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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Humming touch points
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squarewhite



Joined: Jun 06, 2007
Posts: 62
Location: Asia

PostPosted: Wed Sep 25, 2013 12:54 am    Post subject: Humming touch points Reply with quote  Mark this post and the followings unread

Hi,

once again I'm attempting to build a 40106 with touch points. this time, i'm just using 4 touch points for 4 of the hex schmitt triggers and 2 pots. THe signals are all mixed with a 10k resistor to 1 output.

I've added decoupling caps of 100nF from V+ gnd and also a 100uF at the power input. I just added it directly to the power jack, not on the board. There seem to be some sort of oscillations going on in the background and at times when I touch the pots or the touch points, i get a hums along with oscillation.
What could be the problem here? Should I add decoupling caps at every output of 100nF to ground?
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PHOBoS



Joined: Jan 14, 2010
Posts: 1795
Location: Moon Base
Audio files: 351

PostPosted: Wed Sep 25, 2013 5:31 am    Post subject: Re: Humming touch points Reply with quote  Mark this post and the followings unread

Quote:
There seem to be some sort of oscillations going on in the background and at times when I touch the pots or the touch points, i get a hums along with
oscillation. What could be the problem here? Should I add decoupling caps at every output of 100nF to ground?

I can't say what the background oscillation is without hearing it but I might know what's causing the humming when using the touchpoints. Are you
using a wallwart (or another mains powered PSU) ? If so then what you're hearing is probably a 50/100Hz oscillation. At least that's what I get when
using touchpoints and power it from a wallwart or have the output connected to my mixer (which is mains powered). The only way I know
how to get rid of it is by powering it from a battery. And if you want to connect the output to something mains powered you'd probably need an
output transformer.

I don't think adding caps from the outputs to ground would help, it might actually be bad for the chip. Although i've been doing something similar
and didn't have any problems with it yet.

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squarewhite



Joined: Jun 06, 2007
Posts: 62
Location: Asia

PostPosted: Wed Sep 25, 2013 8:06 am    Post subject: Reply with quote  Mark this post and the followings unread

actually the hum is a high pitch whinning sound. BUt using a battery helps to kill the high pitch whine. Thanks for the tip.

But touching the points doesn't kill the hum when connected to a mixer. yes i think it's a electrical hum. What is an output transformer and how can i diy one myself? Also about adding decoupling caps, did it work for you? Should I add decoupling cap at the output of each oscillator or just at the summed output?
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PHOBoS



Joined: Jan 14, 2010
Posts: 1795
Location: Moon Base
Audio files: 351

PostPosted: Sat Sep 28, 2013 9:18 am    Post subject: Reply with quote  Mark this post and the followings unread

An output or audio transformer is similar to an ordinary transformer but more suitable for audio frequencies. And it's used to isolate signals and/or
impedance matching. You don't diy one (although you coud I suppose) but you might be able to salvage one out of ehm,.. someone mentioned a
modem I think, or an old radio. Or you'd have to buy it.

I didn't use it as a decoupling capacitor,. but I did make some (pulse delay) circuits which also had capacitors added between output and
GND. The problem is that the current might get too high when the cap is almost empty or fully charged. But like I said I didn't have any problems
with it yet. Cool

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squarewhite



Joined: Jun 06, 2007
Posts: 62
Location: Asia

PostPosted: Sat Sep 28, 2013 7:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ok I see. I will look around for one and test it out putting it at the outputs. I think I have seen small output transformers with 2 cables on each side.
Can I just solder one end at output of chip and the other end at output to mixer, and the other cables to gnd? Do i need to add any resistors?
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