Earlier last month, a friend decided she was getting a turntable and asked me for advice. Looking at phono stages, I drew a blank. So, we’re going to make one. Some general requirements 1. There should be no hum, hiss, or buzz. Kind of ruins the experience, no? This means the circuit should have excellent PSRR, or an excellent power supply, or ideally both. This also means no ground loops and no EMI. 2. It should also sound like a single ended phono stage. That means more second order harmonic distortion than third, more third than fourth, and so on. There should be no oscillation either, and we’re going to do our best to minimize sources of distortion that can impart a “solid state” sound. 3. RIAA equalization should not rely on feedback. Feedback can be a very lovely thing, but not so much in RIAA equalization. Notably, feedback RIAA works by varying the feedback ratio according to frequency, such that you have a higher ratio at high frequencies and a lower one at low frequencies. An unfortunate consequence is that your input and output impedances are also frequency dependent. So is your distortion. This seems like a bad tradeoff. 4. The user should have full control over cart loading, and be able to plug into any reasonable load. Alternatively, this means that input impedance should be very high, and output impedance should be low. We can then tailor the input impedance to our needs with loading resistors and capacitors. But, we should have the freedom to load the cart with no more capacitance than what the cables provide. 5. The whole thing shouldn’t draw more than a few watts. For goodness’ sake, it’s a phono stage. Radiated noise is proportional to current, so it’s really in our best interest to keep AC currents down anyway. We’re also going to be building to a price point, and strictly for MM. With compelling options in the $200-500 range, it does not make sense to blow that much on fancy Lundahls or Duelunds. Even though Lundahls are lovely. Meet the cascode For high input impedance and gain, you’d be hard pressed to do better than a cascode. A cascode is a common emitter/source/cathode stage working into a common base/gate/grid stage. They might look like this: The lower device is a transconductance amplifier. The current through the cascode is modulated by applying a voltage signal between the +In and Ref (In) nodes. The current is then converted back to a voltage using a resistor, and we can sample the output voltage across it on the -Out and Ref (Out) nodes. The top device is a transimpedance amplifier. Its role is to provide a low impedance load for the lower device such that no voltage signal appears on its collector/drain/plate. It typically takes the bulk of the dissipation, and it’s this device that’s going to swing the signal voltage; so, figures of merit would be a low input impedance, a high output impedance (i.e. its current is not affected by the signal), and good linearity vs current. Here's an example of an all-triode cascode. The Muscovite is one of the few phono stages that does this. Let’s say the triodes are ECC88s, which have a mu of 33, a plate resistance (rp) of 2.7K, and a transconductance (gm) of 12.5 mA/V. In this circuit, the cascode gain is 8 kΩ * 12.5 mA/V, or about 100. Not bad, considering mu is a third of that. Tube folks like to solve their circuits with curves. We can do that too, but probably not with the curve that you have in mind; we use the Ia/Vg curve instead of the familiar Ia/Vp curve. Linked. Manufacturers of fine semiconductors will also include a similar graph in their datasheets. Below is the Vgs/Id curve for the 2SK170. For both devices, linearity goes up with current, but the tube really is the more linear of the two devices. Mind you, this isn’t really a problem for a phono stage; the signal magnitude is so small that we shouldn’t need to swing more than a few hundred microamps. If you mix device polarities, you can throw together a folded cascode. In its simplest form, it might look something like this. The principle is the same: the upper device (PNP) presents the lower device (NPN) with a low impedance such that the lower device develops no voltage gain. Instead, it conveys signal current into an I/V resistor, which, in contrast to the regular cascode, is referenced to ground. An interesting benefit is that the output can be at the same DC voltage as the input, which makes for easy DC coupling. If I were Nelson Pass, I might connect the -Out back to the +In through a feedback resistor and call this SuSy. Or if I were Rod Coleman, I might replace the bottom BJT with a triode and call it a “shunt cascode,” then use that to drive a 300B. There are caveats, though. There are now two paths to ground, so you need twice as much current. Biasing the whole thing isn’t trivial, as it’s a balancing act between gain, idle current, and voltage drop across the devices and gain resistor. I’ve had troubles with oscillation that I couldn’t track down. Unlike the regular cascode, it can be a real cow to linearize, too. Our first stage: a “current shunting” cascode With all that in mind, we can throw together a first stage. We’ll be stealing a trick from Gary Pimm, and load our cascode with a current source. Typically, this isn’t possible; the cascode is a current source, so we would be putting two current sources in series. They would fight to define bias, until the lower impedance of the two is pinched off. The trick is to throw in a shunt resistor, which both defines the DC bias condition and limits the gain to something reasonable, and reasonably independant of frequency. Here's what it looks like, simplified. At the heart is a parallel JFET – BJT cascode. The rest is just there to make sure it turns on. The cascode sinks a current set by the JFETs and their source resistors. In order to define the plate voltage, we run a small current through the 20k load resistor. What’s the benefit over our normal cascode? There are two big ones. If we were to run a normal cascode at 40mA, with a 20k load, the resistor would need to drop 800V. There is nothing wrong with 800V supplies, but if we are even considering one, we should be doing something more interesting with it than heating our listening rooms. The other advantage is that the output signal and input signal are both referenced to ground; the power supply is completely removed from the current loop, and the power supply rejection goes from essentially zero— probably the biggest strike against the cascode— to something usable. Other than the noise benefit, parallel JFETs let us run the cascode at higher current, because the current that each JFET can sink is limited by Idss. Why is this beneficial? The impedance that the JFET drain sees is dominated by the BJT’s emitter resistance, Re, which is a function of current and temperature. For the lowest impedance, we want the current to be high, and the temperature to be cool. So how much gain can we squeeze out of it? With 68 ohm source resistors, we’re looking at a gain in the range of about 50dB. Just enough to use a single stage. A quick word on bias. A good bias supply should have a low impedance and noise across the frequency range. You might think the best bet is to use one of TI’s fancy shunt references, but it turns out they are unilaterally terrible. Instead, we’ll use a Zener. On a bad day, a 6.8V Zener will put out 20µV of broadband noise; this is actually less than an LM4040, which the datasheet specifies at 80µV. Depending on current running through the diode, its impedance is anywhere from 2 ohms to 700 ohms, but, unlike the LM4040, it is constant with frequency because it does not rely on feedback to keep its impedance down. Zener diodes do present a small inductance, so we’ll bypass it with a small capacitor for good measure. Next time: the RIAA network and second stage.