Need help with power supply schematic

I just purchased and built a low power variable power supply kit to power a solid state synthesizer I built years ago from an article in Popular Electronics. The synthesizer uses 50 mils at 12-15 volts DC to produce sounds resembling those of a steam engine. It's used with a model railroad. When I use the power supply with the synthesizer it sounds very garbled and noisy. I'm wondering if the power supply has enough filtering. Any suggestions on how I can modify the attached schematic for better results?

You can add additional cpacitance across C2 (observe polarity). If the leads from this supply to your synthesizer are long, you can add the capacitance at the synthesizer input. Not knowing how your system is physically laid out, I would also twist the output wires from the power supply around earch other and make sure they are routed away from the power infrastructure that runs your model railroad. Also make sure the leads to the potentiometer are short and you can try a cap (0.1uF to 1uF) from the output of the potentiaoleter to ground (i.e. transistor base to ground).

This power supply really isn't suited to audio applications. Not enough ripple rejection. I would replace the transistor with a 78L12 Voltage Regulator, remove the Pot and short the wiper line to ground (i.e. you want the ground pin of the 78L12 to go to ground). This will make the supply no longer variable, but it sounds like you don't need a variable supply for your application. If you do, then I would use an LM317L and rewire the pot and limit resistor (i.e. the variable voltage divider) as required for the LM317L (check the LM317L datasheet if you're not sure how to do that). With the LM317L, though, it won't adjust to 0 (more like 1.25V).

Another alternative is to try something like a 3300uf cap from the base of Q1 to ground. The size of this cap depends on the Beta of Q1 and how much ripple noise can be tolerated. Basically this turns Q1 into a capacitance amplifier. You might be able to go as low as 600uf.

Here's my reasoning: IT=CV so C=IT/V if I=Ic/Beta and the Beta of Q1 is, say, 70, then I=50ma/70=0.71ma T is approx 6ms and V is whatever maximum voltage will not produce perceptible noise in your Train Whistle Synthesizer. Lets say 1mV. So, C=0.71ma*6ms/1mv=4260uf. Round down to the next standard value and you get 3300uf. Best case might be a Beta of 100, and a ripple voltage tolerance of 5mV: C=(50ma/100)*6ms/5mV=600uf.

This is all linear approximation math, but it should get you in the ballpark.

exactly what Steve said - replace the transistor circuitry with an LM7812 or LM7815 and make sure you have at lease 1000 uf of capacitor for the bulk cap (output of the rectifiers - input to the regulator) and then a 0.1 uf close to the output for HF decoupling of the regulator so its stable and 470 uF or so on the output capacitor (make sure voltages are high enough on the caps) this should reduce the ripple and give you nice smooth regulated quiet supply for audio work.
The datasheets for LM78xx regulators abound with application circuit ideas and technical tips. if you measure the output with a meter it will be close to either 12 or 15 volts (depending on what you chose) and look at it with a scope ac coupled with the gain turned way up and a 1x probe and the noise should be low.

you can also buy a DC output regulated wall wart (or find one you are not using for anything at the present time and use the output of it and you get all the global safety requirements met too! I keep a box of them around for just such occasions.

Good luck

Thanks, Todd. I'll try this first before I move on to Steve's and Kevin's suggestions. I sure appreciate the help from the three of you - I've always been facinated by electronics and can read schematics and solder but that's about it.

Thanks for the great suggestions, Steve. These forums are fantastic. As I mentioned to Todd, although I'm facinated by electronics, I'm only familiar with the very basics. I'll give Todd's ideas a try before replacing the transistor.

Thanks for fast reply and detailed approach, Kevin. Hopefully I won't have to do too much rework but it's nice knowing what to change if I have to. It's reassuring how much knowledge is available in these forums and how willing you guys are to help - it's very much appreciated!!!

I also have a similar problem. My synthesizer is also not working properly and I think it is due to my old generator which I use for powering the synthesizer. I guess I will "check a power rental(check a power rental)":http://www.aggreko.com/northamerica/products---services/power-generation-solutions.aspx and then will rent a new generator in order to see if this will help. Hopefully it will.

Place a capacitor from the wiper of the POT (base of the transistor) to the negative rail. 220 uF will cut the 120 Hz ripple by a factor of 40.

Thanks, David. As a matter of fact, I haven't had a chance to date to implement the changes the others suggested so I thought I'd try yours since it seemed to require a minimum of effort. I used a 220uF cap and it works fine. Thanks!! Again, this is a great forum - thanks to you all!

Have a look at these links as well All really standard now for variable power supplies and easy to construct

If you want higher Amperage then consider using an LM317K National semiconductor devices with a series pass transistor

If you want simple overvoltage protection or crow protection consider using this in your design as well
http://www.ieeta.pt/~alex/docs/DataSheets/MC3423.pdf

http://www.diy-electronic-projects.com/p57-LM317-VARIABLE-POWER-SUPPLY
http://www.eleccircuit.com/power-supply-regulatable-0-28v-20amp-by-lm317-and-2n3055/
http://www.electronics-project-design.com/UniversalPowerSupply.html
http://www.k7mem.com/Electronic_Notebook/power_supplies/lm317.html

Now you know why all these so called mains adaptors that you buy for your radios and other battery appliances are rubbish

Simply take one apart and all is revealed You might be lucky enough to see a regulator inside but in general they comprise a full wave bridge rectifier No over volts protection at all No Over current protection at all and maybe if its truly wonderful you might just get what it say’s on the super shiny label on the cheap plastic casing Total cost when it goes wrong , One radio , One keyboard and one horrible bill

But not to worry as my dear friend say’s Its cheaper In China!! hmmmmm Europe can manufacture everything for us We are so brilliant that we don’t need to be able to distinguish a bad desin from another After all It’s a throw age “Yeah OK , what ever !! “ Im sure you know best ha , ha ha !!

Of course this doesent quite end here This will no doubt be the same person who walks into your bussiness and say's "Oh please , please can you fix this for me " Aaaah !!

No worries after all we dont need any designers any manufacturing We just get everyone else to do it for us " Say no more !

You have to laugh don’t you at this

Hi Todd,
can you help me out in few questions? they are pretty basic .
question: the effect on the 5V supply of doubling the value of the 4700UF capacitor is to :
a. Increse the RMS ripple volatge when a current of 100 mA is drawn.
b.have no effect on the ripple voltage
c. increase the output volatge

can you tel me which one is the right answer?

I'm not sure what circuit you are referring to. It sounds like a general question with regard to the output capacitor of a regulated power supply. Regulators charge the output capacitor as well as supply the load. Current flow in the capacitor changes creates a change in voltage across the capacitor i = C * dV/dt, or rearranging terms, dV = i * dt / C. So for a given current, a larger C will result in less dV. Charging the capacitor creates positive dV and the load draws current out of the capacitor creating negative dV. The two sum together to give peak-peak ripple voltage.

There are other subtle aspects of doubling the output capacitor. You have to watch the ESR rating of the capacitor, usually increasing value in the same size case causes an increase in ESR. Current flowing in and out of the cap generate additional ripple voltage due to the ESR. Also changing the capacitor affects the loop bandwidth of the regulator and depending on how your load draws current, ripple voltage can increase due to the regulators inability to keep up with rapid changes in load on its output (outside the loop bandwidth).

Hope that helps.

One trick for dealing with the increase in ESR (or doubling the filter capacitance) is to use two capacitors in parallel. The ESRs of each cap will combine like parallel resistors (if each the same ESR, then total ESR = ESR/2). So, instead of one 4700uF cap, use two 2400uF caps.