Audio Circuit - Supply Noise

Question

i'm designing an Effects pedal, (for guitar, mic, bass, etc.) to be used on stage.

The effect has several stages, which none of them really matter to this question except for a high gain stage.

I'm deciding the kind of supply i'll use for this pedal. My idea is to keep it simple and small so i want to do it with an external DC supply, but here comes the trap.

There's a tube involved in the circuit, typical 12AX7 double triode, so the heater already needs 12V - 150mA.

Ok, so i think of a typical 12V - 500mA supply to be sure BUT! i want to use 20 to 30Vdc for the rest of the circuitry. I wanted to avoid using a transformer with several windings, so i came up with this:

I though of a simple solution with a step-up built with a 555 timer to double those 12Vdc, o even go an get a simple switcher IC. Found this:

I chose the 555 circuit because of its simplicity, its cheap dont need to supply a lot of current (<20mA), and because i don't have a switcher IC at hand at the moment.

The circuit worked like a charm (changed C2 and C3 to 470uF, and changed R1,R2 and C1 to 4k7, 22k and 390pF to work at around 70kHZ, way above audible frequency and low enough to use simple components like 4148 diodes. (besides, if R1 was 100ohms like circuit says it will probably be set on fire). I went ahead and built the pedal on a breadboard.

It worked pretty good overall except for (what every one here already guessed) ... NOISE. Half way Gain stage pot, amazing amplified supply noise.

Measures i took to solve the problem:

• Right next to the output of the supply, a Fe Bead and then a passive double stage low-pass filter (with a cutoff freq. of 0.3Hz).
• On every IC Vcc pin, a small 100pF cap to ground.

I still get noise with the gain set to 1/2 and above (a lot less than before, you could actually use the pedal now, but i HATE noise).

After all this explanation i want to ask two questions:

First: Can i possibly eliminate this kind of noise when it comes to a switching supply for audio, maybe with a better filter or something with simple components, or do i have to go running back to the old transformer+linear regulator for this kind of application? (which i really don't want to do)

Second: When it comes to a using this kind of devices on stage, usually there are a lot of sources from noise (electromagnetic, ground loops, etc..). Is a Transformer+Regulator more or less immune to this kind of noise than a switching supply? Please explain

Thank you, regards.

EDIT: I lowered the switching frequency as adviced by @Jhon D. Its working at 30kHz. The noise came down just a little bit, bit still it appears. To clearify some comments, the diodes are 1N4148, and transistors are BC337 and its complement. So they couldn't be the ones causing the noise, at least not in regards to the working frequency.

I give you two pictures, first the 24Vdc rail. Hellish

Now the Output of the pedal, with NO input (input is grounded), Volume = Max, and Gain = max.

Answer 1

First: Can i possibly eliminate this kind of noise when it comes to a switching supply for audio, maybe with a better filter or something with simple components, or do i have to go running back to the old transformer+linear regulator for this kind of application? (which i really don't want to do)

Well I may have missed what your initial source of the 6-12V is. But regarding the higher voltage supply, NO... you don't have to go back to a brute force supply. The noise can be eliminated or at least minimized. But I've dealt with this kind of thing quite often, and here's what you're in for.

1. the fact that you're hearing frequencies well below your original oscillator frequency of 70Khz does indeed point to parasitic oscillations. The bad news there is that there may be points of instability in your high gain circuit that will "ring" for a short duration at 400kHz every time a strong "impulse" signal is applied. So if "hash" noise pulses from your 70khz is continually supplying the impulses through the supply lines, the "triggered" parasitic will always be there. The bad news there is that you could find that even with the two isolated supplies, certain music signal conditions could still cause those same parasitics to rear their ugly head.

2. It is still possible that the 400khz or other parasitic frequencies are being generated within your booster circuit. They may be very small so you don't see them on the scope, but you did say you were dealing with a high gain circuit. I recently worked with a pre-made variable "boost regulator" whose internal oscillator was around 170khz, and still got horrendous "hashing noise" (more like white noise then a single tone" in my target circuit.). But the principals of propagation of the noise don't change. They are basically electric, propagated through the power supply wires themselves, and magnetic, propagating through the air, and radiating from the power supply wires. If your target circuit has ANY inductors, that radiation can be a major nuisance. To reduce it, you can start by adding a PI filter to the output of your switching supply, consisting of a shunt capacitor followed by a series inductor, and then another shunt capacitor. If the supply wires feeding the 555 circuit are of any length you may need to add an inductor there too. The capacitors should be the fast and low ESR kind, such as polypropylene metalized film. Its hard to pinpoint the perfect values, and even when you do they almost always need to be further tweaked. But a good starting point would be 100uH for the series inductor, and 1uF for the caps. That should at least audibly reduce both the electric and magnetic sources. However, if magnetic field transference is indeed part of the problem, you may have to go the extra steps of making the supply lines into a twisted pair, keeping them well away from any inductors in your audio circuit, and in some cases increasing the distance between your 555 based power supply and the audio circuit. Finally, while most IC manufacturers push for the very fastest switching possible in their ICs, you might lose some headaches in this case by going to a slower version of the 555. If it has a slew rate capable of building an oscillator in the multi-megahertz range, then those edges can cause you problems even with a slower clock speed.

Oh and by the way, here's a little known trick... if you place an ordinary silicon diode in your 555 oscillator circuit, such that the anode connects to pin 7 and the cathode to pin 6, you'll be able to get to a nearly perfect 50% duty cycle with both R1 and R2 the same value! I don't know why this trick is so seldom published, but it works. You can then play with different switching frequencies by just changing the timing capacitor.

Now for this part...

Second: When it comes to a using this kind of devices on stage, usually there are a lot of sources from noise (electromagnetic, ground loops, etc..). Is a Transformer+Regulator more or less immune to this kind of noise than a switching supply? Please explain

It really won't make much difference, if any. If you get to a quiet situation with your current supply setup, and the physical setup doesn't change on stage, any additional noise you get at performance time will likely be due to another source. At performance venues there are a whole slew of issues, and the biggest one related to power supplies is "ground loops". These occur when the power supplies of two or more separate pieces of equipment somehow allow small amounts of 50/60Hz AC to reach their "ground" points. If the equipment is then plugged into different AC sources that are even a few volts different, the ground shield in any connecting cables conducts the AC from one device to the other, coupling the "hum" right into your audio stream. To avoid contributing to this problem, its more important to understand the leakage between your AC source, through your power supply components, that can end up coupled to what you HOPED would be ground. isolation is key. You'd never want to use a supply circuit that didn't have transformer isolation somewhere before you got to the AC mains. And it never hurts to plug as much equipment as possible into the same power strip, fed from a single AC source.

And then of course there is radiated electromagnetic noise. Are you performing anywhere near a 10,000 volt neon sign? Pickup from that kind of noise can't be helped by your power supply. Only adequate shielding, and in some cases "nulling" techniques (such as hum-bucking pickups) can help you out of that headache.

Good luck man!

Answer 2

I'm just a hobbyist. I should start out with that. I have zero electronics training. I read and I try to learn from my betters in this subject. So what I'm going to offer is based on my meager imagination about where this problem might come from and not from experience making a Dickson multiplier like this one.

An obvious thought that came to mind, looking at your scope trace of the power supply output, was inductance. I mentioned it in one of my comments, thinking then that it might be related to the transistor diffusion capacitance and some inductance (it's high frequency hash there.) But another thought crossed my mind and I decided to test it out with LTspice to see if it could replicate your results. And it can. I merely added \$100\:\textrm{pH}\$ of series inductance to your capacitors. That's all it took. I get about \$400\:\textrm{mV}\$ peak-to-peak oscillations with a load of \$1\:\textrm{mA}\$ that die down gradually. Adding just a little bit of series resistance (say, \$1\:\textrm{m}\Omega\$) to damp these out and it gets pretty close.

In this first image I modified the capacitors to have parasitics of \$100\:\textrm{pH}\$ of series inductance and \$1\:\textrm{m}\Omega\$ of series resistance and kept the load of \$1\:\textrm{mA}\$:

But in this second image, I used the exact same circuit but changed the load to \$10\:\textrm{mA}\$ so that you can see the effect of additional loading on the circuit response:

These images don't replicate the rest of the noise you also see. But this is a clean-room simulation, so to speak. Any real circuit will pick things up from the surrounding environment, as well. Shielding may help a little here. Better bypassing, still more. So this gets into the real engineering issues.

I'm thinking @pipe is right about the use of this supply. It's gnarly (technical term, hehe.) You could consider working on reducing any inductance in the switching part of the circuit. Very short leads. Adding lossy beads may help some, as the frequencies are in the area where they may do something useful. It's difficult to get rid of high frequency stuff like this, because it passes right through any trace capacitances. So it does need to be nipped in the bud. Adding a low inductance capacitor at the output might also help some. Adding a carefully constructed \$\pi\$ filter (or two) may also help. I'm not sure about adding a linear regulator, as I can't imagine that it has the speed to respond to these. So I'd focus on finding a passive filtering solution of some kind to kill this before it gets any further.

But first and foremost, if you really insist on working this Dickson multiplier into shape, is that you need to focus on really good layout practices and tighten up everything. If you can work out the details, it will then be worth going to SMT because that will reduce your parasitics still further. But that's only worth it if you can get something reasonable, before that step. And you really need to make sure you've done your very best at keeping all leads as short as possible and to use as thick of wiring as you feel able. I think I'd try to dead-bug this and use heavy copper wires, just to see if I could get close.

I have no idea how this impacts the following circuits you are using and how it then translates into tones or just noise you hear. But the fact that you've tested things with a different power supply, and found that the problems broadly speaking go away, says that, yeah, this stuff is somehow beating down with enough energy left in the audio for you to hear it.

I think a good EE might be able to make some better suggestions, assuming you want to keep using this Dickson multiplier architecture. But I think they might also suggest an entirely different topology and approach. If so, you'd need to give them specifications about what your load looks like, what kind of ripple you can accept and at what frequencies, what kind of load and supply regulation you need to support and more. You haven't yet provided details about how much current is used, for example. That will be needed for any specific design.

Answer 3

If you used an external ("wall-wart") AC supply, you could easily step the AC back up to a high voltage for the firebottle circuit. And, of course, generate whatever positive (or negative) DC voltages needed in the rest of the circuit. I have taken the small transformer out of a wall-wart to use "backwards" to safely generate a high voltage internally but using a low-voltage AC supply externally.