We’d like to present a slightly different topology of hybrid design, using the same two basic components of a tube and an opamp. Both Batman and Mains_hum have built the amplifier I’m about to describe and can comment on its sonic characteristics.

We’ve called this amp the SOHA.

The design of the amp started when Batman started pestering me and Mains about building one of the YAHA amps (YAHA Amps). Being mostly interested in tube amps I didn’t want to fool with the design, but both Batman and Mains began to suggest requirements for the amp and some options for the input tube. Eventually, we came up with the SOHA. I was so ticked at how much they both had bugged me that my first drawing of this amp was called the Stupid Opamp Hybrid Amplifier. The name, as a running joke, eventually stuck and we finally are just calling this amplifier the Stoopid. Batman and Mains will take this up later. But, they tell me that they like the name because it sounds Stoopidly good.

Most of the hybrid amps that have appeared on these pages have used the same B+ for both the tube and the opamp. Some of these amps are designed to be portable enough to run from some kind of battery, but most are really constrained to a low voltage DC supply of some kind, in many cases AC supplied.

It is clear, however, that tubes that are not designed for low voltage use will not perform well at 12-24V. And so we decided to try to provide the tube with higher B+ to get better performance while still keeping the voltages fairly low. This meant that the amp could be small and portable, but would require AC power. Like the other hybrid amps, the Stoopid is designed to give the sound of tubes while avoiding the high voltage risk that some builders don’t like.

Using a common B+ actually does two things to a hybrid amp: 1)It puts the opamp in a single ended configuration where it needs an output cap to block half the B+ and 2) It forces the B+ on the tube to be low.

So, the first thing to do with this amp was to decouple the B+ for the tube and sand. Doing this makes it possible to use a standard bipolar supply for the opamp, eliminating the large output cap and requiring only a small coupling cap between stages.

To keep the tube as linear as possible CCSs are used for plate loads. The opamp, OPA2134 for this version, is run in unity gain mode. One nice feature of this opamp is that it current limits to 40mA.

Batman and Mains built slightly different versions of the amp. Batman used the FET CCS that I originally designed but Mains wanted a BJT CCS. The next incarnation of the amp will probably use JFETS for the CCS.

Both schematics are here:

Batman's Stoopid

Mains_hum's Stoopid

[Edit] After a few more builds of the Stoopid and may posts later, here are the recommended Stoopid configurations. They are listed in order of preference. Note that the heater supply is now regulated DC.

Stoopid - Preferred Configuration

Stoopid - CC Diodes

Stoopid - JFETS

[End Edit]

[Edit]From pages 24 and 25 of this thread I am backdating more schematics. These show mu follower arrangements that will allow you to drive opamps that exhibit more offset currents and lower input impedance. YMMV.

Stoopid with JFET Mu Follower

Stoopid with MOSFET Mu Follower

[End Edit]

The key to this amp is the power supply. It uses an easy-to-acquire 30VCT/200mA transformer. Since the tube is a 12AU7 (12.6V/150mA) the tube filament is taken directly across the AC secondary using two dropping resistors.

The bipolar supply is a conventional regulated supply. Mains chose to use adjustable regulators and set his voltage to about 10.6V. Batman chose fixed 12V regulators. Looking at the schematic you’ll see that the opamp configuration with its power supply is very conventional.

The trick to the amp is that the PS uses a 1.5X full-wave voltage multiplier to generate the B+ for the tube. To make the doubler, the entire secondary of the transformer is rectified through a pair of coupling capacitors and bootstrapped on top of the V+ of the bipolar supply. With a typical transformer with 25% regulation and with no load on the B+ for the tube, this generates over 80V (this is marginally dangerous and will give you a pretty good sting so be careful). The 12AU7s are running at 1mA each. When the B+ is loaded with the tubes, it pulls down to between 55-60V. This means that there is plenty of headroom in the B+ to run the tubes at 40V while still leaving space for driving 7-10V into the opamp. And this seems to give very good performance.

The input stage has a gain of about 12 making it possible to exceed the input tolerances for the opamp with 1V at the input. The current design uses 1N4148 protection diodes, but you can probably skip these to simplify the build if you’re careful how you drive it.

Batman and Mains_hum have breadboarded the Stoopid and seem to like it very much. The operating points for the tube and the bootstrapped PS made it impossible to establish some of the component values correctly in the first draft design. Batman and Mains did all of the tweaking on the amp through their breadboarded prototypes. I guess it’s time for them to talk.

[Edited by runeight on 01-09-2006 at 09:42 PM.]

I like that very much.
It is like seeing children grow up.

[Edited by runeight on 10-25-2005 at 07:12 AM.]

We're still at the prototype stage and here is a pic of the first iteration of the amp on a perfboard.

The amp (like the YAHA) is stoopidly simple and has less than $50.00 worth of parts and no lethally high voltages. It would make an ideal tube headphone amp for those who'd like one but fear high voltage.

OK, how does it sound, well, in a word, stoopidly good. When I first powered up the prototype, the plate voltage was only 17V and the amp sounded decidedly solid state, kinda "steely" and just tonally "off". As we raised the plate voltage the sound became more lush and tube-like and I began not to believe my ears. This thing easily rivals my Cavalli-Jones which cost over three times more to build! It's got decent amounts of nice bass,classic sweeet tube midrange and plenty of top end extension. Also,the soundstage is extremely wide and respectably deep. This thing is just plain stoopid fun to listen to!

Another remakable thing is that this thing can drive cans of any impedance. It sounds really reamarkable with my Sennheiser HD600's but can also drive my Shure E3C (28 Ohms) and Sennnheiser HD280 Pro (64R).

I've cased up the prototype in a 4 X 5.5 inch can that I had lying around just to show how small the unit is. We're now concentrating on making it even smaller so it will be a fitting compliment for the Rio Carbon and other such players. While it will never fit a mint tin, we ought to be able to reduce its size considerably to, pehaps, 3 X 5 inches. If anybody's got good suggestions for enclosures (besides the Hammonds), I'd like to hear about them.

[Edited by Batman on 10-30-2005 at 05:12 AM.]

I guess what we need now, Batman, is for you to do a wiring layout for us DIY dummies in the crowd. (Please.)

Very cool, guys, and my hat's off to you.

12au7 certainly isn't an expensive tube, but which tubes can easily be substituted?

Other 12v heater high-mu double triodes i have are 7025, 7058, and some russian tube - the designation of which escapes me - which i was told was equivalent to iirc an ecc82? I'll have to dig it out.

http://www.nj7p.org/Tube1.php?tube=7025
http://tdsl.dun...ow.php?des=7058

I want to build one of these but I'm not in a hurry as I've decided to start construction on Bender's 6n1p OTL amp as soon as i have my M3 enclosed, and perhaps my multihybrid rebuilt.

[Edited by ericj on 10-25-2005 at 12:19 PM.]

my concept was a bit different tho. the amp as a whole getting only sigle supply and the buffer stage feeding on virtual ground circuit.. and i talked to myself, "do i need another buffer for ground?" soon i got frustrated - am a beginner after all.

Here's the original prototype that assured us that it is possible to make a suprisingly good performing amp utilising a valve, yet be cheap, easy, and reasonably electric shock free.
The transformer is a very cheap 0-15-0-15 unit available in the UK and other EU countries.

After altering some of the PS capacitor values, CCS resistor value etc. etc. we ended up with something that was stable and fun to listen to for extended periods.
Yes, it applies some compression but that's part of its charm, and it would be a rather sterile sounding amp without the 12AU7 (ECC82) altering the sound the way it does with 40V anode volts.

I for one hope that folk attempt to build this amp, suggest/implement improvements, alterations, whatever ....

Now to go build a "proper" one.

btw, i put a diamond buffer module on my yaha and i say it deserves a try. perhaps u might want enough space around the sockets for further mod.

[Edited by ryanjun on 10-25-2005 at 12:52 PM.]

Anyway... this looks very promising.
I can't wait to see how it turns out to be. Nice work, and hat off to you guys.

Gotta order some tubes and parts.

[Edited by seaside on 10-25-2005 at 01:33 PM.]

seaside,
this amp is different to other amps we've cooked up in the past as it allows experimentation with valves with a much reduced risk of being thrown across the room if you touch something you shouldn't

Does this mean at last we get you folk to "try a tube" ?

I am wondering if it might make more sense build a Stoopid first. It will cost less than $50 and it won't have the extremely high voltages. If it sounds good, well then maybe you don't have to take the next step. Or, even if you do, you'll have a nice portable amp to listen to while you're building the next one.

Also, if you decide to build an all tube amp, might I suggest that the 6922 CJ would be an easier build.

---

quote:

[By runeight] ericj, here's a thought for you. Batman and mains_hum tell me that the Stoopid sounds as good or better than the Cavalli-Jones amplifier. The Bender OTL (which, I actually modified to work correctly in the article) is identical to the CJ except for two things: 1) it uses 6N1Ps and 2) it needs a 350V B+. This means that it is likely to sound about the same as a CJ.

I am wondering if it might make more sense build a Stoopid first. It will cost less than $50 and it won't have the extremely high voltages. If it sounds good, well then maybe you don't have to take the next step. Or, even if you do, you'll have a nice portable amp to listen to while you're building the next one.

---

Well, here's the thing.

1: I already have a very nice potted transformer with a 285v 95ma secondary and 6.3v 2.25a secondary.

2: I already have nearly all of the high voltage caps required for the 6n1p-otl. All I'm short, really, are a handfull of resistors and the .47uf 400v caps. And two 6n1p's, as there is currently only one in the closet. This is not the result of an intentional collection of parts - i just turn out to have them already.

3: 6n1p-EV are ubiquitous and much cheaper than more sought-after 6922.

---

quote:

[By runeight]
Also, if you decide to build an all tube amp, might I suggest that the 6922 CJ would be an easier build.

---

Can you explain why?

OTOH, it looks like you have everything. Are you building the one with variable feedback?

---

quote:

[By runeight]

OTOH, it looks like you have everything. Are you building the one with variable feedback?

---

That sounds like a good idea but i haven't precisely nailed down where that variation is described. Do you have a url?

For that matter, I'm unsure where the CJ amp is documented. People refer to that amp constantly, so a google search is pretty much worthless. Just one of those things that everybody already knows, I guess.

6N1P CJ

[Edited by runeight on 10-25-2005 at 06:58 PM.]

Thanks for the pointer. I apologize for my ignorance.

I do still plan to build a Stoopid - as you know it's not so much the having of the amp as the fun of building it. If i end up with too many amps i can sell them or give them away as gifts.

Once we get the design finalized, I'll draw a three-layer pictoral drawing so that even newbies can build this amp. I've almost got a parts list completed as well, which I'll also post.

Nothing wrong with the 213x, but I'm curious what was meant by saying it limits current to 40ma? Not saying i need more than that, just wondering what made it an advantage.

When i build this, I'll be tempted to make it a home for one of my 2107's, since I can't use those in lower voltage projects. But i have plenty of 2132P's, too.

But, BJT input opamps or unity gain buffers have to be tested with this amp because they might load down the tube too much. Buffers will have too much offset voltage at the output and since a solitary buffer won't be within an opamp feedback loop, it will have offset.

OPA2134 and its relatives are fairly common opamps for audio applications and we had to start somewhere. But it would be nice to know how other opamps perform.