Ideas for powering Morley PFV (+28,+15,-15 VDC)?

[robotmonster]

Hi guys, please help me find the best solution for powering my old Tel-Ray Morley PFV Phaser/Volume pedal.

Currently it runs on 110VAC power chord, going into two 30VAC transformers inside. First transformer is supposed to output 28VDC via diode, resistor, cap circuit to power two 28V/40mA lamps, and second +/- 15VDC via diode bridge to power the rest of the circuit. I measured +23,+14,-14 inside though.

This is barely readable schematic might help:
http://www.morleypedals.com/pfapfves.pdf

I would like to get rid of 110VAC power chord and internal transformers if possible. My knowledge of electronics is too basic to be able to figure out a good solution, but I can read schematics and solder well enough. I want pedal to run on 220V, but I can not find matching replacements for internal transformers size-wise. I have gone through many manufacturers datasheets, and only found matching 110VAC versions. Step-down transformer is something I would like to avoid completely. Drilling additional holes and running of 220VAC power chord would be my last option.

Is it possible to take a standard, replaceable wall wart of some sort and then make a little circuit inside with a few components to get those 3 voltages I described above? I looked at LT1054 charge pump examples to get bipolar +/-15V out of a single 9V, and that made me curious about taking that direction. I am not sure what to do with third 24-28V voltage that drives the lamps inside though because I do not understand why did they put two transformers inside.

How about getting 12VDC wall wart, use charge pump to get +/- 24VDC, use +24VDC to drive the lamps, then regulate it down to +/-15? Would this work at all? Any better ideas?

Many thanks for any help.

[wavley]

Well, charge pumps are probably a good solution, I don't have much experience about how much current they put out.  I do have a PFL, but I couldn't tell you about the current requirements.  What I can tell you is the lamps are VERY important in these pedals, both for the sweep of the pedal and for the LFO.

Before you do it, you might want to talk to the guys over at the Tel-Ray forum, maybe somebody else has already solved your problem there. http://telrayoilcanaddicts.yuku.com/

[robotmonster]

Thanks for the reply. I searched the Tel-Ray forum but I could not find any relevant info, apart from few guys trying to source specs for transformer replacements, so I double posted with a link to this thread.

Yes I noticed that those lamps drive photo sensors, foot pedal actually moves a piece of cloth to control how much light gets to the photo cell/s pretty cool concept. Each lamp takes about 40mA, rest of the circuit should be very low current, so I imagine it is less than 100mA total. I will breadboard LT1054 circuit, but I am afraid to hook it up to the pedal to DC power points, until someone confirms that it is safe to do so. Still do not understand why did they use two separate transformers, have not seen this before inside the pedal. Is there something about those lamps that they should be hooked on a different power source from the rest of the circuit? Thanks

[armdnrdy]

I looked at the schematic and this is do able.

I would use an 18vdc wall wart, feed a LM7815 regulator, then feed a LT1054 to create the bipolar +15/0/-15 volts for the main circuit.

Then go back to the 15vdc output of the LM7815, feed an additional LT1054 set up per the data sheet as a doubler. That will give you around 30VDC. I don't know how sensitive the circuit would be to 30 volts rather than 28volts, but I suspect with the tolerances of older transformers and such, it probably won't make a difference. You can always negotiate the voltage down with a few additional components.

I've had great result with the LT1054 in various builds requiring a bipolar supply. (Jet Phaser, Attack Decay, Mutron Phase II)
It's my go to charge pump because it can handle a larger input voltage and has a respectable output current. Also, the notorious charge pump "whine" is easily conquered by adjusting a ceramic cap.

Let me also mention that the above circuit description will do better with the biggest filter caps you can fit. Start around 220uf (I usually fit smaller value monolithic or tants in parallel with the large electrolytics. Better for high frequency filtering and lower ESR)

[robotmonster]

Awesome info, thank you very much.

[armdnrdy]

You're welcome,

Check out the data sheets for the aforementioned components. The infomation you need is all there.

Let me know if you have any questions.

[moosapotamus]

RE: LT1054 voltage doubler
I don't have any experience with the PFV circuit, and I certainly don't mean to be a wet blanket, at all. But in my experience with relatively "power hungry" circuits, on it's own, the datasheet schematic for the LT1054 voltage doubler will do just that - double the input voltage. But, depending on the load from the circuit, you could easily loose an additional 2 to 3 volts (maybe more?). I could certainly be wrong, but my guess is that, with the PFV circuit, you will be lucky to get as much as 28V out from a 15V input. Just my hunch. YMMV... hopefully.

~ Charlie

[armdnrdy]

The LT1054 data sheet shows voltage loss data for different value and make up of external input and output capacitors. The sheet states at 100ma, with 100uf tant. input and output caps, 1.10 loss typ. and 1.60 loss max.

Now there are a few variables such as frequency and the fact that 100uf tant caps are most likely not going to be used. That's one of the reasons I suggested larger filter caps in parallel with smaller tants. (Maybe 10uf)

The tants make up for the electrolytics shortcomings.

I've powered circuits demanding 40+ ma at +15/0/-15 with roughly a 2% voltage drop using the type of circuit that I suggested.

[R.G.]

24V output wall warts are pretty common in both AC and DC output configurations.

If it were me, I'd find an 18Vac-24Vac wall wart rated at 200ma or more. This should not be difficult to find, and probably for free from older electronics.  Then, I would (re) read "Power Supplies Basics" at Geofex. With 18-24Vac available from a wall wart, you can use two diodes and two capacitors to make +/-25 to +/-33Vdc respectively.

+25Vdc is ideal, so if you can find the 18Vac wall wart, much better. This gives you your +25V voltage ready to use, albeit with high ripple. The higher voltage AC wall warts let you use a 7824 three terminal regulator to get your +24V clean, which may or may not matter.

The +/-voltages also let you use a 7815 and a 7915 regulator for +15 and -15 respectively, or you could copy the resistor/zener setup from the original setup. I don't like zeners for applications like this, and the 3-terminals are probably easier to find. 7812 and 7912 could also be used in this circuit and it would work fine as well. The opamps don't care, and the transistors probably don't.

[armdnrdy]

This is a great way to accomplish the power requirements as well!

When I'm designing supplies for builds for my personal use, I tend to sway away from adding additional transformers into to mix. My pedal board has one incoming AC source that feeds a DC brick which supplies (7) 9volt and (3) 18 volt outputs. I tend to design everything around what I have available.

With that being said, I think that R.G.s suggestion would be a great way to go!

Here's R.G.s link:
http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm

And here's another link:
http://metroamp.com/wiki/index.php/Half_Wave_Voltage_Doubler

The 18VAC transformer is definitely the more efficient way to go but if you choose the 24VAC transformer you can use the same voltage regulators (heat sinked) for the + and - supplies and use a LM317 adjustable regulator (adjusted to 28VDC) for the lamp supply.

LM317 PDF:
http://www.fairchildsemi.com/ds/LM/LM317.pdf

[PRR]

Since the pedal is Classic and very Vintage, _I_ would suggest a simple 230V:120V external step-down.

The total demand is under 5 Watts. Such a converter may be no larger than the wall-wart you propose. If kept with the pedal, you are covered anywhere you (or the pedal) goes.

This is much bigger than you need, but has much better Amazon reviews than the 50-Watt jobs, and isn't all that large or expensive.
http://www.amazon.com/Simran-SMF-200-Converter-International-Countries/dp/B000W9DJ1Q

[robotmonster]

Thanks very much guys, I built charge pump circuit with two LT1054 chips thanks to armdnrdy's post above:

I used nicely regulated 15vdc power supply to begin with. First LT1054 was used as an inverter to get something close to -15vdc, then used same +15vdc with second LT1054 following positive voltage doubler circuit example from the datasheet. That gave me a bit above +28vdc which awesome. Voltage drop in my case is useful, because I do not need exact 30vdc. I hooked all three voltages to the points in the circuit and pedal was running fine, drawing something above 100-150mA total from the 15vdc psu. I suppose this is fine across two chips. I measured that +28V gets converted down to only about 20V something pretty early in the pedal's circuit to drive the lamps anyway, so I guess it is not super-critical value to have. Phaser works without it, having all the lamps off, and I can modulate it with any light source (desk lamp for example).

I started testing audio, and turned gain on my little vox transistor amp all the way up, just to test background noise. I could hear a tiny whine mentioned in the post above. Connected back to mains cable / original transformers for A/B comparison and the background noise was a tiny bit more lush sounding so to speak, less synthetic and no audible whine in it. This noise compared to guitar signal was extremely quiet, and barely audible, still I was wondering if I can fix it somehow, because I could hear it well after-all.

armdnrdy's post above gave me clue that pin7 can be connected to ground or pin2 via tiny capacitor to tune LT1054 oscillator's frequency. I connected 18pf cap from pin 7 to pin 2 at the Inverter chip to increase the frequency and it did help calm the whine down. I have also used 220uF with 10uF tantalum in parallel for Cin and Cout at the inverter but that did not change much I must say. Maybe because I already have decent filtering in the circuit with additional 100nF caps across dc rails at various points, plus pedal has huge filtering caps inside. I could not see from the datasheet how to bump up oscillator frequency for the second voltage doubler chip though. If I connect pin 7 to pin 2 the output voltage drastically drops, so I guess it should be done some other way?

That said, I am suspecting it is a sum of two chips oscillator frequencies making this noise, because when I disconnect voltage doubler's +28v from the pedal, with or without cap, first chip alone sounds fine and not whiny. Together they sound whiny. With first chip tuned to higher frequency it sounds very acceptable and non audible, but still maybe a bit more synthetic sounding and compressed background noise (hard to explain) compared to original AC PSUs noise. This post describes similar audible oscillation. Could this maybe be a reason why they used two separate ac transformers in original circuit, and not one with voltage regulators, do you think it was just cost-effective approach at the time?

This is a tiny issue because noise floor is very low, and totally acceptable to use the pedal with. It is only audible when phaser is on at certain sweep positions producing more harmonics. Conclusion is that pedal can be powered fine with 18-24VDC/200mA wall wart regulated down to 15VDC and two LT1054 chips. If I manage to tune voltage doubler chip's oscillator frequency somehow to find a sweet between two this would be a perfectly fine solution indeed. This might be hard though, to find a good sum between two high frequency oscillators. I would recommend a single LT1054 in a circuit to anyone though, to get bipolar or doubled voltage, alone it sounds perfect. I am considering to use a single LT1054 to get +/-28v now, and then to regulate it down to +/-15 with two regulators that may help with the background noise.



Thanks for other kind suggestions, but I would like to avoid using step-down transformers for other reasons. I have a good quality and very heavy 1kw unit already but it gives me weird background noise and strange whine with some gear, not PFV though. I decided to get rid of it for that reason, and to make all my gear switchable between 110 and 220v, or simply to use 110-240v wall wart of some sort which is easy to replace (think one-spot adapter). I am not an expert, but I believe people generally prefer to have things organized that way to avoid any weird ground loop issues when mixing between 110v and 220v? I could still not explain why my good quality 220/110v 1kw step down transformer makes some gear whine or pickup more noise when used with other 220v gear. Still learning all the theory behind this stuff.



Next challenge for me is to learn from RG's articles and try to build something with 24VAC input, just to compare it with 2xLT1054 circuit noise wise. Need to get my hands on 24VAC transformer first Will report back.

Again, thanks very much for all your help.

[armdnrdy]

Hey robotmonster,

Glad to see you made some headway.

Try adjusting the frequency with different size ceramic caps between pins 2 & 8 to negotiate between whine and voltage loss. Also try sticking a .1uf monolithic ceramic cap close to the 1054 from +V to ground. Another thing to try is increasing the size of the charging capacitor. (across pins 2 & 4)

Hopefully, one or more of these things will help.