This thread is an offshoot of the excellent and eye-catching work of F.A Schmidt, Sijosae and others on creating hybrid amplifiers.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.]
posted 10-24-2005 10:41 PM CST (US) 
