PSU in a spring reverb

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ozlow.own

Member
Joined
Jun 11, 2017
Messages
6
Hi everyone  :)

I'm trying to build a mono rack-mounted spring reverb, but I get stuck in PSU questions.
As seen as reverb tanks are pretty sensible devices and I'm not confortable including main wiring in my build, i'd like to use an external PSU for this build. I need a +/-18VDC  (no idea about Amp needed, but it's a push pull amplifier, nothing more). Also, the reverb will have balanced in/out.

So i planned to use a +18VDC wall wart PSU, convert the voltage with 7818/7819 regulators inside the reverb and then power up everything.
Searching stuff i read the Jensen application note AS085 (http://www.jensen-transformers.com/wp-content/uploads/2014/08/as085.pdf) about proper grounding/shielding. Everything seems pretty clear to me but i'm not in the same situation: here you got the IEC connector right into the chassis, so you have the earth connection right there.
In my case i've got only two wires: the + and the - of the 18VDC floating wall wart supply. This means that the chassis is connected to earth only when it's connected to other chassis earthed device(i.e. mixer).

QUESTIONS CORNER:
Is that a problem about safety/noise?
The chassis will be floating at undefined voltage respect to earth if not connected to anything?
There is a better PSU/strategy  that i could use?
 

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I don't know about "better strategy", but first of all, the 78xx regulators need the input voltage to be at least 2V above whatever their nominal output is.

Also, have you decided (or found out) whether you need a +/-18V supply for the circuitry, or can you get by with a +18V only? I know the numbers are the same, but the devil being in the details, there's the niggle of that little "+/-" thing.

Or if you were planning on just using a +18V DC wall-wart, how, pray tell, are you gonna get the negative rail for the (i presume) 7918 regulator, without a switchmode voltage inverter? :)
 
Yep, 18VAC wall wart supply.  Basically a trasformer out of the rack.
The 18V is not a constraint, anything between +/-18 and +/-12V is suitable as well.

Also, have you decided (or found out) whether you need a +/-18V supply for the circuitry, or can you get by with a +18V only?

You mean using a single split supply, bias everything and then AC decouple?

 
Forums are always useful. I already learned three things from your answer:

1) Check two times before posting;
2) Think three times before posting;
3) Be more general.

I need two symmetrical rails (from +12VDC to +18VDC and -12VDC to -18VDC) to power up a mono, rack mounted spring reverb with balanced in/out. I got no idea about current drawing. I don't want to involve main voltage inside the rack. I want to provide proper earthing/shielding to the unit and keep noise levels at minimum (obviously). How would you do?
 
Current consumption will not be very high. You don't need Watts to drive a reverb tank!
I would do it like this: use two diodes (1N4007 or similar) to create one positive and one negative voltage.
Buffer it with two capacitors (1000 or 2200 uF) and add a pair of 7815/7915 regulators.
18 V AC should be more than ample to create an almost ripple free DC voltage for your circuit, even with a single diode rectifier.
Since those wall warts are usually 'double isolated' I would not connect a ground wire.
This is the easiest way to prevent ground loops!
Check if the regulators run hot. If so, add a small heatsink to them.
 
If you really wanted to "gild the lily", you could replace the 78xx/79xx with a pair of "capacitance multipliers", for super-smooth DC ;D
 
No, two diodes!
ARh9O.gif
 
Matt C said:
I still don't see how you're gonna get + AND - 18VDC from a single 18VAC source.  Voltage multiplier?
While at Peavey we used a wall wart to power numerous rack products for several years . IIRC it was a 20VAC-1A and covered almost every op amp application easily delivering +/-15v using half wave rectification.

For higher current we could full wave rectify the supply but that only delivers one voltage rail...  In one high current SKU I split that single rail into + and - by actively driving an error  voltage amp into the ground to evenly split the two rails above and below ground.  Higher voltage can be generated with doublers or DC to DC switchers.

I recall back in the 80s seeing prototype lump PS that delivered split supplies through 3 wires and even regulated output voltages but they were more expensive and not supported by cheap standard 2-circuit power connections.

I probably used millions of that one common wall wart. It was cheap and easier to get regulatory approval but the customers hated them, and the customer is always right.  :-[

JR
 
I wonder, would doing two reversed full-wave rectifiers off the same single AC voltage work? Hmm... I'll have to sim that one.
 
That will only work if you don't need a common '0'.
At the moment you try to create a +/0/-  voltage, the AC voltage will be 'shorted' by two diodes in series. (Of the bridge rectifiers.)
 

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<brainfart> My bad - for some reason, i thought i saw the two diodes pointing the same way, in the schematic you attached earlier.

I gotta get some (more) sleep...
 

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