Posted 20th December 2016 at 05:47 AM byrjm (RJM Audio Blog)
Updated 21st December 2016 at 01:52 AM byrjm
I've come this far so I might as well complete the trifecta.
This is the X-Reg circuit, with a new layout and component numbering to match the new S-Reg and Z-Reg boards. Each is a drop in replacement for the other.
Like its siblings, the X-Reg is a low current voltage regulator for line level audio. The output is about 9 V (adjustable up to 12 V) and the maximum load current is 50 mA without heatsinks, 100~150 mA with small heatsinks on Q1,2.
It is not a true regulator as there is no fixed reference and instead the output voltage is defined relative to the input voltage. The high open loop gain of the op amp is harnessed for very low noise and very high ripple rejection. It is necessarily a high feedback approach.
So, there you are: three mix-and-match power supply options for all your low voltage, low current audio needs.
Posted 19th December 2016 at 01:55 PM byrjm (RJM Audio Blog)
Updated 21st December 2016 at 01:49 AM byrjm
Companion regulator to the S-Reg. Same board dimensions and connection layout. For line level audio.
Same idea as before rectified 2x12 VAC input, +/- 11 V output. 50 mA max output current unless the transistors are heatsinked.
There is a small amount of over-current protection afforded by R3,4 but do not short the output for all but the shortest of transients.
Eagle/Gerber files attached, so you can go get this made yourself, optinally with whatever modifications you need.
I've use this regulator circuit in my Sapphire headphone amplifier. It's a simple and modest circuit but I feel it works really well in practice as long as the audio circuitry it powers has reasonable PSRR. No feedback means no out-of-band noise or instability, even as the output impedance remains low.
Posted 17th June 2016 at 02:44 PM byrjm (RJM Audio Blog)
Updated 20th December 2016 at 12:01 PM byrjm
I'm not totally sure this would work as advertised, but I can't see any obvious reason why it would not...
It's pretty much the same circuit as I used in the CrystalFET, which started out in a previous blog post in the Voltage Regulators for Line Level Audio series, but here I've replaced the MOSFETs with bipolars. It is shown configured to deliver 20 mA @ 12 V, split supply. Enough to power an op amp phono stage for example, or a preamp, or the voltage gain stage of a headphone amplifier.
Posted 20th February 2016 at 01:49 AM byrjm (RJM Audio Blog)
Updated 22nd February 2016 at 09:35 AM byrjm
A while back I did a series of blogs on voltage regulators. Back with a new entry today: The Crystal M, configured here for 40 V DC output and a 25 mA load.
The circuit is based on two p-channel MOSFETs, the top one is a constant current source, the bottom one a constant voltage source. As the load current changes, the voltage source adjusts its current to balance.
I trick, I discovered, to getting it to work nicely - the attached screencap shows it well-behaved while handling a full-swing output current pulse - is the source resistor R10. This resistance dials-down the current gain of the MOSFET, damping out the overshoot.
The ripple rejection is about 70 dB over the audio bandwidth. The output impedance is about 0.05 ohms over the same frequency...
Posted 19th February 2016 at 12:14 AM byrjm (RJM Audio Blog)
Updated 7th April 2016 at 07:58 AM byrjm
I've never put everything into a single LTSpice worksheet like this before: I find it fascinating. You can really pull apart a circuit to see what makes it tick, before solder ever hits the iron.
Power supply ripple, frequency response, gain, and crosstalk can be established. You can look at turn on and turn off transients, inrush currents, and conductance angle, and check peak currents in the filter capacitors. It's all there if you care to peek in and poke around.
I'm such a huge fan of LTSpice...
The only problem, really, is it is too perfect: all devices are perfectly matched, every part value is exact, and the temperature is always 25 C. Ground loops, wiring inductance, and thermal runaway do not exist. So no, of course there are no guarantees - but as a tool to get you 90% of the way there with the minimum of fuss and bother it is truly indispensable.
Actually I find the more experience you have the more useful LTSpice...
Posted 22nd February 2015 at 02:24 AM byrjm (RJM Audio Blog)
Updated 28th February 2015 at 07:17 AM byrjm
I've added an additional RC filter stage (R3, C4 in the schematic below) before the Zener diode, substantially reducing the amount or ripple on the transistor base by cleaning up the voltage applied to the Zener reference. (The original Z-reg is described here.)
Circuit shows C2 with a value of 300 uF. Typically much larger values are used. I kept the filter capacitance to a minimum here to show circuit working with a reasonably high ripple (1 V p-p) on the input. The rectifier diodes used here are of no particular consequence, I just wanted the simulation to generate a realistic sawtooth for the input.
OK, this doesn't do as much as I originally thought. The improvement is mostly below 100 Hz, whereas the ripple is mostly in the 100Hz-1kHz band. There's perhaps 3 dB less output ripple, but that's about it. You can verify this yourself in LTSpice, just cut the wire between C4 and the junction or R1-R3 and rerun the sim.
The GeminiPS is another discrete series voltage regulator, with a Zener reference and bipolar pass transistor. It's an old circuit, published in Practical Electronics in 1970-71, and written by D.S. Gibbs and I.M. Shaw. I happen to have a reprint, but there's a nice overview here.
For reference it might be worth checking back to the two transistor regulator. The GeminiPS circuit is related in the sense that it is a more sophisticated take on the same basic principle. With just a handful of components we have a stabilized, 30 W output with soft turn on and short circuit protection. The circuit can be scaled up and down relatively easily, and the complimentary (negative output) version is an easy modification.
The pass transistor (TR2/3, Q2/3) is between the circuit common and the rectifier anodes. This may seem odd, but it was relatively common back in the day when high voltage transistors were both expensive and rare. The...
I've been meaning to take up shunt regulators for some time. I've never got around to building one myself to try, so I'll have to make do by playing in simulation.
Today's circuit is the shunt analog of the Z-reg series regulator: no feedback, Zener reference, single transistor regulation. The output impedance and ripple rejection-characteristics are similar too, with about 40 dB of RR and an output impedance of just a few ohms. It can be built equivalently from either an pnp or pnp transistor. (See attached LTSpice .asc files.)
The difference between shunt and series regulation can best be explained by considering the upstream power supply: In a series regulator an increase in current demand by the load causes the regulator to increase the current to compensate. In a shunt regulator an increase in current demand by the load causes the regulator to decrease the shunt current to balance, so there is no net change in current flowing...
So you have a small handful of parts and want to build a (simple) discrete voltage regulator instead of using an IC. What to do?
For line-level audio circuits, especially op amp based (IC or discrete) preamps with high PSRR, something like the Z-reg is generally sufficient. Robust, works well, has enough ripple rejection to cut power line noise from the preamp output.
If you add just a couple more parts, however, you can add feedback to the Z-reg circuit, a simple error amplifier in the form of an additional transistor Q2, with the output-sampling voltage divider R1,R2.
The ripple rejection is not vastly superior to the circuit without the feedback unless some additional bypass capacitors are added as shown in the first version of the circuit below. The output impedance, however, improves from a few ohms to a few tenths of an ohm as a result of the feedback. Which could, in principle, be of use.