Sharing: My Musical Fidelity A1s mod...

AC439 · 26th January 2012, 02:54 PM

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

Osvaldo de Banfield · 26th January 2012, 03:15 PM

May be that the output stage be too biased, so it goes near to class A?

AC439 · 26th January 2012, 03:29 PM

It is class A. Quiscent current is 700mA. You can keep your coffer hot all day on top of the heat sink.

AC439 · 12th February 2012, 03:21 AM

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

ixigen · 28th February 2012, 07:01 PM

I have the Musical Fidelity David (german tuned version of the A1) and an A1-X which I want to mod.
Can you upload some high resolution photos with the mods? or a schematic?

AC439 · 28th February 2012, 08:48 PM

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.

26th January 2012, 02:54 PM	#1
AC439 diyAudio Member Join Date: Jun 2007	Sharing: My Musical Fidelity A1s mod... 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

26th January 2012, 03:15 PM	#2
Osvaldo de Banfield diyAudio Member Join Date: Dec 2011 Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina	May be that the output stage be too biased, so it goes near to class A? __________________ A veces pongo un circo y me crecen los enanos... "Que se llama soledad", Joaquin Sabina, el capo.

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26th January 2012, 03:29 PM	#3
AC439 diyAudio Member Join Date: Jun 2007	It is class A. Quiscent current is 700mA. You can keep your coffer hot all day on top of the heat sink.

12th February 2012, 03:21 AM	#4
AC439 diyAudio Member Join Date: Jun 2007	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

28th February 2012, 07:01 PM	#5
ixigen diyAudio Member Join Date: Jul 2006	I have the Musical Fidelity David (german tuned version of the A1) and an A1-X which I want to mod. Can you upload some high resolution photos with the mods? or a schematic?

28th February 2012, 08:48 PM	#6
AC439 diyAudio Member Join Date: Jun 2007	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.

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