L'Amp: A simple SIT Amp

Member
Joined 2009
Paid Member
I played again with the SK82 and a CCS version with a R100. The rival was a R085 instead of the SIT.

The gain with the R085 was ugly high, about 25-30x and you could already hear the hiss from the semi with my 86db Lowther.
Sound was heavy and good, but not spectacular.

The sound with SIT was much more detailed, but very thin.

ZM advised me to seek for the reason not in the CCS, but in the lower part.

First I thought that the different transconductance makes the difference.

Around 5-6 Siemens against 0.5 Siemens. Or was it the big amplification difference hat fooled me....
Later I read Mikes article again, and stopped at the 'load line' term.
Indeed I had a combination of voltage and current for the Sit that was not on the loaddline Mike showed.

I chosed the 19V/1.6A point and I have the strong impression that the sound now lost the overly bright character.

Am I fooled by my exspectations? Does anybody have similar experiences?

Or is it simply the different combination of K2 and k3 that pleases me.
What is the advantage or disadvantage to be on the loadline with the SIT?

Is it possible that the importance of loadline is less important with the R085?

Any answers welcome!
 
I believe that the K82s need to be matched a bit, following the loadline in Mike's article.
I dont have a tracer for high voltage/current, but I can tell for sure that there is a lot of variance among the sony SITs. I have 4 units, two are decently close, but the other two are way off (at the extremes).
 
Pass DIY Apprentice
Joined 2001
Paid Member
The R085 is more pentodey than a 2SK82 so there's less opportunity for working the loadline in OTL territory. Interestingly, the R085 (I posted some curves somewhere recently) is probably more well suited to an SET coupled design as it's more triodey at low-current high voltage. You probably couldn't go real high on the primary impedance, but a 64-150 Ohm transformer is probably workable, e.g. NP's Arch Nemesis.

You might also want to try a bit of degeneration with the R085, if the f-word doesn't bother you.

For the second part of your question: with a SIT, working the loadline to get the harmonic flavor you like best is where all the fun is. In my experience when the second and third harmonics are about the same magnitude things start to sound more threadbare.
 
diyAudio Chief Moderator
Joined 2002
Paid Member
The R085 is more pentodey than a 2SK82 so there's less opportunity for working the loadline in OTL territory. Interestingly, the R085 (I posted some curves somewhere recently)

Here your traces again, annotated.
 

Attachments

  • R085_Id_Vds.png
    R085_Id_Vds.png
    41.5 KB · Views: 533
Member
Joined 2009
Paid Member
I want to thank you all for your help!

So the price for being outside the 8R loadline is mainly a higher distortion?

In Wikipedia the loadline principle is explained as a process of matching a non linear device (SIT) to a linear device (resistor) . Of course we all know that speakers aren't Ohmic.

Yes I will play with a source resistor for the R085, in my build a 1R source resistor reduced gain already under 20db.

Of course I will try Nelson's conditions mentioned in Buzz R085 thread too(42V/2A).

Building a circuit is one pleasure, the more refined pleasure seems to be the trimming!

:)

@ZM and buzz what is ACA?