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?

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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RE: Colorsound Power boost noise issue

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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

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

SHEET 1 2600 692

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WIRE 240 320 240 160

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SYMATTR Value 50k

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SYMATTR Value 50k

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TEXT 48 -408 Left 2 !;ac dec 50 20 200000

TEXT 616 -392 Left 2 !.tran 1s

MK