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Ortofon MCA-10 info

Jim Audiomisc said:
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. :)
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I got that, just unusual to have VCB so low. Certainly avoids avalanche noise! Will have very low input impedance, below 10Ohms, which will cause some HF roll off in cartridges with higher inductances.

I am also not wild about big electrolytic caps at the input; they have significant leakage current (at least several uA), and the leakage can have all sorts of weird excess noise. I bet these took a while to come on song when they were powered up as the caps reformed.
 
Arkless Electronics said:
ERm.... in what way?? It's textbook conventional. Schematic is a simplified overview as there is an inverting amplifier just labelled "M". Suspect some errors also as in above form it has a gain of x550!
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I was puzzled by that - I wonder if the 0.4 should be say 4r7, which would give x55, which would be more normal for a head amp. Would raise a low output Ortofon from say 100uV to 5.5mV, which would be ideal for a conventional MM amp.
 
Such a weird circuit I simmed it.... It WORKS! (didn't expect it to work in sim as expecting areas of transistor model vital to operation may not have been modelled).

35dB gain, THD 0.016% with 500uV input (0.003% with 100uV input), -3dB @ 4Hz - 22MHz (with 47pF at output removed)

3mV in for 0.1% THD and 5mV in for 0.2% THD so dynamic range/overload point adequate. It doesn't really clip... just soft clips progressively rather like a zero NFB push pull triode amp. The big electrolytics at the input will be the SQ killer in practice...

Edit: Using 0R source there!

More... "native" input impedance is 10 Ohms and collector current 1.3mA
 
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PigletsDad said:
I was puzzled by that - I wonder if the 0.4 should be say 4r7, which would give x55, which would be more normal for a head amp. Would raise a low output Ortofon from say 100uV to 5.5mV, which would be ideal for a conventional MM amp.
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Thanks for that. I've always wondered about this thing that makes my antique MC20 sound like a stout (but posh) MM into any MM input I've used it with. Back when I started playing with MCs, headamps and transformers were the only real options available. I've got a couple of the vintage Ortofon xformers, as well, which really don't sound bad considering how cheap these used to be.
 
I think the Ortofon MCA-10 was tailored for use with those antique Ortofon MCs. That's why the native impedance is so low.

The reason why I found the schematic was that an MCA-10 was for sale here, and I wanted to find out which carts it would work with. I had forgotten about this thread.
 
Arkless Electronics said:
Such a weird circuit I simmed it.... It WORKS! (didn't expect it to work in sim as expecting areas of transistor model vital to operation may not have been modelled).
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I modified it for the design in the 730. Felt that worked better. More like the version used circa GHz RF amps.

However it did fall foul of the usual peril - daft reviewer! The problem being that *sustained* excessively high input levels cause the gain to wind down. Martin Colloms duly measured the 'overload' level at the *output*. So got it wrong by orders of magnitude! Effect doesn't happen with music from an mc cart. They can't compete with a sig gen in terms of sustained sinewave levels. 8-]
 
I used a "Jeremiah Braithwaite" Headamp, designed and built by Stan Curtis of Lecson amplifier fame.

Moving Coil Preamplifier...

http://lecsonaudio.com/?page_id=76

Worked very well with my Ortofon SL15e MC.

Main system: Thorens TD125 / SME3009 imp, Radford SC24 SPA50 Pre Power amps, Celef SPT Loudspeakers.
 
PigletsDad said:
I am also not wild about big electrolytic caps at the input; they have significant leakage current (at least several uA), and the leakage can have all sorts of weird excess noise. I bet these took a while to come on song when they were powered up as the caps reformed.
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I think 'my era' in HiFi was a lucky one in terms of components available. Superb high-end pots from Alps, and really good electrolytics from Nip Chem (If I recall the name right). I was very happy with them. And with the choice that may now also surprise people - an IC for all the gain stages in the preamp (except for the MC -> MM level). This was the HA12017 designed for RIAA, but also great as a gain stage more generally.

Alas, I designed the amp to work well, not to cope with the reviewers. :-/
 
The drawback of the MCA-10 design is noise, which is not specially good. 1.3mA is good operating current for a BC550 / BC560, its en is just past minimum, and from BC550.pdf (dzsc.com) about 5nV/rtHz. Their rbb is OK, but not special. Two devices effectively in parallel gets you 3dB back, so you might get something round 3.5nV/rtHz (and much worse in the 1/f region). This is enough to be intrusive on 70uV cartridges. I suspect that if you put a pair of the Hitachi epi devices in, you could make it much quieter, although their optimum en lies at higher currents, round 5mA.
 
Jim Audiomisc said:
I chose some nice japanese transistors for the MC pre. Can't offhand recall their noise specs. But they seemed OK.
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That would make all the difference. My goto parts used to be 2sa1084 /2sc2546, but Renesas don't make them any more. I have a little stockpile that they will have to prise out of my cold dead hands.
 
PigletsDad said:
That would make all the difference. My goto parts used to be 2sa1084 /2sc2546, but Renesas don't make them any more. I have a little stockpile that they will have to prise out of my cold dead hands.
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My circuit diagram of the 730 says 2SA1085 and 2SC2547 in the circuit, but that is a bit different to the MCA-10. Also, I may have changed them after the diagram was drawn. Not looked inside the amps for ages.

http://ukhhsoc.torrens.org/makers/Armstrong/700_Range/Circuit_Diagrams/730Preamp.gif
 
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