Contemporary tradition breaking amp schematics.

[JZatopa]

Although there has always been a divide between "sand" and "glass", there has also been a divide between tube guys. One group feels that the traditional ways must be used to get a good sounding amp, the other has no problem using some SS parts. What I would like to know is if there are any truly contemporary designs that have cropped up over the last couple of years. The meteor and power-drive come to mind but I was wondering if there might be others. I guess I am just wondering if there are other "new school" designs out there.

 

[jan.didden]

Bob Cordell's C-70 amp would surely qualify but you would need to order Linear Audio Vol 3 for the article.
He uses tubes in the amp channel and SS in the power supply regulators for all stages, plus designed the amp based on solid engineering rather than religious dogma.

jan didden

 

[Stormrider]

Looks like the C-70 is actually in Vol. 2.

 

[pauldune]

Some people will disagree, but I think my contraption qualifies:

http://www.diyaudio.com/forums/tube...plifier-2a3-d3a-ccs-coupled-new-topology.html

And no, it is not a "srpp".

Paul

 

[JZatopa]

Pauldune that is a very interesting design, did you ever make a final version? How would you compare it to other 2a3 amps you have heard?

The bob cordel design is interesting as well. I wonder if you could over build the outputs of other tube amps and use higher resistance output transformers to lower distortion and improve dampening further?

 

[Tubelab_com]

When I first posted the PowerDrive concept om my web site the reaction was mostly negative. Over time the reaction has changed and quite a few people actually use the concepts in their own designs.

I have continued to design my amplifiers from a purely engineering perspective, choosing the best component for the job even if it means sticking a silicon device in the middle of a vacuum tube amplifier. Many tube amp builders have grown to accept this, but every once in a while there will be an extremely negative voice with a loud PA system crying foul!

There is a thread about designing a guitar amp using less than $100 worth of parts. The "rules" weren't clearly stated, so I asked early on if there were any objections to a PowerDrive style mosfet follower. The replies agreed that it was OK.

About 1000 posts later up comes "is that a semiconductor I see in the middle of your design carrying AUDIO?" (post #1131) The $#*& storm started in post 1139 and it went downhill from there. Obviously there are still some strongly sandophobic amp builders out there that still share their opinion.

http://www.diyaudio.com/forums/instruments-amps/190738-hundred-buck-amp-challenge.html

About 5 years ago I built the proverbial "bridge too far" I not only stuck some sand in a tube amp, I wrapped a DSP around it! A SE tube amp is terribly inefficient. 5 to 10% plate efficiency is common, with 33% being the theoretical max in class A and maybe 50% in class A2.

It is common in the cellular phone industry to use DSP techniques to improve RF power amplifier efficiency by adjusting the supply voltage on the final RF amplifier in real time according to the instaneous power demands of the device. In other words you change the supply voltage on the fly to keep the amplifier just above clipping, with zero headroom. In a phone this improves battery life during a call.

We can do the same thing in a tube amp. You need a power supply that can be varried from 100 to 400 volts at an audio rate. A dsPIC chip can do that. This allows the dissipation in the output tube to drop....a lot. Tube dissipation is the limiting factor in most of our tube amps, and all of the SE amps. If we can reduce the dissipation, we can increase the power output. Does it work? Well how else can you get 20 watts fron a single 6AS7 in SE?

The device was built for a magazine contest where it won a prize. I wrote an article for the magazine explaing it, but I don't think it was well understood by the typical embedded systems engineer that reads Circuit Cellar. 5 years have passed and the article and associated information have vanished from Circuit Cellar's web site. There is a photo and brief discription here.

MiniTron

The article did get me a phone call and a few emails from the owner of a well known guitar and amp manufacturing company. I explained that that particular design wasn't well suited for guitar amp use since it was very clean up to the point where the dynamic power supply runs out, where it snaps into gross distortion. A few discussions never went any where, but a few more cool and unique circuits are coming.

 

[JZatopa]

Tubelab I always love seeing my fellow FAU alumni doing awesome things and I think that you are on the right track and have been for quite some time. I understand the purist approach to things but as I have grown I have found that to be truly limiting. It does have it's allure though. The fact is that if you want to make the best "X" you can, you need to use all the tech and tools available to you.

That minitron sounds like it is doing what some car amps do to keep cool yet have big output. Aside from power output, how was the sound compared to a conventional power supply? What are the chances you would rebuild the minitron using an Arduino for DSP? If you were to do that, I think you would grab the interest of a large number Arduino users.

 

[smoking-amp]

"The bob cordel design is interesting as well. I wonder if you could over build the outputs of other tube amps and use higher resistance output transformers to lower distortion and improve dampening further?"

Do you mean higher primary impedance was used, or higher winding wire resistance? Over building a tube output stage is certainly do-able, but will run the price and power consumption up.

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The "anti-triode" scheme (P-P emulation of SE) was used in an amp at one of the European triode festivals and won an award in a SE amp contest. Revintage entered it I think.

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You don't need a DSP for the power supply control, just use a conventional SS amp connected in series with the output of a tube amp. The feedback for the tube amp comes off the combined sum then (by special design, and special OT) so as to produce overall tube sound using only a few tube Watts. It is effectively a class G or H amplifier then.

Image from an old thread: (and another oldee image for paralleling tube + SS to unload the tube amp, using a high Zout, current output SS amp)

 

[TheGimp]

Wayback machine ..

http://web.archive.org/web/20090530164821/http://www.circuitcellar.com/microchip2007/winners/abstracts/MT2209_Abstract.pdf

I don't know if the link will last, but one can go to the wayback machine and insert the url in George's post and it will take you there.

 

[pauldune]

Pauldune that is a very interesting design, did you ever make a final version? How would you compare it to other 2a3 amps you have heard?

Yes, I did build a final version of it. It is my main amp for almost 2 years now.
Since then I had the opportunity to compare it to a lot of other sytems, including a few 300B and expensive PP tube amps.
It is difficult to compare complete systems with eachother, because of all the variables that change.
But this I can say, it is still the best amp that I ever heard.
It has it shortcomings, though. Efficiancy is not very good,and it is only about 6 watts.
Distortion figures are a lot better than the prototype, because of the better linearity of the 2a3/300b. I'm using a TJ 300b/2.5 meshplate now.

I didnt post much of it, because it seemed almost nobody was interested.

I'm thinking about a rebuild for some time now, i have a few ideas to make it even better. Dc filament heating, higher B+, and maybe the switch to GM70 as powertube.

Paul

 

[grufti]

The link worked for me just fine. That is one amazing design.

Wayback machine ..

http://web.archive.org/web/20090530...hip2007/winners/abstracts/MT2209_Abstract.pdf

I don't know if the link will last, but one can go to the wayback machine and insert the url in George's post and it will take you there.

 

[Fenris]

Broskie had an interesting Super Triode:
SuperTriodes & Inverted ES Speakers

I'm about 85% done building one, albiet with a few minor changes.

I just built another amp that has relatively straightforward tube topology, but uses tubes for voltage gain and a MOSFET as a current amplifying device, kind of like PowerDrive taken to the next level:

http://www.diyaudio.com/forums/tubes-valves/211352-new-amp-13de7-se-hybrid.html

 

[Michael Koster]

Here are a few I have tried:

http://www.diyaudio.com/forums/tubes-valves/118725-another-kind-hybrid.html

http://www.diyaudio.com/forums/tube...sist-totem-pole-current-mirror-pp-hybrid.html

http://www.diyaudio.com/forums/tubes-valves/136179-schadeode.html

http://www.diyaudio.com/forums/tube...r-triode-strapped-pentode-12.html#post2207735

http://www.diyaudio.com/forums/tubes-valves/144767-class-a2-direct-mosfet-coupled-se.html

http://www.diyaudio.com/forums/tubes-valves/163940-g1-g2-mu-scaled-drive-strawman-design.html

 

[Tubelab_com]

That minitron sounds like it is doing what some car amps do to keep cool yet have big output

I haven't kept up with the current state of the art in car amps, so I can't say, but my design is effectively a class H amp. There are three ways to make a high efficiency audio amp. Class D, class G and class H.

Big power with high efficiency can be obtained by using a fully digital implementation. This is offten called class D and is basically a large agile SMPS with a low pass filter on its output. The power supply voltage is varried at an audio rate and then the audio is recovered by low pass filtering. This offers the best efficiency. There can be problems with interaction between audio harmonics and the switching frequency and filtering issues, but modern implementations have become very good. TI has a "chip amp" that cranks out 600 Watts!

A class H amp has one or two (bipolar) power supply rails that are agile (modulated at an audio rate) and applied to a conventional (typically solid state) audio amplifier. The audio amp remains fully analog and if it is designed with good PSRR the implementation can be an excellent performer. This requires a conventional audio amp of good design and at least one complex power supply, so it can be quite complex. The efficiency is less than class D, but can come close.

A class G amp has a conventional audio amp and multiple sets of supply rails that come from conventional power supplies. The amp switches between sets of supply rails depending on instaneous demand. There is another variation that uses a pair of output transistors on each set of supply rails and the instaneous demand determines which set of output transistors are engaged. The original Carver M-400 cube amp worked this way. It had a +/- 30 volt supply, a +/- 50 volt supply and a +/- 80 volt supply with a complementary set of transistors across each supply. The efficiency is the lowest of these choices and switching issues and power supply problems lead many of these amps to early failure. I got one of the first ones as did many others in the audio club at work. Most blew up within a year or two but mine was still alive when I recently gave it away despite me using it in bridged mono mode (about 700 watts) as a guitar amp.

You don't need a DSP for the power supply control, just use a conventional SS amp connected in series with the output of a tube amp.....as to produce overall tube sound using only a few tube Watts. It is effectively a class G or H amplifier then.

I tried the circuit in your top schematic when you first posted it. I could never get it to work. The SS amp (I used a LM3886 chip) overpowered the output impedance of the tube amp. I never tried to really work on it though.

I now believe that it is possible to build a simple agile SMPS without a DSP, but the whole purpose of the design contest that provoked the amp in the first place was to find new and unique applications for Microchip's new dsPIC chips that were optimized for SMPS power supply use.

Aside from power output, how was the sound compared to a conventional power supply?

The output stage in most SS amplifier designs is based on a pair of complementary emitter followers. By design an emitter follower has good PSRR (power supply rejection ratio). This means that noise, crud, and other imperfections on the power supply won't pe passed on to the speaker as long as the headroom demands on the output stage are met. If we are going to be messing with the power supply knob in real time while the amp is playing, we need lots of PSRR. The typical common cathode tube stage won't cut it. The only useful choice is a cathode follower. So I designed and built a few tube amps with cathode follower output stages. My experiments were all SE but P-P can be done too. Details here:

http://www.diyaudio.com/forums/tube...amplifier.html?highlight=cathode+follower+20W

The cathode follower amp designed above has a different sound than a typical SE tube amp. The sound is more dynamic, and the bass is solid. Like many SE tube amps, this amp can be pushed into clipping without the obvious harshness found in a solid state amp. Also the distortion characteristics are typical. The more power it makes, the higher the distortion. I still have it and now that I have a big power supply I must hook it back up.

The cathode follower amp with the DSP modulated power supply sounds similar up to a point. The measured distortion characteristics are atypical. The distortion settles at about 0.5% once the output power is high enough to overcome the noise, about 200 mW. The distortion stays nearly constant rising to 0.9% at 10 watts. The output stage sees a constant voltage across it, so it delivers a constant distortion....until the power supply can't go any higher, then things get ugly fast. It sounds like a solid state amp hitting clipping.

What are the chances you would rebuild the minitron using an Arduino for DSP?

For this application.....zero. The Microchip dsPIC chosen for the Minitron (and the Microtron and Megatron that only existed in the simulator) had some specialized hardware inside it that was designed for SMPS use. The agile buck converter that ran the power supply could run on its own with software needed only for initialization and setup. The rest of the chips processing power could be used for other tasks. There is no other microprocessor with this capability.

The original project submission (not the abstract) outlined a feature set that included auto bias, auto tweaking, several distortion profiles, and more. I never finished the code for all of that stuff, but some amp company in England had the balls to patent some of those concepts AFTER I published the article. They even used some of my verbage. That is what drew the emails from Peavey.

I have been working on refining those concepts on and off in the 5 years since I wrote those articles. I will eventually post some designs using those concepts. My current design is a guitar amp controlled by a microprocessor. It does not use power supply modulation, but there are some new tricks that I have not divulged yet. I did not choose an Arduino for this project, BUT I did use one of these:

Digilent Inc. - Digital Design Engineer's Source

Think of it as an Arduino on roids. It has the same form factor as an Arduino Mega, and can be programmed with Arduino sketches.

If you were to do that, I think you would grab the interest of a large number Arduino users.

I don't know how many Arduino users would also want to build a bad azz tube guitar amp, but.......

 

[SY]

Look at any of the schematics at Tube amplifiers for high-end audio by The David Berning Company for totally nontraditional approaches. David did solid state followers driving sweep tubes long before any of us ever even thought to use these parts. Look at the EA2-150- sweep tubes, screen drive, digital autobias, switching supplies... and in 1977!

 

[Tubelab_com]

David did solid state followers driving sweep tubes long before any of us ever even thought to use these parts.

I knew that Berning did screen drive amps. I didn't realize WHEN he did them. In 1977 I was making audio amps, and big ones at that......they just didn't glow. All were sand based life forms back then.

I kind of drifted away from tubes in the early 1970's and didn't find my way back until I traded a car for a Scott 272 integrated amp in the 80's. I still have the amp. The car has been crushed and melted. I may be selling the amp soon though. It hasn't been plugged in in about 15 years.

In 1977 I had discovered the microprocessor and was attempting to make music with them. Of course working for the company that designed the 6800 and 6809 chips helped too.

 

[SY]

In 1977, I was building very conventional tube amps. My first exposure to David's work rocked my world.

 

[smoking-amp]

"I tried the circuit in your top schematic when you first posted it. I could never get it to work. The SS amp (I used a LM3886 chip) overpowered the output impedance of the tube amp. I never tried to really work on it though."

If the tube amp is lower power than the SS amp you have to use an OT with a lower output Z than the usual 8 Ohms. Basically the tube amp only contributes a fraction of the total output voltage at the same current when connected in series, so you can figure the secondary impedance (versus 8 Ohm) from the Watts ratio required. Typically the secondary will be under an Ohm, so the effective OT primary/secondary ratio is higher than normal, which then makes it a lot easier for the tubes to drive.

 

[JZatopa]

I don't know how many Arduino users would also want to build a bad azz tube guitar amp, but.......

If you build it, they will jam!

 

[JZatopa]

Look at any of the schematics at Tube amplifiers for high-end audio by The David Berning Company for totally nontraditional approaches. David did solid state followers driving sweep tubes long before any of us ever even thought to use these parts. Look at the EA2-150- sweep tubes, screen drive, digital autobias, switching supplies... and in 1977!

Those are some of the coolest amps I have seen in a very long time. I would love to hear how they perform in person. It looks like he also designed the Milbert BAM car amplifier, an amp I have wanted to own for a very long time. It's a shame it's so expensive.