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Old 29th June 2014, 01:42 PM   #1
wicked1 is offline wicked1  United States
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Default Modular Synthesizer with Tubes

Tubelab Said:
In 1970 I saw ELP and became infatuated with the Moog Synthesizer. All synthesizers in 1970 were analog, voltage controlled. I got an idea for a completely digital synthesizer and in 1971 I started building it. It took about a year to make sound, after all I was using free IC's obtained from the dumpster behind the Coulter Diagnostics (blood counters) factory. They were all RTL. This was before CMOS was invented! Yes, there are about 400 chips on those boards, all hand soldered PTP style. Yes, I still have them after 40+ years. I have started work on a new synthesizer, and several other music generation related devices. No tubes in these projects with the exception of a vacuum tube VCA / Envelope Follower and compressor. This is not just another copy of one of the $$$$ classics. There are plenty of these out there already.

And I said:
George, I was very in to building electronic instruments. I built a big modular. In contrast to Tubelab, when I got in to synths was the late 80's and everything was digital, so I built an analog modular. I've been thinking about getting back to it..
I've been reading up on tube analog synths, and am thinking about building one... But they're difficult to control (if you want standard tuning and keyboard control).

And Tubelab said:
I have been investigating that as well. A true 1V/OCT scale is hard to do with an all tube design, and keep the tracking and drift issues to a minimum. I decided that the VCA would be the easiest circuit to tackle first since errors only affect amplitude, not pitch. It is also a path I need to conquer in order to design a new vacuum tube audio compressor. A compressor is a VCA, a controller, and an envelope follower. There were several "reference standard" tube compressors used in the broadcast industry that have become cult classics commanding stupid price tags today. ALL of these used the 6386 tube which has become rather hard to get....Even Sovtek knockoffs cost over $100 each!!!!! I have about a dozen from the 1950's and started characterizing their control characteristics about 2 years ago. You get about 50 db of gain control from a CV that goes from -50 volts to zero. I am still looking for a cheap TV tube that can do the same gain range on a 5 volt CV swing.

Reality probably dictates that there will be some silicon, and maybe even some digital assistance in whatever "tube" synthesizer modules I design. You can find high voltage opamps that can scale a 1V/OCT CV into whatever needed to control a tube VCO, but tracking is still a problem, not to mention thermal drift.

I have been experimenting with a PIC microcontroller and a 16 bit D/A converter. Use the A/D inside the PIC to read the CV, then a lookup table applies the needed correction and sends the data to the D/A. The lookup table can be different for each VCO, and one PIC chip can handle a dozen SPI or I2C D/A's, each with their own VCO, AND feed a hand full of DDS chips.
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Old 29th June 2014, 02:07 PM   #2
wicked1 is offline wicked1  United States
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George, keep us updated. I'd love to work on a tube based synth (would be modular in my case).

Personally, I only care if the voicing is tube. VCO, VCA, VCF. Interfacing w/ standard CV is definitely a goal. Using silicon to do so, is probably the best option. Then your VCA tube doesn't need to give a certain range based on 5v.. Amplify or shift that 5v cv w/ some opamps, and feed the tube whatever it needs to give the range of gain control you want.

And for control, 1v/oct, I was also looking to a microcontroller. (Unfortunately, I have almost no experience w/ programming microcontrollers, so would definitely need assistance at this point in the project)

There's not a lot of info on tube synths out there, so you've probably found this already.. If not, here are some designs to take ideas from:
Audio Synthesis via Vacuum Tubes
Synth schematics--::--Vacuum Tube VCO

I haven't read those pages in about 10 years, since before I got in to tubes... I can probably make some good progress now.

Last edited by wicked1; 29th June 2014 at 02:10 PM.
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Old 29th June 2014, 06:05 PM   #3
JMFahey is offline JMFahey  Argentina
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Why make a tubed analog synthesizer?
The earlier experimenters on the 40s and 50s would have *murdered* to get some 741 Op Amps , go figure.

Their only option was some:
Click the image to open in full size.
Each cost like a small car and any 741 would **destroy** it , spec wise.

Why build pyramids this way:
Click the image to open in full size.
when now you can build them this way:
Click the image to open in full size.
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Old 29th June 2014, 06:47 PM   #4
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Because in subtractive synthesis distorted signal sound a lot more interesting than clean ones. A sine wave is completely useless as a signal, you must get the harmonics somehow so the filter can get its teeth into something and distortion is just as viable as choice of different waveforms. The option of both would be ideal.

I suppose the OP knows about this lot:
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Old 29th June 2014, 08:44 PM   #5
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I have known about Metasonix ever since Eric announced his vacuum tube synthesizer at the NAMM show several years ago. Eric Barbour was the applications engineer for the original Svetlana corporation, before that operation shut down and Mike Matthews bought up the name.

Eric has done several vacuum tube devices including some guitar amps with rather interesting names and graphics on their covers......get small children out of viewing range before checking out his web site....unless it has changed since I last looked.

Vacuum tube synthesizers.....this is the original. A Hammond Synthesizer from 1939!

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Old 30th June 2014, 04:31 AM   #6
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I started one of these when I was a first year apprentice in 1973.
ETI 4600 Website

It has now been 99% complete for 40 years. Happy Birthday to my ETI4600 project.

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Old 30th June 2014, 12:04 PM   #7
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It's been so long ago that I nearly forgot, and it seems that the web has already forgotten....

Surplus electronic shops used to be very common. They were more common in cities that had a large tech base. We had good ones in the Melbourne Florida during the 60's and 70's "space race".

There were a lot of good surplus shops in the Boston area during the dawn of the computer age. Sometimes the surplus shops even created electronics kits from their surplus to augment sales.

There was a Boston area surplus shop that designed and sold a music synthesizer kit. The company name was BNF, which is still listed as a Peabody, Mass. corporation. They made a synthesizer kit in the late 70's or early 80's. I bought one at a hamfest, had it for a few years and sold it at another hamfest.

It was an impressive looking modular with a separate keyboard, but this particular one never worked. The guy I got it from was an electronic tech, but not the original owner. He gave up. I got sound out of it, but it was temperamental, and very unstable. I was convinced that there were severe power supply problems, but never made it work right, and eventually sold it.

I can not find any info on the web. Does anyone else remember it....did anyone ever make one work?
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Old 1st July 2014, 01:20 AM   #8
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Don't remember BNF, but a friend built up a modular system from another 70's Boston area modular kit vendor, Aries, which eventually worked pretty well after a bit of massaging on my part.

One of my first real electronics gigs was at ARP, ca. '77, and doing an all-tube build was definitely discussed from time to time on the tech line. Most of us were more interested in figuring out how to implement digital technology to greater effect at that time though.
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Old 1st July 2014, 02:27 AM   #9
kevinkr is offline kevinkr  United States
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Don't know what became of BNF, but they had a pretty close tie in with MIT and sold kits for a variety of things designed by people in the engineering school and labs. I was there once in the 1980s with a friend who purchased a speaker kit designed by someone at MIT, not Dr. Bose.. lol

Eli Hefron was another surplus outfit located in Cambridge IIRC.. Abbott Electronics in Woburn had a lot of HH Scott surplus - bought quite a lot of it when I realized what it was for repairing Scott amps.
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Old 1st July 2014, 04:44 PM   #10
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a friend built up a modular system from another 70's Boston area modular kit vendor, Aries
I found pictures of the Aries on the web, and that was indeed the unit I had. Trying to think that far back isn't always possible but I believe that there was a connection between those two companies, or Aries may have evolved from BNF's surplus operation. I had a scan of the manual somewhere, but I don't know where right now since I am in the process of moving.

Google finds a 1977 Aries catalog. The AR-300 is the machine I had. I see no obvious BNF connection, so maybe it's all inside my head?????
Too much power is almost enough! Turn it up till it explodes - then back up just a little.

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