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Voltage Regulators for Line Level Audio. Part IX : Two Transistor Series Regulator

Posted 20th February 2014 at 11:50 PM by rjm
Updated 4th March 2014 at 01:56 AM by rjm

Part of a series.

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.

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Voltage Regulators for Line Level Audio. Part VIII : The Teddyreg

Posted 19th February 2014 at 01:44 AM by rjm
Updated 19th February 2014 at 01:46 AM by rjm

Part of a series.

The circuit is described here.

I'm not going to spend too much time on this one. The idea is to increase the input impedance of the pass transistor by buffering it with a jFET so it will support a high-impedance passive CRCRC filter section that generates a low noise reference voltage. The reference is defined not by a Zener or diode stack, but by a simple voltage divider. There is a LM317 pre-regulator on the front, but it is traditionally configured and works independent of the following circuit so it is omited here together with the additional transistor that speeds up the charging of the reference voltage filter capacitors.

The basic problem is that lowering the noise of the reference cannot lower the output noise indefinitely. After a point the output noise is defined by the performance of the pass transistor instead.

As you can see from the simulation, the circuit has the same ripple rejection as the filtered zener-referenced one-transistor regulator,...
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Voltage Regulators for Line Level Audio. Part VII : The k-multiplier

Posted 17th February 2014 at 11:55 PM by rjm
Updated 18th February 2014 at 12:08 AM by rjm

Part of a series.

This circuit is from this page by Kean Token, also referenced in his recent blog post.

Two versions are presented, one with all the protection diodes and a simplified version with extraneous components removed.

LTSpice simulation shows so-so performance into a light load, with about 70 dB of ripple rejection and a fairly high output impedance, but the drop out voltage is respectably low and we must factor in - coming directly from the Jung Super Regulator - that this is just a two transistor circuit, with no error amplifier to provide feedback.

As a frame of reference, it is quite similar in performance to the Z-reg we looked at back in part III.

The k-multipler is of a class of voltage regulators where the output is referred to the input voltage, rather than to ground. It provides "X volts less than the input", rather than the traditional regulator which provides "X volts above zero"....
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File Type: asc voltageregulator-kmult-simple.asc (2.1 KB, 109 views)
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Voltage Regulators for Line Level Audio. Part VI : The Jung Super Regulator

Posted 13th February 2014 at 04:21 AM by rjm
Updated 14th February 2014 at 10:52 AM by rjm (clean up)

In the next part of the series, I'll be presenting various published regulator circuits.

Today we have the "Jung Super Regulator" (2000 version) on deck, thanks to Tangentsoft's excellent write-up.

In translating the circuit to LTSpice, I've made some concessions. While I have kept the protection diodes so as to be consistent with the original - even if they do nothing in this simulation - the op amp, transistors, voltage regulator and reference have been substituted with working equivalents from the LTSpice libraries. I've been approximate in the resistance and capacitance values, and tuned the circuit to output 10 V at 10 mA to keep in line with the previous circuits I've uploaded.

It works though, and, under simulation at least, it works extremely well. Putting it together in LTSpice gave me a new appreciation for just how much work and refinement went into its design. Now, its an open question whether such over-the-top performance...
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