124B Pentode Dual Tube Breakout Board. Not a fun Post.

 

Sad to say that AudioDiWHY has not been fun over the last few weeks. A pervasive sadness has crept in and my pychiastrist girlfriend says I need a break.

I finished working on the tube timbre trasher (part one here).....where I am designing and prototyping a synthesizer waveform distortion module using a Soviet-era low voltage pentode, the 1J24B. For this post, let's focus on the breakout board needed for the prototype. I'll post the rest in a few days.

Tube audio! Fun stuff right? The Soviets—they knew tubes!! Long gone and good riddance?  Let’s take some tiny, low voltage tubes used in USSR fighter jets, turn swords into plowshares, and use them for audio! 

Until….until....when cleaning out my lab, I mindlessly dug up the box the 1J24B's came in:

 

I redacted the Ukrainian vendor's name and address, but after searching the web, as far as I could tell, the Ukraine was the primary source of the worlds' NOS, affordable 1J24B's. 

Needless to say these parts, which were really easy to obtain 2 months ago, are no longer available on Ebay, and perhaps anywhere else, due to heartbreaking world events.  

Specifically, the 1J24B's I had been using for my prototypes came from the same Ukrainian city you see in the war correspondence here. 

The news from this part of the world is maddening, confusing, and at times, terrifying. My psychiatrist girlfriend, usually tough as nails, was about as scared and upset at the news of the conflict as I've ever seen. The battle at Zaporizhzhia made her frightened for her kid, who is studying abroad, not too far away from the conflict, as well as for herself, for the world, and for me. 

This made me want to stop working on this for-fun project and spend my time doing more useful and constructive things. But I am still stuck at home due to the global pandemic, and found myself continuing to work with my remaining 1J24B's.

 

Anyway....putting politics and emotions temporarily aside, well before I realized where I sourced the parts from, I created a two tube version of the 1J24B breakout board you see in previous posts here and here. I posted the files for this board on my faithful sponsor, PCBWAY's project page, here. 

The design requires you have some 1J24B’s already or can get some.

UPDATE: 5-25-23 Tragically: the war in Ukraine drags on, but these tubes are available again from Western retailers. Synthcube for instance has them in stock as of today it appears: go here.

If so, you can use this PCB to breadboard a low cost, low voltage dual tube design, say a push pull amplifier or a tube based op amp. 

Cathodes for this BOB require ground and -2 to -15VDC. See the post here for how the 1.5V direct cathode power works.

Under different circumstances, I’d be all over further experimentation; finding audio uses for these extremely interesting tubes would be a lot of fun.

 

Now, not so much. Due to the war, it's not much fun experimenting with these parts.  Maybe it's just me.

Damn. 

 

Differential Op Amps--Analog Math Fun! Simple Curve Tracer!

The folks at the incredible Bell Labs were on fire right?  

I see that the C programming language came from those guys, along with the transistor.  

They were pioneers in electronic music creation, even.  

But, arguably, we audio DIWHY folks' favorite Bell Labs creation is the op amp--the handy 25cent  IC commonly used in a negative feedback loop for analog data processing.

Yes, Bell labs invented that as well.  

Op amps are everywhere! I use them everywhere in my designs....so does everyone else--adding two signals, changing a DC offset, inverting an audio signal, as a comparator.... 

Question: Can we use it for simple subtraction? 

Of course. It's easy.

Simple math fun: subtract two voltages--if R1 = R2 = R3 = R4, the output is V2 - V1.  

By setting R1 = R4 and R2 = R3, you can gain up V2 - V1; the relationship of R2-R3/R1-R4 gives you your gain.

You can add more inputs and do more math, but it starts to get a bit more complex fast (Hope I got that equation on the right, right?)

So what else can we use the difference amp for?  How about a very simple mic preamp (here)? 

Can we use a differential op amp in test equipment, to measure the voltage drop across a shunt resistor ? Yes.

Indeed. This is an extremely common circuit fragment in audio--you can find a differencial op amp in the output stage of YuSynth's popular VCA design, for instance:

    

Pun Intended 

I took a "different" approach; I wondered if I could use a difference op amp to form a basic V-I component tracer (a good video about how to build a simple tracer--without op amps--is here).  

To motorize this pursuit, I could have breadboarded a $2 curve tracer, but I hate breadboards. Instead, I crafted a very simple differential op amp PCB and sent the gerber off to my sponsor: PCBWAY.  Here we go again with the shameless plug: Help out this blog and check 'em out OK?

The board came back to the USA really fast....here it is.

Populating it took about 30 seconds....give or take.....

Ready to hook up +/- 15V and ground and start testing things....

To trace a diode I put a 100hz 10V P/P triangle wave from a Siglent 1025 waveform generator into "FG-in"; put a diode under test between TEST1 and TEST2 wirepads, soldered a 1K resistor into R4, and jumpered R1. 

Next I tied IN1 to J1 and In2 to J2 (or IN1 to J2/In2 to J1, depending on how I wanted my scope trace to appear along the X axis--both work). 

I increased the gain of the circuit by increasing R2 and R3 to 10K. This made the range of the traces easier to see on my scope. 

To wire the PCB to my scope (a Siglent 1202XE--for X-Y mode, press the "acquire" key and then using the soft buttons below the display, set "XY"  to "on") I ran the FG-IN signal to Y input terminal and the output of the op amp to the X input terminal.

Some meatballs with my spaghetti?

For a diode--the result is a quasi-decent curve:

4004 diode....

5V zener....

The problem I faced: op amp rails max out at maybe +/-15 or 20V, but for a useful trace cursor, I needed a much wider voltage range, maybe +/- 60V, which would fry any ordinary op amp.  This single differential op amp design cannot accommodate the high voltages needed to create traces for parts like the one here. Oh well.  

I also needed a staircase generator to generate traces for transistors (example schematic can be found here, but I'd probably use an Atmel 328 MCU and a MCP4911 Digital to Analog converter instead) , as well as a switch or relay to compare the curves of 2 different transistors under test. 

Maybe in the coming months I'll work on this.....

Anyway if you are a fellow breadboard hater and would rather get this very simple differential op amp configuration on a thru-hole PCB, you can get the gerber, Eagle files, BOMs, PDF's etc. from PCBWAY's project page, here.

Coda 

I am curious: what happened to Bell Labs?  They got sold to Nokia. I didn't know that--you learn something every day. So much for American Ingenuity right? The trust busters broke up Bell in 1984, so the biggest monopoly we're left with are these guys.  Has Nokia made the transistor obsolete? Come up with a more popular programming language than C? Not yet. But! They have given the planet a ubiquitous ring tone. 

Al Fine 

In the immortal words of the Chambers Brothers: Time! Time to start wearing real shoes again. Time to shave every morning....time to go back to my 1.5+ hours a day work commute--after two years--wow.  

It was real, it was fun, but it wasn't real fun.

Sequencial Quad Sample/Hold

I've not had time to post to this blog lately; COVID stats are improving here in the Northwest US, and thus I am commuting to my day job again. 

This is good news I guess--the "health is improving" aspect, anyway? 

During the (hopefully) final few weeks of our collective two year COVID house arrest I found myself digging through my box of old audio IC's: which puzzle to solve next?  

The Curtis ElectroMusic System's CEM3394, intrigued me: an "all in one" synthesizer chip used in the Sequential Multitrak, an instrument I owned and loved back in the late 90's.

I don't know what happened to the Multitrak.....I sold it? Loaned it? Couldn't remember.....

I had four new old stock CEM3394's in my junk box. Could I make a single board, low parts count synth out of these IC's? 

Where would I start?

Like the popular CEM3340 IC, the CEM3394 required analog control voltages to function. I wanted to understand how the internal control voltages ("CV's") were implemented in early digital/analog synthesizers--and use that in a future 3394 project--since I wanted to stay true to the spirit of these venerable old designs.

I'd need a to pick an MCU for the project--the 3394 chips were designed to be microprocessor controlled. Some old synths used Z-80's, but would an Atmel328 work?

I started to research and came across this post. Short answer: yes, this could work!

(Aside: The post's final comment from "Grumpy Mike" makes me sad, it's the kind of fear and loathing and BS I really dislike; besides, he's wrong. Sequential CEM based gear from that era didn't use 16 or whatever DACs in a single synth, and when they worked correctly had internal CV's that didn't sag--you could hold a single chord for hours and it was solid). 

How did Sequential avoid putting several (expensive) digital to analog converters ("DACs") in every instrument? And how did they keep critical MCU controlled CV's, like oscillator V/octave, rock solid?

Turns out they used multiplexers, or MUX's (for the Prophet 5, CD4051 IC's) and had a Z-80 processor refresh a DAC that in turn fed many MUX'd sample and hold capacitor + op amps circuit fragments, quickly, over and over--sequentially.

That way the SH's outputs would not droop, and very few MCU and DAC's--relatively expensive components then and now--I see just one of each in the Prophet 5, right?--were needed for a complete, complex microprocessor controlled analog synth. 

Prophet-5--thing of beauty!

No Holds Barred

The Prophet-5's service manual helped me understand Sequential's no-droop sample and hold. Get its PDF here; an excellent explanation of how the MPU > DAC > S/H mux's work is on page 29.

But!! As a curious DiWHY dude, why read when you can build? 

I designed and laid out a simple quad sample and hold board, which I called, predictably, "Sequential Quad SH". 

Coded it--built it.

Worked first time (WFT).

MCU control was from a "Minimalist" Atmel328 C dev board; you can get that from sponsor PCBWay's project site, here.  

For the Digital to Analog Converter (DAC) I chose a SPI 12-bit Microchip MCP4921; download an embedded C library for it and its 10 bit cousin for AVR328 processors here. 

I used Eagle to lay out the PCB:

The PCB has provisions for an External DAC feeding the 4051 MUX, if you use the MCP4921 make sure to jumper the 2 "DACLINK" pins in the middle of the board; if you want to use a different DAC IC (for a CEM3394, an AD5792?), tie your external DAC's output to the right pin of DACLINK and omit the MCP4921.

The Eagle gerbers were uploaded to this blog's enthusiastic, burning-the-midnight-oil sponsor: PCBWAY.  Please help support this blog and check 'em out.

Soon the PCB was back, so I built it....

Board, breadboard and minimal Atmel 328 dev board....next, hook it all up....

Let's try some different caps....

Ready to test!

The board worked pretty much first time....I made a few dumb mistakes during fab--I forgot to solder on some caps, but once I corrected that mistake this was a win.

Code Me Up Scotty! 

Crafting the Embedded C firmware for the Sequential Quad SH was pretty easy. I used my usual toolchain: Atmel ICE and Atmel Studio 7.  I leveraged my minimalist AVR board (here) for the brains of this proof of concept but most Arduino development boards with AVR processors should work with little to no modification to the code. I uploaded code for the SH project to Github--repo is here.  

I also created repos for the drivers used in the project, each with its own main.c. Get the DAC code here. For the 4051 MUX the .c and .h files are here.

The main.c's infinite loop sent hand-coded values to each channel of the mux. For real-world applications I would change the demo code to read values from the outside world--from the MCU's ADC's for example.

    while (1) 

    {

write4921(4000);

_delay_ms(2);

        single4051_out(0);

        single4051_inhibit();

        _delay_ms(1);

write4921(0);

_delay_ms(1);

single4051_out(1);

        single4051_inhibit();

        _delay_ms(1);

write4921(2500);

_delay_ms(2);

single4051_out(2);

        single4051_inhibit();

        _delay_ms(1);

write4921(1000);

_delay_ms(2);

single4051_out(3);

single4051_inhibit();

_delay_ms(1);

}

I had to experiment with timing--the Sequential manual says one should inhibit MUX operation whenever the DAC value changes, and I found that doing so got me the cleanest looking DC output on a scope.

Here's one of the 4 S-H outputs, with a 12 bit DAC value of decimal 4000:

And yes, the CV never sagged as long as the PCB and Dev board had stable DC power. I went away for coffee and came back a half hour later--spot on.   

Bust a Cap

One of four sample and hold subcircuits. Voltages are captured in C9; since the non-inverting input of the opamp is almost infinitely high impedance, as is DAC2 sourcing the voltage, the voltage is held until changed.

I didn't know what type of capacitor to use for each sample and hold subcircuit; I have heard techs (passionately) discuss what is the best and most stable capacitor for sample and hold applications. 

I tried poly film, polystyrene axial, and mylar .01uF's. No difference was seen on my scope for any of these. 

I ended up using .01uF poly box film caps because I had a lot of them lying around.

Samples to Hold

If you want to play along at home, get the embedded C code, Eagle files, gerber, drawings and so on from Github (here). The project can also be found on a PCBWAY project page (here).  

Overall, this was a fun proof-of-concept to research and build. 

Once finished, the circuit provided a simple way to get a lot of horsepower out of a single inexpensive DAC. I figure with a reasonably fast MCU this idea can be expanded to accommodate 40 CV channels, maybe more; for more ideas see the Sparkfun page here.  

I can now see why MUX's were so critical to the Prophet-5's design as well as other classic computer controlled synthesizers: mux IC's are relatively inexpensive; DACs, not as much.

It would be pretty easy to turn this design into a Euro 4- or 8-channel "no sag" S/H Eurorack module, but I don't have time for that now, with my post-Covid19 day job ramping back up. 

Overall, with affordable multichannel DAC's like the MCP4728 available the CD4051 MUX approach may not always be necessary, but it's a good tool to have in one's toolbox. But in general, this MCU > DAC > MUX paradigm, I figure, will be find inclusion into upcoming audio DiWHY projects.

Hats off to Sequential for providing rock solid CVs that don't sag, in a manner that doesn't break the bank, presented in a the service manual is clearly written--so much so, even I can follow it!

Sample well, Luke. See ya next time.