I've just finished a NE5534 buffer feeding into a couple of NE5532s and everything works as advertised except..when I touch the chassis, I get a sports radio channel through the speakers! The station is well tuned, sounds like AM and comes on and off by touch.
I implemented an input filter before the DC blocking cap with fc=72KHz (1k/2n2 - I ran out of 1nF).
The circuit is fed from single supply 12v. There is 100uF on the rail as well as 100nF directly at pins 4 & 7 of the NE5534 (pins 4 & 8 of each NE5532).
The PCB has a single sided ground plane which is covered by solder mask (but might have incidental contact with the chassis). The input / output / power are attached to a plastic faceplate. The housing is a small Hammond enclosure:
This is the first part of the schematic:
Any suggestions on how to get rid of this radio station?
I implemented an input filter before the DC blocking cap with fc=72KHz (1k/2n2 - I ran out of 1nF).
The circuit is fed from single supply 12v. There is 100uF on the rail as well as 100nF directly at pins 4 & 7 of the NE5534 (pins 4 & 8 of each NE5532).
The PCB has a single sided ground plane which is covered by solder mask (but might have incidental contact with the chassis). The input / output / power are attached to a plastic faceplate. The housing is a small Hammond enclosure:
This is the first part of the schematic:
Any suggestions on how to get rid of this radio station?
Attachments
Oops my bad. The Ti datasheet is pretty vague regarding external frequency compensation. (*Edit - I just found the series datasheet that has the information regarding compensation).
I'll have a dig in my spares box and see if I have any 22pF!
Would the lack of a compensation cap have anything to do with the issue of receiving radio or just oscillation prevention?
I'll have a dig in my spares box and see if I have any 22pF!
Would the lack of a compensation cap have anything to do with the issue of receiving radio or just oscillation prevention?
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Grounding the case has eliminated the AM reception when touched.
Strangely, I have a plastic potentiometer knob that when touched causes the station to play again though (but at a lower volume). The potentiometer externals are not grounded so I guess it will be easy enough to put a little runner to ground.
I've not used this particular Hammond case before - the sizes I usually use have metal front and backs, and I often ground boards via mounting holes on larger chassis, so I've not come across this issue before.
Thanks for the tips, especially the 22pF and reminder to ground out the box.
Strangely, I have a plastic potentiometer knob that when touched causes the station to play again though (but at a lower volume). The potentiometer externals are not grounded so I guess it will be easy enough to put a little runner to ground.
I've not used this particular Hammond case before - the sizes I usually use have metal front and backs, and I often ground boards via mounting holes on larger chassis, so I've not come across this issue before.
Thanks for the tips, especially the 22pF and reminder to ground out the box.
Place the housing of that Potentiometer with .5mm wire to ground panel or to ground where Audio GND is connected, this will remove that antenna of the Poteniometer rod. Make sure that the wire is not too long, with not too long I mean about 4 inch or 10 cm..the shorter the better.
Regards another Chris
Regards another Chris
If there is a powerful AM station around, ALL metal parts must bond together. Very preferably metal all around the amp, and possibly tied to wall-outlet "earth".
My early audio career was stunted by living in the shadow of 50,000 Watts. Didn't need an amplifier to pick up the ball game and beer ads.
My early audio career was stunted by living in the shadow of 50,000 Watts. Didn't need an amplifier to pick up the ball game and beer ads.
Chris H, thanks I'll have a look and see if I have any ferrites here I can use or I'll chuck some in my next order. This version has PCB mounted potentiometers but the next will be chassis mounted with flying leads so I think it will be worth trying for that version especially.
Thanks Chris. I suspect my finger is the antenna and the metal mounting body of the pot is receiving the signal because the potentiometer rod and knob are both plastic.
I'll put a short ground runner and see if I can get it onto the ground plane close by.
The circuit is only 12v so I've been using SMPS and all of those I have on hand are double insulated with a barrel plug. (No PE)
Thanks Chris. I suspect my finger is the antenna and the metal mounting body of the pot is receiving the signal because the potentiometer rod and knob are both plastic.
I'll put a short ground runner and see if I can get it onto the ground plane close by.
The circuit is only 12v so I've been using SMPS and all of those I have on hand are double insulated with a barrel plug. (No PE)
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