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Old 11th October 2008, 03:52 PM   #261
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HI

Im a total newbie.

I would like to start off with the transformer. Im living in Singapore. Is this transformer most suites me with a 8 ohm speaker?

http://singapore.rs-online.com/web/s...2238229#header

If i would get a lower one about 250va, what is the difference?

Thx
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Old 11th October 2008, 04:29 PM   #262
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2 x 0-30V AC secondary is too much; 25V is a reasonable maximum you should consider.
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Old 11th October 2008, 04:57 PM   #263
AndrewT is online now AndrewT  Scotland
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Hi,
if you are selecting a transformer for a power supply near the chip limit you must take account of both the transformer specification AND the normal range of voltages available to you.

eg a 230:25Vac transformer used on a 240Vac supply produces 26.1Vac.
A 240:28Vac transformer used on a 220Vac supply produces 25.7Vac.
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Old 11th October 2008, 05:10 PM   #264
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Ops..

So this should be the best...

http://singapore.rs-online.com/web/s...duct&R=2238207
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Old 13th October 2008, 10:50 PM   #265
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Thank you for this excellent thread Peter!
I assembled one of the Audiosector LM3875 classic kits last week and this was my first attempt at building an amplifier and thanks to the instructions and pictures here it made it very easy.

I'm currently using the amp to power a pair of older Infinity SL30 bookshelf speakers with 6.5" woofers in my computer desktop sound system and it sounds fantastic. The Infinitys are 6 ohm speakers and the amp drives them easily. Very lively high impact sound with the avel-lindberg 250VA 25+25V transformer.

I am having one minor issue though, my source is toslink optical out from my computer to my Zero DAC/headphone amp, then to a TC-754 preamp from Ebay which has done very well for me, and then from the pre to my gainclone. Without the RCAs from the pre connected to the amp it is absolutely dead silent, but when I connect the RCAs I get a low hum which can only be heard from about 4-5" away from the speakers, and gets slightly louder with the volume on the pre turned to max. It doesn't seem to effect the sound at all, but I'm wondering if this is an issue with my amp or if it would be somewhere else.

I located the star ground as far as possible from the transformer in the small aluminum chassis I am using, but I do not have any resistors in the ground path, could that possibly help?
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Old 15th October 2008, 12:54 AM   #266
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Quote:
Originally posted by Coreyk78
Without the RCAs from the pre connected to the amp it is absolutely dead silent, but when I connect the RCAs I get a low hum which can only be heard from about 4-5" away from the speakers, and gets slightly louder with the volume on the pre turned to max. It doesn't seem to effect the sound at all, but I'm wondering if this is an issue with my amp or if it would be somewhere else.

I located the star ground as far as possible from the transformer in the small aluminum chassis I am using, but I do not have any resistors in the ground path, could that possibly help?
With RCAs connected, if you turn the volume completely down and there is no hum, it means that the noise is generated by a previous stage.

If the hum is still present though, it may be caused by less than optimal ground connections in your amp. The preferred method of wiring was presented here: http://www.diyaudio.com/forums/showt...69#post1518369
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Old 15th October 2008, 11:22 PM   #267
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Quote:
With RCAs connected, if you turn the volume completely down and there is no hum, it means that the noise is generated by a previous stage.

If the hum is still present though, it may be caused by less than optimal ground connections in your amp. The preferred method of wiring was presented here: http://www.diyaudio.com/forums/show...369#post1518369
I have no volume pot on the amp itself, I'm using the preamp for volume control, I don't know if that makes any difference in this case.

I currently have the ground wires from the rectifier board on one ring terminal and the ground wires from the amp boards on another ring terminal. Both of those ring terminals are connected to the chassis at the same point, along with the safety ground. So I should join all 4 of these wires from the boards and have only 1 wire connected to the chassis and that should remove the hum?

Also I have the ground wires soldered to CHG pad rather than OG on the amplifier boards because it was simpler for me to do. Would it make any difference to solder those wires directly to the OG pad?

I'm going to do some rewiring tonight, hopefully I can kill the hum and have an amp I am fully proud of!
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Old 16th October 2008, 12:24 AM   #268
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With volume located in a preamp, this may apply as well, but will greatly depend if volume is located at preamp's output (rarely) or preamp's input (most common). Depending on volume setting, a noise from a source component may be also attenuated.

It's recommended to connect ground wires as in my previous link; using either OG or CHG for chassis ground connection shouldn't make much difference.
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Old 16th October 2008, 02:47 AM   #269
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Success! I rewired the ground as you showed and my hum problem has almost completely disappeared.

I tried listening to some other equipment I have and noticed that both my onkyo receiver and a pair of self powered monitors produced the same low hum. I hadn't noticed before because I had never been listening for it I suppose, hehe. So I think it may be something to do with the house wiring causing the last little bit of hum that I can't get rid of, oh well, it's an old house and I'm very happy with the way the amp sounds.

Thank you for your help Peter
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Old 31st October 2008, 02:42 PM   #270
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Hi All

I received my LM3785 kit today. So far I have made up the boards as monoblocks and have kept the supply and amp boards connected. I am using 300A 2 x 25 toroids and I'm going to build two plywood chassis with aluminium face and back plates.

I have rescued a pair of heatsinks from old pc cpus, and am wondering if they'll be enough.

I'm thinking that I'll keep the trafo at the front, maybe just run the LED to the front panel to show power on, and keep the amp connections as close to the inputs as possible. Is this a good idea, or should I keep the AC lines short?

I'm thinking that a third aluminium panel between the trafo and the pcb would reduce AC interference.

This is my first DIY project so any advice gratefully received - you wont offend...

Click the image to open in full size.

Matt
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