I just want to share my recent experience modding my MF A1S. I bought this amp brand new many years ago from Hong Kong. After I moved to US, the voltage change was a challenge. I don't want to use external step down tranny so I opened the box and wired it for 110v use but ended up having the original tranny toasted after a few months. Put in a replacement and kept burning fuse. The amp was then put in storage for 10+ years.
I revisited the amp lately and decided to do something about it. I put in a slow blow 2.5A fuse (verses a 1.5A normal fuse) and the amp stays on without problems. Then I decided to mod the preamp and rewire the volume pot. Most designs use op-amp for the preamp but I decided against it. I never like the sound of op-amp and I think less is more (in terms of components in the signal path). I decided to go with MPF102 jFET. I picked the power supply design from tnt-audio.com's Zener + emitter follower, which turns out to be a great dead silent power source. Then put in a single stage stereo MPF102 preamp without bypass cap on the source resistor. Power supply to the preamp is around 17v and I set the Vd at around 9.5v. I reused the volume pot and wired it the conventional way after the preamp and before the main amp. Everything else is original (definitely more mods to come).
The amp still runs very hot (by design) but the sound improved a few levels up once it has warmed up. It definitely has it own magic. I listened to it late into the night one music after another (post modding) and was not aware it had past 1am in the morning. I also have class-T amp (TA2024). While I like the TA2024 very much, there is certainly some magic with the A1s that the T-amp doesn't have. I was thinking about selling the A1s via online auction as a repair/parts unit and glad I did not.
The next thing I need to do is to put active cooling with a 120mm super quiet computer fan........It does run very hot !
-AC
I revisited the amp lately and decided to do something about it. I put in a slow blow 2.5A fuse (verses a 1.5A normal fuse) and the amp stays on without problems. Then I decided to mod the preamp and rewire the volume pot. Most designs use op-amp for the preamp but I decided against it. I never like the sound of op-amp and I think less is more (in terms of components in the signal path). I decided to go with MPF102 jFET. I picked the power supply design from tnt-audio.com's Zener + emitter follower, which turns out to be a great dead silent power source. Then put in a single stage stereo MPF102 preamp without bypass cap on the source resistor. Power supply to the preamp is around 17v and I set the Vd at around 9.5v. I reused the volume pot and wired it the conventional way after the preamp and before the main amp. Everything else is original (definitely more mods to come).
The amp still runs very hot (by design) but the sound improved a few levels up once it has warmed up. It definitely has it own magic. I listened to it late into the night one music after another (post modding) and was not aware it had past 1am in the morning. I also have class-T amp (TA2024). While I like the TA2024 very much, there is certainly some magic with the A1s that the T-amp doesn't have. I was thinking about selling the A1s via online auction as a repair/parts unit and glad I did not.
The next thing I need to do is to put active cooling with a 120mm super quiet computer fan........It does run very hot !
-AC
Active cooling update: I put a 120mm 12v quiet computer fan with rubber feet on top of the A1s. I literally do not hear any noise from the fan. It drops the temp down a lot. Before the fan, I was unable to put my hands on the top of the amp for more than 5 seconds. Now, I can put my hands on it for as long as I want. The temperature now feel like in the 40'F. (I think it was like 70'F without the fan).
-AC
-AC
Sorry, I didn't take any pitcures when I did the mod. I'm not going to reopen the amp and get into the thermal grease mess again.
You can search the web for "MPF102 jFET preamp" and will be able to find tons of schematics. You build two of these for stereo. Then follow my above information to build the power supply and wire the volume pot the conventional way and that's it. It is a simple mod, the challenge is how to mount the new component to the chassis. I disabled the original preamp by yanking out the op-amp IC.
You can search the web for "MPF102 jFET preamp" and will be able to find tons of schematics. You build two of these for stereo. Then follow my above information to build the power supply and wire the volume pot the conventional way and that's it. It is a simple mod, the challenge is how to mount the new component to the chassis. I disabled the original preamp by yanking out the op-amp IC.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.