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 Elliott Sound Products Project 102 

Simple Pre-Regulator
Rod Elliott (ESP)

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Introduction

There will be many times where it is desirable to use the P05 supply module from a higher voltage source. For example, if you want to add balanced inputs to a power amplifier, then you need a +/-15V supply, but the amp's supply voltage will be much too high for the regulator ICs.

This project is about as simple as they come, and is very cheap to build. It is designed for exactly this purpose - to reduce the amplifier supply voltage to a safe value for regulator ICs. It's worth noting that even though the LM317/337 (for example) have a rated voltage differential of 40V (the actual voltage across the IC), this is really the absolute maximum voltage that should ever be applied. Ideally, it will be kept much lower, and I recommend a maximum input voltage of around 25V. This reduces device power dissipation dramatically, and ensures that the ICs are well within ratings at all times.

While the datasheets claim that these regulators can be used at higher voltage as long as the voltage differential between input and output is within the ratings, they will fail if the output is shorted, or even if the output caps are too large (acting as a 'temporary' short). Use of a pre-regulator minimises the chance of failure.

In some cases you can use the pre-regulator by itself (with reduced output voltage of course) as the power supply to opamp based circuits. You don't need regulation with the majority of opamp circuits and their power supply rejection is usually very good. If you can reduce the main supply ripple by 50-60dB, no noise will be audible. Naturally, this is something that you must test for yourself - there are too many possibilities in circuitry, and no one solution is suitable for all applications.

All the circuits shown can be improved, potentially dramatically, by adding current sources to replace the resistors. However, this increases the complexity, and makes a simple circuit less simple and far more difficult to put together. The idea is to provide a means to reduce high power amp rail voltages to something cleaner (less ripple) and more regulator friendly. The circuits are not intended to be regulators in their own right, but in some cases they may be all that you need.


Description

The basic circuit is shown in Figure 1 and it is very simple indeed. You will need to make a few simple calculations to determine the resistor values, and these are explained below. There is also an 'enhanced' version that provides even better ripple and noise rejection - see below. Finally, there's a version using MOSFETs which some may prefer.


Figure 1 - Basic Pre-Regulator Schematic

The circuit shown uses the 24V zener diodes (D1 and D2) to regulate the output voltage to a little under 24V. This is a perfectly safe input voltage for standard 3-terminal regulators, and using this circuit will provide even better regulation and supply noise rejection then normal. Using MJE3055 and 2955 transistors will allow for supply voltages up to 56V quite safely, but they will need to be mounted on a heatsink (with insulating washers). If you have a supply voltage of more than 56V, use transistors with a higher voltage rating.

In some cases it might be necessary to include a 10Ω 1W resistor in the GND connection from the main supply. This is something that you may need to experiment with if a ground loop (and subsequent buzz) causes a problem. In most cases you won't need it, but sometimes there will be no alternative. It can be useful to bypass the resistor with a 100nF polyester capacitor so that RF noise is properly grounded.

I suggest that R2 and R4 should be rated for 1/2W, and 1W zeners are recommended. Optionally, you can add a 100nF high frequency bypass cap in parallel with R2 and R4. Don't expect a difference though, unless RF interference is common in your area.

The only calculation is to determine the value for R1 and R3. First, measure the power amp supply voltage (V1). The resistor value is calculated to provide a maximum zener current of 20mA, and this will ensure sufficient base current for the pass transistors for up to 100mA or so output current at ±15V. If the current drain of your preamp is greater than 100mA, you'll need to allow for more base current for the series-pass transistors. Be careful that you don't reduce the resistance value to the point where the zeners dissipate more than 75% of their rated power or they will run far too hot.

V2 = V1 - Vzener   (Where V1 is amplifier supply voltage, and a Vzener is the zener voltage used)
R1 = R3 = V2 / Izener   (R1 and R3 values are in kΩ, Izener is zener current in milliamps - 20mA is ideal)
P = V2² / R1   (P is power dissipation of R1 and R3 in mW)

Let's assume a supply voltage of ±56V for an example calculation ...

V1 = 56V
V2 = 56 - Vzener = 32V
R1 = R3 = 32 / 20 = 1.6k (use 1.5k)
P = 32² / 1.5 = 680mW = 0.68W (use 1W)

The dissipation in Q1 and Q2 may also be calculated, but you need to know the current drawn by the external circuits. For example, if the external circuitry draws 50mA, the transistor power dissipation is ...

P = V2 * Iout = 32 * 50 = 1600mW = 1.6W (they will need a small heatsink)

That's it for the basic version - it could hardly be simpler. Your regulator ICs are safe, and have around 30dB less input ripple to contend with. This means that if the main supply rails have a typical 6V of ripple and voltage variation due to varying current drain, this will be reduced to about 150mV total variation.

If you need more output current you can use Darlington transistors, such as the TIP140 (NPN) and TIP145 (PNP) or their higher voltage versions if needs be. These can easily supply over 1A while allowing you to use the resistor values as calculated above. With a typical gain of around 1,000 you can even increase the resistor values if desired. Remember that the zener current should be set for about 20% of its rated maximum current (allow between 10mA and 20mA for 24V zeners). The pinouts for the TIP140/145 are the same as shown in Figure 3.


Add Some More Noise Rejection

While the basic circuit shown already has quite good noise rejection, some applications might need the maximum possible noise rejection. If this is the case, you can use the version shown below. The Value of R1 and R3 are calculated exactly as before, but R1A and R1B are half the value calculated, and the same for R3A/ R3B. Power dissipation in each resistor is half that calculated above for the 'basic' version.

In this case, you'd use either a 50V or 63V cap, depending on which is easier to get and cheaper. The resistors (R1A/B and R3A/B) would be either 680 or 820 ohms. One is a bit lower than the total calculated value and the other is a bit higher, but either will be fine. Increasing the capacitor value will give even better noise rejection, but the supply will take a lot longer to reach full voltage. With the 100uF cap and 680Ω resistors for R1A/B and R3A/B, it will take around 80ms before the output voltage stabilises.


Figure 2 - Enhanced Pre-Regulator Schematic

The capacitor voltage is determined by the following procedure ...

Vcap = Vzener + ( ( V1 - Vzener ) * 0.5 )

Using the same voltages from above, we get ...

Vcap = 24 + ( ( 56 - 24 ) * 0.5 )
Vcap = 24 + 16 = 40V

The added capacitor ensures that there is reduced ripple in the zener current, so the output voltage will also have lower ripple. You can expect an additional ripple voltage reduction of around 15dB with the values determined here (for a total of 45dB). Increasing the capacitor value will improve things further, but any regulator IC can easily handle the output of the circuit shown. As an added benefit, the output voltage is also relatively free of high-order harmonics, because the added capacitor acts as a low-pass filter.

C3 and C4 are optional. They do help, reducing high order harmonics further and reducing the overall ripple by about another 6dB. Whether you consider the added cost to be worthwhile is up to you - personally, I wouldn't bother because the caps will typically be mounted close to the zeners so will get hotter than normal and may have a reduced life. However, the AC ripple current is tiny so a bit of extra heat is probably not a major problem. As you would expect, the extra capacitance does increase the time before you have full output voltage.

As before, you can use Darlington transistors if more output current is needed, or if you want to use higher value resistors for better noise filtering with the same capacitance.


MOSFET Pre-Regulator

If you need a circuit that can either pass more current or that would benefit from the lowest possible ripple, a MOSFET is a good choice. The output voltage is less predictable because of the gate-source threshold voltage (it can vary by several volts, and depends on output current). This means that although you can get very high ripple rejection even with low value capacitor(s), the output voltage will vary with current. The circuit diagram is shown next. The suggested MOSFETs are rated for 100V, and more current than you will ever need. They can be replaced by almost anything else that you may have in your parts bin, provided that they have suitable ratings for your application.


Figure 3 - MOSFET Pre-Regulator Schematic

You still need to provide a reasonable current through the zener diode, and the calculations shown above are still required. The biggest difference is that the output voltage will be up to 5V lower than the zener voltage (D1, D2), so higher voltage zeners are used. There is no gate current, so no allowance is needed for the base current for a bipolar output. The second pair of zener diodes (D3, D4) is necessary to protect the MOSFET gates, which will be destroyed if the gate-source voltage exceeds 20V or so.

The current through the 24V zeners should not be less than 4-5mA, so R1A/B and R3A/B would be 2.7k for a ±56V input. The output voltage will be around 4-5V less than the zener voltage. Ripple can be expected to be less than 1mV, and does not change appreciably with output current. Ripple can be reduced further by increasing the value of C3 and C4 in parallel with the zener, but there is probably no point because output ripple will typically be less than that from the Figure 2 circuit.

Heatsink requirements are determined in the same way as for the other circuits shown. MOSFET dissipation is determined by the voltage across the MOSFET and the current though it, and for most preamps (under 100mA), and will usually be less than 5W.


Construction

Construction is non critical, and the resistors, zener and power transistors can be mounted on a tiny piece of Veroboard or similar. There are no stability issues, and you only need to make sure that the transistors have an adequate heatsink. Mounting to the chassis will normally be quite sufficient - even a steel chassis will keep the temperature well within limits. Remember that the transistor cases must be electrically isolated from the chassis, and Sil-Pads will be fine due to the low dissipation. Do not use tantalum capacitors in any of the circuits shown - they are the most unreliable caps ever made, and I don't recommend them for anything.

A suggestion for assembly is shown in Figure 4. This construction method will be quite acceptable for most applications. The transistor cases must be isolated from the heatsink with silicone washers. The assembly shown is for the Figure 1 circuit. It includes the optional resistor and capacitor that can be added if earth/ ground loop hum is experienced. The network may not work in all cases, and may require experimentation to obtain minimum hum levels.


Figure 4 - Construction Suggestion

The above does not include the additional capacitors shown in Figure 2 and can't be used for the Figure 3 circuit. If you want to use either of the alternative circuits, you'll need a piece of Veroboard or other prototyping board to mount the extra parts. A heatsink will be needed in almost all cases, unless your preamp only requires a very low current. With a 56V input and 20mA output, dissipation will be over 0.6W, so only the most basic heatsink is necessary - a piece of 1mm flat aluminium will suffice, or use the chassis. You need to determine the dissipation with your circuit (based on the current drain and main supply voltages) to work out how much heatsink you will need.


Testing

Connect to a suitable power supply - remember that the supply earth (ground) must be connected! When powering up for the first time, use 100 ohm to 560 ohm 'safety' resistors in series with each supply to limit the current if you have made a mistake in the wiring.

There is very little that can go wrong (other than wiring mistakes), so any fault you may find is easily rectified. Note that extreme care is needed against shorting the outputs of any of the circuits shown, as there is no short circuit protection and failure is almost guaranteed. While protection can be added, it's no longer a simple circuit because of the additional parts needed. In service, a short is highly unlikely unless there is a catastrophic failure somewhere, and many other parts will need to be replaced anyway. Adding a couple of cheap transistors/ MOSFETs is the least of your problems.


 

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Copyright Notice.This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2003. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.
Page Created and Copyright © Rod Elliott 03 Jul 2003./ Updated Nov 2012 - added Figure 2 and text to suit./ Dec 2016 - added MOSFET version.