Re: Re: Re: Cgd
Hi Edmond,
I think we agree on the concern in regard to dramatic increases in Cgd at low Vds. If you feel that keeping the MOSFETs 5V from the rail is not the best way to deal with the problem, I understand. My amps don't oscillate anyway even when I let them go to the rail. I just like Baker clamps as a general rule. For one thing, if the output tries to slam against the rail, one does not get polluted rail noise and ripple into the signal when using a suiably designed Baker clamp.
The other thing is that those extra five volts in a high powered amplifier don't really mean a lot in the real world, so why kill yourself to get them. An amplifier that can swing to a 50V peak will produce 156 watts into 8 ohms. One that can swing to 55V will produce 189 Watts. This is only 1 dB of difference. Or, putting it another way, the extra beef in the power supply and heat sinks to get the same peak power without going within 5V of the rail is rather insignificant.
For similar reasons, I am also a big fan of soft clipping in amplifiers.
Cheers,
Bob
PS: don't ignore the increase in Ccb of BJTs at low Vce.
Edmond Stuart said:
Hi Edmond,
I think we agree on the concern in regard to dramatic increases in Cgd at low Vds. If you feel that keeping the MOSFETs 5V from the rail is not the best way to deal with the problem, I understand. My amps don't oscillate anyway even when I let them go to the rail. I just like Baker clamps as a general rule. For one thing, if the output tries to slam against the rail, one does not get polluted rail noise and ripple into the signal when using a suiably designed Baker clamp.
The other thing is that those extra five volts in a high powered amplifier don't really mean a lot in the real world, so why kill yourself to get them. An amplifier that can swing to a 50V peak will produce 156 watts into 8 ohms. One that can swing to 55V will produce 189 Watts. This is only 1 dB of difference. Or, putting it another way, the extra beef in the power supply and heat sinks to get the same peak power without going within 5V of the rail is rather insignificant.
For similar reasons, I am also a big fan of soft clipping in amplifiers.
Cheers,
Bob
PS: don't ignore the increase in Ccb of BJTs at low Vce.
Re: Re: Re: Re: Cgd
Hi Bob,
According to my simulations, your amp (and mine) severely rings when Vds < 2.5V. OK, it doesn't oscillate, but it isn't very stable either. So we do have a serious problem.
If you allow yourself to trivialize the waste of 5V (or 33W) by calling it a difference of only 1dB, then you will certainly allow me to exaggerate the case by giving another example:
Suppose we have single supply of only 12V (from a car battery for example) and we clamp the amp at the rails minus 5V, then 2Vpk-pk is left or 1/8W rms into 4 Ohm. If we allow the MOSFETs to let them go to the rail voltage minus 0.5V, then we get 11Vpk-pk or 3.78W rms into 4 Ohm. That's 30 times more output power and a little bit more than only 1dB.
Cheers, Edmond.
Bob Cordell said:
Hi Bob,
According to my simulations, your amp (and mine) severely rings when Vds < 2.5V. OK, it doesn't oscillate, but it isn't very stable either. So we do have a serious problem.
If you allow yourself to trivialize the waste of 5V (or 33W) by calling it a difference of only 1dB, then you will certainly allow me to exaggerate the case by giving another example:
Suppose we have single supply of only 12V (from a car battery for example) and we clamp the amp at the rails minus 5V, then 2Vpk-pk is left or 1/8W rms into 4 Ohm. If we allow the MOSFETs to let them go to the rail voltage minus 0.5V, then we get 11Vpk-pk or 3.78W rms into 4 Ohm. That's 30 times more output power and a little bit more than only 1dB.
Cheers, Edmond.
Hi,
these last few posts have reminded me of JLH's 80W mosfet amp.
His schematics show 5V extra on the output FETs than on the supplies to the volt amp stage.
I had thought this was a misprint, but you now have me wondering if the late JLH had deliberately staggered these supply voltages because it sounded better that way.
these last few posts have reminded me of JLH's 80W mosfet amp.
His schematics show 5V extra on the output FETs than on the supplies to the volt amp stage.
I had thought this was a misprint, but you now have me wondering if the late JLH had deliberately staggered these supply voltages because it sounded better that way.
AndrewT said:
Hi Andrew,
The answer to that is "no" it wasn't done in any attempt at improving the sonic results of that amp.
Interestingly, if you look at his earlier 80-100W (W.W. Aug.1982) quite similar design, he showed the difference the opposite way around, with 65V rails for the front end, and 60V for the output devices.
In private discussions, JLH made it quite clear to me that these voltage variations were more 'accidental' than due to any deliberate design decisions on his part.
At around that time (i.e. late '80s) I carried out a lot of intense listening tests on JLH's amp designs at his suggestion, and reported the results to him frequently. I found that varying this relationship either way within sensible reason (just a few volts DC) didn't appear to have much effect on the sonics of those amps.
I found plenty of other circuit changes which most certainly did, though!
Regards,
abzug said:
Yeah!
The fact that they dribble a little bit, usually smell like wee, blindly and haphazardly drive their Volvo's and Nissan Pulsars at a top speed of 6 mph and generally suck society dry via their pension benefits doesn't mean that they are silly.
Lumanauw, that's an awfully unkind thing to imply.
Nelson Pass said:
lumanauw said:Happy birthday, Edmond
I'm surprised. I don't know anyone age 65 so good with simulators and so many ideas.
Hi Nelson, Lumanauw Bobby & Glen,
Thank you!
BTW, Don't worry, I don't take lumanauw's words the wrong way. I know him a little bit longer and I'm sure these words were meant as a compliment.
Cheers, Edmond.
abzug said:
So which model Volvo do you drive then?
Edmond Stuart said:
Hi Glen,
You're right, if geezers "Urinat In Ventum" they do smell like wee.
Fortunately, homo sapiens.....
Cheers, Edmond.
Knowing very little Latin, I did an Internet search of your signature a little while ago. I really wish I hadn't clicked on the link entitled "pictures of........"
BTW, Happy Birthday
Cheers,
Glen
Edmond, even though I studied Latin in high school, 50 years ago, I could not completely figure out what the phrase meant, but the internet told me immediately.
I recall on a mountain in Norway with a 50 MPH wind and having to relieve myself, when I was even younger than many of our critics on this website.
Of course, I TRIED to go, away from the wind, but the vortex blew it back onto my sports jacket. What a mess! Reminds me of myself, today.
I recall on a mountain in Norway with a 50 MPH wind and having to relieve myself, when I was even younger than many of our critics on this website.
Of course, I TRIED to go, away from the wind, but the vortex blew it back onto my sports jacket. What a mess! Reminds me of myself, today.
john curl said:Edmond, even though I studied Latin in high school, 50 years ago, I could not completely figure out what the phrase meant, but the internet told me immediately.
I recall on a mountain in Norway with a 50 MPH wind and having to relieve myself, when I was even younger than many of our critics on this website.
Of course, I TRIED to go, away from the wind, but the vortex blew it back onto my sports jacket. What a mess! Reminds me of myself, today.
Hi John,
That's quite a story
Corollary:
Homo Sapiens Non Urinat In Vortex
Cheers, Edmond.
