The Supra-Supa Phono stage thread.

[DRONE7]

I have recently finished building the Elektor Supra Phono stage (circa 1982 ?)and attempted to install it with my current phono cartridge.

I have learnt a considerably amount thanks to the kind and informative contributions of the DIYAudio community so thought to collect here as much information on this circuit as I could glean.

Firstly the circuit..
http://www.pcb-audio.com/pcb-audio_com/riaa-super-low-noise-preamp-circuit.jpg
from a Group-buy for the PCB(called 'Supa' here)....
http://www.diyaudio.com/forums/group-buys/266887-supa-phono-preamplifier-pcb.html

An earlier thread...
http://www.diyaudio.com/forums/analogue-source/91497-bc550-bc560-very-low-noise-riaa.html
and a later one..
http://www.diyaudio.com/forums/elektor/121825-sln-pre-amp-elektor-aug-1982-a.html
and my adventures in loading for MM...
http://www.diyaudio.com/forums/analogue-source/275759-help-cartridge-loading.html

and lastly, the commercial offering..
Hormann Audio - Hormann-Audio-Produkte (note this takes a long time to load...😉)

I am going to hazard a guess that this design was originally for a very high gain MC phono preamp.( that could be easily modified for MM operation) and was presented as a MM preamp that could also be used with MC by the Editors of Elektor so as it may have more universal appeal.

It's major drawback for MM use is the very high input capacitance of 280pf. Add some cable capacitance and tone-arm cabling and you can very quickly hit a total way west of 550pf !

If you have an Ortofon, a Shure, a Grado or similar or intend to alter it for use with a MC cartridge then you should be good to go..otherwise read on.

As the circuit is presented C1 @4.7pf is merely a place-holder for substitution to increase capacitance ! The multiple Transistors used already contribute a given of 280pf.

By changing R14 to the designers recommended 27R and R1 to 100R then the preamp can be used with MC cartridges which, happily, do not appear to suffer to any great degree from capacitive loading.

In this thread http://www.diyaudio.com/forums/elektor/121825-sln-pre-amp-elektor-aug-1982-a.html
AndrewT and Shaun both argue for a splitting of the gain stages and the commercial offering appears to have done just that with two stages of the same circuit, presumably with gains adjusted and the RIAA omitted in the second. (image attached)

Part one finished.. all comments, corrections, and suggestions are most welcome..🙂

 

[tiefbassuebertr]

For this approach there are small signal transistors necessary, where perfect results possible at very low idle currents. With usual available types like BC550/BC560 this isn't possible, because for optimal results at this type quiescent currents between 8 and 15 mA for each transistor is necessary. This is a great drawback resp. disadvantage.
Which vintage small signal transistor type was available in the seventies for such audio applications optimized for low current ?