Slewmaster - CFA vs. VFA "Rumble"

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#3,721
It has got drivers now.
Bimo, can you try to increase the followers' current to, say, 20mA (by decreasing R110) in your simulation?

Cheers,
Valery

P.S. My Lynx card is coming today, so I will be able to measure THD, IMD, etc. on my live prototype
 
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#3,725
I would also try different loads (4, 16, 32 ohm) and the whole thing but without MOSFETs (even with some high load - 100 kohm - or no load). Would be interesting to see dependencies...

In addition, I will PM you the SPICE models used in Multisim (licensed ones - non commercial use only) - worth to try and see if they are different...

Cheers,
Valery
 
#3,727
bimo said:
I try. Thank you for the suggestion.
Click to expand...

Higher the rails, mosfets needs much more headroom, approaching the clipping normally rises THD, also Rds ON has to be considered as a factor. Driver's bias at least 30 mA or more on a low Rgate-to-gate clamping resistor.

My real measurements shows:

THD (100 Wrms/4 Ohm, 10 kHz)
0,004 %

Now even better with the latest version. 😉
 
#3,728
LC, does your new design have drivers or is it driven straight from the VAS like VSSA. I finally finished my VSSA from the official boards, driven from a single 500VA transformer and MrEvil cap multiplier. I think there is a grounding problem. Do you suggest a ground lift?
 
#3,729
Lazy Cat said:
Higher the rails, mosfets needs much more headroom, approaching the clipping normally rises THD, also Rds ON has to be considered as a factor. Driver's bias at least 30 mA or more on a low Rgate-to-gate clamping resistor.

My real measurements shows:

THD (100 Wrms/4 Ohm, 10 kHz)
0,004 %

Now even better with the latest version. 😉
Click to expand...

Ok, thank you LC - good to know 😉
 
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#3,730
Ranchu32 said:
Love your work Valery.

I'm waiting to receive your tube boards from Terry; when I do, I plan to assemble them with this vertical mosfet Slewmaster 🙂
Click to expand...

Cool :up: 😎
Meanwhile think about the right power-up circuit - good practice is to switch-on the tube heating a bit in advance. In my implementation the sequence is: Heating on - 20 seconds - all-the-rest on - 10 seconds - check DC offset, if ok - speakers on.

Cheers,
Valery
 
#3,731
Ranchu32 said:
LC, does your new design have drivers or is it driven straight from the VAS like VSSA. I finally finished my VSSA from the official boards, driven from a single 500VA transformer and MrEvil cap multiplier. I think there is a grounding problem. Do you suggest a ground lift?
Click to expand...

In VSSA thread you could find the info, in short 10 Ohm GND lift resistor on VSSA PCB has to be shorted with wire. Fast-on GND connector on PCB serves two wires only, one connecting GND to PSU, second connecting GND to -SPK terminal. Input pads are connected to isolated RCA only, no other GND connection whatsoever.

First One has drivers, VSSA has high VAS bias.
 
#3,737
vzaichenko said:
Not bad at all already...
Another option to try - use the second EF, which is part of original schematic, with NJW0281/0302 transistors and 68 ohm (R113) between them. A REAL driver 🙂
Click to expand...

I will have some time later today to get back to my build and that was my exact intention - to use the board as originally configured only with MOSFETs - and so using a 2EF driver for the gates.
 
#3,740
I change driver using 2SC4793 and 2SA1837. Driver current is ~36mA. Resistor gate is 22 Ohm. THD rise a bit but slew rate increase significantly. This is the result:
PM 79 GM 16
180W/8Ohm, 1kHz -> 0.003693%
180W/8Ohm, 20kHz -> 0.012127%
70W/8Ohm, 1kHz -> 0.000439%
70W/8Ohm, 20kHz -> 0.003299%
317W/4Ohm, 1kHz -> 0.000787%
317W/4Ohm, 20kHz -> 0.007792%
SR ~328
 
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