SMPS considerations for guitar pedal effects

[Potato Cakes]

So the added components in relation to ground did not seem to solve the issue. I've attached images to compare noise between the SMPS with the capacitance multiplier and the components added in relation to the mains earth (PEN). This test was done with a wah pedal connected to the Hi-Z input of a MOTU interface. The SMPS image shows the 315 Hz tone that I am getting which is clearly audible through an amp. This was taken with the heel all the way down (lower frequency range of wah). With the toe down, this level decreases by about 6dB. With the Danelectro wall wart (control test) there is a 400 Hz tone that is about 15 dB lower than the SMPS, however, this frequency does change with the sweep of the wah's potentiometer. In use, I can not hear a constant tone through amp with the wall wart and it sounds perfectly fine.

Regarding the SMPS build being discussed, I have wired this with connecting the -VDC directly to mains earth and by floating it, all with the same noise results. I also get a constant tone with some of my digital pedals as well, albeit much lower. I'll grab those next and take some measurements as well and see if get similar results as above.

Thanks!

Paul

 

[Potato Cakes]

Well, now I have more things sort out. I did the same test with a Meris Polymoon pedal (digital) and the noise differences were almost the same. I do not know if the Meris pedal has more power filtering built in but since it's digital I imagine it does.

 

[Potato Cakes]

I added a 1000uF cap to the output of the SMPS before the CM to take it to near capacitance load and for the wah pedal it moved down the frequency of the oscillation. It some what improved the noise floor for the other pedals.

 

[Admiral-dk]

Attached is the schematic. This was made with a lot of help from @thor.zmt. I had some very small boards of this made to use in future projects. This combined with a SMPS that is not clean enough for audio use yielded an exceptionally low noise floor in a full 1073 type project. Without this board the noise and oscillations are ridiculously high and completely unusable in pro audio circuits. So I used this same circuit with a small 9V SMPS but I got varying levels of oscillation/hum depending on the type of pedal. So now I'm back to scratching my head.

Thanks!

Paul

Sorry - but Q1 is connected wrongly - you need to swap pin 1 & 2 (Collector & Emitter) in the schematic ...!!!!

As shown it will mostlikely destroy Q1 + it don't do what it is supposed to do .... That said it will probably run on just Q2 (but not as well, as it could) - again as shown.

Per

ps - I don't get the idea about using R4, R5 & R6 - in my book they deterioate the function of the circuit ...?

 

[Potato Cakes]

That schematic is working perfectly for me. Without it the noise from a standard SMPS is very high in use with pro audio gear. With it is almost non-existent. There is no extra noise induced in the circuit and nothing burns up.

I did a whole other thread regarding SMPSs and a preamp on which I was working. The above schematic is what came out of that discussion:

https://groupdiy.com/threads/smps-class-a-microphone-preamp-discoveries.82639/
Thanks!

Paul

 

[thor.zmt]

The SMPS image shows the 315 Hz tone that I am getting which is clearly audible through an amp.

Too light load burst mode from the PSU?

Try adding a lamp or other dummy load and increase the output capacitor.

Thor

 

[thor.zmt]

Attached is the schematic.

In the schematic Q1 has emitter and collector mixed up?

Did you build it right?

Thor

 

[ppa]

Attached is the schematic. This was made with a lot of help from @thor.zmt. I had some very small boards of this made to use in future projects. This combined with a SMPS that is not clean enough for audio use yielded an exceptionally low noise floor in a full 1073 type project. Without this board the noise and oscillations are ridiculously high and completely unusable in pro audio circuits. So I used this same circuit with a small 9V SMPS but I got varying levels of oscillation/hum depending on the type of pedal. So now I'm back to scratching my head.

Thanks!

Paul

the schematic has a mistake, the BD140 is connected on the contrary.

 

[Potato Cakes]

That is wildly embarrassing.

Somehow the circuit still kinda works. I will make the correction and test again.

 

[thor.zmt]

That is wildly embarrassing.

Somehow the circuit still kinda works. I will make the correction and test again.

Not really "works".

The wrongly connected Transistor still works as transistor, just very, very low beta (single figures).

Voltage handling will also be compromised, but here this doesn't matter.

Also, do provide a minimum load to the PSU. If you are scientific, connect the Mic in of your interface via coupling caps to the PSU output and use some Audio analyser Software to see what goes on.

With no load you typically see a lot of garbage. Dial up the load in 3dB steps until the noise lowers and stabilises. I found that drawing ~10% of rated power quietens down most SMPS that are noisy at no load. Maybe add some LED's to light up the pedals on the floor?

Of course, some SMPS are brutally noisy, there the only thing you can do is 0.33R (ideally an inductor with ~0.33R DCR) +10,000uF (++) filtering.

With 0.33R & 10,000uF you get a 50Hz low pass first order. Place the appx. 10% dummy load before the 0.33R resistor. Follow with the (correctly) connected capacitance multipler and things should quieten down.

One big problem with many commodity SMPS that they are not engineered with noise as priority.

I design a really quiet SMPS, which is available in 9V and while overpriced, not really that expensive, if you count your time messing with cheap stuff.

It's called the iFi I power and used to be 50 bux for 9V/1.5A, which includes a loparity inverter to have center negative for pedals.

It went up to 70 Bux before being discontinued in favour of a "new and improved" version for double the money with a new case design and pretty (or not) lights for double the money.

Shame that. I could use a large company bankrolling making some musician specific fixers and these powersupplies in volume, which bring price down and give may musicians and studio people struggling with noise affordable (and safe) solutions.

Thor

 

[Potato Cakes]

Made the correction. Much better results. Can't believe I didn't catch it earlier. The wah pedal no longer generates that 315 Hz. And the Meris pedal is quieter than I can ever remember.

I try to keep the dyslexia to a minimum but sometimes it gets the best of me. Thanks for everyone's help pointing out what should have been glaringly obvious.

Thanks!

Paul

 

[ppa]

That is wildly embarrassing.

Somehow the circuit still kinda works. I will make the correction and test again.

You were probably misled by the fact that the classic capacitor multiplier is made with a darlington, while here there is a different one. But I see that this circuit has a smaller voltage loss between in and out, we speak of around 0.8V vs. 1.2V of the darlington and lower out impedance of it.