Group DIY - Power Supplies

Power Supplies

Please check with me first as this page was under Construction so grab what you can.

For our purposes we need three basic types of power supplies, which reflects the three circuit topologies that audio tends to use.

The early audio circuits used Tubes or Valves and the power requirement were varied. Usually high volts and mostly the AC was rectified using a Tube. The Heaters were AC and the power transformers were multi tapped to cater for these situations. This is all very difficult and the supply of these parts have dried up except in the retro world, which can be expensive. However there are work arounds and some of which are an improvement anyway.

As Tubes gave way to discrete components the need for high volts dropped to a more manageable 24 volts. There are variation like the 1176 which has+24V and -10V just to be difficult. The current started to climb but regulated supplies were quite normal in many circuits not just audio circuits. We now take advantage of this with simple regulators in easy to use cans. With the introduction of Computers and mobile phones the regulators chips are getting smaller and lower in volts. One day it might be hard to get a 24 volt 3 amp regulator as, it seems, the switch mode supply is the favored way to go. The switchmode is an option for us but perhaps more on this later.

For the last 30 years audio circuits have been based around the op-amp and so a Balanced Supply is typical. The op-amp usually requires plus and minus volts, say 15 volts positive and 15 volts negative. This situation can allow a circuit to be DC coupled, that is no capacitors or interstage transformers are required to cure DC offsets. Positive and negative rails are equal and balanced around the zero position - Balanced Supply.

Another area that will interest us is phantom power for microphones. In the past phantom has been anywhere form 24 volts to 52 volts for various reasons but for us, generally, 48 volts is what we require.

It probably can't get any more simple than a Box full of batteries. Not meant to be a definitive power supply but it is hum free and stable and is floating so can take ground loops issues away from the power supply. Great for when you are shaking down a prototype for the first time. Doubles up as a portable supply for location recording. The phantom may be a little low at 36 volts but you can add an extra 9 volt battery to bring it up to 45volts with dry cells and perhaps a little more with some of the NiHD batteries.

Regulated 15 volts using a 7815 and 7915 in a T092, T0220's or T03's. Phantom can be regulated with a TL783KC if you have the transformer volts - and - DC to DC converter if you don't.

Usually I have the bulk of the power supply including the power transformer on the main PCB of the project. When the project gets larger and the current consumption is higher and I still want it to fit into a 1RU box I have to use a small toroidal transformer. An 18 or 30 VA PCB mount transformer will easily fit into a 2RU box. Many of my projects are 2 or 3RU as I just love to use those big VU meters.

Occasionally I will use a toriodal power transformer and a separate Power supply board not for the current capabilities but for a lower noise floor. Mis-pre's and power amps seem to favour this approach. Some will argue and some don't like toroidal transformers but I have had good results so I continue to use them.

Below is my prototype opto compressor. It is a single channel unit with on board power supply. I know it is a little difficult to see and I do hope to improve things but I think you should be able to make out the power supply transformer in the bottom left hand corner. The AC power gets into the box via an IEC power inlet. The IEC inlet is modern and has a built in fuse, I tend to use one of these on all my projects. Above the power transformer you can see the two main smoothing capacitors. What is difficult to see just below the capacitors are two W04 bridge rectifiers.... yes two bridge rectifies. Perhaps I'll explain this later. Above and after the smoothing capacitors come the Voltage regulators and I think you can just see one of them if you look closely. In this case they were a 7815 / 7915 combination.

All power supplies follow the same basic layout and they get bigger for more current. A higher voltage regulator requires more volts going in and therefore a higher voltage transformer. The more more volts a voltage regulator need to 'throw away' the more power it will need to dissipate and that means hotter. Sure you can use a heat sink but it is better to select more appropriate values or devices. Sometimes I will use a more robust TO3 package even if I don't need the current.

Here are a few more pictures that show the On board and Off board transformer styles:

The above pictures show my typical dual bridge and larger filter cap arrangement followed by the 7815 / 7915 regulators flanked with 10uF tants.

Peter C has been a favored contributor to Group DIY from day one. We can thank Pete for bringing the Green-Pre to us. He has also provided a power supply for this project. Balanced 15 volts and 48 volts phantom. His method was similar to above with the 7815 and 7915 but Pete chose to use 4 12 zener diodes to set the 48 volts phantom.
http://1176neve.tripod.com/1176/id11.html

Here is a good starting point for a balanced supply. There is both as a fixed and a variable to chose from.
You will find schematics and a PCB design with overlay for each.
There is also a link to some data on the LM317 and LM337.
http://orca.st.usm.edu/~jmneal/dual_psu/psu.htm

Keith Andrews who is know to many of us as SSL Tech has provided a good looking +/-18volt with phantom supply. Based on 7818 and 7918 regulators with an adjustable LM317 for phantom there is nothing too special BUT and interesting arrangement of two bridge rectifiers make this an excellent choice. The positioning of two caps before the phantom rectifier turns this into a voltage multiplier.
The simple solutions are often the sweetest.
http://www.beatbazar.com/guests/ssltech/kps-1/index.htm

Single Ended :- Neve things.

24 volts regulated with a 7824 or adjustable with a LM317. Voltage doubler for phantom then regulated with TL783KC.
Perhaps the phantom circuit from the G9 tube mic-pre

Thomas Kristiansson (TK)
Power Supply schematic for a Neve Clone using an 24V AC adapter.
TK power supply.pdf

Zappo :- for tubes.

When too many volts is barely enough and be warned and be careful. 300+ Volts for the HT and regulated and adjustable DC heater voltages for heaters with a LM317. Again phantom can be doubled/tripled or juggled with windings and then regulated with a TL783KC.

A good example of power for tubes can be found in the G9 mic-pre power section.

Power Supply PCB PDF1

Power Supply PCB PDF2

Power Supply Schematic PDF1

Below is my driver board for the Pultec clone EQ. It is a variant of Jakob's design but I have put the power transformers on the driver board. The two PCB mount power transformers (blue and grey) are from Farnell and I'm sure many suppliers will have a similar product. The first TX provides the low volts for the heater which is regulated. The regulator has a heat sink and is hidden by the Tube. The Ac volts from the first transformer is then stepped up to the higher volts via another PCB TX used in reverse. Yes backwards, you must take the time to look at Jakob's circuit to better explain this. I had to experiment with the voltages to get the result I wanted as when used in reverse the voltages don't quite follow. The larger and thicker wired toriodal TX's were more predictable than the small PCB mount ones. For this project I ended up with 240V to 9V at 6VA (then regulated for the heaters) .. followed by 6V to 240V at 4VA(remember it is reversed). It still only got up to about 225Volts. For those in the US these PCB mount TX have the 110V option. Next time I think I will use a higher VA rating on the first TX as it does get quite hot.

More tube things are at:
http://w1.521.telia.com/~u52107110/schematics/pre/pre_8703_03.jpg

Phantom :- for mics that need it.

Jakob posted these at RO and I though it would be useful here.
The first two are form Eddie Ciletti and Walzingbear. These two sites should always be you first when you need something and you can't find it at RO.

http://www.tangible-technology.com/articles/frying.html
http://w3.one.net/~robgrow/circuits/powersupply/phantompower.html

http://www.rane.com/pdf/ms1bsch.pdf
http://www.paia.com/phantsch.gif

There must be an easier way that can save money especially when there is only one or two Mics to power.
A voltage trippler circuit could be the go here.
Here is something my Dad found at the local library.
I can now give you the reference.
"Radio Theory Handbook for Amateur Operators, Second Edition, 1991".

Voltage tripler

This is the circuit diagram of a voltage tripler. This device has similar applications as the voltage doubler. However, the output voltage produced is triple the peak of the input half-cycle. On the first half-cycle of input, C1 is charged negative, C1, D1, R1, to positive. On the next half of the input cycle, C2 is charged to twice the source voltage. The source voltage is in series with the voltage across C1 and extended to C2 via D2. As the output voltage consists of the voltage across C2, which is twice the source voltage, and the source voltage extended in series, the output voltage to C3 via D3 is approximately three times the secondary voltage of the transformer. On the next half of the input cycle, C2 is charged to twice the source voltage as it sees the polarity from the transformer and the charge of C1 via D2 in series.
At the same time the charge across C2 is applied to the output in conjunction with C3 producing three times the source voltage.

A word from John Klett

I stole the following from John Klett's forum. John can be found at ProSounds Recpit.
http://recpit.prosoundweb.com/index.php

I have time for genuine curiosity and people who do some homework.

When someone asks about something very basic like phantom power... the question of how to do this has been asked and answered many times... so I guess I feel like... 'hey - if this guy would just search this forum or others like it, or many other places for that matter...

Google - look for microphone and the exact phrase "phantom power"

http://www.sounddevices.com/tech/phantom.htm

The phantom power circuit I like is 52volts feeding a 0K5 to 1K0 ohm series resistor that dumps into a 47mF/63vdc cap that filters to ground. Off that cap you have two matched 6K8 resistors that feed the hi and lo (pin 2 and pin 3) at the xlr. You can add a switch before the first series resistor and add an LED with series resistor to ground if you want a light to tell you if it is on or off.

Phantom Power is not as much voltage critical as people think. You can use lower voltages with lower drop-on resistors - so a couple 9 volt batteries CAN do the job... but at some point the phantom power drop-on resistors will be "seen" as a load by the mic. The microphones are for the most part taking phantom in as a current source so as long as enough current is available (based on the voltage and drop-on R's) the mic will be happy... now this is not universal and the standard that everyone designs to today is 48 volts with 6K8 drop-on's... best to stay with that. The maximum amount of current any oone phantom powered line can draw from the 48 volt supply is 14.1mA (that would be if the hi and low are shorted to ground. A 48 input console only needs a 0.75 Amp supply to do the job. I buy 1 Amp supplies for this and anything bigger is fine - overkill, but fine for the job. A shorted mic should never be able to "blow out" a phantom supply.

Thanks John ... I don't think I can say it any better or add any thing more to that - Kev.

That Sony DC to DC converter that I have spoken of ? This is what it looks like and how it is packaged:

Another point of view to a 48 volt supply from a guy at prosound ( bryson )
http://members.aol.com/ecc81/ps48.html

Some very general rules :-

This is only meant as a guide to help get the ball rolling on that design for a power supply.

Voltage is easy to identify for each component and usually there is volts to spare but for electrolytic caps it is best to leave some headroom. For the main smoothing caps I think it is best to have not less than 1.5 times the volts required. Since caps come in set voltages things can shift little but go for higher than required. For an example the required voltage might be 15 volts then a 25 volt cap will be fine but at 24 volts you will need to be up to the next voltage, say 35 volts and if this is not available then you may end up as high as 50 volts.

The more current, the larger the caps will need to be to keep the ripple in check. My simple rule is 2 times the current required. 1 amp or 1000mA requires 2000uF. So the nearest common value is 2200uF. So all you have to remember is milli (mA) leads to micro (uF).

Rectifiers come in a variety of shapes and sizes, that is if you don't go for the simple 4 diodes. IN4004, IN4007 for 1 amp or IN5404. IN5408 at 3 amp. As for the Bridge Rectifiers , WO4, 1 to 1.2 amp and PO4 around 6 amp. Different manufactures have different spec so you just have to check. There are more packages to chose from, inline and dual inline and spade terminals on the very high current stuff. So as you can see when it come to rectification there are many options.

Regulators also come in only a few basic sizes so things should be easy but there is a catch. A regulator needs to drop the voltage down to the required level and the only way to do that is by throwing energy away. The more volts thrown away means more heat. The more current also means more heat. You will soon know if you make a mistake because the regulator will turn OFF or fail if the going gets too tough. The TO-220's are generally good for 1 amp and the TO-3's are anywhere from 1.5 amp to 3 amp and sometimes 5 amp. These figures are at the top of the range and will require heatsinks etc. For very small projects or low power parts in a larger circuit the very small TO-92 package is good for up to 100mA. These can be recognised by the 'L' in 78L12. An example can be seen in the SSL Clone.

So here are a few quick thoughts

Caps:
uF = 2 times mA required.
Volts = 1.5 times volts req.

Rectifiers:
WO4 = 1 amp.
PO4 = 4 amp +

Regulators:
TO-92 100mA
TO-220 1amp
TO-3 1.5 amp ++

Here is an interesting article about turning a fixed regulator into a variable reg.
http://www.rason.org/Projects/projects.htm

If you need details and there isn't enough here.......

OR you want a variation to the above just e-mail me.

The Group DIY Power Supply collection :-

Under development.

This collection of power supplies will share a common theme and style and so should make things easy for those new to DIY. The first is aimed at the simple op-amp based Mic-pre requiring +/- 15 volts and 48 volts phantom. Current should be a problem but it is worth double checking.
As an example we can look at Peter C's Green Pre. One module with no signal draws approx 18mA per rail.

Voltage is easy to identify for each component and usually there is volts to spare but for electrolytic caps it is best to leave some headroom.

Group DIY PS 01

Refer to the Dual KDMP project.