• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Sonic Frontiers SFP-1 Phono Stage

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Jebem

Jebem

Member
Joined 2011
sites.google.com/site/sunajebem/home
#1
Hi,

I have this idea of using a JFET as a Phono Head Amp for my MC Lyra Lydian cartridge (500uV).
Currently I use a Musical Fidelity The PreAmp A3 (shunt power regulation, LM394 BJT matched pair head amp, and VQ1000J MOSFETs), and I want to beat it.
I know that one AD797 IC as it is based on BJT's can give better S/N ratio, but I don't like to see so many transistors in the signal path... I want a minimalist design.
And why not mix JFET with Tubes?
First time I saw this design was in the 90's from Sonic Frontiers, when I used to buy quality components from The Parts Connection.

So, I share here the Sonic Frontiers SFP-1 Phono Stage schematic (partial, due to upload space constrains) so you can comment on it.

The JFET in use are marked as J310. Anyone have information on this one?

+B1 AND +B2 = 210VDC (for the 12AT7 output tube and 6DJ8 RIAA driver)
+B3 = 187VDC (for the JFETs)

015.jpg. 016.jpg

Cheers.
 
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#5
Jebem,

For the hybrid circuitry approach, I advise you look into Allen Wright's cascode, with SS "downstairs" and a triode "upstairs". Allen, who sadly is no longer with us, used a JFET in a single ended setup and a twin BJT in a balanced setup.

AFAIK, AW's company VSE is still in business and they will sell you appropriate "sand" for the project.
 
#6
Thanks a lot DF96 for Your good input (by the way, I like the "D" series valves, really... I have got a few valve portable radios from eBay UK, like Bush TR82C's or the KB RHAPSODY DE LUXE PP251 to mention but a few... it reminds me when I was 6 years old and started poking around with my dad's Philips "D" series radio... ).

Thanks a lot Eli for your suggestion.
I'm learning from You that Allen has passed away... How sad. This is one big loss, as I really liked his ideas, diverging from the standard stuff. I used to visit his site some years ago, and reading his articles in Sound Practices USA magazine as well. Maybe I will test one of his circuits one of these days.

Cheers,
Jose Mesquita
Lisbon
http://sites.google.com/site/sunajebem/faca-voce-mesmo-1/amplificador-211se
 
#12
I'm bumping this old thread in case there are still some SFP-1 users around, and the schematics are already here for viewing. I have had an SFP-1 in my main system for nearly 15 years now and it has been rock solid, until a couple of weeks ago. I have always used the MC input and have never switched it to MM. Recently I have been hearing a random popping and crackling in the left channel only. My first thought was a bad tube as the ones in the unit have been there for over a decade. I tried re-seating all of the tubes, no change in the noise. I then swapped the left and right channel tubes, but the noise still remained in the left channel. Strange. I then cleaned the MC/MM internal switches on both channels, cleaned the tube sockets and pins, but there is still noise in the left channel. Now I'm thinking it's not likely a tube issue. To confirm this I switch both channels to MM mode and there is no noise. OK, on to the MC FET stage for the left channel. I first replaced the B+ supply resistor that drops to the B+ to ~10V, no change. I then swapped out the J310 FET with a new replacement, still no change. Now I'm confused as to what the issue could be. I touched up all of the solder joints in that section and it did not help. Now I'm down to replacing all of the resistors in that stage, but before I do I'd like some guidance as to what might be the cause. Is it in any way possible it could still be a tube issue, even though they are quiet in MM mode? The MC signal goes through the same stages, so I'm not sure how that could be.

Thanks!
 
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