| Rafal |
Hi Folks,
I'm re-doing my chipamp. :smash: :smash: 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)?:confused:
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 |
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| 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. :hot:
I suggest the 12awg route for proper protection. I hope this helps.
John |
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| 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. |
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| 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. |
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| 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. |
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| maxro |
| quote: | 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 |
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| 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. |
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| Redshift187 |
| quote: | 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? |
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| 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. |
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| pacificblue |
| quote: | Originally posted by AndrewT
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. |
The safety earth does not need to be permanently connected, if the connector is designed in a way to guarantee that the safety earth is always the first to make contact and the last to lose contact. This can e. g. be achieved with a sufficiently longer pin for the earth than for the other leads. |
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| Redshift187 |
| quote: | Originally posted by pacificblue
The safety earth does not need to be permanently connected, if the connector is designed in a way to guarantee that the safety earth is always the first to make contact and the last to lose contact. This can e. g. be achieved with a sufficiently longer pin for the earth than for the other leads. |
This is what I was getting at. IEC connectors always have a longer pin for safety earth. |
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| pacificblue |
| quote: | Originally posted by Redshift187
This is what I was getting at. IEC connectors always have a longer pin for safety earth. |
Yes, as long as you make sure that it cannot accidentally be plugged into the wrong place like a wall outlet or an IEC distribution box like these. |
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| ! |
You do not want to think in terms of wire current capability versus transformer rating. You want to think in terms of the additive impedance of the wire after that of the capacitors for the PSU.
Go as low as you reasonably (affordably) can in wire gauge unless you're deliberately trying to make some kind of filter with the wire. Granted, the wire will be short so it won't matter much but on the other hand given the time and expense of everything else, what's an extra couple dollars to go with a lower gauge wire? I'd use 8 ga. at the most, or if you really want to use what you have, parallel the runs with two conductors each +- and four for ground. |
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| troystg |
Steve at Apex.jr used to have (may still) some 3 conductor twisted shielded cable that was Silver plated Tefl.... (he can't say that anymore) insulated.
I bought some and use it for low current power cords. CD players, Pre-amps and such.
It would be ABSOLUTELY FINE for a chip-amp....
Speakon power connectors on the ends would make it quick detachable and professional also... |
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| ! |
| quote: | Originally posted by troystg
Steve at Apex.jr used to have (may still) some 3 conductor twisted shielded cable that was Silver plated Tefl.... (he can't say that anymore) insulated.
I bought some and use it for low current power cords. CD players, Pre-amps and such.
It would be ABSOLUTELY FINE for a chip-amp....
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That's a pretty big stretch to make considering a power amp uses maybe 10-100X the current. The gauge of wire used should be constrained by three things. What the PCB or mounted connectors will allow, what the budget will allow, and keeping the cord at least thin enough that it will flex as needed to travel the course from one components block to the other. Granted, it's just another case of diminishing returns that beyond some point dictated by current used, it won't make much if any audible difference but considering the total amp budget and time spent, there's no real reason to cut corners on this aspect of the build. |
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| troystg |
I didn't mention the ga. on purpose, but I have some twisted, 3 cond 18 ga. and 2 cond 16 ga. Both with heavy braid shields.
Most of the time I use the 3 x 18 and tie the shield to the source plug side and the 2x 16 occasionally when the device is not a three conductor cord such as cd players.
I find the shield DOES make a difference. More so than the Silver plating or the Tefl.. insulation. |
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