Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 14th December 2011, 10:57 AM   #1
Michael Rothacher is offline Michael Rothacher  United States
diyAudio Member
Michael Rothacher's Avatar
Join Date: May 2001
Default L’Amp: A Simple SIT Amp: Part Deux


In the first L’Amp article, we built-up a simple Class A amplifier using a not-quite-unobtainable Static Induction Transistor. It’s a nice amp but doesn’t have a lot of power or voltage gain. In this little vignette, we’ll examine a simple option for improvement.

An Inconvenient Truth

Apparently incandescent light bulbs are bad. Wicked, naughty light bulbs. When I went to the hardware store to pick up bulbs for the first article, there were little signs all over the shelves reading STOCK UP NOW! It seems incandescent bulbs are being phased out in order to save the planet, etc. (not that there’s anything wrong with that).

As if audio hobbyists weren’t already neurotic enough about obsolete parts, now we have to worry about light bulbs. Of course, we could always use a big power resistor on a big heat sink with a big carbon footprint. Actually, in the US, 300 Watt bulbs are exempt from the current legislation which covers bulbs from 40 to 100 Watts so we can relax for the time being. However, there is an interesting alternative if you’re so inclined.

L’Amp Gets Inducted

So, maybe you’d like to see a little more gain from our amplifier and while we’re at it some more power would be nice too. And whether you use light bulbs or a resistor, the amp isn’t terribly efficient (not that there’s anything wrong with that) so maybe you’d like to try and improve that figure as well. Oh and maybe you’d like to accomplish all of that without adding complexity to the circuit or more transistors.

You’re very demanding, but okay we can do that. Many of the old audio textbooks had amplifier examples which used inductors for the Collector load (Drain for MOSFETs). I asked about this on the forums many years ago and Nelson Pass responded that he’d used a big spool of magnet wire in a Zen-type circuit with good results. A big spool of magnet wire, eh? Again, Pass comes through with a nice hardware store solution. So, I bought a couple of spools, and as happens in this hobby, they went on a shelf and stayed there for several years until recently.

While writing the first L’Amp article, It occurred to me that the middle part of the 2SK82 curve might be interesting with the right load, so an 11 pound roll of 16 gauge magnet wire came down from its shelf. What’s so special about using an inductor as the Drain load? Well, the impedance of an inductor is frequency dependent. The impedance at a given frequency can be calculated like this:

In this formula, f is the frequency in hertz and L is the inductance in henries. We can see that an ideal inductor will have high impedance at high frequency, low impedance at low frequency, and zero resistance at DC.

A big spool of magnet wire isn’t exactly an ideal inductor and there will be some DC resistance and other losses. My spool of magnet wired measured around 110 millihenries and the DC resistance was 5-6 ohms. Lower resistance and more inductance would be nice but we work with what we have.

You may recall from the first article that the resistance of the two parallel light bulbs was around 13 ohms and the effective amplifier load was the bulbs in parallel with the 8 ohm speaker or about 5 ohms. If we replace the bulbs with the spool of magnet wire as in Figure 1, the effective load of the amplifier will be:

Since the impedance of the inductor will be quite high compared to the load at audio frequencies, the effective load is approximately equal to the speaker load or 8 ohms. An 8 ohm load line would be a bit flatter and might just do the trick in the middle of the 2SK82 characteristic curve so we plug in the inductor (Figure 1) and search for the sweet spot.



Our 1-watt distortion is about the same, but now we’re reaching 12 watts with less than 2% distortion (Figure 2). The inductor stores energy in its magnetic field which effectively increases the supply voltage (double in theory). This increases our maximum voltage swing and you may have noticed, I lowered the supply voltage to 35 volts. Our efficiency has improved from 4% with light bulbs to around 20%. The gain has gone from 6 dB to approximately 8 dB, which isn’t a lot but it’s about as much as the original 1994 Zen.

How’s that for a slice of fried gold!

More Measurements

Figure 3 is the distortion vs. frequency and the higher distortion at low frequency is evident. A bigger coil would help but we’ve seen worse.


The frequency response was -1.12 dB at 20 Hz and -1.72 dB at 60 kHz. The output impedance was approximately 8 ohms and the amp didn’t have a significant turn-on thump. Figure 4 is the square wave response at 40 kHz.


For fun, here's the load line:

Heavy Metal

This is a heavy amp which may appeal to you, audiophiles like heavy equipment. One of my monoblock channels with transformer, inductor, heat sink, and case weighs over 25 pounds giving it an impressive audiophile-approved weight to watts ratio. Each spool of magnet wire will set you back twenty times the cost of a pair of light bulbs and about half the cost of an output transformer. The magnet wire I used is made by Essex, and each spool is wound with approximately 1,400 feet of heavy-insulation 16 gauge with a resistance of approximately .5 ohms per pound.

Figure 5 is the complete schematic for a monoblock channel. I used a transformer from Antek with two 28 volt and two 15 volt secondary windings. The negative bias supply is just a typical application note circuit using an LM7912 negative voltage regulator. You can try something much fancier if you like. P1 adjusts the bias voltage and R2 sets the minimum bias value.


Once everything is assembled test the power supplies first without the amp connected. If everything checks out, then adjust P1 fully clockwise and bring up the power. You should measure approximately -12V from the wiper of P1 to ground. Now slowly adjust P1 counterclockwise (towards zero volts). You’re looking for approximately +23 volts from the JFET drain to ground. The idle current (about 2 amps) is equal to the voltage across the coil divided by its DCR. The bias supply will be around minus 5-6 volts. If all goes well you’re ready to connect them to speakers and try them out.

NOTE: The author suggests that you should always have a speaker or dummy load connected when powering the amp on and off. Furthermore, you may wish to install a 200 Ohm, 3 Watt bleeder resistor from output to ground to safely discharge the coupling capacitor in case you should forget the foregoing.

So, does this one sound tubey? There are good tube amps and bad tube amps; so when we compare transistor amplifiers to tubes, we’re making a sweeping generalization about the sound of tubes. But eventually the question will arise. Assuming we’re talking about attributes like richness and warmth, good depth, and sweet highs, I found this amp to sound much like the original but (and I could be psyching myself out here) I’d say this one sounds even tubier. Slightly.

We’ll Meet Again, Don’t Know Where, Don’t Know When…

I think it’s fair to keep the name L’Amp. The L in this case stands for Inductor. This little upgrade is a pretty good example of just trying something out and seeing what happens. It may be a little over the top but this is DIY after all and we’re generally undisturbed by extreme measures to achieve incremental improvements. I wouldn’t view this as a mandatory update, Rather it depends on your particular requirements and spirit of bold curiosity.

Well, that’s all for this pair of articles; I’ve had a lot of fun preparing them. It is plain to see that Static Induction Transistors are very interesting devices and have lots of potential as audio amplifiers. I hope you are inspired to try them.

I think we’ll be hearing a lot more about SIT transistors in the not too distant future, as I’ve read that First Watt has begun releasing amps into the wild for evaluation. Thanks again to Nelson Pass and friends at the Pass Labs Forum on diyAudio.

Now go build something!

A thread about the various L'Amps is here:
L'Amp: A simple SIT Amp

© Michael Rothacher 2011

Last edited by Michael Rothacher; 17th October 2013 at 11:53 AM.
  Reply With Quote
Old 3rd February 2012, 07:00 AM   #2
Zen Mod is offline Zen Mod  Serbia
Official Court Jester
diyAudio Member
Zen Mod's Avatar
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain

my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM
  Reply With Quote
Old 3rd February 2012, 10:19 AM   #3
nicoch58 is offline nicoch58  Europe
diyAudio Member
Join Date: Oct 2004
When an output transformer ?

Great work ,thanks !

Last edited by nicoch58; 3rd February 2012 at 10:38 AM.
  Reply With Quote
Old 3rd February 2012, 11:07 AM   #4
Michael Rothacher is offline Michael Rothacher  United States
diyAudio Member
Michael Rothacher's Avatar
Join Date: May 2001
I know a couple of readers had their hearts set on something active, so here you go:

L'Amp CCS Addendum
  Reply With Quote
Old 3rd February 2012, 11:29 AM   #5
Michael Rothacher is offline Michael Rothacher  United States
diyAudio Member
Michael Rothacher's Avatar
Join Date: May 2001
Ah yes, the coveted "Fugly". Thanks ZM
  Reply With Quote
Old 3rd February 2012, 01:41 PM   #6
Tea-Bag is offline Tea-Bag  United States
not politcally affiliated
diyAudio Member
Tea-Bag's Avatar
Join Date: Feb 2006
Location: Kennebunk
for 16R speaks - 700ft of wire?
Did you try the R085's in place of the IXYS's in the addendum?
Did you have eggs or oatmeal for breakfast?
  Reply With Quote
Old 3rd February 2012, 02:03 PM   #7
Michael Rothacher is offline Michael Rothacher  United States
diyAudio Member
Michael Rothacher's Avatar
Join Date: May 2001
I'd still want a coil at least as big as the one in the article. You really want 100+ mH with low DCR. More inductance and less DCR would be even better. The coil in the article represents a readily achievable happy medium.

I haven't yet tried the R085.

Eggs, with turkey bacon
  Reply With Quote
Old 3rd February 2012, 02:17 PM   #8
Antoinel is offline Antoinel  United States
diyAudio Member
Join Date: Jan 2012
Location: Pennsylvania
Voice Coil of a high power woofer; to be used while inside or outside the magnet? Heat transfer?
  Reply With Quote
Old 3rd February 2012, 04:44 PM   #9
nicoch58 is offline nicoch58  Europe
diyAudio Member
Join Date: Oct 2004
lundhal have nice filament choke but $
  Reply With Quote
Old 4th February 2012, 11:55 PM   #10
bohica2xo is offline bohica2xo  Qatar
diyAudio Member
Join Date: Dec 2008

Almost looks like a class E RF amplifier.

  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
L'Amp: A simple SIT Amp ra7 Pass Labs 2828 13th August 2017 08:18 PM
L’Amp: A Simple SIT Amp: Part 1 Michael Rothacher diyAudio.com Articles 23 1st September 2014 01:02 AM

New To Site? Need Help?

All times are GMT. The time now is 01:38 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio