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Commercial Gainclone kit- building instructions

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.
 
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?
 
Coreyk78 said:
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/showthread.php?postid=1518369#post1518369
 
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! :D
 
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.
 
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 :)
 
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...

P1000025.jpg


Matt
 
Rnfb premium kit question.

Hello all,

This question is for anyone who has built Peter's premium 3875 kit.

It seems there are multiple options to mount the Rnfb resistor on the board. I can place the caddock in the spot marked "Rf" on the pcb or it seems I can mount one leg on the tiny hole by pin eight and solder the other leg to pin 3.

What is the preferred method?

Thanks!
 
Wiring the transformer???

I'm building my first AudioSector LM3875 Dual Mono Classic Amp. I believe it's a version B. I'll be using an Avel Lindberg Y236750.
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=122-640
I looked at the spec sheet and that didn't help.

I'm a bit unsure about wiring the primary. It's got two pairs of wires. I'm in the States, so I'll be running off 115VAC. Do I only use one pair and does it matter which one? If I need to use both pairs, how do I wire them?

Secondly, Is there an advantage to using two different rectifier boards, or should I run both amps off one board? It appears that both rectifier boards can supply two channels of the needed voltage.

Thanks.
Mike
 
Re: Wiring the transformer???

mikje said:
I'm building my first AudioSector LM3875 Dual Mono Classic Amp. I believe it's a version B. I'll be using an Avel Lindberg Y236750.
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=122-640
I looked at the spec sheet and that didn't help.

I'm a bit unsure about wiring the primary. It's got two pairs of wires. I'm in the States, so I'll be running off 115VAC. Do I only use one pair and does it matter which one? If I need to use both pairs, how do I wire them?

Secondly, Is there an advantage to using two different rectifier boards, or should I run both amps off one board? It appears that both rectifier boards can supply two channels of the needed voltage.

In States you need to wire primaries in parallel (115V operation); it's been explained on page 5 of specs sheet: http://www.partsexpress.com/pdf/avelspecs.pdf (blue/violet wires together and gray/brown together).

When using single transformer I always recommend a single rectifier board.
 
Thanks Peter,
What about the secondaries? It shows them in parallel too; however, I'm thinking I run one pair to one rectifier and the other pair to the other rectifier.

Sorry if this seems like a no-brainer of a question, but since this has to do with where the power is being applied I want to make sure I do it correctly for obvious reasons.

My other question was about the rectifier boards. It appears they both have two rectified outputs. Do I need to use both boards or should I run both power outputs off one board? Since I'm using a dual output transformer, do I treat it as though it is two transformers and use the jumpers as you mention on the first page of this forum?

One more question; are the LM3875s insulated or do I need the mica pieces between them and the heatsink?

Thanks again for your help and for this project.

Mike
 
mikje said:

What about the secondaries? It shows them in parallel too; however, I'm thinking I run one pair to one rectifier and the other pair to the other rectifier.

Sorry if this seems like a no-brainer of a question, but since this has to do with where the power is being applied I want to make sure I do it correctly for obvious reasons.

My other question was about the rectifier boards. It appears they both have two rectified outputs. Do I need to use both boards or should I run both power outputs off one board? Since I'm using a dual output transformer, do I treat it as though it is two transformers and use the jumpers as you mention on the first page of this forum?

One more question; are the LM3875s insulated or do I need the mica pieces between them and the heatsink?

The secondaries should be used separately, as you mentioned.
And indeed, you run power to two amp boards off a single rectifier board (as it provides pads for dual outputs)

Although transformer has dual secondaries, you cannot treat them as dual ouputs as the amp uses symmetrical power supply (+/- V) which makes it single transformer. Connection diagram as here: http://www.diyaudio.com/forums/attachment.php?s=&postid=1508798&stamp=1210694958

The LM3875 that come with my kits are insulated, so no mica pieces needed (only thermal grease).