## 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:

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:

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.

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:

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!