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 Forum index » DIY Hardware and Software » Circuit Bending
Bending radio: getting silent output safely
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jorgen



Joined: Jul 30, 2013
Posts: 4
Location: Ireland

PostPosted: Tue Jul 30, 2013 7:17 am    Post subject:  Bending radio: getting silent output safely
Subject description: Looking for another way than telephone mic on speaker to safely get sound from bent radio
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Hello!

I've just gotten started with circuitbending and I'm having fun with some radios. I use a telephone mic on the speakers to record sound without fear of dying from electroshock.

However I'd like to route the sound through some software effects, without getting the pure sound from the radio itself (which is superloud, that's cool but a bit unpractical). On a workshop I was told I should never ever connect the output of the radio to a soundcard etc; no direct connections between my fingers on the circuitboard and something plugged in the wall..

Is there any way to do this?
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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Tue Jul 30, 2013 7:30 am    Post subject: Reply with quote  Mark this post and the followings unread

I think the best/safest way would be to use an audio transformer. They are available with a low input impedance of 8 ohms (primary) and high output
impedance >1K (secondary). So instead of the speaker you use the primary winding of the transformer and your secondary would be your line out.

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jorgen



Joined: Jul 30, 2013
Posts: 4
Location: Ireland

PostPosted: Tue Jul 30, 2013 7:41 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
I think the best/safest way would be to use an audio transformer. They are available with a low input impedance of 8 ohms (primary) and high output
impedance >1K (secondary). So instead of the speaker you use the primary winding of the transformer and your secondary would be your line out.


Thanks for the reply! So this would keep any strong current from entering through the audio transformer and into the radio?

EDIT: rephrasing question
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PHOBoS



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PostPosted: Tue Jul 30, 2013 8:30 am    Post subject: Reply with quote  Mark this post and the followings unread

the transformer would replace the speaker so if you have enough room for it then put it inside the radio. But it might be usefull to put it in a seperate box
so you could use it for other devices aswell. If you allready have a seperate audio output on the radio you could use that too, but you might need a
different transformer.

as for safety, I think the only way safer would be some sort of optical connection,. it's pretty safe. It's actually the same as what you do with a
pickup coil: the speaker coil is the primary winding, the pickup coil the secondary.

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jorgen



Joined: Jul 30, 2013
Posts: 4
Location: Ireland

PostPosted: Tue Jul 30, 2013 9:54 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
the transformer would replace the speaker so if you have enough room for it then put it inside the radio. But it might be usefull to put it in a seperate box
so you could use it for other devices aswell. If you allready have a seperate audio output on the radio you could use that too, but you might need a
different transformer.

as for safety, I think the only way safer would be some sort of optical connection,. it's pretty safe. It's actually the same as what you do with a
pickup coil: the speaker coil is the primary winding, the pickup coil the secondary.


Thanks a lot! I'll go get myself some audio transformers then.

EDIT: Another question: At a local store I found an audio transformer with the following specifications:

Application: Output
Primary impedance: 1.2kΩCT
Secondary impedance: 3.2Ω

Do you think this would work? I'm asking because it seems this is the "opposite" of what you recommended, wrt primary and secondary
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richardc64



Joined: Jun 01, 2006
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PostPosted: Tue Jul 30, 2013 5:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

Wait a second! WHAT kind of radio? If it's tubes ("valves" to you UKers,) then yes, don't touch the innards while it's plugged in. If it's "Solid State" (transistors, ICs, etc.) it almost certainly has a power transformer to step-down the AC ("mains") voltage -- just like the power supplies we build. There's no electrical hazard.

FURTHERMORE, if the radio's output uses a transformer to power the speaker, you can connect directly to that transformer's secondary, perhaps through a switch that disconnects the speaker.

Really, one can't call oneself an electronic hobbyist until you've gotten zapped a few times.

Paging RF.

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knives and bearskins." -- Spock to Edith Keeler
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jorgen



Joined: Jul 30, 2013
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Location: Ireland

PostPosted: Sat Aug 31, 2013 3:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
Wait a second! WHAT kind of radio? If it's tubes ("valves" to you UKers,) then yes, don't touch the innards while it's plugged in. If it's "Solid State" (transistors, ICs, etc.) it almost certainly has a power transformer to step-down the AC ("mains") voltage -- just like the power supplies we build. There's no electrical hazard.

Paging RF.


Thanks for this info! The radio must be solid state I'm pretty sure (It's a relatively new, standard one with a cd player).

The reason I'm a bit careful is I went to this workshop with Nicolas Collins and he said to never connect through the output of the radio in case a rouge current from the wall socket ends up in your hands.. I mean that's a bit more than a tiny shock. Was he being extra super careful saying that, like you seem to imply Richard?
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richardc64



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PostPosted: Sun Sep 01, 2013 8:35 am    Post subject: Reply with quote  Mark this post and the followings unread

jorgen wrote:
Thanks for this info! The radio must be solid state I'm pretty sure (It's a relatively new, standard one with a cd player).


Ah, I see. Then it's almost certain to have a power transformer, and it would take a pretty massive failure for the mains voltage to appear in the radio's guts. Caution is all well and good, but can be taken too far.

Less certain is how the speaker is connected. It might be thru a transformer or, more likely, direct or thru a capacitor. If by transformer you could switch the output to a low ohm resistor -- 100 or so and capacitor couple to whatever external circuit. For complete isolation in either case, use a transformer with low impedance on both primary and secondary. You could suck one off an old dialup modem card.

Or just use the radio's earphone jack. I just now tried that type of radio/CD/cassette using that method thru my PC's Line In. No problemo.

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knives and bearskins." -- Spock to Edith Keeler
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