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Old 8th April 2012, 08:59 AM   #11
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Thank you Mooly for your explanation. I'll take the time and go through the information you have provided, however, if the reactance of the 0.1uF cap is only 160 ohms at 10kHz then would a lower value cap have a higher reactance? Or perhaps this is just not the way to solve the problem.

I'll go through the data you sent me and see what I can figure out. Thanks for your help with this project.

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Old 8th April 2012, 11:28 AM   #12
Mooly is offline Mooly  United Kingdom
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Connecting a cap directly across an audio output is normally a "no no".

The major concern with connecting a small capacitive load across an output is that it can cause instability and oscillation. Normally that's a problem even for low values of C. Some opamps are oscillation prone even driving the capacitance of a few metres of cable which would be in the 10's to 100's of Pf. Many opamp data sheets show the recommended method of driving capacitive loads which usually involves a low value resistor after the opamp output. But that's for small low values of C

0.1uf to an opamp output is just swamping it. If you had a scope you could see for yourself what it really does. The opamp would swing less and less voltage at the output as the frequency increased and the transient performance would be non existent.
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Old 8th April 2012, 01:51 PM   #13
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Originally Posted by vielle568 View Post
I have found a solution; I don't know if it's a good solution but it works well enough for my needs.

I added two 104 caps going between ground and the analogue switch input and output pins. I can only assume that these act as decoupling caps and bring the DC voltage levels on these two pins at more or less the same level (0V) and as a result the 'click' from switching has now become almost inaudible.

Thanks everyone for your help and suggestions. They were greatly appreciated.

Vielle568
Ok, others has already said what I would have been saying. my suggestion would be, remove the switch IC. physically. replace it with a tactile switch (mechanical switch). connect 10M resistor parallel to the switch. Now try switching it (manually).

If this works probably we can use different switching ICs. do listen it properly when switch is in off position and the click noise too.
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Old 8th April 2012, 06:28 PM   #14
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OK Mooly, I understand what you're saying and I'll take the caps out. I've tried out the circuit a few times and the output signal doesn't have the loud clicks but there appears to be a mild effect like a phaser that is detectable. I do have a scope but it isn't capable of doing spectrum analysis so I'll take your word for the effects on my signal output.

I'll read through that info you left me yesterday and see what I can learn from that.

Meanwhile, Acosticraft has suggested a 10M resistor in parallel with the switch. I assume that this is to suppress the audio signal from passing but to allow the DC levels either side to be at the same level. Do you have any comments on this approach Mooly? It certainly won't take long to wire it up and see what happens. I'll let you know....

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Old 8th April 2012, 06:36 PM   #15
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You would see the effect on your scope if you looked at say a 10 or 20khz sine wave (square would show it even better) and then just dabbed the cap across the output. It will reduce the amplitude dramatically as the opamp current limits. Try it at say 10 volts output

If you haven't a signal generator then a CD with test tones is good too.

The 10M across the switch ! I wouldn't like to say really. It shouldn't make any difference, the only real effect being a tiny amount of signal being allowed to pass. It all depends on what the switch actually feeds into (impedance wise) and what the static conditions of that input are.
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Old 8th April 2012, 07:08 PM   #16
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This shows an opamp driving a 0.2uf cap at 10 khz. The green trace is the input signal and is also how the output of the opamp should be driving any normal load within its limits. The blue trace shows the actual output of the opamp. You can see how distorted the signal is as the opamp can not drive the cap.

Although the opamp here is a different type it is representative of what happens.
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Old 8th April 2012, 07:40 PM   #17
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Have you tried to connect a resistor from the output of the switch to ground? This may help to keep the same DC level on both sides of the switch. Perhaps a 470kohm or perhaps smaller. Since we don't know exactly what is connected to the output of the switch is is difficult to give exact guidance.
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Old 8th April 2012, 07:51 PM   #18
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Maybe replace the switch with a jumper. When you open and close the jumper, do you still have the click? That should narrow things down. If you don't. try softening the edge of the switch pulse with a small resistor and cap, maybe 50 ohms in series and .01uF to ground. Also, disconnect the switch pulse and pulse it to be sure you don't have a cross coupling issue elsewhere, like through ground.
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Old 8th April 2012, 10:03 PM   #19
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I can understand what you're telling me Mooly. No, I don't have a signal generator but I can see clearly from your signal plot the deformation you've been describing. Those caps have now been removed from my circuit.

I did try putting the 10M resistor across the switch terminals. I used a mechanical switch initially as Aucosticraft suggested. There was no audible click when the switch was toggled; just a change in hiss (probably due to all the temporary cables wiring up the circuit; nothing shielded at the moment). However, when the analogue switch was put back in place the click was still audible, especially at low signal levels.

In reply to JensH the analogue switches in this circuit are placed after a preamp and signal splitter. They are controlled by a microprocessor; the pulse output passes through a Schmitt trigger inverting buffer to ensure a clean signal to control the switch. After the switch the pulsed audio signal passes to an output connector and is then cabled to a mixing table and power amp. There are three similar circuits in this design that run in parallel. And yes, I have actually tried connecting resistors of various values between the switch terminals and ground, but it didn't seem to have much effect on the clicks being generated.

In reply to Conrad Hoffman I have just replaced the analogue switch with a simple push type switch and tried pulsing the signal manually. The output is actually very clean and there are hardly any clicks when the button is pressed; the ones that can be heard from time to time I would assume are due to switch bounce and can be ignored. If this is so then the problem narrows down to the control signal. The manufacturers state that the switch is a special model made for audio signal with a gradual ramp on ramp off break before make contact; it still clicks loudly however. I'll try Conrad's suggestions and see what happens......

Vielle568

Last edited by vielle568; 8th April 2012 at 10:07 PM.
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Old 9th April 2012, 12:28 AM   #20
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Quote:
Originally Posted by vielle568 View Post

I did try putting the 10M resistor across the switch terminals. I used a mechanical switch initially as Aucosticraft suggested. There was no audible click when the switch was toggled; just a change in hiss (probably due to all the temporary cables wiring up the circuit; nothing shielded at the moment). However, when the analogue switch was put back in place the click was still audible, especially at low signal levels.


Vielle568
This is definitely because of charging of capacitor.
Can you add a 10E resistor in parallel with the coupling capacitor (the one at the output of NE5532 ? See what happens?

If the above solution did not work would you be ok replacing existing switch with CD4016 (its not pin compatible). If you intent to go with CD4016 I can suggest you how to configure it to avoid click.
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Last edited by Aucosticraft; 9th April 2012 at 12:37 AM.
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