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ra7 2nd December 2011 03:00 PM

L'Amp: A simple SIT Amp
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Seeing as there isn't a thread in Pass Labs for this wonderful little amp, I thought I'd start one.

I have all the key parts. In fact, the circuit is identical to the Delite, except the actual transistor. I will be building this soon.

Mike, what's in store for part 2?

Roscoe Primrose 2nd December 2011 03:16 PM

Michael Rothacher 2nd December 2011 07:14 PM

1 Attachment(s)
To kick off the thread, here’s some bonus material with special thanks to a few early beta testers.

1. Use a properly sized or oversized heat sink, period. Don’t skimp here.

2. I used 50 Watt Dale power resistors attached to small heat sinks for both of the 1 Ohm resistors. 20 Watts or larger is recommended. I like to comfortably oversize these parts. They’re cheap on Ebay.

3. I liked the sound with no source degeneration best, but I knew some beginners might want a mostly plug-and-play option. If you build Figure 11, you want the KD-33 grade parts sold by Acronman, or the circuit probably won’t work correctly. If you have the skill and confidence, I would recommend the version with the separate voltage reference for the bias and no source resistor (Fig. 10).

4. I haven’t tested any other grades and I have no data on them. If you have definitive information on the grading please share it here.

5. Until I started experimenting with these devices gate leakage wasn’t something I’d had to worry much about. I bought four of these VFETs and they all measured around 1 uA, which is pretty manageable, but I don’t know if this is representative of all 2SK82’s. You may need to lower the value of R1. Try 10k and see how that goes.

6. The operating points as published are for 8 Ohm speakers.

7. I’ve attached the diagram for the negative bias supply I used. You could probably do something simpler, or perhaps more complicated. It’s up to you. It doesn’t require much current, but it needs to be clean. A certain wizard I know posted a diagram for a negative bias generator which derives from a single B+. Here’s the link:

8. By the way, this amp has a really great “Glow Factor”, not too bright, not too dim, but just right for those late night listening sessions.

This one’s a lot of fun, and once you have all the parts, you could build it over a holiday weekend.

More later…


Roscoe Primrose 2nd December 2011 07:30 PM

When my K82s get here, I'll be building one kinda like fig 11, but with V+ of about 22v and a 1:1 transformer in place of the light bulbs.... I'll most likely lose C1 as well. Will report when it's up and running...


CanAm Man 3rd December 2011 01:25 AM

I can see it now, Mike......

In a few days, folks will be asking you if you can cascode your SIT's in this design, etc, etc, etc......! ;)

ra7 3rd December 2011 01:30 AM

The first 'improvement' can be substituting the bulbs for a CCS. It usually results in more power and lower distortion. It'll be interesting to see what Roscoe comes up with using the 1:1 transformer. Keep us posted.

stefanobilliani 3rd December 2011 11:42 AM

Very nice article thanks !
I have just ordered a couple of those Sony's pearls .
Wonder how good it'll sound with a negative Vgs ....

Michael Rothacher 3rd December 2011 11:55 AM

In Part Deux, we'll improve the gain and efficiency a bit, but it's an easy mod if you choose to try it, so I'd go ahead and start building. :) No spoilers.

The thing to know about these devices is that they cry out to be used in really simple circuits. I'll be sticking to the simple stuff.

stefanobilliani 3rd December 2011 12:02 PM


Originally Posted by Michael Rothacher (
No spoilers.

The thing to know about these devices is that they cry out to be used in really simple circuits.

Oh so good to hear that. Actually can't wait to receive them , in the meantime
I am listening the same little amp with the LU1014D , no degeneration , no cascode . So sweet .

Michael Rothacher 3rd December 2011 12:18 PM

1 Attachment(s)
And here are some of the bonus materials.

Here's a pic of the makeshift curve tracer I used:

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