SS CFP MC Head Amp

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JamesD
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#1 SS CFP MC Head Amp

Post by JamesD »

I need a head amp for my new MC cartridge - its an Ortofon SL-20E soit has a very low impedance coil and very low output - 2.5R coil and 70uV output... so the MC head amp input has to be very low noise and have a loading of around 22R input impedance - a gain of 43 gets the output up to 3mV and something happy with a 47K loading would be nice.

A transformer might work but my only mc step up has gain of 15 and would load the cartridge with 200R so could work with a 22R across the primary but it would be light on gain...

Knocking up a head amp isn't trivial for this one as the low coil impedance means it needs a very low noise gain stage to not make the input noisier than the cartridge - fortunately there are some extremely low noise transistors made by ZTX - I also have a stock of low noise hitachi trannies from the 80s that are really good - 2SA1085 and 2SC2547 from back when I designed the Tesseract Audio mc head amp. I'm starting with the ZTX ones. The design uses a Complementary Feedback Pair (CFP) running of a nominal 9V battery - the design is happy down to 6V for about 300+hrs use. Gain is 46-50 times and noise is predicted at around 500pV/root(Hz) with the 22R input resistor. The circuit is ac coupled in and out using 1000uF Panasonic FR series capacitors...

Here is the circuit
CFP MC Head Amp.png
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Nick
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#2 Re: SS CFP MC Head Amp

Post by Nick »

Given the battery supply and input and output caps, could you float the battery and lose the input cap?

Though I guess you may be using one battery for both channels.

Ignore that, stupid idea.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
JamesD
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#3 Re: SS CFP MC Head Amp

Post by JamesD »

I intend to knock up the richard lee duraglit style head amp as a floating design to see how different it sounds from my version above. The reference I used to inform me about various modern takes on the design is https://hifisonix.com/mc-head-amp-circuit-compendium/ that documents various designs and also provides the LTSpice models to play with for the designs. The corresponding DIYAUDIO discussion is https://www.diyaudio.com/forums/analogu ... d-amp.html

Other references I consulted for various aspects are:
http://www.dicks-website.eu/low_noise_a ... part1.html
https://www.beis.de/Elektronik/LNPreAmp/LNPreAmp.html
http://www.janascard.cz/PDF/Design%20of ... ifiers.pdf

I like the Compound Feedback pair and have used it many times since it was shown to me in the late 70s - a special case of this is the Sziklai pair that has 100% feedback so unity gain or very close to it - its the CFB version of a Darlington with better gain (marginally) but lower distortion and much better thermal characteristics - see this page for a good run through the Sziklai Pair https://sound-au.com/articles/cmpd-vs-darl.htm.

I am not aware of any good single reference for the generic case but examples are used regularly - the particular case of the triode/transistor compound pair is often known as a Supertriode and Tubecad has a few examples of this such as https://www.tubecad.com/2009/12/blog0178.htm - scroll down to thetriode/ p-channel mosfets for the exact analogue ...

I would summerise the main advantage of the CFP as being low distortion with high gain and bandwidth - the main disadvantage is a tendency to burst oscillation that can be difficult to halt on a conventional circuit board implementation - bird nest construction can help :D

ciao

James
JamesD
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#4 Re: SS CFP MC Head Amp

Post by JamesD »

Having slowly knocked up the CFP head-amp and stabilised it against it tendency to burst oscillation by tweaking the resistor and capacitor on the base of the second transistor, I have been delighted to realise that my step up transformer was limiting the mc cartridges I use and that they had better frequency extension, better detail and better timbre reproduction than the MM cartridges that I have as well as that sense of performance being better. The only downside is that my two mc cartridges have very different output voltages and this caused me pain to reset the gain every time I changed cartridge...

Reading through my cartridge head amp notes from when I designed the Tesseract Audio head-amp in the 70s, a current input head-amp- I realised that my two cartridges had similar current outputs and that a current input head-amp could be set to just one gain setting for both cartridges - a possible solution if sounds good. It should sound good as the Tesseract head-amp when used properly was exceptional...

My two cartridges are a good old boy Denon 103 - 300uV into 100R from a 40R generator and a Ortofon SL20E - 70uV into 20R from a 3R generator and so we have between 3uA and 3.5uA generated = close enough to the same signal for me.

Looking through the Hifisonix compendium linked in the post above I settled on modifying the Chadwick circuit for my use. I'll draw up my modified schematic this week. I have all the components so I just need to find the energy to build it...

James
Andrew
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#5 Re: SS CFP MC Head Amp

Post by Andrew »

Interesting stuff, James.

Curious about your bird's nest, can you share a picture?
Analogue, the lost world that lies between 0 and 1.
JamesD
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#6 Re: SS CFP MC Head Amp

Post by JamesD »

Hi Andrew,
Will do. I'm in the sleepy time after my last session of chemo so it will be a day or two before I get it done.
ciao James
Andrew
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#7 Re: SS CFP MC Head Amp

Post by Andrew »

No worries, James, hope you're on the mend :!:
Analogue, the lost world that lies between 0 and 1.
JamesD
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#8 Re: SS CFP MC Head Amp

Post by JamesD »

Sorry for the delay in posting a photo of the birds nest but it is inside a copper foil wrapper that is soldered closed and a sealed paper wrapper for insulation from the earthed copper foil, I haven't had the energy to open it up yet after my last dose of chemo. Getting my energy back but not there yet...

ciao

James
Andrew
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#9 Re: SS CFP MC Head Amp

Post by Andrew »

No worries, James. I was just curious.
Analogue, the lost world that lies between 0 and 1.
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