+- power supply

hi all, found this forum recently and im very impressed by the amount of expertise. seems like there is much to learn from here!

i have a simple question - i'm building my first power supply. it will be for opamp based spring reverb amp.

could someone direct me to schematics for a simple one?
the opamps require roughly +15v and -15v DC. i've got two 12,5v AC transformers, voltage regulators, caps, resistors, diode bridges etc.

like i said never done power supplys or AC electronics before so something simple would be preferred.

many thanks
jouni

Hi, Jouni-

> power supply. ...for opamp... a simple one?

Purists will faint, but this always worked great for me:

EEeeek----smelling salts....

(lol)

Hi jouni, welcome to The Lab!

PRR's design is a basic but clever design to get a split supply from a single AC supply (i.e. one of your 12.5V transformers) Not to be sniffed at- you'd be surprised how many pieces of equipment you open up and see running with this setup (e.g. Rane) Give it a go, wire it up, and check out how it performs- there are pitfalls in certain situations.

Other simple ideas- zener regulator, "amplified zener"- zener diode followed by emitter-follower etc. Seeing as you've already got these bits:

[quote author="dharma one"]....i've got two 12,5v AC transformers, voltage regulators, caps, resistors, diode bridges etc.[/quote]

...it may be worth taking the design a bit further. What type of regulators have you got? If they're 78xx/79xx types, there's a very simple PSU design here from SSLTech:

http://www.beatbazar.com/guests/ssltech/kps-1/index.htm

This design supplies phantom power too, but just leave out all those components and you've got a nice simple +/- supply. You may be stretched to get 15V from your 12.5V transformers, once you take into account the 2x 0.7V diode-drops from the rectifier, and the drop-out voltage of the regulators. You can replace 7815/7915 with 7812/7912 for a +/- 12V supply. You could always build the +/- section on a piece of Veroboard/stripboard etc. Just watch out for the pinouts of the 78xx and 79xx regulators- they are different!

Here's a link to the 78xx series voltage regulators, there's some application info, and an example +/- supply on page 7:

http://www.sunmark.com/datasheets/7805.pdf

Good luck with it,

:thumb:

Mark

much appreciated. your schematic is just what the good doctor ordered PRR.

nice one mark mark, that one looks easy enough too. what would the benefits be in using regulators? ive got 7812s as far as i recall

cheers
jouni

Well,

A voltage regulator is an active device which has at its heart a series-pass transistor. The output voltage is constantly sensed and kept to the rated output voltage (within limits) The series-pass transistor acts as a "variable resistor" (but of very low impedance...) in series with the input voltage, and the excessive voltage from the input is dissipated as heat. This is why you need a heatsink on a voltage regulator. The greater the load current, and the greater the input-output volatge drop, the greater the dissipation (i.e. heat!)

Advantages- well, you will be guaranteed the rated output voltage as long as:

1. the input voltage stays a few volts above the desired output voltage (i.e. a voltage reg can't make a voltage higher than it's input- that's a different type of reg entirely)

and:

2. that the output current you draw stays within the limits of what the regulator can supply. If you draw too much current, the regulator will overheat. But all modern regulator IC's have an overcurrent shutdown, which protects the regulator and your driven circuit as an added bonus!

Also, the voltage regulator works by feedback, and this inherently reduces any noise in the input voltage. Noise in a power supply line can be seen as the voltage you want (e.g 12V) varying up and down. This can happen slowly (e.g. 100Hz/120Hz- ripple from inefficient smoothing caps) or MF/HF noise within the audio band and up to RF from mains-born interference from other equipment such as motors, old SMPS etc plugged into the same mains circuit. The voltage regulator can cope with these voltage fluctuations and minimises them (not completely- it will attenuate them by so-many-dB depending on PCB layout, decoupling caps etc)

PRR's circuit will work absolutely fine. But, the output voltage is load-dependent. Those 220R resistors will cause a varying voltage drop as your load current increases. Also, due to half-wave rectification, the frequency of the ripple in each supply line is 50/60Hz compared to a full wave rectifier which has ripple of 100/120Hz. This is because from the incoming sinewave, alternate peaks and troughs are fed to the + and - rails. The smoothing caps have to work twice as hard to "fill in the gaps" inbetween the peaks. So the caps have to be larger, and as load current increases the ripple voltage will increase.

Mark

ok i see. so out of these circuits -

http://www.accutronicsreverb.com/schemat.htm

- would it make sense to build the constant current one (assuming i'm using PRR's power supply)? given that there will be varying voltage drops if the load current fluctuates

thanks,
jouni

dharma one, that "constant-current" one is something of a misnomer. It is a voltage-to-current converter, in which the instantaneous current in the spring driver is determined by the signal input voltage at that moment. It does not have constant power supply drain, except at zero signal.

Having said that, it does have, probably, plenty of power supply rejection, which means that fluctuations in the supply won't have much effect on the output.

To add to that though: for this and all equipment fed from power supplies, one needs to know how much current will be demanded of the supply. So, for this reverb application, you need to know the maximum signal those drive coils are going to handle and work out what the associated power supply drain will be.

There's also the issue of real audio signals versus lab test conditions. PRR's simple supply will sag under heavy load but will take a while to do so.

> there will be varying voltage drops if the load current fluctuates

Enough yaky-yak. Bcarso, off the floor.

What IS the load current?

Two to four op-amps is 3 to 20mA.

The lowest-Z Accutronics coil saturates at 28mA peak. This will be around 10mA of DC in the op-amp or booster driving it. They tell you not to saturate because it gets muddy. With normal speech/music peaking at 28mA, the caps can handle those peaks and the raw supply only has to be good for about 1/5th of that, or 2mA average DC. However, Fender guitar is a popular application for spring reverb and can have low peak/average ratio (clipped plucks and compressed sustain), so split the difference, 5mA maximum DC current due to load.

So a couple low-power opamps with no signal might be 3mA. A quad of hungry opamps driving an 8Ω coil might be 20mA idle, 25mA rocked-out.

This is not a large current or a large change of current.

We have a "12.5VAC" transformer. It is probably at least 0.3 Amps, and we are only asking for 0.050A AC, so it is more nearly no-load than rated load. At no-load it will rise maybe 20%, to 15VAC. Peaks are 15V*1.4= 21V. Diodes drop 0.6V, we could have 20.4V peak on the first caps. Ripple is well under 0.1V, so we take the DC as 20.4V.

The smallest load, 3mA, will drop less than a volt in the 220Ω resistors. Problem: this is 19.8 volts, in excess of normal chip ratings. I assure you that 99.9% of "36V" chips will take 40V for a very-very long time, probably longer than the spring's useful life. Or you could use 5532 chips, which are rated 44V, eat more current and will suck-down to 18.7V, and can drive the 28mA coil rating without boosters. Also low-noise, which can be an issue for the pickup end of the spring.

Or you could up the 220Ω resistors to (20.4V-18V)/0.003A = 800Ω (use 1K).

The largest load, 20mA-25mA, would drop 4.4V to 5.5V in the 220Ω resistors, giving 16V idle, 14.9V rocked-out.

The drop from no-signal to full-signal, a volt or so, should not affect chip opamps. The drop is symmetrical. These chips are not fussy about rail symmetry or voltage as long as they can pass the signal swing. The low-Z coils only need 0.224V drive! Even with a build-out resistor, only 1.3V. As long as the chip has 5V of rail-power, it will be fine. The hi-Z coils could need 13V drive, so we might barely miss "spec" with a 14.9V rail and some losses, but the rated drive current is not a precision thing.

The ripple is doubled with this half-wave contraption, but we just have to double the capacitance to "fix" it. At ~470uFd~1,000uFd 25V, double the uFd is much less than double the price, and far cheaper than another transformer (a second 12V or a 24VCT for true full-wave). Yes, it is 120Hz (per rail) and more annoying than 60Hz ripple, but the second R-C filter knocks that down a lot. Using rough math, I get 1 milliVolt on the rail and 1 microVolt in the chip. That won't amaze fans of super-regulators, but in a REVERB application I bet you have more than 1 microVolt of room-buzz on the spring pickup. (If you do suspect power-ripple, like when you unplug it the buzz vanishes in the second it takes the caps to discharge, use a separate chip for the pickup amp and give it a 3rd R-C rail filter.)

If you like regulation, aim for +/-12V DC from 12VAC windings. 15V DC is far too close to the 17V-21V DC you will get from 12V AC.

Somebody stop me! Clearly I have too much time on my hands (that situation looks as if it will change soon).

PRR, you got the 120 and the 60 reversed. But hey...

thanks for the thorough explanation PRR.

seeing that youve got experience with the accutronics tanks, i'd like to ask how do i match their coil impedances to line level input & output signal? i'm building this as an effect unit, not for guitar..

cheers
jouni

> seeing that youve got experience with the accutronics tanks

Who, me? I haven't touched those things in so long I forget what I knew. All I did just now was look on Accu's site.

> how do i match their coil impedances to line level input & output signal?

"Match" implies we know what we are doing. With these spring reverbs, we don't.

When in doubt, plagiarize. There were a few "good" spring units. And there are a lot of guitar-amps you can study.

One basic concept is: they like a high-impedance drive. They are a heavy electro-mechanical device with an audio low-pass effect. They also have some inductance. The simple way to get fairly-flat response is hi-Z drive. You can use a tube (see Fender) or transistor, or stick in a series resistor about 5-10 times higher than the rated impedance, or use a complex feedback trick. Given 10-cent op-amp chips, it seems simplest to get the 150Ω coil and stuff about 1K in series: chips drive that fine.

The amount of gain needed at input and output depends a LOT on your line levels, the type of sound, how you feel about hiss, etc. The best way is to prototype variable gain input and output amps, use them a while, and modify to suit you. The basic plan on Accu's site is a good starting place. Driver has a gain of 10 or so and an input pot so you can trim gain from 10 to 1/10. Output amp trimmable from 1 to 100. Start there.

Reverbs always need some EQ, to help flatten (or less-slant) the basic response and because reverb tonal balance "says a lot to the ear" and must suit the music. Most reverbs toss in a few fixed caps, though some have 4-band EQs. Because of the way the spring saturates, you may even want a high-boost going in and a high-cut coming out. Or vice versa.

The practical signal-to-saturation range of a common spring reverb is not huge. In pop music that is always playing, the music and the reverb tend to mask the hiss, but it can be a problem if the music pauses. Low-noise recovery amp is a good plan. Some reverbs include limiting, linear or distortion, so the average spring level can be high (allowing less recovery gain and hiss) without socking the peaks.

The Accutronics springs are as simple and predictable as violin-building. They are really very complicated things. Fortunately it is easier to swap resistors and caps than to shave fiddle-wood and gut those cats.

[quote author="PRR"]and because reverb tonal balance "says a lot to the ear" [/quote]
Genius.

how come I can never be this economical with words and still convey so much?

:thumb:

Keith

built PRR's schematic with a 12.5VAC transfomer, blam works straight off, sweet! i measured of the + and - rails with the 0v rail, the multimeter shows just slightly over +/- 25VDC. is this alright for the opamp, if not, whats the best way to drop it to around 15VDC?

cheers
jouni

> slightly over +/- 25VDC. is this alright for the opamp,

No!

Use the AC voltmeter to check the "12.5VAC" winding. Either it is making a lot more than 12.5V +20% regulation, or something else is wrong.

You should also throw a 2K2 reistor on each output and see if the voltage comes down to more like 18V. It might be all spikes that don't carry real power.

Re-check your DC voltmeter against a fresh 9V battery. I don't mean to be rude, but there was a case here where we went nuts trying to diagnose wrong-voltage issues, and finally traced it back to a sick voltmeter.

ok i measured the AC transformer with 2 different batteries, it gives 18.40VAC with both batteries.. this is without load. weird.. i guess i'll just have to buy a new 12VAC transformer then!

edit:
i forgot to mention. the 1000uF caps are rated 25v, is this fine?

If you have 15 volt rails a 25 volt capacitor is fine but I would use 35 volt caps on 25 volt unregulated rails.

Sorry to be ambiguous. I actually meant: measure the voltage of a fresh battery, and be sure the meter says the right thing. 1.56V on a fresh new "1.5V" battry, a bit over 9V on a "9V" battery, etc.

> it gives 18.40VAC

Assuming no odd fault in the meter: are these maybe door-bell transformers?

PRR: "are these maybe door-bell transformers?"

I was thinking that these have to be about the poorest regulation transformers I've ever heard of.

I have a story but the margins of my notebook are unfortunately too small to contain it.