On Sunday, July 5, 2015 at 10:16:27 AM UTC+2, Tauno Voipio wrote:> On 4.7.15 23:30, bitrex wrote: > > So as a hobby project I'm thinking of building a tiny little theremin > > using some of those Soviet subminiature pentodes that are really cheap > > on eBay. Unfortunately, it seems that the most complicated part of the > > project would be the frequency mixers, as I don't think they really made > > subminiature heptodes and a "proper" mixer would probably require two > > tubes. > > > > I'm thinking about going solid state for the mixers in the first > > iteration at least...since the RF is so low I could probably just use a > > dual opamp with sufficient GBW. There was this article but of course > > all the links to the schematics are broken: > > > > http://electronicdesign.com/analog/make-frequency-mixer-op-amps > > > > Can anyone suggest an opamp mixer topology for the low 100s of kHz? > > > Forget tubes or opamps. The semiconductor solution is called a > Gilbert cell (Google for it). With unsorted transistors from > the same batch, you will get a far better mixer than with tubes. > Do not forget the low-pass filter after mixing.Barry Gilbert now works for Analog Devices, and they do produce some rather nice Gilbert-cell-based integrated circuits - I've already listed the AD633, AD834 and AD835 in this thread. There are others - Barry has exploited the idea with some enthusiasm. -- Bill Sloman, Sydney
Opamp frequency mixer
Started by ●July 4, 2015
Reply by ●July 5, 20152015-07-05
Reply by ●July 5, 20152015-07-05
On 4.7.15 23:30, bitrex wrote:> So as a hobby project I'm thinking of building a tiny little theremin > using some of those Soviet subminiature pentodes that are really cheap > on eBay. Unfortunately, it seems that the most complicated part of the > project would be the frequency mixers, as I don't think they really made > subminiature heptodes and a "proper" mixer would probably require two > tubes. > > I'm thinking about going solid state for the mixers in the first > iteration at least...since the RF is so low I could probably just use a > dual opamp with sufficient GBW. There was this article but of course > all the links to the schematics are broken: > > http://electronicdesign.com/analog/make-frequency-mixer-op-amps > > Can anyone suggest an opamp mixer topology for the low 100s of kHz?If you insist on tubes, look for 7360 beam-deflection tubes for mixers. They were the best of their kind at their time. Also, you'll a hefty sensitivity for magnetic disturbances in addition of the other tube quirks. -- -TV
Reply by ●July 5, 20152015-07-05
On Sat, 04 Jul 2015 16:30:24 -0400, bitrex <bitrex@de.lete.earthlink.net> wrote:>So as a hobby project I'm thinking of building a tiny little theremin >using some of those Soviet subminiature pentodes that are really cheap >on eBay. Unfortunately, it seems that the most complicated part of the >project would be the frequency mixers, as I don't think they really made >subminiature heptodes and a "proper" mixer would probably require two tubes.If you just need the difference between the fixed and variable frequency oscillator, any envelope detection (AM detection) circuit should work. Feed the detector with both frequencies to the rectifier. There is a beat (amplitude variations) between the two frequencies. Using a half wave rectifier will create a DC biased waveform with the beat (the difference frequency) riding on top of it and some high frequency noise above that. Remove the DC bias with a capacitor and the original frequency and their sum frequency with some low pass filtering. This should be perfectly sufficient when there are only exactly two frequencies sufficiently higher (say 50 kHz) above the desired audio difference frequency.
Reply by ●July 5, 20152015-07-05
On 2015-07-04 4:37 PM, bitrex wrote:> On 7/4/2015 5:40 PM, Joerg wrote: >> On 2015-07-04 2:24 PM, Tim Wescott wrote:[...]>> >>> You could make a 180 degree shifter and summer by making an appropriate >>> transformer (probably at least mildly resonant at good toob impedances) >>> from signal A, with the secondary center-tapped and fed with signal B. >>> Feed a pair of triodes with the transformer outputs, and poof! you >>> have a >>> single balanced mixer. >>> >> >> For 180 degree shifting all you need is one triode as a follower. Make >> the anode resistor the same as the cathode resistor. Then the anode >> carries the 180 degree signal. > > Only problem then is that the two outputs have wildly differing output > impedances, which could be a problem... >In that application it's not such a problem because the anode resistor is very low and it's value is the impedance of that node now. The tube doesn't amplify the signal voltage, it can't. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●July 5, 20152015-07-05
On Saturday, July 4, 2015 at 4:30:29 PM UTC-4, bitrex wrote:> ... There was this article but of course > all the links to the schematics are broken: > > http://electronicdesign.com/analog/make-frequency-mixer-op-amps >Links are not broken, click of "Fig. 1" and not "opamps".
Reply by ●July 5, 20152015-07-05
On Saturday, July 4, 2015 at 1:30:29 PM UTC-7, bitrex wrote:> So as a hobby project I'm thinking of building a tiny little theremin > using some of those Soviet subminiature pentodes that are really cheap > on eBay. Unfortunately, it seems that the most complicated part of the > project would be the frequency mixers, as I don't think they really made > subminiature heptodes and a "proper" mixer would probably require two tubes. > > I'm thinking about going solid state for the mixers in the first > iteration at least...since the RF is so low I could probably just use a > dual opamp with sufficient GBW. There was this article but of course > all the links to the schematics are broken: > > http://electronicdesign.com/analog/make-frequency-mixer-op-amps > > Can anyone suggest an opamp mixer topology for the low 100s of kHz?The transconductance OA has been mentioned already---and if I was insisting on OAs, I'd probably go with that. The Gilbert (or Jones) cell method was mentioned, and is very effective, but it is not an OA. One comment on your link "complained" about the need for a LPF on the output. I don't fully get the complaint. After all, LPF is always needed unless "image reject" double balanced and linear mixing is done. But even that is too hard to do for obtaining very high rejection. If considered non-OA methods, a simple analog switch in chopping mode will do it. That would be "singly balanced." IIRC, one can differentially drive four analog switches, that are also differentially chopped, and obtain double balance. I/O + - O O | | +---+ +---+ | | | | o o o o \...|...|...\.....O LO o o o o SW/Relay control | | | | | \ / | | X | | / \ | | | | | +---+ +---+ | | O O + - I/O The "X" is simply a wire cross for the wires drawn at 45 degrees.
Reply by ●July 5, 20152015-07-05
On 7/5/2015 2:03 PM, Simon S Aysdie wrote:> On Saturday, July 4, 2015 at 1:30:29 PM UTC-7, bitrex wrote: >> So as a hobby project I'm thinking of building a tiny little theremin >> using some of those Soviet subminiature pentodes that are really cheap >> on eBay. Unfortunately, it seems that the most complicated part of the >> project would be the frequency mixers, as I don't think they really made >> subminiature heptodes and a "proper" mixer would probably require two tubes. >> >> I'm thinking about going solid state for the mixers in the first >> iteration at least...since the RF is so low I could probably just use a >> dual opamp with sufficient GBW. There was this article but of course >> all the links to the schematics are broken: >> >> http://electronicdesign.com/analog/make-frequency-mixer-op-amps >> >> Can anyone suggest an opamp mixer topology for the low 100s of kHz? > > The transconductance OA has been mentioned already---and if I was insisting on OAs, I'd probably go with that. The Gilbert (or Jones) cell method was mentioned, and is very effective, but it is not an OA. > > One comment on your link "complained" about the need for a LPF on the output. I don't fully get the complaint. After all, LPF is always needed unless "image reject" double balanced and linear mixing is done. But even that is too hard to do for obtaining very high rejection. > > If considered non-OA methods, a simple analog switch in chopping mode will do it. That would be "singly balanced." IIRC, one can differentially drive four analog switches, that are also differentially chopped, and obtain double balance. > > > I/O > + - > O O > | | > +---+ +---+ > | | | | > o o o o > \...|...|...\.....O LO > o o o o SW/Relay control > | | | | > | \ / | > | X | > | / \ | > | | | | > +---+ +---+ > | | > O O > + - > I/O > > The "X" is simply a wire cross for the wires drawn at 45 degrees. >There's a very simple circuit for a four-quadrant multiplier in the LM13700 application notes. I'm not sure if it will work at a few hundred kHz, however. The LM13700 is a very useful chip and works at low supply voltages as well. It's unfortunate they don't make better OTAs that would be suitable for much higher frequency applications.
Reply by ●July 5, 20152015-07-05
On 05/07/2015 19:59, bitrex wrote:> > The LM13700 is a very useful chip and works at low supply voltages as > well. It's unfortunate they don't make better OTAs that would be > suitable for much higher frequency applications.You might find the LM13700 especially useful in your Theremin project for the volume control circuitry. That VCA stage can be a problem area in Theremin design. I find the old RCA design which modulates tube filament power to vary the volume delightfully zany! piglet
Reply by ●July 5, 20152015-07-05
On Sat, 04 Jul 2015 19:27:27 -0400, bitrex <bitrex@de.lete.earthlink.net> wrote:>On 7/4/2015 6:23 PM, Phil Hobbs wrote: >> On 7/4/2015 5:53 PM, John Larkin wrote: >>> On Sat, 04 Jul 2015 16:30:24 -0400, bitrex >>> <bitrex@de.lete.earthlink.net> wrote: >>> >>>> So as a hobby project I'm thinking of building a tiny little theremin >>>> using some of those Soviet subminiature pentodes that are really cheap >>>> on eBay. Unfortunately, it seems that the most complicated part of the >>>> project would be the frequency mixers, as I don't think they really made >>>> subminiature heptodes and a "proper" mixer would probably require two tubes. >>>> >>>> I'm thinking about going solid state for the mixers in the first >>>> iteration at least...since the RF is so low I could probably just use a >>>> dual opamp with sufficient GBW. There was this article but of course >>>> all the links to the schematics are broken: >>>> >>>> http://electronicdesign.com/analog/make-frequency-mixer-op-amps >>>> >>>> Can anyone suggest an opamp mixer topology for the low 100s of kHz? >>> >>> Any tube will mix: diode, triode, pentode, CRT, PMT. Armstrong didn't >>> need any fancy-pants megagrid tubes when he invented the superhet: >>> >>> https://www.google.com/patents/US1342885?dq=armstrong+1920&hl=en&sa=X&ei=LlSYVfrkLNPcoATEo47YDw&ved=0CE8Q6AEwBw >>> >> Armstrong was an amazing guy--he invented the oscillator! >> >> Also the superhet, the superregen, FM radio, and many other things. >> >> One of my technical heroes. >> >> Chees >> >> Phil Hobbs >> >> > >I've read about the story of Armstrong and it's very sad. I often found >myself wondering why, at some point, someone didn't urge him to cut his >losses, come to some kind of settlement with Sarnoff and RCA, and then >just walk away. > >It would have been far better for his health in the long run. What good >is pride if the end result is a 12 story swan dive off a hotel balcony?A lot of creative inventor types are bipolar and compulsive and a little schitzoid. Not survival traits. Business types and sociopaths and evil VCs often take advantage of them.
Reply by ●July 5, 20152015-07-05
Like I said, you can use a pair, if you don't mind the count of two. They're cheap enough to throw in anywhere, so who cares? I don't think they made a heptode, at least in the CKxxxx series for example. I see some battery op types (1AE5, 2G21/22), but those might be incongrouous with your system (or an alternate direction to go in, for the whole thing, if you buy a case of filament-type pentodes instead). Or you can "cheat" the submini theme by tossing in a regular "miniature" socket and use a 6BE6 or whatever. I don't see that as a sacrifice; it'd be like using a SOT-223 LDO (instead of a DFN or CSP variety) in a circuit that's mostly TSSOP and QFN. It's not like it's terrifically different, and if it works, it works. I've got a 6AL5 in my submini SW radio, since I don't have any submini diodes handy. If you're going to give in and throw SS at it, you might as well do the whole thing in SS, in which case I'd recommend making/using SBMs like the CA3028, or DBMs like the MC1496, etc. Compare with my volume mixer: http://seventransistorlabs.com/Theremin/Images/Mixer1.png BTW, all of your timbre (tone quality) comes from the oscillators and mixer: their distortion/purity, locking behavior (defines how rough the low notes are), and distortion/IMD in the product. So if you're looking for a "particular sound", that's where you need to do it. SS vs. toob will have some play there. You can always distort a sine wave more, so you can instead make as clean an audio output as possible, then distort it with add-ons. You can literally use guitar pedals (and I would recommend it), with one caveat: try to get a fixed amplitude audio signal, then distort it, then volume-control it. So you'll be putting the pedal on a "patch" loop on the unit, not the output. Putting the final (variable amplitude) output into a guitar pedal (or some circuit like that) may have undesirable results (attack/decay, change in timbre in the process), depending on the circuit. But that might also be a good thing, so see what works for you (when you get there). Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com "bitrex" <bitrex@de.lete.earthlink.net> wrote in message news:55984260$0$17453$4c5ecfc7@frugalusenet.com...> So as a hobby project I'm thinking of building a tiny little theremin > using some of those Soviet subminiature pentodes that are really cheap on > eBay. Unfortunately, it seems that the most complicated part of the > project would be the frequency mixers, as I don't think they really made > subminiature heptodes and a "proper" mixer would probably require two > tubes. > > I'm thinking about going solid state for the mixers in the first iteration > at least...since the RF is so low I could probably just use a dual opamp > with sufficient GBW. There was this article but of course all the links > to the schematics are broken: > > http://electronicdesign.com/analog/make-frequency-mixer-op-amps > > Can anyone suggest an opamp mixer topology for the low 100s of kHz?
