Reverb power supply

[Ben79]

Hi Folks,

I've been out of the game for a while, but I'm about to embark on my longtime dream of building my own spring reverb!

I've cheated and bought the Surfybear PCB but I'm going to do my own power supply.

I found a 12v transformer on the street, now I need to rectify that sucker....

The circuit draws about 4 watts apparently. Assuming a sensible overbuild margin can anyone recommend me a bridge diode/smoothing cap combo that should do a good job? I guess 1n4004 for the bridge but I'm not sure about the caps.

Is there any advantage to running a regulator?

Should I keep my power supply away from the circuit board? How far?

Hope you're all keeping well. I'm hoping I can get a workspace going at some point and start building some pedals again. I woke up in the night a while back with an idea for a modular fuzz, maybe it's been done already but I want to give it a go.

[crochambeau]

I took a peek at the surfybear site, but it read more like a user manual than a technical document.

Regarding your transformer. The secondary is 12 volts AC. Is the secondary simply two wires, or is it center tapped?

The answer to that question will determine what diode configuration will work best. 1N4004 are more than plenty for a 12 volt line, so long as your draw is less than an amp (if 4 watts from 12 volts is accurate you're completely safe). I'd feel comfortable with 1N4001 in that slot due to their 50 volt rating.

Regarding transformer placement. You're driving a spring tank, correct? It's more important to keep the transformer away from your tank than the circuitry. Your circuitry is not going to pick up a hell of a lot of hum, the transducers in your tank will. I'd place the transformer closer to the tank INPUT, where that transducer is being driven by an amplifier. The tank OUTPUT will be a low level signal about to be amplified to signal level, and any hum here will ruin your day.

I'd build it without a regulator and add one if loose power rails are being a problem. I like my reverbs a bit reactive and sloppy though, it's really a taste thing.

[Ben79]

Thanks Curtis!

The tranny is a 2 wire secondary 3.4A job.

I've got 1n4001s aplenty already so that's good news. I was considering sticking a 9v regulator in there and wiring in a fuzz too, an Escobedo Tripple I think....



What would you recommend smoothing cap wise for the 12v portion?

[crochambeau]

4 watts from 12 volts indicates around 300 mA current, the 1N4001 is good to 1 amp.

I'd play with capacitor value, starting with a range from 100-1000 uF. Capacitance that is too low will simply result in hum, and shouldn't damage anything, capacitance that is too high may result in inrush current that could stress your diodes. I would avoid going into the tens of thousands of uF, if you are having trouble killing hum either a regulator or a choke in a π filter configuration should sort you out... even a low value (high wattage) resistor in the π filter can eliminate a lot of noise.

I plugged 15 volt, 18 ms, 0.4 amp into this calculator and it returned 480 uF as optimal value: https://electronicbase.net/smoothing-ca ... alculator/ (I realize all of the numbers I plugged are a bit large).

They also show the full wave rectification bridge that you'll want to employ with the two wire secondary, but you can read up on it more here: https://en.wikipedia.org/wiki/Diode_bridge

[Ben79]

Great, thanks, I'll get on this tomorrow and keep you guys posted.

[imJonWain]

To follow up what crochambeau said about transformer placement don't forget about PT's physical rotation/angle. The PT has a magnetic field and you may find your reverb tank picking up noise when it's oriented a certain way relative to your PT. I had a guitar combo with obnoxious reverb hum but if you removed the tank and rotated it 90 it was dead silent even within 8" of the PT.


Didn I mail you a reverb PCB i did some time ago? I know someone in France bought one.

[Ben79]

You did indeed IJW! It's still sitting with my other PCBs waiting to be used one day.

Thanks for the tip. I decided to build an external power supply in the end, that way I have no hum worries and can take advantage of the spare amps with a couple of regulated 9v outs for powering pedals. I rarely use more than 2 pedals.

I've been hacking away with bits of junk I've found on the street to build the enclosures and I've soldered up the rectifier/filter board. Putting the Tripple circuit together today. I'm actually on an old communist-era housing estate in Germany now and I'm restricted in when I'm allowed to use power tools so I have to wait til tomorrow to carry on with the wood work. Nice to be building again...

[imJonWain]

haha that's awesome! What are you doing in Germany? How do you like it? Are you guys locked down?

[Ben79]

A few things, about to start a kind of paid state volunteer position, some casual tea packing for a small business and I'm about to finish the first issue of an english language magazine I'm producing for the city, which is Jena. If the magazine sells I guess I'll carry on with it. I like it a lot here. My German is basic so I'm limited communication-wise but it's a nice peaceful, pretty city with some arty bits going on, always a view of the hills and the forests so you don't feel immersed in buildings. Food is cheaper than in France and an alcohol free beer is 80 cents from the supermarket! Rent is relatively high in Jena, though still low compared to the UK, but in other towns and cities around here it can be very low and you can buy places in the sticks for almost nothing. The state unfortunately has the second highest AFD (far-right) following in Germany (24% I think) but Jena is leftish.

[Ben79]

I built my first power supply!

Rectifier diodes and filter caps for the 12v for the reverb and a regulator for a couple of 9v for fuzzes....

But it blew up!

Don't know how I managed to do this but I got my + and - rails after the rectifier the wrong way round so the caps fried and now my transformer secondary is reading 0.3ohms....

Shit. Always check your circuit before plugging into the MAINS is the lesson I've learned here. At least it was mostly found parts.

I have a 12v laptop power supply hanging about, I guess I'll be using that then if it's not too noisy.

[crochambeau]

What did your transformer secondary read before you used it to BBQ some caps?

[Ben79]

I don't know. It might not be toasted?

[crochambeau]

I don't know. It might not be toasted?

Yeah, without an earlier measurement a low DCR measured does not prove fault. 0.3 ohms does seem low, but it might just be larger wire for more current resulting in less copper loss.

I like using a variac or known step down transformer to test an unknown or suspect power transformer. Another way is to feed it something like (a measured in operation) 2.2 volts at your line frequency from an audio circuit so you can determine ultimate output voltage, but 100 times smaller.

Or simply wire it up to a meter and remote toggle mains with a power strip and let 'er rip.

[Ben79]

OK, maybe I can run some AC through it from an analogue synth and see what it does.
Thanks!

[crochambeau]

OK, maybe I can run some AC through it from an analogue synth and see what it does.
Thanks!

If you're feeding it from a sound source, make absolutely certain you directly measure the voltage of the source while under load, because a lot of things do not like driving a transformer directly and the output levels will drop. Headphone or amplifier output is probably a bit more reliable than a line level (I've noticed a few of my synths do not like driving my main mixer directly, as it has 600:600 ohm transformers across all the inputs).