positive reference off the negative rail and vice versa might lead to supply rail modulation, although I doubt this really matters in practice). JLH also did a 1989 acticle where he changed the regulator pass trs to FETs.
I did find it very beneficial to improve the VAS regulators though.
I got the chance to compare my version with the offical Hart one at Shaun's house. I thought mine was slightly better and Shaun thought his was slightly better (predictable?), the offical one was a bit more laid back sounding.
Oh and BTW, I built mine without the active balance control and with a gain of 50 IIRC.
Unfortunately I dissasembled mine to use the parts for other projects. It would be interesting to hear it again, in comparison with my later projects.
I agree it is unnecessary high for most of today's sources.Upupa Epops said:Is free, if it is 42 or 45 dB, both values are too high...
Can be lowered. Not too difficult.
We must remember that when first version of this amp came out,
there was not sources with CD-standard or any higher output voltages.
It was tape recorders and turnatables with phono amp circuits.
Use of nominal input of amplifiers was often 0.3 - 0.5 Vrms.
And input impedance was generally 100 kOhm and mostly at least 47 kOhm.
Later on came the the standard to be like 1 Vrms input into 22 kOhm,
because the voltage and current drive capabilities of sources was better.
Use of good output stages of CD-players and the CD output standard 2 Volt rms
has changed the need of gain and input impedance of power amplifiers.
Use of separate preamplifiers/output amplifiers has adopted to better drive capability of most sources that connects to power amps.
If John Linsley-Hood should be able to re-release this power amp today
he would change value of several components, including the gain of this amplifier.
Now he is dead.
Hi Upupa,
your comment in post50,
My Sugden P128 monoblocks have done exactly as you have said. Volt amp stage 8V higher (81Vdc) than output stage (73Vdc) and planted in series on output rails.
So when output rails sag under load they bring down the volt amp rails in sympathy. But why is this necessary?
What would be wrong with regulated rails for the volt amp stage and unregulated (sagging) for the output stage?
your comment in post50,
Why?
My Sugden P128 monoblocks have done exactly as you have said. Volt amp stage 8V higher (81Vdc) than output stage (73Vdc) and planted in series on output rails.
So when output rails sag under load they bring down the volt amp rails in sympathy. But why is this necessary?
What would be wrong with regulated rails for the volt amp stage and unregulated (sagging) for the output stage?
It is good for stability of amp by clipping, Andrew. I'm using in VAS antisaturation circuit, which cause, that near the clipping fall voltage gain of VAS down and VAS is not able to drive all structure of amp into deep saturation. Condition of correct function of this circut is, that at the first must go into the clipping VAS, not output stage ( you must imagine, that at the clipping is amp like by disconnect feedback, which cause rising of gain of all amp and all structure is " pushed " by OLG into deep saturation, certainly only by " normal " connection without antisaturation circuit . Result is, that clipping is quite clear, without any oscilations and you can to use high OLG ( which is good for low distortion ).... But condition of correct function is good designed proportion of both voltages ( at VAS and OS ).... Antisaturation circuit is very fine thing and I can't imagine good designed amp without it....
Hi Upupa,
not taking the output devices into deep saturation allows them to respond more quickly when the amp comes out of clip. Correct?
What if the amp is never allowed to clip? Does the rail voltage argument still hold? Does it apply at all?
Does the deep saturation only apply to BJT output devices? or does it also apply to FET devices?
not taking the output devices into deep saturation allows them to respond more quickly when the amp comes out of clip. Correct?
What if the amp is never allowed to clip? Does the rail voltage argument still hold? Does it apply at all?
Does the deep saturation only apply to BJT output devices? or does it also apply to FET devices?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.