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Old 1st May 2012, 02:30 AM   #1
Bonsai is offline Bonsai  Taiwan
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Default Musical fidelity A1-X

I have just had the pleasure of repairing a friends 18 year old MF A1-X class A amp that has been out of commision for a few years. The 0.47 Ohm PSU filter resistors were burnt out. I replaced them and viola - working again.

I looked at Mark Hennesy's excellent page about this amplifer for the circuits, and must admit I winced - that volume control, the double feedback network, 'lytics everywhere. Well, those are my engineering prejudices. Reality is a bit different.

Hooked up to my B&W 702's this little amp blows the Marantz PM7000 away - very smooth sweet sound and I am most impressed. Of course its no match for my Ovation amp but that is a 250W monster. I could happily live with this for a second amp for jazz and light chamber music. Heatsink temperature after about 40 minutes was 55C BTW (measured with a gun type IR thermometer)

I real little classic - awsome!
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Old 1st May 2012, 08:27 AM   #2
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I've not seen inside these before, thanks for the pics, Bonsai.
What are the thick, square sections - heatsink extrusions?. Even at 2 x 10W, it must run hot throughout but that's nothing new for MF.

Are you contemplating a 250W version for a shoot-out?
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Old 1st May 2012, 11:02 AM   #3
Bonsai is offline Bonsai  Taiwan
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Hi Ian,

Yes, that's the A1 sitting on top of the 250W Ovation. That's got 5 pairs of 21193/21194 biased up at around 120mA per pair, so it's got a few watts of class A!

The A1 has an inverted U section inside and the output devices are mounted on this. The inverted U section is then coupled to the top plate with some Heatsinks compound and 3 screws.

Surprisingly good sound for such a little amp, and especially the idiosyncratic line stage.

I returned the amp to the owner today an he was pretty pleased.
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Old 1st May 2012, 11:37 AM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Bonsai View Post
That's got 5 pairs of 21193/21194 biased up at around 120mA per pair, so it's got a few watts of class A
would it help understanding if we that know referred to maximum ClassA output current rather than referring to maximum ClassA power?

5pr @ 120mA bias gives a push pull output stage a maximum ClassA output of just under 1.2Apk.
I would finish the sentence from above with "so it's got 2.3Apk of ClassA".

That seems unambiguous and informative.
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Old 1st May 2012, 12:03 PM   #5
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Well...Audio Amplifier Design and Circuits | hifisonix.com has a bit more info for others if Bonsai doesn't object.

'Wonder how it sounds (or not) since the big bang and dash for SSRs?
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Old 1st May 2012, 12:53 PM   #6
AndrewT is offline AndrewT  Scotland
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Default OOPs

I doubled the ClassA current twice.
I can't do that.
It should have said
"so it's got 1.1Apk of ClassA".
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Old 1st May 2012, 02:35 PM   #7
Bonsai is offline Bonsai  Taiwan
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It's fixed and singing again Ian.

The cause of my mishap: two 2 pin mains sockets next to each other. One wired for 110 the other for 220. That's why it fried up.

I found out because I have a 1kva transformer that I wired up to take in 100vac in Japan and give me 220vac out. Well, I tired the same trick here and the transformer buzzed and the mains tripped. I checked my wiring, it was ok, checked the voltage on the input and it was 218vac.

Luckily, after I repaired my amp, I had plugged it into the LHS socket.

The CD player is universal mains input, and this was plugged into the 220vac socket, so I never suspected.

BTW there are four 220 sockets in the house. I would have thought they would have at least used a different type of socket.
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Old 1st May 2012, 02:36 PM   #8
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Quote:
Originally Posted by Bonsai View Post
....and must admit I winced - that volume control, the double feedback network, 'lytics everywhere. Well, those are my engineering prejudices. Reality is a bit different.
Just curious, what is it about that volume control ?
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Old 1st May 2012, 02:38 PM   #9
Bonsai is offline Bonsai  Taiwan
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Andrew, c. 600mA bias in total is indeed just under 1.2A in class A.
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Old 1st May 2012, 02:41 PM   #10
AndrewT is offline AndrewT  Scotland
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So the question from my viewpoint is:
Does stating the ClassA current provide more information than stating the ClassA power?

Which way would do better service to the DIYaudio community?
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