Opamp + tube RIAA board art

A recent discussion on diyaudio.com reminded me of the opamp-based RIAA idea I shared last summer. It turns out that someone else has done something similar and reports very good results. Koifarm was after a more integrated build with phono, streaming, and line level all in a box, but the basic idea of using an opamp to perform the RIAA corrections and a tube to provide some/all of the voltage gain is the same. We differ just in how it’s integrated: I’m after a simple RIAA module with outboard tubes while Koi was going for an all-in-one.

Here’s where I’ve landed so far on a board to contain the opamp bits and bobs:

The opamp runs from the 6.3Vac heater winding that would be included on any tube-centric power transformer, meaning there are no special windings or an extra transformer to power the solid state section. The output of the RIAA module would be fed to a simple tube gain stage of your choice. Here’s a grounded cathode application, but keeping the tube off the board means there’s tons of flexibility.

So, will it work? Koifarm thinks so and he’s a pretty prolific phono preamp builder. I’ve also already tested the same RIAA correction scheme in the battery powered phono project. I’m saving up a few designs to place a board order, but I’m hoping this RIAA module would make for a relatively simple and fun build this year.

Phono Preamp Heaters

Heater supplies, even with indirectly heated tubes, are a potential source of hum with high gain circuits like phono preamps. In a grounded cathode gain stage, the tube will amplify any signal it sees between the grid and the cathode. The tube doesn’t particularly care if that is an audio signal or an induced signal from some other part of the build. Indirectly heated tubes have a cathode sleeve around the filament heating it. The close geometry creates a happy little environment for coupling between the two. Eliminating this source of noise may require running heaters on DC rather than AC.

Here’s a simple schematic adapted from something Eli Duttman suggested for his modified RCA phono preamp:

12V dc heaters

This circuit (now on a PCB waiting for a phono build) uses a voltage doubler to turn a common 6.3Vac input into ~16Vdc which is then regulated to 12Vdc by a LM7812. The regulator is limited to 1.5A, but this is probably enough for any sane phono preamp’s heater demands (the pair of 12AX7 in the El Matemático require only 0.3A). This is one way of producing a DC heater supply.

I was recently discussing truly budget-oriented tube phono preamps with another builder. They proposed a $100 parts budget. The first place I’d look to start cutting costs in such a build is on the relatively pricey purpose-built power transformer needed for tube projects. In the case of a simple phono preamp like El Matemático, I’d try the following cost-cutting measures to the power supply:

  • Solid state 1N4007 rectification
  • Use a 115/230V isolation transformer like Triad N-68X in reverse (115V in, 230V out) for B+ @ $16
  • Use a 12V SMPS like Meanwell EPS-15-12 for heaters @ $7
  • Triad C-1X choke @ $10 and 220uF 350V+ caps @ $4 ea as CLC filter
  • Add RC to end of CLC filter to lower B+ and/or clean up residual ripple

We can greatly lower the cost of the B+ supply with the isolation transformer trick but it leaves us without a heater supply. Rather than a separate 6.3V or 12.6V transformer followed by a regulator circuit like the one shown above, I’d be tempted to experiment with a switch mode power supply like this Meanwell unit:

EPS15-12

The EPS15-12 supplies up to 1.25A at 12Vdc with 80mV of ripple (peak to peak). One need just supply it with mains voltage (85-264Vac). Power supplies like this switch at a very high frequency, which is why their transformers can be made so small. If that switching is audible, capacitively coupled between cathode and heater, additional filtering may be needed. Meanwell does not specify the switching frequency, but it’s very likely well above the 20hz-20khz range.

The final, potentially very affordable power supply, would look something like this:

very cheap psu

More Opamp RIAA

I’ve detailed some very simple RIAA math for opamps in a past post and even did a little PCB board project to test the calculations. The image above is from a Patreon patron who built a battery powered phono from the same batch of PCBs. I’m very happy with the beginner-friendly nature and sound of this 9V-powered opamp phono preamp. The $25 bill of materials is nice, too. But, it doesn’t have a tube.

Now that I know the RIAA math and combination of passive and active equalization works, I’ll move on to phase 2. The battery powered two-stage preamp has about 40db of gain (60db if you count what’s needed for the RIAA correction). What if we only asked the opamp to perform the equalization (without the extra gain)? Having an opamp-based RIAA correction module eliminates the pesky RIAA math, but still lets us roll our own for the rest of the circuit.

Here’s a quick take on the circuit:

unity riaa signal

This brings the low frequencies from the phono cartridge up and the high frequency levels down to create a ‘flat’ signal. All that’s left is to make up the 40db or so of gain to get around 1Vrms output. A stage or two of grounded cathode tube amplification is the simple answer. There’s no urgent need for high Mu here, either: just about any tube could work. Note R16 still allows for some gain to be set at the opamp, so even a single tube stage can get a little help.

Keeping with the theme of simplicity, the opamp circuit would be powered from a common 6.3V winding:

unity riaa power

The heater supply is voltage doubled and regulated with a common IC. We can also use a rail-splitter to create a virtual ground and improve the performance of the single-supply opamp circuit.

In theory, the above looks like a fun and simple way to build a tube phono stage. The tube type(s) used would be extremely flexible and the RIAA portion adds no real complication to the build. The builder needs only focus on their tube fundamentals.

This is on my short list for the next batch of test boards!

One little project comes to fruition

Early this year, I wrote a post about simple RIAA correction with opamps. Although it doesn’t involve tubes (yet), I recently completed a PCB-based build based on this post. This was both to test the calculations/theory as well as good practice in PCB design.

This ultra-simple phono preamp runs on just a pair of 9V batteries for power and utilizes a mix of feedback and passive EQ for RIAA correction. The batteries should last about 24 hours (playing time), but a bipolar AC-derived supply could be substituted without trouble. Gain is easy to adjust with just a couple of resistors (set at 40db in my build). The bill of materials runs about $25 with 5532 opamps and 5% tolerance WIMAs.

I’m planning on building a couple of these with coworkers and basing build instructions and any revisions on the experience. I do have some extra boards from this first run. Shoot me an email if interested!

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.

stop stop its dead already.png
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!

DIY tube RIAA calculator sheet

All-In-One RIAA Calculator

Here’s a spreadsheet I built for calculating RIAA values in two stage tube phono preamps. When comparing results to other published designs using the same filter network, everything looks correct (within a few percent due to estimation of Rp). I used this sheet for El Matématico.

If you want to estimate values for something like a CCS loaded stage, you can set Rload on the appropriate stage to 1M or thereabouts. If you’re looking at using a cascode, mu follower, SRPP, etc 1st stage, you’ll need to make sure the Zout figure the sheet uses (cell I6)  reflects the Zout of the topology because it is used to calculate R1. Same thing goes for cell I3 (Miller capacitance of 2nd stage) if you use a gain stage after the filter that affects this (cascode, grounded grid, etc).

Go build a phono preamp!

“All-tube” MC phono preamp (continued)

Not long ago I wrote a short post about MC carts and the noise contribution of tubes when amplifying such tiny signals. I focused on step-up transformers as the solution to noiseless amplification, but there is another approach.  If you don’t like solid state, stop reading. Ok, now that you stopped reading and checked out the going prices for step-up transformers, you’re back. Good. Don’t worry, this approach uses the tubeyist solid-state device: the JFET.

A cascode is a compound amplifier in a totem pole arrangement. Here’s a great explanation by Valve Wizard Merlin. This allows you to achieve huge amounts of voltage amplification with fairly economic current usage and without coupling capacitors or multiple phase inversions. The driving force in this arrangement is the transconductance of the lower tube. The lower tube and upper tube do not need to be the same, nor do they even need to be the same type of device.

JFETs (junction gate field effect transistors) are voltage controlled devices, just like tubes. In fact, they bias in a very similar way: Rsource in the above raises the n-type JFET’s source voltage above the gate, similar to the way a cathode resistor in a grounded cathode amplifier raises the cathode above the grid. On the other hand, even the lowliest JFETs have a higher transconductance (gm) than the mightiest small-signal tubes. Icing on the cake is that JFETs, properly chosen and cared for, are lower noise devices. As such, they make a great lower device in a hybrid cascode.

The overall gain of a cascode simplifies to approximately:

gm(lower) * Rload

This equation is a simplified expression of the total gain of both devices:

[gm * (Rp + Rload) / (Mu + 1)] * [(Mu +1) * Rload / (Rp + Rload)]

AKA [JFET gm * load divided down at tube’s cathode] * [grounded grid gain of tube]

Rp and Mu are characteristics of the tube upper device. The choice of upper device affects how much of the voltage gain is performed by the JFET by affecting the load it sees. A high Mu and low Rp upper tube (i.e. high transconductance) presents a lower load as divided down at its cathode, thus less voltage amplification by the JFET (and more voltage amplification made up by the tube due to the higher Mu). A low transconductance upper tube does the opposite. But regardless of the tube (assuming an appropriately sized Rload), the overall gain remains the same: ultimately the transconductance of the JFET multiplied by the load on the upper tube.

So where’s this headed? Obviously there’s a full design coming to try out this idea, but the takeaway is that a hybrid cascode is potentially a great way to step up the tiny signals from a moving coil cartridge with very low noise and hand the now-larger signal off to a tube amplification stage without multiple supply voltages, coupling caps, or an expensive step up transformer.

The catch? Cascodes have poor power supply noise rejection and a fairly high output impedance. But there are ways to minimize these factors, too.

Further recommended reading: 1, 2, 3, 4

Letters to WTF: All-tube MC phono preamp?

Q:
 ­
Why not build an all-tube MC phono preamp?
A:
 ­
I haven’t built an all-tube MC phono preamp.  I build MM stages with the assumption that MC users will use step up transformers. Generally, its much easier to keep the whole thing quiet that way. This is a simplified explanation, but I hope it gets the general idea (SNR) across:
 ­
Tubes impart noise in a few ways but they’re all usually tiny.  MC carts need around 60-70db of gain to bring them up close to line level.  This is multiplying the MC signal from the cart by 1000-3000x before it gets to your amp. In contrast, a MM cart usually needs 40-50db, which is an amplification of 100-300x.**
 ­
The more tubes used, the more very tiny sources of noise get introduced.  Although the noises are tiny, they are amplified by the preamp, just like the signal. So the more gain required to get the signal to a desired listening level, the closer the tiny noises get to being audible as well.  Several stages of tubes for a very large amount of gain can lead to unwanted noise for this reason (guitar amps and phono preamps are both good examples of this).
 ­
The step up transformer cannot practically do more than about 20db-25db of gain without having some undesired consequences, but they don’t have ‘moving parts’ and are very good at rejecting noise/hum.  The 25db of gain from a transformer is enough to lower the tube gain needed from 3000x (70db) to 175x (45db), meaning any noise from the tubes is amplified much less. It’s kind of like giving the tube part of the preamp a head start in the race against noise.
 ­
So that’s why I haven’t built a MC phono preamplifier. If I were to build one, I’d most likely add step up transformers to a MM preamp like the El Matematico or similar.  To me, this is the most practical approach with the highest likelihood of success.
 ­
 ­
**At the risk of muddying the waters, phono preamps actually need about 20db more than the numbers mentioned here but this is then attenuated by the RIAA correction filter to result in ~40db or ~60db net gain, MM and MC respectively.