I’ve built a (Deacy-style) guitar amp from the output stage of an 18v 1.5w portable radio (Hacker Democrat Rp34). I’m powering it using a 5v/2.4a powerbank and USB buck booster module set to 18v. I also have a Range Master treble booster in the guitar signal line that can be swicthed in or bypassed. See attached diagram.
The amp is quite noise free when the guitar is silent but when you play notes on the strings behind every note you hear a sympathetic fizzing, fuzzing buzz. This buzz decreases in volume when I touch the inductors on the buck booster while playing. And it goes completely when the guitar strings are silenced.
With the treble booster engaged a loud squealing noise is added in, which seems to be triggered by the lightest of touches on the guitat string and sometimes it will continue to squeal even when the strings are silent.
I’ve tried powering the amp using a 12v DC wall supply the sound is a lot cleaner (compared to the buck booster also at 12v). I am fairly certain that the buck booster module is responsible for the noise and I think I need to filter the power coming from it.
1. I’ve read that a Pi filter - capacitor-inductor-capacitor - might be what I need but despite some research my very limited knowledge of electronics is holding me back. Could anybody recommend a very general starting point for the values of these components? [powerbank is 5v/2.4a, buck booster is 3w max output set to 18v, amp is 18v 1.5w going into a 16ohm speaker]
2. Also I've read somewhere that Pi filters are "not suitable for varying loads" - an amp is a varying load, isn't it? The same website recommends an L section filter "as its output voltage do not vary largely with load current.” Is that correct, should I actually be making one of these instead?
3. Am I missing something else?
The amp is quite noise free when the guitar is silent but when you play notes on the strings behind every note you hear a sympathetic fizzing, fuzzing buzz. This buzz decreases in volume when I touch the inductors on the buck booster while playing. And it goes completely when the guitar strings are silenced.
With the treble booster engaged a loud squealing noise is added in, which seems to be triggered by the lightest of touches on the guitat string and sometimes it will continue to squeal even when the strings are silent.
I’ve tried powering the amp using a 12v DC wall supply the sound is a lot cleaner (compared to the buck booster also at 12v). I am fairly certain that the buck booster module is responsible for the noise and I think I need to filter the power coming from it.
1. I’ve read that a Pi filter - capacitor-inductor-capacitor - might be what I need but despite some research my very limited knowledge of electronics is holding me back. Could anybody recommend a very general starting point for the values of these components? [powerbank is 5v/2.4a, buck booster is 3w max output set to 18v, amp is 18v 1.5w going into a 16ohm speaker]
2. Also I've read somewhere that Pi filters are "not suitable for varying loads" - an amp is a varying load, isn't it? The same website recommends an L section filter "as its output voltage do not vary largely with load current.” Is that correct, should I actually be making one of these instead?
3. Am I missing something else?
Attachments
The diyAudio Store sells an SMPS filter kit for USD 10.00, which includes a PCB and all electronic parts. It's rated for 3 amperes of maximum current, 48 volts of maximum voltatge. Maybe that would be a cheap and easy experiment, that you could try very quickly (?)
SMPS DC Filter Kit :: diyAudio Store
SMPS DC Filter Kit :: diyAudio Store
Also, connect the power switch just after the power bank, to eliminate noise
when the buck/boost converter is off. Or else turn off the power bank.
when the buck/boost converter is off. Or else turn off the power bank.
1. the germanium transistor amp has ground on the positive rail.
2. The treble booster has gnd on the negative rail. That is probable why there is no connection to gnd at all. Cannot work.
3. The L-pad just eats all the power even at low volumes.
It isn't the voltage booster it is the amp that oscillates because of the different gnd polarities
2. The treble booster has gnd on the negative rail. That is probable why there is no connection to gnd at all. Cannot work.
3. The L-pad just eats all the power even at low volumes.
It isn't the voltage booster it is the amp that oscillates because of the different gnd polarities
Thanks, this talks about power coming from wall warts, mine comes from a usb buck booster...is this still relevant?
The circuit shown looks a little complex and I don't know how to get hold of one of those pcbs. I suspect I could make a simpler version with just two caps and an inductor. The part list includes 2 x 470uf caps and a 1 x2.2u inductor. If I arranged these in a Pi filter would they achieve a similar result?
- 1 & 2 - But it DOES work, apart from the noise with this power supply. And using a different power supply there is no noise. The two circuits are powered separately and do not share a ground as far as I understand.
- 3 - Yes, the L-Pad eats power and that is exactly what I want it for, to reduce the volume as there is no master volume. It works exactly how I want it to.
Do you have any suggestions for the ratings of the components for a Pi filter?
With the treble booster engaged a loud squealing noise is added in, which seems to be triggered by the lightest of touches on the guitat string and sometimes it will continue to squeal even when the strings are silent.
You call that working?
You call that working?
When the treble booster is bypassed (and unpowered) the signal going into the amp still has the fuzz and buzzing.
And it doesn't do that with a wall wart power supply. You might be right and there's a grounding conflict when the amp is used in conjunction with the treble booster but I'm happy to leave that till I've figured out the amp's noise problem.
(I can't see way the treble booster and amp's relationship is any different to a treble boost pedal output going into an amp's input. A negative ground powered FX pedal can feed its output signal into an amp that happens to be a positive powered ground, can't it?)
And it doesn't do that with a wall wart power supply. You might be right and there's a grounding conflict when the amp is used in conjunction with the treble booster but I'm happy to leave that till I've figured out the amp's noise problem.
(I can't see way the treble booster and amp's relationship is any different to a treble boost pedal output going into an amp's input. A negative ground powered FX pedal can feed its output signal into an amp that happens to be a positive powered ground, can't it?)
The PCB manufacturing files ("Gerber" files) of the SMPS filter sold by the diyAudio Store, are freely downloadable. Follow the link provided by member rayma in post #2.
Five minutes ago, I got price quotations to purchase a batch of ten of those PCBs. I used the wonderful and free website
PCBShopper – A Price Comparison Site for Printed Circuit Boards
I asked for quotes on 54x36 mm, green, 0.6mm thick PCBs, to be shipped to the UK and paid for in GBPounds. Here are the first two (of twenty) price quotes, from two PCB manufacturing houses in China. If you don't mind leisurely shipping, you can get ten boards for five quid! Jeepers. See attachment below. Click on it for full magnification.
Of course these are only the bare PCBs, you'll also need to buy the electronic components ... which is not an especially easy task during a global supply squeeze. But think of the money you'll save by not having to pay the diyAudio Store shipping fees!
_
Five minutes ago, I got price quotations to purchase a batch of ten of those PCBs. I used the wonderful and free website
PCBShopper – A Price Comparison Site for Printed Circuit Boards
I asked for quotes on 54x36 mm, green, 0.6mm thick PCBs, to be shipped to the UK and paid for in GBPounds. Here are the first two (of twenty) price quotes, from two PCB manufacturing houses in China. If you don't mind leisurely shipping, you can get ten boards for five quid! Jeepers. See attachment below. Click on it for full magnification.
Of course these are only the bare PCBs, you'll also need to buy the electronic components ... which is not an especially easy task during a global supply squeeze. But think of the money you'll save by not having to pay the diyAudio Store shipping fees!
_
Attachments
Thanks Mark, I'll look into building it. Do you think I could build a simpler Pi filter using 2 x 470uf caps and a 2.2uh inductor from that parts list, and that might help?
The gnd issue is something very complex and difficult to understand. To explain what I mean would take a lot of text, but I try.
When you look at your voltage booster you will see that the negative rail is directly connected to your power bank. Even inside the power bank there is electronics to switch off or regulate, but not in the negative rail.
Shielding and filtering will need the lowest possible impedance path to be effective, and that is in modern parts the negative rail. All current has to flow back to where it comes from in this case the lithium cells in your bank.
For higher frequencies the output capacitor in your supply will present a low enough impedance path but still for DC the positive rail has a much higher impedance than the negative.
This also applies for the wall wart, but the loop is much smaller there , so less crap is being induced.
It would be similar if you exchange the gnd and live wires of your guitar cable. Somehow you would say it "works" but...
How to solve the issue?
get an amp with negative rail to gnd or put an audio transformer in front of the amp like from a DI-box.
This would also solve the issue with the treble booster, and a proper gnd can be connected there.
Hope this helps
Bansuri
When you look at your voltage booster you will see that the negative rail is directly connected to your power bank. Even inside the power bank there is electronics to switch off or regulate, but not in the negative rail.
Shielding and filtering will need the lowest possible impedance path to be effective, and that is in modern parts the negative rail. All current has to flow back to where it comes from in this case the lithium cells in your bank.
For higher frequencies the output capacitor in your supply will present a low enough impedance path but still for DC the positive rail has a much higher impedance than the negative.
This also applies for the wall wart, but the loop is much smaller there , so less crap is being induced.
It would be similar if you exchange the gnd and live wires of your guitar cable. Somehow you would say it "works" but...
How to solve the issue?
get an amp with negative rail to gnd or put an audio transformer in front of the amp like from a DI-box.
This would also solve the issue with the treble booster, and a proper gnd can be connected there.
Hope this helps
Bansuri
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Thanks for your time and your persistence in trying to educate me, Bansuri.
I would like to keep the amp so I will try your audio transformer route but this throws up a few more questions for me.
- "put an audio transformer in front of the amp" does that mean between the guitar signal input and the amp OR between the powerbank and the amp power input?
- If the latter, should I still be using the buck boost module or is the transformer replacing it?
- How do I figure out the rating/value of the transformer?
Thank you
I would like to keep the amp so I will try your audio transformer route but this throws up a few more questions for me.
- "put an audio transformer in front of the amp" does that mean between the guitar signal input and the amp OR between the powerbank and the amp power input?
- If the latter, should I still be using the buck boost module or is the transformer replacing it?
- How do I figure out the rating/value of the transformer?
Thank you
I've found this module, I wonder if that would work as you suggest? Not for sale anywhere as far as I can tell.
+/-18V Isolated DC-DC Converter Dual Supply Output from USB 5V Power Input - Electronics-Lab.com
+/-18V Isolated DC-DC Converter Dual Supply Output from USB 5V Power Input - Electronics-Lab.com
Followed your advice and tried to get the SMPS filter kit. Out of stock. What do you recommend now? Thanks.
The audio transformer would go at the input of the 1½ W amplifier.
A capacitance multiplier would require conceding a voltage drop. Also, a passive filter can provide better high frequency attenuation.
Recommend keeping all grounds connected -- switch only the signal lines.
Your diagram shows no ground connection to the Treble Boost module.
Also agree with the previous poster that gain control belongs at the signal level, not output. Battery energy is a limited resource; it strains reason to waste it driving an L-pad.
What is the purpose of the 'Treble Boost only Output'? Usually picking a signal out of a chain doesn't require disconnecting the amp its connected to.
Cheers
A capacitance multiplier would require conceding a voltage drop. Also, a passive filter can provide better high frequency attenuation.
Recommend keeping all grounds connected -- switch only the signal lines.
Your diagram shows no ground connection to the Treble Boost module.
Also agree with the previous poster that gain control belongs at the signal level, not output. Battery energy is a limited resource; it strains reason to waste it driving an L-pad.
What is the purpose of the 'Treble Boost only Output'? Usually picking a signal out of a chain doesn't require disconnecting the amp its connected to.
Cheers
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why buck boost?
just use the wall adapter.
the booster is noisy, likely barely enough current
not sure if this is supposed to be portable.
easier to use 18 volt battery pack.
or even small agm 12 volt
just use the wall adapter.
the booster is noisy, likely barely enough current
not sure if this is supposed to be portable.
easier to use 18 volt battery pack.
or even small agm 12 volt
Still trying to get the SMPS filter kit. Out of stock. What do you recommend now? Thanks.