Mint Tin Dreams

Or are they night-terrors?  I haven’t been able to get the tube amp in a mint tin idea out of my head. Assuming a suitable sub-mini tube type exists, the first obstacle is generating a B+ (48V or more) from battery power. For that, a DC boost circuit is in order. Of course, you should also have an easy way to recharge because whatever you build will probably eat batteries with complete abandon.

Here’s something untested and completely silly on the topic:

charger and boost board

Muchedumbre XL

The regular Muchedumbre is an ultra-simple buffer with few parts and straightforward operation. It’s a great beginners circuit for high voltage tube applications.

If you’d like something with just a little more nerd sprinkled on top (and an extra 12AU7), try the following. This XL version uses a White Cathode Follower buffer for about half the output impedance of the vanilla version. It requires a few extra resistors and caps and the heater reference voltage is tweaked just slightly so I can sleep better.

For ideally symmetrical drive ability, the series resistor in the anode of the upper 12AU7 should be calculated as:

Rseries = (Rp + 2 * Rload) / Mu

Plugging in values for a 12AU7 and a (worst case) 10k input impedance gives you about 1k5 (rounded so that it’s an easy to find value). You can optimize this for the input impedance on your amp using a plate resistance number of about 7k and Mu of 20. Just keep in mind that the resistor is in series with the tubes and so it drops B+ voltage based on the current at the bias point.
I am assuming a low input impedance on the amp and so the calculated value is also on the low side, but this preserves operating voltage and overhead. The actual drive current required in a typical preamp output stage is very small, so even a loosely optimized WCF is plenty capable. When in doubt, use smaller values for the series resistor for line level. If we were trying to drive something needing lots of current swing like a bunch of parallel output tubes or headphones, we’d be pickier about Rseries.

Muchedumbre XL

If we break down the circuit into a cathode follower (upper triode) and a grounded cathode amplifier (lower triode), we can see that this creates a nifty push pull circuit. The cathode follower is non-inverting, so it’s pulling the output in the same direction as the input signal. The lower triode is a grounded cathode amplifier and so it inverts the input that it sees. But the input that it sees is from the anode of the upper triode, which is already inverted. You invert the inverted and you get non-inverted (same ‘direction’ as the upper triode). Tada!  Push (lower) pull (upper).

 

A different way to RIAA

Phono preamps can be tricky builds due to the need for high gain with low noise. In tube land, linear high gain is not too difficult to achieve even without feedback. Power-supply-based noise can often be brute forced with extra filtering, actively regulated B+, and/or DC-powered tube heaters. High PSRR topologies (eg differential) also have an advantage in the early amplification stages.

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building phono PSUs be like

The place where most DIY builders are probably tripped up is the mysterious RIAA voodoo.  Because the physical limitations of the vinyl medium and cutting process require a limiting of low frequencies and a boosting of high frequencies, we need to reverse this EQ on the playback end in order to get back to ‘flat’ frequency response.

At it’s most basic, the RIAA equalization standard defines three frequencies: 50hz, 500hz, and 2122hz. We should have a 20db boost to 50hz, a -20db/decade transition from 50hz to 500hz, flat playback from 500hz to 2122hz, and a -20db/decade falling response above 2122hz. Note that 20db/decade is equivalent to 6db/octave, so these are not especially steep filters.

Splitting the RIAA requirements between low (<1khz) and high (>1khz), the low frequency manipulation requires at least 20db of gain from whatever device we are using. This type of EQ is commonly referred to as a shelving filter. The high frequency portion is only reducing the response and so it doesn’t require gain (ignoring the overall gain needed to get to line-level signals). This reducing of the high frequencies can be as simple as a first order low pass filter (just a resistor and a cap).

Tubes, with their fairly high output impedance and finite Mu, complicate RIAA frequency-dependent impedance calculations. Operational amplifiers, on the other hand, make filter maths fairly straight forward. Here’s an example:

opamp riaa

Starting at the output, the R1 and C1 combination form a simple low pass filter. Because the output impedance of opamps is so low, our equation need only involve the cap and resistor:

f(-3db) = 1,000,000 / (2 * Pi * CuF * R), rearranged as:

R1 = 1,000,000 / (2 * Pi * 2122hz * C1uF)

Begin with a tight tolerance capacitor (say 0.1uF) and you’ll get a resistor value that may come off the shelf or be created with a parallel/series combination (in the case of a 0.1uF C1, the resistor would need to be 750 ohms). The resistor appears in series with the output, so large values may require a high input impedance in the following stage.

The shelving filter created by R2, C2, and R3 appears in the feedback circuit of the opamp. Because we need 20db of gain, we know that the ratio of R2 to R3 should be approximately 10:1 (a 10x voltage gain difference corresponds to 20db). The 50hz point is set by the combination of R2 and C2 and is found with the same kind of capacitor reactance equation as the low pass:

R2 = 1,000,000 / (2 * Pi * 50hz * C2)

Again, start with the cap value because caps have fewer options and are harder to find in a tight tolerance. A 0.047uF cap gives an R2 of about 68k, meaning R3 should be about 6K8. The overall gain of the stage is further set by R-gain (Av = 1 + R3/R-gain).

So that’s a pretty simple way to EQ your vinyl to flat. More gain to bring the signal up to line level could be added by following the EQ/opamp stage with a ‘normal’ tube stage or two. Expect to see some more on this topic in a future project!

Board prototypes on the way

Although I love wiring tubes point to point, there are times where some TO92 or other small parts are needed. These often benefit from short leads, making layout and spacing critical. One of my upcoming projects makes these kinds of demands. Having dealt with death-by-soldering iron and oscillations when trying to point to point wire small parts in the past, I decided to try my hand at some small boards to make things easier on myself.

I still believe that for tubes there are real advantages to p2p wiring and turret strips. After all, they’re fairly large robust parts and part of the fun of building something is scavenging enclosures, optimizing the layout and grounding, etc. But where a small solid state circuit is needed, a modular board is great to have.

More to come on these boards once I’ve been able to test them and use them in builds.

ccs board finalreg board finalshunt board final

Soviet Tubes

Soviet Tubes Pic.png

I have spent way too much time this week researching soviet tubes but at least I have a reference sheet to show for it. These are the tubes that seem to me to be the most interesting and commonly available through either NOS vendors or current production. Lots of interesting stuff here and many are still very cheap (<$5 USD). I’ve got a couple packages on the way already.

Soviet Tubes Reference (PDF with datasheet links)

Letters to WTF: how do I know what size potentiometer I should use?

This is a dang good question. Different values of pots don’t attenuate any more or any less than one another. A 1M pot is not inherently quieter/louder than a 10k pot. So why the eff are there so many values of potentiometers? It has to do with input and output impedance.

A pot is a made up of a resistance and a wiper making contact with the resistance. The point where the wiper makes contact divides the resistance into two parts. We usually take our output at the wiper and connect one end of the resistance to our source and the other end of the resistance to ground. As we turn the knob, the wiper moves to make one resistance larger or smaller relative to the other. You can think of it just like a resistor divider:

generic poteniometer

The arrow is our wiper, R1 is the portion of the resistance between the wiper’s contact and the input, and R2 is the portion of the resistance between the wiper and ground. The resistor marked “A” is standing in for the output impedance of the source connected to the pot. The resistor marked “B” is standing in for the input impedance of the next stage or amp.

From the perspective of A (the source), the pot’s value looks like R1 in series with R2 and B in parallel:

Zin = R1 + R2||B

When you have the volume very low, R2 is very small and so the impedance is basically R1 (or close to the pot value). When you have the volume turned up high, R1 is very small and so your impedance is dominated by R2||B.  This is where it gets more interesting.

If B (the input impedance of the following stage) is large relative to R2, the input impedance of the pot is basically R2 (again, close to the pot value). If B is a relatively small value, the term R2||B is closer to B. Because the volume is all the way up, R1 does not add to the input impedance of the system as a whole. The input impedance in that case would be about the value of B.

Now, what about output impedance of the pot? From the perspective of the wiper, R1 and R2 are in parallel, so the output impedance is R1 || R2. If we also include the output impedance of the previous stage, the output impedance of the pot from the perspective of the wiper is:

(A + R1) || R2

The worst case scenario for the output impedance is when the two terms A + R1 and R2 are equal.  This gives the largest value. If A is small relative to R1 and R2, the highest output impedance of the pot is therefore at half of its travel (linear pot). Here, the value of R1||R2 is 1/4 the value of R1 + R2 (or 1/4 the pot’s value). As A gets larger, the maximum possible output impedance also gets larger.

We want the pot value (R1+R2) to be much lower than the input impedance of the next stage or device (B) because that will let the pot behave most consistently in terms of impedance. We also want the pot value (R1 + R2) to be much larger than the source impedance of the preceding stage (A) because it keeps the maximum output impedance at or below 1/4 of the pots value.

Think of it all as a system. We want B much larger than 1/4 the pot value and the pot value much larger than A. A factor of 10x is pretty good. So, if you have a 25k input impedance on the stage following the pot (B), you’d want a pot value (R1 + R2) no larger than 10k. If your pot’s value is 10k, you’d want the source impedance feeding (A) it to be no larger than 1k.

Or, just add buffers to everything and let impedance math be damned.

 

 

 

 

(power) supply chain

Postponement is a powerful supply chain concept employed to minimize inventory/capital for a business by delaying configuration of a specialized product until as close as possible to delivery to the end-user. For great examples, see Dell’s made-to-order computer business or Edcor’s made to order transformers.

In vacuum tube land, the transformer is a critical component. Tubes come in all shapes and sizes, requiring a variety of voltages for optimal operation. This has lead to many different power transformers and filter configurations for various circuits. We even have transformer companies whose entire business strategy is founded on servicing the myriad of transformer configurations and custom options.

What if we could find one power transformer that could be used with any circuit? What would it look like? Well, if I were to build one it would probably look something like this:

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Looks pretty simple to be universal, doesn’t it. But what’s up with the 50V winding? It’s not a heater tap but it could be used for bias, I suppose. The use I have in mind is something like this:

In conjunction with the 300V winding, the 50V winding will allow you to create 250V and/or 350V outputs (all voltages AC of course). Using the extra winding and some filter math, you could easily tune in any target B+ from low max voltage tubes like 6V6/EL84 to higher max voltage tubes like EL34 or KT88. Careful attention would need to be paid to phase labeling and any power supply would use a bridge rectifier, but those are pretty small prices to pay for more flexible parts.

A transformer like this with a 250mA current rating might be the only transformer a builder would ever need for a variety of projects. Fewer parts means less money tied up in iron for users and fewer SKUs means more economies of scale for transformer manufacturers. That’s the beauty of postponement.

Bench update:

  • Aikido Headphone amp is underway
  • Push-push octal monoblocks are in design phase
  • Working on write-up for latest peanut watt SET “El Cacahuate”