Filtering AC hum on guitar pedal power supply

[Bonzai_Musik]

Hello,
I recently put together a BigMuff style guitar pedal, and it appears that my cheap 9V power supply does a bad job filtering AC hum, at least I think so.
I then found this little project called "The Power Pellet" made by DIYGuitarPedal:

Which I built with an added switch to turn the supply on and off. But once I plug the rest of the supply to my pedal, they don't turn on at all. Yet when I measured after the filtering (red connector), I get 9,05 V DC.
Do you see something wrong here ? Or should I check my connectors going to my pedals ? (The soldering side is upside down on the photo).

 

[Bo Deadly]

I think that drawing might be a mistake. Only one of the diodes is rectifying. The other is just like maybe a protection diode. But it's very close to being like this (Two Diode Full Wave Rectifier Circuit):


if you had a transformer with a center tap.

But it should still work. Albeit probably not very well. With only one diode rectifying you're getting half the current. So maybe the wall wart transformer doesn't supply enough current. Post a pic of the transformer. What sort of current does the pedal require?

Without a center tap on the transformer, your best bet would be this circuit (Bridge Rectifier Circuit):


This will full-wave rectify which will supply more current and the filter cap and RC will work much better.

 

[ccaudle]

Only one of the diodes is rectifying.

It appears to be reverse polarity protection in case you connect a supply with the wrong connector polarity. This is a filter for a DC supply with excessive ripple, not a rectifier for an AC input.

Yet when I measured after the filtering (red connector), I get 9,05 V DC

That is measured with the pedals connected to the supply?

I can't see the wiring to the output connector very clearly from that angle. Are you sure you did not accidentally reverse the polarity to the output connector? Put in a plug into the output connector and make sure the center is positive (assuming that is what you want, center negative if that is what your pedals require).

 

[JohnRoberts]

Guitars hum... you can test using a 9V battery to confirm that hum is coming from PS or the guitar.

JR

 

[David Kulka]

If the power supply is all or part of the problem, consider adding a 9V regulator.

 

[Bonzai_Musik]

It appears to be reverse polarity protection in case you connect a supply with the wrong connector polarity. This is a filter for a DC supply with excessive ripple, not a rectifier for an AC input.



That is measured with the pedals connected to the supply?

I can't see the wiring to the output connector very clearly from that angle. Are you sure you did not accidentally reverse the polarity to the output connector? Put in a plug into the output connector and make sure the center is positive (assuming that is what you want, center negative if that is what your pedals require).

Here's a photo of the ouput's wiring, it's supposed to be straight.
Also a photo of DC measurement at the output, I get a good 9V but once I plug my pedals it goes to a couple of mV, is this normal ? I'm fairly new to electronics.
Thanks for your help.

 

[JohnRoberts]

no it isn't normal.

JR

 

[Bonzai_Musik]

no it isn't normal.

JR

So could it be that my pedal connectors are shorting the 9V supply to ground ?

 

[JohnRoberts]

to help troubleshoot add a resistor in series with the power supply. If the output is shorting it you will see the drop across the resistor and it will heat up. If the voltage just goes away may be a problem with open circuit connector.

JR

 

[ccaudle]

I get a good 9V but once I plug my pedals it goes to a couple of mV

That could be reverse polarity protection on the pedals (if they just have a diode across the power input as protection).

I'll ask again: have you verified whether your pedals need center positive or center negative on the power input, and have you verified that your connector wiring matches what the pedals need? It looks like center negative is the most common configuration, but you have to check each of your pedals to make sure.

 

[zzzzz]

Following this -- I recently had a Truetone One Spot Pro pedal power supply that started introducing some weird hum into the pedal chain. Might crack it open and see if I can jam some more filtering in there / see how it's even set up in the first place.

 

[Bonzai_Musik]

That could be reverse polarity protection on the pedals (if they just have a diode across the power input as protection).

I'll ask again: have you verified whether your pedals need center positive or center negative on the power input, and have you verified that your connector wiring matches what the pedals need? It looks like center negative is the most common configuration, but you have to check each of your pedals to make sure.

Well since I wasn't sure I just opened up the pedal and copied the wiring,
On the picture of the pedal DC connector, one red wire is connected to a 9V battery and the other to the +9V on the PCB.
And of course the red wire on the DC connector of the filtering is the +9V side of the power supply.
So this seems right to me, let me know what you think.

 

[ccaudle]

this seems right to me, let me know what you think

I think if it seems right but doesn't work, then you have to get your voltmeter and dig deeper. There is only so much to be guessed from a couple of pictures.

Put a battery in your pedal without the power supply connected, and measure the voltage at the power connector inside the pedal. You did indicate that the battery voltage is also connected to the power input connector, correct?

Take out the battery, and connect the AC-DC power supply without your filtering circuit. Measure the voltage at the connector again, verify that it is close to the same as the battery. Verify the pedal is working with no battery installed, just the power supply connected.
This is just a sanity check to verify the pedal is still working and that you definitely know where the positive power connection comes in on the connector.

Connect the power supply to your filtering circuit and your circuit to the pedal. Measure the voltage at the connector in the pedal again, and note whether it is close to the same voltage, same voltage but opposite polarity, 0V, small voltage of same polarity, or small voltage of opposite polarity. If I understand correctly your original description it is expected to be a small voltage, but you never described checking the polarity inside the pedal connector to make sure the entire chain of connector wiring and cable was correct from end-to-end.

Same voltage: congrats, you had a bad cable or loose connection, probably working now. Not expecting this, just adding for completeness, because sometimes it happens, especially if the cable connector is not quite the correct size but is close.

0V: cable is not connecting somewhere. Maybe the connector is the wrong size to fully mate. Not expecting this either since if I understand the original description correctly the voltage was pulled down even inside the filtering circuit.

small voltage, opposite polarity: a connection is backwards on one of the connectors, and either intentional reverse polarity protection is pulling the voltage down (parallel diode), or an intrinsic junction diode behavior was turned on by the opposite polarity power and having the same effect.

small voltage, same polarity: Make sure you did not accidentally install a large value resistor in place of the 100 ohm filtering resistor. I cannot see the colors well on my computer screen, is that brown-black-black-black on the resistor you used? Also verify the current draw of the pedal. I think a Big Muff style pedal with four transistors should just be a few mA. The 100 ohm filter resistor will drop 500mV at 5mA (just V=IR Ohm's Law) so should still be fine, you would still have 8.5V available to run the pedal. If your Big Muff style pedal is actually something different than an original style design, e.g. using some op-amps, all the current has to pass through that 100 ohm resistor so you will get a larger voltage drop across the resistor. Unlikely to be the problem since you said it dropped to a few mV, but would be a possibility if you were measuring e.g. 6V or 8V.

9V, opposite polarity: obviously doesn't match your description of the voltage dropping to a low value, but I just include this case here in case someone else is working on a similar problem. In this case either the reverse polarity protection in the pedal is a series diode rather than parallel diode, so no current is flowing, or the power supply was able to supply enough current to destroy a component so no current is flowing at that point and the voltage went back up to 9V.

 

[Newmarket]

I think that drawing might be a mistake. Only one of the diodes is rectifying. The other is just like maybe a protection diode.

D2 does look like its a polarity protection diode - but it seems unnecessary since there is already a series diode D1 to block any reverse polarity connection ???

 

[Tubetec]

Like David said a regulator is a good plan ,
The usual Boss style 9v supply has a regulator , the extra smoothing in the pedal enclosure wont be enough alone to make the unregulated supply quiet , the output voltage will also vary depending on current . You'll need at least a few extra volts supply so the regulator can drop drop to 9volts for it to regulate properly . It may be worth adding a small heatsink to the 7809 depending what current is required and what the supply voltage is after the rectifier.

Theres plenty of off the shelf rectifier/regulator boards that will do the job fine .

 

[Brian Roth]

I always thought this was an interesting product:

https://www.tedweber.com/wpdlxfmr-2/
Eight 11 VAC windings. Plenty of AC voltage from each winding, then to mutliple full wave bridges, each into a medium sized filter cap, then followed by a 3 terminal regulator on each branch.

Bri

 

[Tubetec]

Hi Brian ,
a seperate supply/reg for each pedal in the chain would be nice , any pedals with different ground reference/polarity could be easily configured .

Theres always some kind of grounding loops going on in pedal boards as you have a ground connection between both signal jack ground/pedal housing and back via the psu ground , I wonder could using patch cables with the screen only terminated at one end be used to eliminate these small inductive loops ,

Its a pain when a guitar players pedal board is all full of honk and buzz due to poor grounding practices , they then end up using gating to try and cure it . In a studio setting you generally only want to include effects your using at the time in the signal path , so well worth convincing the guitarist to ditch the usual collection of wall warts and splitter cables in favour of something better approach to grounding.

 

[Newmarket]

Hi Brian ,
a seperate supply/reg for each pedal in the chain would be nice , any pedals with different ground reference/polarity could be easily configured .

Theres always some kind of grounding loops going on in pedal boards as you have a ground connection between both signal jack ground/pedal housing and back via the psu ground , I wonder could using patch cables with the screen only terminated at one end be used to eliminate these small inductive loops ,

With all analogue pedals you may be okay with non-isolated power. At least for live work and rehearsal. Although remember that 'analogue' delays have clocks in them so are a bit digital in that sense. With 'digital' units most likely a problem if sharing a 'daisy chain' connnection or similar.
Don't go with disconnecting the screen at one end. Unlike 3 Wire balanced connections the unbalanced connections on the pedalboard will then be relying on the power 0V for signal return and whilst it might work okay in a particular instance, it's chaotic and any changes in setup are likely to have unpredictable effects on hums , buzzes and allsorts.
You could try the 'tricks' like putting a resistor (eg 100 Ohm) in the screen connection. And possibly a capacitor (eg 100nF) in parallel with that.
But you'd probably need to do that in the plug body. So if you are using flat 'pancake' jacks...
But the real answer is 'real' isolated power as you say and also allows for polarity as required and stacking outputs to give eg 18V.

Beware commercial products that claim to be 'isolated' - they often have a common ground and are only isolated in the sense of having a dedicated regulator etc such that eg a short circuit on one output doesn't stop the other outputs working.
Note that this (link) is galvanically isolated. I have a couple of a previous version.
Harley Benton PowerPlant Junior
but that the 'non-junior' version isn't isolated
Harley Benton PowerPlant

 

[JohnRoberts]

I always thought this was an interesting product:

https://www.tedweber.com/wpdlxfmr-2/
Eight 11 VAC windings. Plenty of AC voltage from each winding, then to mutliple full wave bridges, each into a medium sized filter cap, then followed by a 3 terminal regulator on each branch.

Bri

They mention that it is hipot tested but no mention of UL approval. UL specifies more than just hipot, but that is better than nothing.

Back last century when Peavey (AMR) was selling numerous SKUs using the same 16VAC wall wart, I sold a 1U rack mount transformer with something like 6 or 8 individual 1A x 16VAC winding outputs. This nicely cleaned up the back of many racks but ultimately was not worth the cost to the customers and didn't sell well.

JR

 

[Matador]

I always thought this was an interesting product:

https://www.tedweber.com/wpdlxfmr-2/
Eight 11 VAC windings. Plenty of AC voltage from each winding, then to mutliple full wave bridges, each into a medium sized filter cap, then followed by a 3 terminal regulator on each branch.

Bri

Can confirm that scheme works well: I designed a PCB for a customer that did exactly that (although if I recall that Weber transformer is too big to fit in a 1RU rack case), so I ended up using a 12VAC toroid, and coupling that to individual power isolation transformers that were PCB mounted.

Such a scheme gives a lot of flexibility, in that you can freely mix positive and negative supplies, stack two supplies to get 18V, stack two supplies and get +- 9V, etc.