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Old 12th March 2011, 10:01 AM   #1
cpitts is offline cpitts  Australia
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Join Date: Jan 2010
Location: Melbourne
Default Musical Fidelity A220 Class A Tweak/Refresh

Hey guys/gals (most likely guys),

Just thought I'd post up a thread of the amp that I'm modding. Amp is:

Musical Fidelity A220
Class A, 50W into 8Ohms, 100W into 4Ohms

I'm fully documenting the process (along with modding my speakers internals) on my own website (pics, parts, reviews, etc) so if you want to see more details (part numbers, schematics, etc) you can waste a lot more time on my personal blog/website Doodads galore! We just luv em..

Either way, the plan of attack is to do the following replacements:
  • Capacitors - Temp rating. Factory units are to 85'C only. (Mundorf purchased as replacement items, Rubycon ZL/ZLH for smaller duties)
  • Capacitors - Speed. Factory units are good, but far from great.
  • Pwr Rectifier Diodes - Speed, faster switching, the faster the better! Soft recovery.
  • Quad Op Amp - Motorola MC33079P, KDO9705 (final OA not decided yet) ideally will be 4 x single OA's on a PCB into factory DIP socket
  • PCB to Spk terminal - replaced with higher quality shielded cables
  • Other cabling from/within PCB - replaced with higher quality shielded cables
  • Shielding of the PSU and Toroid away from the PCB (where possible).
  • Earths - Power socket to chassis binding post. Thicker cable required.
  • Central Earth Point - for speaker and RCA cable shields
  • Speaker Binding Posts - Upgrade to something thicker (not 100% sure this will be done yet, OEM are OK but may be worth rewiring while soldering them)
  • RCA Plugs - Upgrade to better RCA plugs for core devices (CDP, Tape, Pre, Vid/Aux)
  • Volume pot - ALPS 422N-50KAX2 to be replaced with TKD 50K log taper
  • Possibly hard wire tape switch out of signal path
  • Upgrade all resistors to 0-0.5% 0.5W Metal Film PRP items
  • Upgrade all links to solid .8mm copper
Have just finished upgrading my speakers with excellent results and the amp's already begun and will soon see the long list above revved up for (hopefully) cleaner highs, even wider and deeper stage and cleaner signal path. The MF is already a very strong amp so hopefully this will make it even faster which is about the only thing it lacks from the factory; what people call the 'smooth/layback Class A sound' from the amp.

I want the amp even cleaner, lower noise floor and just a little more rapid in it's response which I think is entirely achievable with the attack I'm making on it.

Either way, here's a few shot of the amp in 1 piece and then ready to go!

Click the image to open in full size.

The PCB itself (I've labeled each component to make it easier):

Click the image to open in full size.

and....... the parts that are going to be fitted..... on a Wii Fit board.... of course!

Click the image to open in full size.

and this is the current state of the amp :-). PCB out and on its side (still have massive heat sink attached but it's all ready to roll with the upgrades.

Click the image to open in full size.

Will keep this thread updated as the project rolls along in the coming days/weeks.
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Old 12th March 2011, 02:11 PM   #2
jaycee is offline jaycee  United Kingdom
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Location: Norwich, UK
Seems like a good plan of attack so far. The replacement of the capacitors alone will bring a lot of new life to the unit. If it's small signal cables, by all means use screened cable, but otherwise dont bother. The power wires to the Phono stage should be twisted together though.

4 single opamps is not going to be worth doing - simply fit a good quad opamp. An OPA4134 would be my choice here. It isn't worth fitting anything very fancy because odds-on it will oscillate. Put some 100nF ceramics right under the chip, from each supply pin to ground. Yes ceramics - these are the best for decoupling. Leave the phono stage opamp alone - in this design, a 5532 is great. Maybe try an OPA2134. Don't bother going nuts.

I'd probably look to replace signal coupling electrolytic capacitors with Wima MKS2 polyesters where I could. These are available with 5mm pin pitch and so usually drop right in.

The power amp itself is basically the Hitachi sample app for their MOSFETs. Not too shabby, but could've been vastly improved - especially for driving 2 pairs! Personally I think those 3300uF capacitors after the resistors are pointlessly large in value. A better way would have been to use separate supplies for the front-ends. Oh well.
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Old 12th March 2011, 07:45 PM   #3
cpitts is offline cpitts  Australia
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Hey jaycee, thanks for the extra tips. There will be a lot of Vishay MKP1837 bypassing occurring as well to help speed things up.

Honestly, I'm not touching the Phono stage as I don't even own a record player, so money there is wasted at this stage.

Cheers for the suggestion on the OPA, was aware of these but thought they were out of production?!? Will double check though, as a drop-in replacement, it's a very easy upgrade...... the beauty of having a DIP socket from the factory, I can change IC's very easy.

Yes, I'd love this amp to have the ability to drive 2 power stages for bi-wiring. Something sorely missed, for sure! But that's no doubt what the more expensive models were there for..... *sigh*

Last edited by cpitts; 12th March 2011 at 07:54 PM.
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Old 3rd June 2011, 08:50 AM   #4
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Default mosfet replacement on musical fidelity a220 ampl.

Any suggestion for buz900p-buz905p remove from heatsink for replacement?
Can i use the irfp240-irfp9540p with appropriate pin oriantation?
I need help please.
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Old 4th July 2011, 02:09 AM   #5
rubli is offline rubli  Mexico
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Default BUZ

they have the originals:

Magnatec | Semiconductors | Discretes | MOSFET | MOSFETs & JFETs - N-Channel |BUZ900P

per suggestion of another member, i would not change for another tyos, nevertheless, if you succeed, please let us know !!


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Old 6th December 2011, 11:34 AM   #6
cpitts is offline cpitts  Australia
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Join Date: Jan 2010
Location: Melbourne
Default And it is done......

Well, it's taken a while..... quite a while, but you can't hurry a big job (when it's getting done by a mate).

All resistors have been replace. (PRP 1%, measured typically at 0-0.05%)
All electrolytic caps have been replaced with Mundorf MLytic caps
Output caps have been replaced with Auricaps
All ceramic caps have been replaced with Silver Mica caps (0.05%)
All power caps have been replaced with MRA wire wound resistors (0.5%)
All cabling has been upgraded to silver plated 6N cabling
All links have been replaced with 0.8mm 6N hard copper links
Volume dial upgraded with TKD 50K log unit
Power reg replaced with IXYS FRED (+extruded copper heat sinks)

To be done:
Quad op amp -> still getting here on a slow boat from USA. *sigh*

Current results:
CLEAN, very, very clean signal.
DETAIL, lots and lots more detail.
Cymbols and vocals so much clearer. This seemed to be missing before, so is what I was after!
Bass is deeper, smoother and stronger.
Frequency balance is going to take ages to get right because I have poly props in the output stage that need to be run in. These alone can take hundreds of hours.

Separation of content in the music is much better. Things don't run together as much. This is the extra speed and clarity I was looking for.
Transitions between frequencies at the speaker is much better and smoother.
Deeper and more granular sound stage aparant.

To pay for this in labour would be rediculous and not worth while. But to do it yourself or in conjunction with a friend, the results take a really good amp and make it about 80 to 100% better! The improvement is that obvious (and it still has to run in properly!!! )

I will say that the TKD seems to make the volume dial quite sensitive so this may not be to everyone's liking.

I'll post up some pics if people are interested in what it looks like now. ( I know you all are. :-P )
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Old 6th December 2011, 11:51 AM   #7
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It's not class A.
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Old 6th December 2011, 11:56 AM   #8
cpitts is offline cpitts  Australia
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Heavily class A biased work for you then?
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Old 6th December 2011, 11:59 AM   #9
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Ok, but that's the same as calling it class AB.
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Old 6th December 2011, 12:05 PM   #10
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Very heavily biased class AB.
I used to have one of these a few years back. The Heatsink gets extremely hot, as in skin burn hot.

Last edited by rob g; 6th December 2011 at 12:08 PM.
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