Introduction, and RF Interference Question

Anything non-technical you want to talk about, talk it over and have beer ...

Introduction, and RF Interference Question

Postby DrRogersA » 16 Nov 2015, 15:32

Greetings, all. I originally tried to post this in the debugging forum, but for some reason it wouldn't let me post a new topic (probably because I haven't posted anything before).

Quick introduction: I am new to building stomp boxes, and I fear I have become addicted. I am actually an Electrical/Computer engineer (BS in Electrical, PhD in Computer). I worked with all this stuff in college and was able to solve test problems and make straight As, but I had no idea how to actually apply this analogue stuff to anything. The vast majority of my professional experience has involved writing software, so until I got into building my guitar and then stomp boxes, I had not looked at an analogue circuit in about 12 years. But now everything is starting to make sense, and I am one happy camper.

So now for a question: what can be done to reduce/eliminate AM radio interference? For my first build, I put together the Tonefiend Bad*ss Distortion (based on the Electra Distortion, http://www.seymourduncan.com/tonefiend/ ... -4-v03.pdf ). I used a 2N5089 tranny. At high gains, I'm picking up a local AM radio station (Tejano music is nice, but doesn't go well with the blues). The interference was pretty bad on the breadboard. I thought that mounting it in a metal enclosure would rid me of that interference, but it did not.

For my second build, I have breadboarded a Bazz Fuss ( http://home-wrecker.com/bazz.html ) using two 2N3904 trannies wired up as a Darlington. Once again, I am getting that same Tejano radio station at high gains. It's not quite as bad as with the distortion build, but I would like to eliminate it if at all possible.

So, what would be some good things to try? I'm guessing a low-pass filter either at the input or output of my circuits. Any suggestions for R and C values? Or any other suggestions?

FYI, I am playing a Gibson Les Paul with relatively hot humbuckers into a Carvin Solid State amp.

One possible contributor to my interference is one of my cables. I have a Fender-brand (I think) cable that is pretty quiet when connected directly between the guitar and amp. My other cable is a cheap Chinese cable that I intend to replace eventually. It's pretty noisy (but no AM radio) when connected directly between the guitar and amp. I have noticed that the AM radio is quieter when using the good cable from my guitar to the pedal and then the Chinese cable from the pedal to the amp. When the cheap cable is going from the guitar to the pedal, the AM radio is louder. (I'm guessing the cheap cable acts as an antenna, and putting it before the pedal is causing the pedal to amplify noise that is already in the circuit in addition to any additional interference the pedal itself is receiving.)

Any suggestions or comments? Thanks much.

Regards,

Austin Rogers
DrRogersA
 
Posts: 3
Joined: 05 Nov 2015, 14:51
Has thanked: 0 time
Have thanks: 0 time

Re: Introduction, and RF Interference Question

Postby DrRogersA » 16 Nov 2015, 15:35

Oh, one more question: is there a way to change my username? I see it is my e-mail address, which I would rather not make public.
DrRogersA
 
Posts: 3
Joined: 05 Nov 2015, 14:51
Has thanked: 0 time
Have thanks: 0 time

Re: Introduction, and RF Interference Question

Postby deltafred » 16 Nov 2015, 16:33

Welcome to FSB.

You come across this quite a lot in stomp boxes, some "designers" have almost zero theoretical knowledge of
analogue design. The main problem is that the circuit often has a bandwidth far beyond the audio range so will amplify anything within it's bandwidth, and pn junctions are notorious for demodulating AM. As a mentor of mine used to say "unwanted bandwidth is just (adding to the) noise", something that a lot "designers" don't seem to realise.

To lessen the bandwidth of your circuit I would suggest a low value capacitor across the input, straight across the jack is a good place because if you can stop the unwanted signal getting into the box then it will be less likely to be amplified. This will shunt high frequencies to ground but leave lower frequencies relatively untouched. The exact value is best found by trial and error, you want a value large enough to supress the unwanted frequencies but not too high as to muffle the tone.

As to your username I would PM modman (the board owner) or one of the moderators, I'm sure it is a easily sorted.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012
User avatar
deltafred
Opamp Operator
 
Posts: 1377
Joined: 06 Apr 2010, 17:16
Location: Yorkshire, England, UK
Has thanked: 296 times
Have thanks: 238 times

Re: Introduction, and RF Interference Question

Postby DrRogersA » 17 Nov 2015, 12:37

A bit of an update: it was that cheap Chinese cable. Removing it from the signal chain lowered the noise floor drastically. I was lucky yesterday in that 100 ft of quality shielded cable was sitting in my mailbox, so I just whipped up a cable to replace the cheapie. Now, the boxed-up distortion effect was very, very quiet, with no Tejano music playing through it whatsoever. The fuzz effect, still on the breadboard, is much quieter, with the Tejano coming through only with the gain and volume cranked to feedback-inducing levels. I tried putting a small cap from input to ground to filter out the noise, but it didn't make any difference whatsoever.

Regards,

Austin Rogers
DrRogersA
 
Posts: 3
Joined: 05 Nov 2015, 14:51
Has thanked: 0 time
Have thanks: 0 time

Re: Introduction, and RF Interference Question

Postby deltafred » 17 Nov 2015, 18:42

I missed that it was on your breadboard, putting it in a grounded metal box will certainly improve it even further.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012
User avatar
deltafred
Opamp Operator
 
Posts: 1377
Joined: 06 Apr 2010, 17:16
Location: Yorkshire, England, UK
Has thanked: 296 times
Have thanks: 238 times

Re: Introduction, and RF Interference Question

Postby DrNomis » 17 Nov 2015, 19:22

Breadboards can be notorious for being a radio-frequency interference magnet, I've built some gain-based guitar FX circuits on a breadboard and had lots of headaches trying to stop the circuits from self-oscillating at radio frequencies, as it turned out the aluminium base the breadboards were attached to was acting as an antenna even when it was connected to circuit ground...... :thumbsup
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.
User avatar
DrNomis
Old Solderhand
 
Posts: 6856
Joined: 16 Jul 2009, 05:56
Location: Darwin,Northern Territory Australia
Has thanked: 123 times
Have thanks: 384 times


Return to Freestomp Café!

Who is online

Users browsing this forum: No registered users and 4 guests