LM7809 noise filtering with capacitance multiplier failure

Thread Starter

bignobody

Joined Jan 21, 2020
97
A little background:

After getting a good result with a custom fuzz pedal for my bass guitar, my friend asked if I could make a Boost pedal for him. Told him I'd try! I found a schematic for the "Alembic Stratoblaster" and built a version of it on protoboard (one I found called "Stratoblaster (Improved)" - not reposting because the image asks that it not be reposted on other sites (even though I found it on another site...)).

The boost worked, but the circuit was not useable due to noise. It's the first time I've experienced audible noise from the LM7809 voltage regulators that I've been using.

It's also the first time I've tried using a JFET (2N5458). Perhaps they are more susceptible to noise than other components I've used in the past?

So I did some research on circuits to reduce regulator noise and found this article:

https://www.edn.com/simple-circuits-reduce-regulator-noise-floor/

The article talks about the LM7805, but I reasoned that this should work for the LM7809 as well since they are in the same family.

I tried Figure 1 (capacitance multiplier) on a solderless breadboard and it seemed to work. So I went ahead and had a PCB made.

I just finished building it and hilariously it seems I've managed the opposite of what I wanted: it's amplifying the voltage regulator noise instead of filtering it... And so far I'm a little stumped to what I've done wrong.

Here's my schematic. C1, C4 and C5 are all tantalum capacitors.

1630077981561.png

And here's my PCB outline in case I've done something wrong that I'm not aware of.

1630078106801.png

I'd appreciate any insights you may have - I'm still pretty new to making PCBs.

In the meantime I'm going back to protoboard to do some more experiments with the LM7809 and noise filtering.

Thanks for reading!
 
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Audioguru again

Joined Oct 21, 2019
3,677
Please describe the noise. Is it low frequency hum from a poorly filtered AC to DC power supply? Is it high frequency hiss? Is it acoustical feedback howling from a nearby speaker that is too loud?
Is the noise still there if a 9V battery replaces the 7809 regulator?
What voltage is Vcc? A 7809 needs a minimum of about 11V at the minimum of any rectified hum.

The original circuit is used everywhere and is very simple. It produces very low hiss.
 

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Thread Starter

bignobody

Joined Jan 21, 2020
97
Please describe the noise. Is it low frequency hum from a poorly filtered AC to DC power supply? Is it high frequency hiss? Is it acoustical feedback howling from a nearby speaker that is too loud?
Is the noise still there if a 9V battery replaces the 7809 regulator?
What voltage is Vcc? A 7809 needs a minimum of about 11V at the minimum of any rectified hum.
It sounds like an oscillator. I measured it with my oscilloscope and it's a 7volt, 60hz signal which is what makes me think it is power supply noise.

It's being powered by a 12V DC wall wart. Other than the addition of the capacitance multiplier parts I've used this power supply setup for several circuits without issue.

I can try doing a battery build, but I don't usually like to power my circuits with batteries.

* Edit: looked again. Not sure where I got 7 volts from. Removed.
 
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Thread Starter

bignobody

Joined Jan 21, 2020
97
Fair enough. I've used the same power supply with other circuits I've built and haven't had any problems like this, though.

Another scope shot, this time with the pedal engaged. Seems like it's amplifying, but only the power supply noise.

1630093419068.png
 

Ian0

Joined Aug 7, 2020
3,529
Do you have a dodgy JFET of dubious origin? We had a small guitar practice amplifier made in China, and many units suffered from an excess of mains hum. It had a J113 JFET as a preamp. Changing the JFET to a reputable one solved the problem.
 

Thread Starter

bignobody

Joined Jan 21, 2020
97
Maybe your wall wart is missing or has a defective output filter capacitor and/or defective full wave rectifier?
Possible! I usually buy them second-hand when I can. Tried another one and I can see the bottom of the wave isn't truncated. Still loud 60 Hz hum and nothing else, though.
1630097033257.png
 

Thread Starter

bignobody

Joined Jan 21, 2020
97
Do you have a dodgy JFET of dubious origin? We had a small guitar practice amplifier made in China, and many units suffered from an excess of mains hum. It had a J113 JFET as a preamp. Changing the JFET to a reputable one solved the problem.
If my circuit looks like it should work, I am definitely starting to consider that it could be a bad component(s).

I got the 2N5458s from a guitar supply store that has been pretty good so far, but I can't really say where they got them from.
 

Audioguru again

Joined Oct 21, 2019
3,677
Wait a minute! A power supply with a full-wave bridge rectifier produces 120Hz hum, not 60Hz hum.
An amplifier that uses an ordinary unshielded audio input cable produces 60Hz hum that the input wire picks up from nearby electrical wiring.
 

Thread Starter

bignobody

Joined Jan 21, 2020
97
Wait a minute! A power supply with a full-wave bridge rectifier produces 120Hz hum, not 60Hz hum.
An amplifier that uses an ordinary unshielded audio input cable produces 60Hz hum that the input wire picks up from nearby electrical wiring.
I can try putting it inside an enclosure to see if I can eliminate some external noise. I'm not really detecting the instrument signal with all the noise, though, so I'm not certain this is the only problem.
 

Thread Starter

bignobody

Joined Jan 21, 2020
97
Make sure you are supplying enough voltage at the 7809's input. Measure it with load.

Been there very recently.
The 7809 seems like it's doing it's job. Black probe on the ground terminal, red probe on the LM7809 input measured 12.5 V with one power supply and 16.5 V (!) with the second one I tried. Output shows a nice 9V.
 

Audioguru again

Joined Oct 21, 2019
3,677
Is the input cable a shielded audio cable? An unshielded wire at the input will pickup 60Hz and other interference.
If you built the simple high-input-resistance circuit on a solderless breadboard then the many rows of contacts and long wires all over the place are antennas that pickup 60Hz mains hum and all kinds of other interference.
 

Thread Starter

bignobody

Joined Jan 21, 2020
97
Is the input cable a shielded audio cable? An unshielded wire at the input will pickup 60Hz and other interference.
If you built the simple high-input-resistance circuit on a solderless breadboard then the many rows of contacts and long wires all over the place are antennas that pickup 60Hz mains hum and all kinds of other interference.
Cables carrying the audio signal in and out of the jacks are guitar patch cords... How good the shielding is, I'd have to cut into them to find out.

I expect a certain amount of noise when breadboarding, but my breadboard version doesn't have anywhere the amount of noise of my PCB build.
 
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