Amp upgrade experiences, MF A370 like

[Bodgit]

Took my MUSE MOSFET 300W amps to John at jsaudiorepairs, he quickly found what the issue was with one being quieter than the other and to make good is not going to cost a fortune, one of the MOSEFTs sas shorting to the case. The 24 MOSFETS (6 pairs in each) need to come out and have new insulation gaskets fitted together with thermal compound. About £400.

He then took me through the design weaknesses inherent in the amps and I have the option of a full monty upgrade. I've had unreliability issues with these amps and they are 20 years old so this could give them a new lease of life and turn them into heavy weight power amp contenders:

- new bigger toroidal transformers, about double the size, £100 each. The transformers are always going to be the limiting factor with the design he said

- bigger capacitors, there are 4 in each @ £100 so that's another £800. Together with bigger transformers, this gives the amps massive headroom

- the Hitachi MOSFETs (2SJ56/2SK176) are working close to their limits, especially if the music is on loud and they could go pop, there is no thermal protection. Two MOSFETs have already been replaced and he suggested replacing with the Profusion ECF10N20/10P20 models which are rated at 200V vs 160v, are more stable and sound better.

That little lot would be the thick end of £2k all in, but would bring them up to a standard which to buy new would cost multi-thousands.

He compared this to what he offers as an upgrade to the MF A370 amp, wonder if any one here has had this kind of upgrade performed on a MOSFET amp and what the improvements were like? I'm very tempted but keen to hear from others before giving him the nod.

 

[MF 1000]

Is that the upgraded A370 in the pic David ? 

 

[Bodgit]

No that's one of my MUSE 300s. Just that the MF A370 is similar in so far as its a big lump of a MOSFET thing.

 

[MF 1000]

I love my 45 Kg lump of MOSFETs

i did speak to Jon not long after I acquired it (2012) and a rebuild like you've described was approx £1000 or more if you opted to go to bigger transformers in an external enclosure - but the effects were apparently night/day etc.  He knows his stuff on MF kit and MOSFETs in general.  He could probably do the upgrade in stages replacing the caps/trannys as a 'Stage 2' 

 

[Bodgit]

Yes I guess the upgrade could be done in two stages, although with the bigger capacitors and trannys he would need to do some layout changes so it might be a bit less cost efficient.

He mentioned the full A370 upgrade was about the £2k mark too,  prices have gone up no doubt in the last 8 years!

My long term ambition was to upgrade the speakers to active SCM50s but I can't see that happening, too much £££. So it got me thinking that upgrading these amps would be a good way of getting the best out of what I have.

 

[plasticpenguin]

Don't know about the A370. I did, many years ago, home demo A300. Gawd it was dreadful. So warm and mushy you cook an egg on it, and that was with bright RS6 speakers. Couldn't wait to sling it back to the dealers.

 

[Bodgit]

Well that's a completely different kettle of fish, nothing like the brutish MOSFET amps like my MUSE and the A370.

My first amp was a MF A100, it ran hot and was temperamental, sounded warm like a valve amp as it was class A. 

 

[MF 1000]

Don't know about the A370. I did, many years ago, home demo A300. Gawd it was dreadful. So warm and mushy you cook an egg on it, and that was with bright RS6 speakers. Couldn't wait to sling it back to the dealers.

Any relation between the A300 integrated and the A370 dual mono power amp is limited to the makers name only really.  Of its genre the A3cr pre and A3 power amp combo was far better and could I believe be monoblocked to run two A3's to good effect.

Always bear in mind the 'house style' of MF amps was a warm almost valve amp like sound as opposed to the clinical strike of Audiolab etc.

 

[plasticpenguin]

Any relation between the A300 integrated and the A370 dual mono power amp is limited to the makers name only really.  Of its genre the A3cr pre and A3 power amp combo was far better and could I believe be monoblocked to run two A3's to good effect.

Always bear in mind the 'house style' of MF amps was a warm almost valve amp like sound as opposed to the clinical strike of Audiolab etc.

Maybe so, but the overriding thoughts were "I wouldn't touch a MF again".

It wasn't even natural sounding; almost like they said 'let's make the warmest amp possible and stuff any other sonic quality'. I have more musicality in my fingernail than that particular amp. bad experience.

 

[It Cost How Much!?!]

Personally I would not spend £2k on upgrading an amp. I would sell it and spend on a better amp or just keep it and buy LPs.

 

[Bodgit]

To buy an equivalent amp to mine upgraded would be north of £10k I believe.

 

[bencat]

To buy an equivalent amp to mine upgraded would be north of £10k I believe.

The important part of this is that you believe it would cost that . I suspect that quite an inprovement could be had for less than the 2 k but only you can decide that as this is your system and your money .

 

[Bodgit]

The capacitors are half the cost roughly, plus a much bigger transformer, upgrading from 4x10,000 micro farad to 4x 33,000 or 47000 micro farad. Thats a lot of capacitance and any amp with the kind of current capability I'm looking at would need similar capacitors & transformer. 

 

[MF 1000]

As I recall for the A370 the best path was less reservoir capacitance ( bear in mind it has 176 000 uF per channel) and increasing the size of the transformers - Jon said MF had heat issues with the original design with this combo and case fans in the 1980s were very noisy etc, but much bigger transformers housed in a seperate enclosure would work wonders and remove a major heat source from the main body of the amp - incidentally this is the route they went with the likes of the A1000, A1008, KW series etc in later years.

 

[Bodgit]

As I recall for the A370 the best path was less reservoir capacitance ( bear in mind it has 176 000 uF per channel) and increasing the size of the transformers - Jon said MF had heat issues with the original design with this combo and case fans in the 1980s were very noisy etc, but much bigger transformers housed in a seperate enclosure would work wonders and remove a major heat source from the main body of the amp - incidentally this is the route they went with the likes of the A1000, A1008, KW series etc in later years.

Sounds about right, I just spoke with John and he mentioned this upgrade with external toroidals. Hopefully getting about that capacitance in mine, standard is 40000uf so it's a big leap. The MUSE circuit lends itself with easy current upgrading and the toroidal is quite small, so both are needed with mine plus higher voltage MOSFETs. My heatsinks are on the limit for 300W.