Design of umbilical cord

[Rafal]

Hi Folks,

I'm re-doing my chipamp. It will have a seperate box for a PS and a seperate for an amp. Hopefully it will also look nicer and it will allow me to try a different PS easily (like a battery PS)

My current design is posted on Brian GT's gallery (the first posting).
http://www.briangt.com/gallery/nigc

I'll post pics of my new design

My current PS consists of a 400va toroidal transformer going to a rectifier with +22, ground, and -22v coming out. I would like to use 18 gauge wires in the umbillical cord (a 3 wire cable) with 1 18 gauge for +22VDC, one 18 gauge for _22VDC and one for ground.

My question is: Is this gauge going to be sufficient or should I try something larger ( if so, how big)?

Also, what have you folks found to be a succesful umbilical cord desing that resulted in very good sound and low noise?

Thanks in advance

[John Medvich]

As you know, VA is Volts/Amps. So a 400VA Trans has 400VA/22V = 18.182A

The NEC (USA) and CSA (Canadian) specify that a 20 amp max load should be carried on a 12awg conductor.

Also note that you can use the 18awg conductor, but you would have to limit the current to the capability of the conductor being used. With this being said 18awg is fine, but you have to have a current limiting device (fuse) rated for 12amp to properly protect the wire. This option basically lowers the available continuous current that you need in long loud listening as you could blow a fuse and stop your fun.

I suggest the 12awg route for proper protection. I hope this helps.

John

[Greg Erskine]

Deleted.

[AndrewT]

Hi,
the output from a transformer rectifier with capacitor input filter has to be down rated to 70%.

The 22Vdc indicates a 15Vac transformer.
400VA / [2 * 15] = 13.3Arms continuous maximum.
After down rating and converting to DC, the maximum continuous DC current output is 6.3A per rail.

18g will be just fine.

However, the transformer has mains on it.
If the mains connection becomes faulty and contacts the low voltage side, then mains voltage can pass down the umbilical.
To protect against this eventuality, the power amplifier must also be protected with a safety earth.
I can see three alternatives:-

1. double insulate the first box and comply with ALL the double insulation standards. I do not know how onerous these requirements are.

2. feed a safety earth wire down to the amp box. The amp box will be connected back to the mains distribution board VIA the umbilical and then the mains cable to the socket outlet. BUT the safety earth must be a permanent connection.

3. Connect the amplifier box to the mains safety earth via it's own socket outlet.

Next,place some smoothing in the transformer box. This must be rated for the high ripple that first stage smoothing imposes on the caps.
Feed smoothed DC down the umbilical.
Put your main ampifier capacitors in the amplifier box. These satisfy the AC and peak load current requirements of the varying output from the amplifier. These will determine the sound quality and should be good (for mid and treble) and adequate if you want bass from your speakers.

[Rafal]

Thanks very much for your replies.

This clears things up a bit. I think, that since I have a 3812gauge wire already, I think I'll just use that to be safe.

Now, I think I'll remove the "heatsinks" (1/8" aluminum plates) from the direct contact with the wooden base to reduce the risk of fire.

[DIAR]

I had my gainclone in separate boxes before. One power supply box with two sets of transformers and rectifiers and the power amp boards were in separate boxes.




I had real problems with some sort of RFI that I couldn't get rid of. I tried every possible tweak that I could think of but nothing helped. Since I put everything in one singe box, the amp has been very quiet.

[maxro]

Originally posted by DIAR
I had real problems with some sort of RFI that I couldn't get rid of. I tried every possible tweak that I could think of but nothing helped. Since I put everything in one singe box, the amp has been very quiet.

Could it be that the seperate boxes were wooden and it's the metal box that is doing the shielding on the one box set up?

max

[DIAR]

I also tried shielding the wooden boxes but it didn't help. But then again I only used aluminium foil (a lot of it) to test the shielding. I also tried two totally different power supply cables but it had no effect either.

[Redshift187]

Originally posted by AndrewT
However, the transformer has mains on it.
If the mains connection becomes faulty and contacts the low voltage side, then mains voltage can pass down the umbilical.
To protect against this eventuality, the power amplifier must also be protected with a safety earth.
I can see three alternatives:-

1. double insulate the first box and comply with ALL the double insulation standards. I do not know how onerous these requirements are.

2. feed a safety earth wire down to the amp box. The amp box will be connected back to the mains distribution board VIA the umbilical and then the mains cable to the socket outlet. BUT the safety earth must be a permanent connection.

3. Connect the amplifier box to the mains safety earth via it's own socket outlet.

I was considering something a little different... What if an IEC power cord (with different connectors than your mains) was used as an umbilical?

[wazzy]

what about an xle or balanced mic plug, have used thes on a bunch of amps i've made and it has a spring loaded clip on it so it can't unplug accidentaly.