APEX FX8 bimo mod

Hi FX8 fans

new Toshiba TTA004B_Q/TTC004B_Q (hfe about 210..others about 85-100)
I did the VAS transistor update at my other FX8 too. Be aware that you have to adjust the bias again because of the higher hfe of this toshiba transistors.
i changed to ceramic caps to the Wima film caps. Sound is now better, give me more chest of the singers and better impulse response.

chris
 
Hi

Additionally i change the PSU rectifier - from KBPC to the active rectifier by Prasi for each rail. Without done the Quasimodo snubber for the transformer i am very happy with this change. Very clear sound as i had the experience with the ACA PSU for a friend. Maybe the transformer snubber in the old configuration should be done more necessary then now with the active rectifier. the sound is very nice. You should try this. additionally i have 2 Volts more at each rail. 😉

LT4320:
here

chris
 
Hello again

i have done some changed inspired by some threads:

  1. gate resistor change at the N ch mosfet 100--330R (maybe too high?)- i looked at a lot of Lateral threads and there are always changes of the gate resistors.
  2. snubber between Drain and Gate - like in the VSQA thread- 220pF and 47R in serie - to avoid HF oscillations.(the smal 0,25W resistor is under the yellow tube)
    its recommended by AKSA that it is for high transconductance MOSFET and/or higher rails but it works with my LATERals (Renesas) and just 34V.

my feedback after listening :
i like the sound of FX8 but the lack of fine high frequencies and compress feeling at higher volume is marked by me as disadvantage. the mids a very good and the bass is controlled and not boomy but somethimes "too controlled". but okay.
this amp can help if you have a boomy room or a loudspeaker which need less low freuency power to have a blanced feeling.

after my changes i compared with the VSQA amp. the difference i noticable. no stress in higher frequencies, flute and singers are clear and feels not compressed.

try it and give me feedback.

kr
chris
 

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Hi Chris,
that reminds me of the old 2SK134/2SJ50 in the Crescendo or something else.
Between the drain and the gate of 2SK134 an extra 33pF was fitted, and between the source and the gate a 330pF capacitor, because lateral mosfet are not equal, but symmetrical driven.
Are you sure the capacitors must be equal ?

Peter
 
May be i am wrong, but i think to use 330pf and 33 pf is the better way, because the pf value of the lateral transistors is different. Your snubbers are equal, i think because the VSQA Amp uses the same power transistors (as i have seen) at the output.
I will try as i wrote. It is the same difference as in the datasheet of the old 2SK134/2SJ50.
If you have success with your snubbers all is fine...🙂

Peter
 
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BTW....i found a adapter by Hoppes Brain....TOP- To3, MT200-TO247😉
 

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Good evening,
anyone interested in experimenting? I have the MIRO360 boards for sale, which I no longer use because I have too many amplifiers.
Best regards.
Guglielmo
Italy