Shielding requirements in active speakers? - diyAudio
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Old 20th June 2004, 07:48 AM   #1
tcpip is offline tcpip  India
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Join Date: Jan 2003
Location: Mumbai, India
Default Shielding requirements in active speakers?

I'm a freshie to diy audio, and have been doing a lot of reading and learning before embarking on my first significant project. I'm interested in building speakers which will have one small power amp per driver, and active xo.

Ideally, I'd like to have just one line-level output going from my preamp to each speaker. I'd like to house the active xo and power amps inside the speaker. And I'd like to have the power supply for all this inside the speaker too. And no, I don't want to use toroidal xfmr for the power amps (availability and price are both problems here). So, the only inputs on the rear of my speakers, ideally, should be (i) an RCA or XLR socket for the signal, and (ii) an IEC socket for the mains supply.

Now my questions:
  • What do I need to do, if anything, to protect the signal-level circuits from interference caused by the speaker drivers whose coils will be less than 20" away? Assume unshielded drivers.
  • What about interaction between the large power transformer and the speaker drivers? Typically, if I'm building a three-way, the large woofer coil will be perhaps 15-18" away from the power transformer.
I intend to build floor standers with the electronics housed in a full-height chamber at the rear of each speaker. The power amps' heatsinks will be in a long vertical strip along the rear surface. (See Pass Labs' Rushmore speaker .... my ideas are very similar, without the Class A orientation.) Therefore, left to myself, I'll probably use one big power xfmr for each speaker, and get it made to order (E+I custom-built is affordable) with taps for 12-0-12 (for active xo and BSC circuits) and 30-0-30V for power amps. I'll place this xfmr right at the bottom of the tall chamber at the rear. Next to it will be the bridges and filter cap banks. Above these, mounted on the heatsink, will be the power amps, and above the power amps, perhaps at a height of 30" or so above the transformer, will be the active xo circuits. So I'll manage to put a fair bit of physical distance between the power xfmr and the line-level circuits, but the speaker drivers will be quite close to the line-level circuits. Hence some of my worries. I also intend to enclose the power xfmr in a box made of thin steel sheet, for EMI shielding.

Any opinions and inputs most welcome.
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Old 20th June 2004, 10:52 PM   #2
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Join Date: Dec 2003
Location: Munich
Hi tcpip!

Don't worry about the small signals which are close to the speaker.
The magentic field of the speaker is statical, so it does not induce to the signal, as long as the wires are not heavily moving....
From theory it could happen that a vibrating wire gets some signal induced, but I never heard about issues with that.

Shielding the XFR with metal is fine.
Generally you may probably not face to much issues
with induced hum from the XFR as you are able to provide
a lot of distance between the signals and the XFR.
You should also try to have much distance between the
signal sockets and the mains socket.

So you may probably "only" face more or less the usual hum issues due to unfortunate grounding structure.
For this you should try to check through the relevant threads
of the forum, if you are not experienced.

Good Luck
Markus
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Old 21st June 2004, 06:23 AM   #3
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Quote:
Originally posted by ChocoHolic
Don't worry about the small signals which are close to the speaker.
The magentic field of the speaker is statical, so it does not induce to the signal, as long as the wires are not heavily moving....
Thanks a lot.

I was actually wondering whether the high-current output of the power amps flowing through the voice coils of the speaker drivers would create a varying magnetic field which could then induce electrical currents in the signal-carrying leads of the active xo and other small-signal paths. I know the static magnetic field is not a concern.

But from what I hear, even this voice-coil magnetism should not be strong enough to worry my circuits, which will be more than a foot away from the coils.

Quote:
Shielding the XFR with metal is fine.
Generally you may probably not face to much issues
with induced hum from the XFR as you are able to provide
a lot of distance between the signals and the XFR.
Yes, I intend to put the RCA sockets at the top of the speaker's rear wall, and the mains input and xfmr at the very bottom. That should give me enough distance.

Quote:
So you may probably "only" face more or less the usual hum issues due to unfortunate grounding structure.
Yes, all those star grounding problems will have to be taken care of in the usual way. That's something separate which I'm trying to address anyway.

Thanks a lot.
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