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Old 8th January 2010, 03:02 PM   #971
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No additional resistors are needed, as you already have series/shunt input resistor divider on amp board (220/22k)

It's best to change the gain with R3, but I wouldn't go over 1k6 or so. If you need additional gain trimming, increase R1 (22k will give you -6dB gain reduction)
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Old 9th January 2010, 09:36 PM   #972
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Quote:
Originally Posted by kele1975 View Post
I think you need to replace your chip.
I've just finished replacing the chip and insulating all the wires so it would happen again. However, the offset at the speaker posts is still way to high (34V). Anyone any thoughts about what could be the case here?

Greets,
Erik
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Old 10th January 2010, 11:28 AM   #973
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What about other parts on the board.If it's not a problem try to desolder them and check.(I'm suspecting on one of capacitors,usually they fail first after the chip). On the rect boards also.
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Old 15th January 2010, 09:24 PM   #974
Harolda is offline Harolda  Netherlands
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Default Grounding of brided LM4780

Hello Peter,

i just recieved my transformers for my amps. I use 4 of your lm4780 in bridged mode, 2 per channel, 1 for highs and 1 for lows with an active cross over. The transformers are 200VA each, and each LM4780 wil have its own transformer and rectifier board, so the amp will be completely dual mono.

xlr input--LM4780 bridged--rectifier--transformer.

Each transformer has 2 x 2x 24 and one central ground. As i understand i need to use a star ground for the PS and connect the signal gnd directly to the LM4780 board. But i still have some questions
- should i also connect the central earth of the transformer to the star ground?
- should each channel have its own star ground? (but i use a full aluminum casing

Some pictures of my housing, first setup. There will be a dual compartment in the front, for each transformer


I also added a soft start and a speaker dc protection (central pcb). The amps will have xlr inputs and get signal from an active cross over and pre amp (both fully balanced).

In the pictures you can also see the transformer i will be using (4 in total, 2 per amp), R-core 200VA

Finally i will use a copper bar to press the LM's to the heat sink and not use the screws like in the pictures.



regards,

Harold
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Old 16th January 2010, 09:32 PM   #975
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Default My first diy amp

Finally I finished to assembly my first chip amp. Not totally true, The only thing missing is the cover. First I thought about perforated aluminum for the cover, but the amp seems to not produce much heat, I think simple aluminum sheet will do the job.

My amp is based on Peter designs and ideas. As the Blackgate N 100 are so expensive, I go with Blackgate K 100 ( it is FK but in more compact version). I just take the integrated version and made it more compact. Just test it with Philip Glass string quartet and it sound great. Now, I need to find a place to anodize the faceplate.
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Old 20th January 2010, 07:35 PM   #976
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etoiledusoir - nice looking amp! Have you shrunk the case dimensions and made your own or are you using parts bought from Peter?
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Old 20th January 2010, 09:58 PM   #977
dasein is offline dasein  United States
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Hello everyone!

I'm just starting to put together my first LM3875 kit! I'm very excited yet a bit worried since the most complicated thing I've built was the Millet Hybrid from the headfi.org world. I think I'm rightfully scared given that I haven't dealt with main line electricity before.

In any case I had four questions:

1) I live in the US and I'm trying to figure out how to safely ground the chassis and everything. I'm building a stereo with one rectifier and transformer. Can any more experienced DIYers in the US suggest what screw size and bolts and gauge wire I should be using to ground things to Safety Earth? Otherwise I was going to emulate P. Daniel's set up at the beginning of this thread to the best of my abilities.

2) I have some left over Blackgate NX 47uF 6.3V. Is there anyway I could use them on the rectifier bridge or is the voltage rating too low?

3) I wasn't sure how to gauge how hot the LM3875 chips would run. I was thinking of using these heatsinks: http://se.mouser.com/Search/ProductD...y532-569003B00 Would they be adequate or do I need something more hefty? I'm planning to use these to drive 8ohm speakers without a pot so I imagine they would run pretty hot.

4) Since I'm not using a pot, do I need to add some resistance there? I'm hoping to get as much power as I can since my speakers are relatively inefficient (86db). I'm guessing that I don't need to and that I can solder the RCA connections directly to the PCB, but I figured it wouldn't hurt to ask...

Thanks! and by the way, very impressed with the helpfulness throughout this thread I hope the kindness will extend to me as well =)

Last edited by dasein; 20th January 2010 at 10:21 PM. Reason: forgot link to heatsinks
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Old 20th January 2010, 10:24 PM   #978
Bill_P is offline Bill_P  United States
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1. I use 14 gauge wire for safety ground crimped into a ring tongue terminal. The screw should be 8-32 or larger and a lockwasher should be used under the nut.

2. Capacitors should be rated 50 volts or more; the 6.3 volt capacitor cannot be used at the rectifier output.

3. If there was supposed to be a link to heatsink data it didn't show up. Use the largest heatsink you can fit in the chassis. Mine is 4"x7" with 2" fins. I see you added the heatsink link - it looks far too small. I am using a 4" section of the heatsink from Ebay item 250509769380. There is a picture in the auction.

4. Peter's amplifier circuit board has places for input resistors, R1 and R2. Additional resistors are not needed.

Last edited by Bill_P; 20th January 2010 at 10:43 PM. Reason: More info
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Old 20th January 2010, 10:30 PM   #979
udailey is offline udailey  United States
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This amp will drive your 86db speakers to about 100db with cdp straight to the amp. Thats loud. Really loud.
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Old 20th January 2010, 10:34 PM   #980
dasein is offline dasein  United States
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Ah ok thank you for the responses. I have a preamp that I'm going to use to attenuate the signal. I just didn't want to attenuate at the gainclone.

2. I thought that the supplied optional caps for C1 and C2 are Panasonic 10V caps. Am I confused?
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