The Hybrid SIT Parafeed

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My journey to build this amp started at you tube.
I watched the Nelson Pass presentation about SITs at Burning Amplifier Festival and so I got inspired.
First I began to search the net for SITs and quickly found the Little SIT amp,
designed by Michael Rothacher. By then it was clear to me that I would get me some of the 2sk82 transistors to play whit it.

I wanted to do a different approach, to higher voltage region,
where the lines look good. The only way I could do it was whit an
output transformer. The output impedance of 2SK82 in higher region is 10-12R. But this was a problem since there is very difficult to
make a good low impedance, high current and sufficient inductance transformer. This way it was done by Lennart Jarlevang, he used the PASS SIT-1 whit output transformer and it worked very nice, only the low end was limited. So after this review the best option was a parafeed topology.
I was familiar whit the high level matching UTC LS-33 and LS-34 transformers. So there was only one thing to solve,
the plate reactor for 2SK82. I talked to megaherz audio in Italy and they made it very well.
So the hybrid SIT parafeed was born.

After first listening sessions I concluded it had extreme potential.
The sound was very relaxed, full bodied and the bass region was extremely good articulated.
So my amp went to at least two revisions. The parafeed capacitors are PIO type. This schematic is the result.
I still want to do some distortion analysis, but will need to wait when my friend is not busy anymore.

374704d1380888199-lamp-simple-sit-amp-uplsitparafeed.jpg


HyridSITParafeed.jpg
 
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Here a better photo.

HybridSITParafeed2.jpg

And some more data.

The A+ power supply has a 500VA toroid transformer whit 60V secondary.
The SIT is biased at aprox. 62V at 750mA. On grid we have -12,5V.
I am using two 33R mundorf suprime resistors in parallel and bypassed whit 4x1000uF 50V Elna silmic 2 and 680uF carafine.
The parafeed caps are obbligato PIO 5x70uF bypassed whit russian teflon FT-3.

6E5P tube is biased at -1,5V, so I could also use a alkaline battery instead of auto bias.
The anode voltage is 132V and current 33-35mA. The output impedance is around 900R.

Its a lot of iron. But iron is good for sound to my ears.
 
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Nice Hybrid... is the Impedance ratio of your OPT is it 53//8r ,and how many watts you squeeze from it?

Did you try using SE OPT only without the parafeed with higher A+(150v or higher) and lower idle current?

I didnt measure it. This needs to be done yet. I assume 10-11W.
I didnt use air gapped OPT. Maybe someone could make it,
but I guess it would be very expensive.
Higer A+ is possible, but you need to watch the power dissipation not more
then 50W for the 2SK82.
Also the input capacitance would be nice to know for this operaion point.
I can only assume that its lower then in high current low voltage operation.
 
Nice Hybrid... is the Impedance ratio of your OPT is it 53//8r ,and how many watts you squeeze from it?

Did you try using SE OPT only without the parafeed with higher A+(150v or higher) and lower idle current?

The impedance ratio of UTC LS-33 is 50R to 7.5R.
Very practical this matching transformer. I can connect speakers and headphones from 1,2-500R impedance.
 
After a good month of listening to the latest revison as pictured in the schematic I finally tried something really different. I loaded the driver tube whit Garry Pimms CCS and used the MU out to the 2SK82. At first I expected a more SS sound but I must say after few days of listening I can say that this is the best amplifier I ever heard. It must be the right combination of harmonics I guess.) The output impedance of the driver stage is way lower whit the MU out and the SIT likes that! I also changed the drivers PSU a little bit. Now I use 20H choke whit 150DCR and the anode voltage is little higher around 178V and 35mA...
I even could lower the input impedance of the SIT to 10K.
I am really surprised whit this CCS.
 
The other day I posted into wrong thread...
But here I am...
I am keeping posting for me as a diary...
I must say I made big improvement.
First of all I lowered the driver stage output impedance whit
CCS Mu follower output. Then I changed the cathode resistor and cap whit a
white diode (its a package in SMD whit two dioded in parallel). Forward voltage is
2.9V, so perfect. I get 180V at plate and 36mA current.
I also tried whit fixed bias on SIT but didnt work good. Its tooo much current drift couse of the unregulated supply...So maybe I will bypass the elnas whit 70-100uF Mkp or similar to get to cover the main frequencies down to aprox. 150hz...Will seee.
But this amp really sounds absolutly beautiful. There is some sort of sinergy grown between the SIT and the driver stage. It sound big and delicate but in no way harsh or SS like. This is a good candiate for a life time low power simple amp....
Now I will listen and will see how I can move on.......
 
scholl

Hello,

Can we get more details on the choke? Inductance, core size, DCR etc.

I know of only one other successful true transformer loaded transistor amp and all my attempts failed. Seems the internal resistance is too low and the transistor shunts the transistor.

Thanks,

Scott
 
Hello,

Can we get more details on the choke? Inductance, core size, DCR etc.

I know of only one other successful true transformer loaded transistor amp and all my attempts failed. Seems the internal resistance is too low and the transistor shunts the transistor.

Thanks,

Scott

Interesting never thougt of something like that....

The plate reactor has core size of 8,1cm * 10,2 cm * 6,9 cm. Its quite big and havy...Its from M6 grain oritented steel I think.
The inductance at 1A is 1H and whit lower currents more...DCR is aprox. 6-7 ohms...

It works well.....But if we talk about perfect world, I am a realist and can tell you there isnt a perfect or ideal world out there, so I am not looking for it...) This amp makes fun and joy...It just pictures the frequencies the way I like it. Thats all! :D
 
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