Colorsound Power boost noise issue

Hi Andy,

You are getting an oscillation that could come from a number of sources. Here is what I observed as differences between the original and the newer units. By mounting the controls on the opposite side you are loosing the shielding that would have been provided by the metal control housings. In the original the treble at least was grounded on the circuit board.  I would tie the metal control housings together and then to a good common ground. Notice how the original control has the input and output lines carefully run down the sides and away from each other. You might even try putting the input lines in coax to help minimizing the chance that they can couple with the output wires. It isn't obvious why the original switch and your switch are different. The difference may be bringing the input and output lines too close together and letting them couple by capacitance. Instead of tying the grounds together on the input and output jacks you might try tying them together on the ground of the circuit board. Hopefully this will give you some experiments to try out. If it doesn't lead to a solution come back and I will try to think of some more.

John

Hi John, thanks for your reply. I'm off to sunny Yorkshire today until Friday, but will check the groundings at the weekend and report back.... In a nutshell, you suspect it's a ground issue?

I'm not 100% convinced that the DC power lines are an issue as this problem occurs using both DC power and batteries. Here is a gut shot of a reissue power boost with an LED, and a DC power socket. This also has a master volume installed

solasound 18v power boost

The difference between the 2 switches is because of the inclusion of the LED. You need the extra pole for the LED.

I think this is purely a  shielding/grounding issue as you pointed out initially.

When I get home I'll pull the board out of the enclosure, and connect all the pots together and ground them to the pcb, I'll then just connect a battery, and run the board out of the enclosure... I'm sure the issue wasn't there previously when I ran the board out of the enclosure, but process of elimination looks like the key to this issue.

Hi Andy,

I just noticed that the original case and the new case are made of metal and your replica is made of plastic. The metal could have a shielding effect that the plastic doesn't give you. I hope the extra ground wires and the separation of the signal in and out wires solves the problem. Keep in mind there are a few possibilities and I may have missed on my guesses.

John

Hi John,

My case isn't plastic, it's a Hammond enclosure made of aluminium.

You are correct the original cases were made of pressed steel.

UPDATE:

I have removed the board from the enclosure, and removed all the wires from the board. I'll now start a process of elimination.... first of all I'll tidy up all the wiring with new wires and ground the jack sockets to the board, and then test the board as it is, and post a video.... coming soon

Ok guys, here's the latest update.

I've removed the board from the enclosure, and the noise is still there. It's been wired as basically as possible. I've wired the treble pot to the ground on the board, as the original pedal had the treble pot grounded directly to the board via the body of the pot.

I've checked and triple checked all soldering on the board and there are no solder bridges that I can see.I'm beginning to think it's the board, and maybe during my design phase I did something wrong somewhere.

Anyway, here's the latest video, i think I need to get this sorted before I stuff it into an enclosure.

Hi Andy,

It is a high gain circuit, perhaps some component values were misread (e.g. red instead of brown) on the resistors? In the photos, some colours look brown on the original, but red on the replica. But it could be lighting conditions.

It would be worth confirming with a multimeter.

As I understand it is an amp with tone controls. According to the video, it is functioning fine when the volume control on it is set low. At that low setting, is the output comparable to the original amp, at the same setting, or at a higher setting? Due to the long wiring, perhaps there is some instability due to the very high gain as that control is turned.

Hi Shabaz,

I can understand your query regarding the resistors values, but the values were taken off the schematic, and confirmed by comparison with the following images. one of these images was taken from a boost pedal that is owned by a forum member over at www.freestompboxes.org. The other is a stock image from the internet.

The schematic was drawn by the forum member (Electric Warrior)

The values were confirmed via a thread on that forum last year. Here's a link to the thread on the forum,

freestompboxes.org • View topic - Colorsound Power Boost 18v

Hi Andy,

I see.. One other thing that could be worth doing (since your circuit appears to work when the gain is set to the lowest setting) is to swap out the 22uF capacitor at the bottom-left of the circuit diagram (i.e. the one connected to the volume pot) and change it to (say) 10uF, and if that doesn't help, also swap out the 4.7uF capacitor at the bottom-right side (the one in parallel with the 1k resistor) and replace that with 2.2uF.

The old capacitors may have not had the capacitance to match the value written on them, whereas modern capacitors are a bit more closer to the value printed on them. Half the capacitance could be a possibility.

Also it could be worth lifting up the variable resistors slightly off the PCB, to see if a few additional mm air gap makes a difference. There could be capacitive coupling if the back of the pots is metal, and that will be reduced if the gap is increased.

Hi Andy,

This is an excellent experiment that you have conducted to solve the problem. It eliminates several of the concerns that I expressed. I have gone back and examined the schematic that you posted. I am not enough of an expert to say anything specifically on this but I have never seen a volume control configured like the one in your schematic. Perhaps someone like michaelkellett can take a look at the schematic. I personally would like to understand how this configuration is controlling the volume. It obviously works fine up to the point where the oscillations start. If we can understand what is going on perhaps we can find a way to suppress the oscillation at high gain.

John

Prior to the latest experiment, the pots has plastic dust covers on them that prevented them from touching the board. i have since removed the plastic covers so that i can ground the metal covers to the board.

John, it's less of a volume and more of a gain potentiometer. Increasing the "volume" pushes the sound into distortion. It doesn't really happen on this little practice amp as the pedal is really intended to be used on powerful valve amps. When i plug it into my big guitar system and switch the pedal on even with the "volume" on it's lowest setting it adds around 20db boost. this is standard for this pedal.....

So, think of this control as less of a volume, and more of a gain.

Here is a YouTube video of a demo... it'll give you an idea of how it should perform.

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John, it's less of a volume and more of a gain potentiometer. Increasing the "volume" pushes the sound into distortion. It doesn't really happen on this little practice amp as the pedal is really intended to be used on powerful valve amps. When i plug it into my big guitar system and switch the pedal on even with the "volume" on it's lowest setting it adds around 20db boost. this is standard for this pedal.....

So, think of this control as less of a volume, and more of a gain.

Here is a YouTube video of a demo... it'll give you an idea of how it should perform.

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I've done a bit more tinkering, and have a few positive things to report.

In DIYStompboxes a forum poster suggested I wire up a jack plug shorting the tip to ground this shorts the input to ground. This should reduce RF pickup if that is the problem. i wired up the plug and tested the board again, no cigar, still the same.

There have been some changes, and I've done nothing!!

1. The noise has reduced significantly by itself, i don't know why.

2. I can turn the volume of the guitar, the amp and the pedal all the way up and the noise is just audible.

3. However, when I back of the volume of the guitar all the way off, the noise increases in volume.

Here's the latest video.

Hi Andy,

It is likely unstable due to feedback. The long tracks, the wires, and and the large bits of metal (the pots) capacitively bridging portions of circuitry could be causing further issues. Plus the input has a high impedance (possibly more than the guitar pickup requires; I'm not knowledgeable on that though). Regardless of grounding the pot, the metalwork seems very close to different portions of the tracks. Adjusting the volume of the guitar is changing the input impedance if the guitar has a pot on the output and that could change the behaviour too.

Furthermore, your gain may be too high due to your capacitors having higher capacitance than the original design; that can be solved by sizing the two capacitors mentioned earlier to be a bit smaller in value (half the value could be a good place to start).

Hi Shabaz,

The feedback is to be expected with this pedal, i'd be pretty annoyed if it didn't feed back at high volume to be honest.

All the capacitors, etc are as per the original design.... However, there were 2 other schematics that they used, a version 2 (mine is the version 1), and also an overdriver schematic, which was done for the US market, and ran on 9v instead of 18v.

here are all the schematics

Hi Andy,

The feed back that shabaz  is referring to is the noise you are complaining about. The put put put is a form of low frequency undesirable feedback.

John

Ahh, i understand. Sorry, newbie to terminology

Hello Andy,

If the noise reduces when the guitar volume control is at min this suggests that it is affected by the impedance to ground at the input, and that it doesn't like low impedance, which is kind of confirmed by your results with the shorting plug.

There are (at least) two ways a circuit like this can be unstable, one is where it actually self oscillates at the lowish frequency you are hearing and the other is where it self oscillates at a very high frequency (that you can't hear directly) but that the oscillation is pulsed at a low frequency. This second effect used to be called squegging but I think the term is less common now.

To work out what is actually going on you need an oscilloscope - any other approach is going to be shooting in the dark - we might get lucky , and we might not.

Where are you - there may be a suitably equipped E14 member who could help out.

MK

Hi Michael,

Thanks for your insight as I was not sure how to help Andy and I still don't understand how that volume control works but tonight the bread board and I are going to duplicate the first stage and look at it with my oscilloscope and see if I can figure it out. Unfortunately Andy and I are 6000 miles apart and my oscilloscope won't be of much help to him.

John

The basic circuit of the amplifier is based on what we used to call a DC coupled pair and was widely used as the magnetic pickup pre-amplifier in HiFi designs of the 70s.

Here's an article about it but it refers to the author's book , Discrete design: 2-transistor RIAA preamp

It uses separate paths for AC and DC feedback, but the Colorsound design rather obscures this in its search for distortion  and a definite sound of its own.

There should be no AC signal at Q2 emitter so the 150k resistor is providing DC feedback to Q1 base.

AC feedback (and some DC) comes via the 12k resistor from Q2 collector to Q1 emitter, where it is shorted out by the 22uF cap and the pot, since the pot can be adjusted to zero there is NO AC feedback at maximum gain.

It should be possible to model it in LTSpice.

MK

Thanks Michael for taking the time to spell it out for me. I will study the schematic and your explanation so that I better understand this design for the future. Also thanks for the link.

John

It simulates OK in LTSpice,

300Hz sine wave, transient simulation, max(almost) gain

Model follows:

Version 4

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MK