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Commercial Gainclone kit- building instructions

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Peter Daniel

Peter Daniel

Member
Joined 2002
audiosector.com
Paid Member
dillmeister said:
Hi Peter, I built an LM3875 kit a few months and have been listening to it daily. I was using a set of old Cerwin Vega U6 speakers that are actually satellites for a subwoofer system. I decided to try out the sub with the system the other night and it made a really nasty humming sound. I triple checked the connections as the crossover goes through the sub and thought perhaps I had erred. The next day I tried a Denon receiver and the speakers worked perfectly. I then changed the connections back to the chip amp and the nasty hum again. I figured ok perhaps the positive and negative were screwed up when I built the chip amps so I tried carefully to reverse one side and the noise was indescribable. Although the two chip amps are in the same case I have completely separate transformers and wiring except for the mains. I did some research and found that the sub is what is called a dual voice coil. Is it possibly because the two mono amps aren't matched close enough that the sub does not work? It's not a huge concern as I can still use my other amp but I would like to find a solution as I plan on building another dual mono for my main system which also uses a sub/satellite configuration. I hope I have given enough information to get some replies. ( : Thanks in advance.
Click to expand...

Try connecting a single coil only.
 
kaputt

kaputt

Banned
Joined 2011
22V DC offset

Hey guys! I need help with DC offset. I built the LM 3875 kit and get 92mV offset on one channel and 22 Volts (not Millivolts!!) on the other. There's no soundoutput whatsoever. No crackling, no hum, no music.
Do you need any more info like certain measurements to give me some help? I checked the schematic a quadruple times and measured for shorts so far. Thanks for any help!!
 
lduarte1973

lduarte1973

Banned
Joined 2009
lanchile07 said:
'commercial amp' definitely has clearer treble and thats where it excels...

Well,I have noticed and compared that these amps are NOT fatiguing as some commercials ones.You can listen to them all day long. I believe is because it's "delicate" trebles.
Some people like a lot treble,but if you are going to listen to it for a long period of time the sound becomes fatiguing.
I had so many amps like:
Marantz,Luxman,Adcom,Arcam,portal panache etc. but I still like the LM3875 GC better.;)
Click to expand...

did you have the luxman 384A ?
 
kaputt

kaputt

Banned
Joined 2011
DC offset update

Update: I took out R1s and exchanged them for wire bridges. Now I have DC offset of 32mV versus 12.2V and quite good sound from the one channel with the 32mV offset. I have a feeling the other chip is dead?


kaputt said:
Hey guys! I need help with DC offset. I built the LM 3875 kit and get 92mV offset on one channel and 22 Volts (not Millivolts!!) on the other. There's no soundoutput whatsoever. No crackling, no hum, no music.
Do you need any more info like certain measurements to give me some help? I checked the schematic a quadruple times and measured for shorts so far. Thanks for any help!!
Click to expand...
 
S

ss007

Member
Joined 2009
Hi Peter, I've been using a ESP preamp that I assembled. For months I've had the 25K Nobel pot still in the GC circuit. I leave the pot all the way up and use the volume control on the preamp. The other day I took the Nobel out of the circuit but it doesn't sound as good now. I would describe it as being compressed or held back a bit, just not as open and airy. Could it be an input imp. issue? Am I crazy? Would love to hear what you think. Thanks
 
A

AndrewT

R.I.P.
Joined 2004
Do you have RF attenuation filter at the signal input of the power amplifier?
Do you have RF attenuation filter at the power input to the power amplifier?
Do you have RF attenuation at the signal input of your source/pre-amp?
Do you have RF suppression fitted to the noise emitting equipment, eg fridge, central heating pump, and other motors, etc.?
 
E

ecosprog

Member
Joined 2010
Just finished putting together one of Peter´s LM3875 premium kits. I powered it up last night and got DC offsets which looked OK but when I connected it to a source and speakers I have less output from the right channel than the left.

The offsets are, left channel @ -66.2mV, and right channel @ between -43 to -46mV. The right channel is constantly fluctuating while the left channel is unchanging. These values are without load. The voltages on the regulator board are +29.51V and -29.51V.

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I will unmount the amp PBB´s today and check the solder joints on the chips but failing that I could use some advice on what to look for.
 
S

syklab

Member
Joined 2009
ecosprog said:
Just finished putting together one of Peter´s LM3875 premium kits. I powered it up last night and got DC offsets which looked OK but when I connected it to a source and speakers I have less output from the right channel than the left.

The offsets are, left channel @ -66.2mV, and right channel @ between -43 to -46mV. The right channel is constantly fluctuating while the left channel is unchanging. These values are without load. The voltages on the regulator board are +29.51V and -29.51V.

awupeu.jpg



I will unmount the amp PBB´s today and check the solder joints on the chips but failing that I could use some advice on what to look for.
Click to expand...

Hi,
What is that circuit you have between the power inlet and the transformer?
 
E

ecosprog

Member
Joined 2010
Peter Daniel said:
What source are you using?

Does the offset change without source connected?
Click to expand...

I pulled the boards yesterday and checked all the solder joints. Re-soldered some, including the chips and the offsets are now better with the right channel now being stable.

The sound has improved and I spent last night listening to the amp. Very nice sound with no hum, or pop when turned on/off. The only slight problem is there is still a slight difference in channel output. The right channel is slightly down on the left.

Today I will take the boards out again and remove all the resistors and re-solder them. I had used silver solder when I put these together and had a lot of trouble getting the solder to flow to my satisfaction. I will redo the joints with standard solder and that should give better connections.

Once I have rebuilt things I will report back.
 
A

AndrewT

R.I.P.
Joined 2004
ecosprog said:
I will redo the joints with standard solder and that should give better connections.
Click to expand...
If you are still learning to solder, then I strongly suggest you use a leaded solder. In particular find a Eutectic 63/37 tin/lead solder with a good flux for electronics work.


Do not use 60/40, or worse 40/60 (plumbers solder).
Do not use any of the higher temperature solders.
Do not use any of the lead free solders.
 
E

ecosprog

Member
Joined 2010
AndrewT said:
If you are still learning to solder, then I strongly suggest you use a leaded solder. In particular find a Eutectic 63/37 tin/lead solder with a good flux for electronics work.


Do not use 60/40, or worse 40/60 (plumbers solder).
Do not use any of the higher temperature solders.
Do not use any of the lead free solders.
Click to expand...

Thanks Andrew for the advice. I should point out that I´m not new to soldering, lots of experience. Problem was that I have a new soldering station, and the tip isn´t one I´m comfortable with. It took a bit of getting use to. I have re-done everything with Eutectic 63/37 instead of the Mundorf silver solder and the joints are all now looking good.

Now I just need to put the boards back and we´ll see how things work.
 
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