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Old 31st January 2014, 06:29 AM   #1
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Default Low Noise Discrete Power Supplies for Phono/Line Preamps

I've been looking at developments in simple discrete circuits for RIAA preamps and line amps for about 10 years. There are a lot of circuits I've designed and built (and documented here and there on this site) and I'm going to do some sort of thread to consolidate them all when I feel up to it. After 10 years, it's time to do some sorting and summing-up and figure out which topologies have the most promise.

At any rate, some lot of the more simple and ultimately promising circuits (esp. for RIAA first stage duty) have poor supply rejection, so they require a quiet, well-behaved power supply in order to shine. I'll be presenting 4 discrete regulator circuits here that look good - 3 shunt regulators, 1 series. Three circuits use an LED voltage reference, one uses a zener diode reference. Consider them as grist for the mill (in advance).

None of the circuits to be presented in this thread use more than 4-5 active devices, so they can all be built discrete-fashion on a piece of perf board and not suffer for it. Several have been already presented on Diyaudio, but were lost in the shuffle. I'll present circuits, simulation results, and where applicable, a link to the original presentation thread. More later - this is just the teaser.
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Old 31st January 2014, 05:00 PM   #2
mlloyd1 is offline mlloyd1  United States
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sounds good.
consider us sufficiently teased.


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Old 1st February 2014, 07:49 AM   #3
h_a is offline h_a  Europe
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Quote:
consider us sufficiently teased.
Discrete regulators get always a warm welcome
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Old 2nd February 2014, 04:56 AM   #4
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Sweet! I'm stoked! I'd like to try a few.thanks.
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Old 2nd February 2014, 05:31 AM   #5
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Subscribed with interest!
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Old 2nd February 2014, 10:11 PM   #6
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Since three of the four circuits to be presented are shunt regulators they'll get discussed first. The lone series regulator in the suite was just ginned up late last week in simulation, and hasn't been built yet. I'll present it here because it has some interesting features that make it akin to the shunts and perhaps a valuable addition to a designer's tool kit.

The shunt regulator circuits I'm showing here are all based on the architecture shown in the attached diagram - a stiff voltage source fed by a stiff current source. A resistor of appropriate value and wattage could be used instead of the current source, but the current source provides extra attenuation/isolation from whatever nastiness is present on the unregulated supply. The current source is set for the maximum current draw anticipated by the circuit you're powering, plus some extra margin to keep the voltage source happy and stable (and conducting) under all load conditions. A good shunt regulator circuit is set such that the voltage source never goes into cutoff under all load conditions, just like a Class A amplifier.
The circuits I'm presenting were designed mostly around requirements for low noise and low output impedance, so that the output voltage does not vary as a result of the audio load imposed on it. Setpoint accuracy and output drift are ok for the circuits presented, but not by any means spectacular. For most simple amplifier circuits, these won't be as important as noise and output impedance. Circuits with poor PSRR such as the Pacific/Simplistic RIAA preamps and the like demand a quiet supply voltage that doesn't jerk around under load, elsewise you get hiss and other antisocial behavior.

The circuits presented here have no IC opamps. To me, opamps (especially the wide bandwidth, premium performance variety) are like thoroughbred horses, pretty and capable of flashy performance, but requiring a lot of pampering to behave properly, much less shine. Those enamored of the thoroughbreds can check out threads on the Jung, Sulzer, and other opamp-based regulator solutions discussed extensively in other threads. The thrust here is that a small pinch of discrete components can provide impressive performance and predictable behavior, with much less of a need for pampering (and a wider range of operating voltage).
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Old 2nd February 2014, 10:41 PM   #7
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For enterprises like this, it's best to get the simple stuff out of the way first. Attached is a set of possible options for discrete current sources. I presented six possible solutions, in the spirit of completeness and perhaps being nice to folks that have fewer options for obtaining the necessary components.

To be honest, though, I rarely stray beyond the simplest solution, "A". This consists of a depletion-mode N-Channel mosfet set up as a current source. Resistor R1 sets the output current, and R2 is a stopper resistor to keep Q1 from inappropriately bursting into song - mosfets operating in the linear region are notorious squealers unless they're dampened down.

For lower currents, the Supertex DN2540N5 depletion mode fet works ok. However, it has a minimum IDSS of 150 ma, which makes it marginal if you want to consider one regulator to power both R and L channels. A much beefier workhorse is the IXYS IXTP08N50D2, which is a 500V, TO-220 device with an IDSS of ~ 800 ma. They don't cost a lot of money, but not a lot of people stock them. If you don't want to go to the trouble, you can use the mosfet/bipolar "ring of two" current sources shown in "B" and "C". I used these circuits before I discovered the IXYS device. I used mosfets for the pass elements in these circuits, as they tend to have a higher output impedance than a bipolar transistor. The bipolar solutions shown in "E" and "F" are cascoded to help get around that problem.

Circuit "D" is one I haven't tried yet, though I see no reason why it wouldn't work OK. It uses a constant current source (Q2) to set up a constant voltage across R2 to act as a voltage reference, which along with R1 and the Vgs of Q1, sets the operating current. C1 bypasses R2 to make the voltage across it more quiet in the audio band. If I remember correctly, Salas used a concept similar to this in his "simplistic shunt".

This is a start. I'll go into more detail later, but I'm nursing a cold right now. Talk amongst yourselves, ask questions - back later.
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Last edited by wrenchone; 2nd February 2014 at 10:54 PM.
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Old 3rd February 2014, 06:05 AM   #8
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Here's some reading to do while waiting for me to gather the wherewithall to write about the next bits. Some of you will already be familiar with this thread, but if not, it's a must-read.

Some noise measurements for LEDs and zener diodes
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Old 3rd February 2014, 05:41 PM   #9
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Here is a link to the thread containing my first shot at a discrete low-noise regulator. This was made when I knew less about LED characteristics than these days. If you read the previously cited thread on LED/zener noise closely, one realizes that this regulator is a candidate for either an LED reference or a 10-12V zener.Two instances of this regulator are currently working in my living room system, one with green GaP LED reference and one with red GaAsP reference. Both as quiet as a mouse, at least in ear-to-the-speaker terms, which is more than I can say of the TL431-based regulator they replaced. I'll be posting an updated schematic and simulation profile as time permits. The link -

Regulator for RIAA Preamp and Line Amp

Last edited by wrenchone; 3rd February 2014 at 05:54 PM.
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Old 15th February 2014, 09:39 AM   #10
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Here is the update simulation of the circuit in the thread referenced in the previous post. The simulation uses a voltage source and a resistor to simulate a string of GaAsP deep red LEDs (low impedance (~1.5 ohms at 10 ma bias), low noise). An exposition regarding this circuit will follow when I've had some sleep, including some general comments on the advantages of a good shunt regulator vs. a series pass solution
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