advertisement


Ortofon MCA-10 info

Jim Audiomisc

Jim Audiomisc

pfm Member
Does anyone have either a circuit diagram of the old Ortofon MCA-10 moving coil 'head amp', or has one they're willing to open and photograph/circuit trace? I've been looking for full details having written this

http://www.audiomisc.co.uk/Armstrong/reviews/finale/1982.html

a while ago where it 'features'. But I lost my copy of the diagrams years ago, and when I asked Ortofon I was told everyone involved has gone, and their records of it have been binned. So although I'm clear about what I recall and wrote I don't have an actual diagram.

Jim
 
The link to the French forum was pretty handy! :)
http://www.audiovintage.fr/leforum/viewtopic.php?f=83&t=28592&start=30
has some photos that immediately confirm what I recalled about the circuit topology and match my memories. I'll see if I can use them to work out the detailed circuit diagram. But if anyone has one they can trace exactly, please let me know. My eyesight isn't what it was, and has always been lousy. :-/

If my French was any good I might have tried asking on the French forum. But the last time I needed any French was in the 1970s. I think I can still recall the words for 'screwdriver' and 'inverter'... but that's about it. Beyond that it's " 'allo 'allo "...

BTW This examples a trend I've been getting more and more worried about. The way info on old designs and manufacturers is tending to 'evaporate'.
 
This, http://users.ece.gatech.edu/~mleach/headamp/ seems cleverer than the Ortofon, so probably not relevant, I remembered the common base and batteries.

I have a clear recollection (so likely faulty) of the circuit appearing as an example among several in an ETI article, possibly Stan Curtis authored. It might be worth emailing him.

Paul
 
I just posted on a Danish vintage forum to see if anyone there would have a complete and detailed schematic. The audiomisc name on the web page did not register at all, not even when I read the link. Must be getting old ...
Glad that my image Googling yielded something useful for a start.
 
The page on the Leach circuits is interesting. I wouldn't have read Audio at the time (which gave me grief for years as i couldn't get a copy of their review of a 600 unit!). And I confess I didn't read ETI either.

FWIW a lot of old issues of 'Audio' are available from
http://www.americanradiohistory.com
so I'll see if they have it. I've been meaning to try contacting Stan for other reasons, so this may poke me into finally doing so.

The basic (pun!) idea wasn't new. I liked it in part because I'd seen a number of low noise microwave amps that did it. I then found it used by the MCA-10 and decided to tweak it a little. I did actually find that some driver transistors worked with low noise, but weirdly, worked better if I used the collector and emitter reversed. This worked, but I decided that it was so weird that people would think I was crazy. I *think* the reason was that they were multple emitter designs to help prevent secondary breakdown, so you got some device sharing advantages in one pack.

Anyway, small signal transistors were cheaper.
 
I was given a faulty MCA-10 ten years ago. I replaced all the capacitors and it was working. I cannot tell how good it sounds since I haven't used it much at all.
 
hifienthusiast said:
I was given a faulty MCA-10 ten years ago. I replaced all the capacitors and it was working. I cannot tell how good it sounds since I haven't used it much at all.
Click to expand...

Any chance of you either:

A) Trace out and draw the diagram

B) doing something like putting the PCB on a flatbed scanner and putting up the result? (And a top down 'map' photo of the coponent side?) That would make tracing the details a lot easier.

As things are, I'll try doing it from the French forum pictures, but they aren't ideal for this. Much easier to trace a diagram if you can handle the board.

Jim
 
Hi Jim.
So far no luck with getting the schematic. Everyone points at your simplified version.
Kind regards,
Peter
 
OK, thanks.

I was contacted by someone from wikipedia a year or two ago when they wanted info for their pages on Armstrong. I was told I was a 'primary reference'. Which sounded wonderful until I realised they meant, "We can't accept what you say until we can find another source." 8-]

We really *do* need to set up a society for collecting and preserving documented info on the history and details of audio equipment.
 
All I have been able to find has been the same pictures and references:

2016-05-30_131307_mca10a.359408les6.jpg


2016-05-30_131334_mca10b.584935verifdessoudures.jpg


I will try another dedicated turntable forum "Pladespilleren.dk"

EDIT: "Basically it's a complementary common base amplifier with only two transistors: BC485 and BC486.
The collectors are connected together and the input goes to the emitters through 2200uF capacitors. The bases are grounded.
The output is taken from the collectors through a 22uF tantalum cap."
 
OK, If I read that correctly, the transistors are operating with Vcb about 0, as the bases are tied together and the collectors tied together. Do BJTs actually give any gain under those conditions? IE is about 2mA (roughly), and looking at Figure 4 in https://www.onsemi.com/pdf/datasheet/bc550-d.pdf, VCe(sat) is about 35mV at 2mA. But we have about 0.6V of VCe (== VBE), so we are not in saturation.

Very odd and unfamiliar corner of parameter space!
 
PigletsDad said:
OK, If I read that correctly, the transistors are operating with Vcb about 0, as the bases are tied together and the collectors tied together. Do BJTs actually give any gain under those conditions? IE is about 2mA (roughly), and looking at Figure 4 in https://www.onsemi.com/pdf/datasheet/bc550-d.pdf, VCe(sat) is about 35mV at 2mA. But we have about 0.6V of VCe (== VBE), so we are not in saturation.

Very odd and unfamiliar corner of parameter space!
Click to expand...

The emitters are pulled negative relative to the bases.

Bipolars generate current gain. The (comon) collector resistor then produce voltage gain.

Its similar to some microwave amp designs that need low noise at high frequencies.

Nice to see this appear after so long from the question. :)
 
You must log in or register to reply here.


advertisement


Back
Top