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Commercial Gainclone kit- building instructions

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#202
Your readings are not reversed, the offset is just lower. I used 50K pot you are using 25K that one reason it's lower. The offset is almost always lower with volume down.

Also, those values are quite typical and that the amp reacts for volume change may indicate that both chips work fine. It's just connection of the signal is somehow lost, check also power supply connetion.

BTW, check this post for offset measured for a batch of chips: http://www.diyaudio.com/forums/showthread.php?postid=1524877#post1524877
 
#203
Peter Daniel said:
Your readings are not reversed, the offset is just lower. I used 50K pot you are using 25K that one reason it's lower. The offset is almost always lower with volume down.

Also, those values are quite typical and that the amp reacts for volume change may indicate that both chips work fine. It's just connection of the signal is somehow lost, check also power supply connetion.

BTW, check this post for offset measured for a batch of chips: http://www.diyaudio.com/forums/showthread.php?postid=1524877#post1524877
Click to expand...


Thanks for your reply.

That's what I figured after I posted, so I edited it out to not look stupid. haha.

I will check everything again, thanks!
 
#204
found the problem!

seems I put too much heat on the right channel #2 pin on the nobel pot, and melted it inside.

edit: I was able to fix it by mashing some solder wick on the plate, but I'm getting 1.8 ohms of resistance.

I guess I need a new pot. What would you recommend? 50k with a shunt? I have a spare 100k noble, can I use that?
 
#206
Just finished making a LM1875 amp following Peter's instructions.
DC offset is about 10mV each channel, no hum and is sounding pretty good.. Great enjoyment😀 .Just need to find a nice case.
 

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#208
Master Piece

Superb Tutorial Peter !
A lot Of new things found here

This is out of my expectation
All you give to forum is your commercial amp
Wonderful !

Maybe you will invention a new Class A Amp ?
Who was take the picture when you soldering ?

A great respect from your friend

regards, jeffry
 
#209
Dear Casiomax

casiomax said:
Torroidal transformer are not available in my country, have search but come to no avail.

A standard transformer with center-tap would be the next choice of mine, how much amp does it need to supply just a mono amp? i wanted to build for my guitar amp. would a 5A with 22v-0v-22v transformer adequate?
Click to expand...

You can find standart transformer in Glodok Harco
But hear that hum in transformer

For Better option, order from our neighbour country
http://singapore.rs-online.com/web/...&searchTerm=nuvotem+300va+2x0-25v+sec&x=0&y=0

email me if you need more information
 
#210
Peter,

I’ve had a quick search, and can’t find an answer to my query. I have built an Audiosector LM3875 kit in a dual mono configuration with a separate passive pre amp. I have used 2 identical 120VA, 18-0-18v torroidal transformers. The amp is used to drive 8ohm speakers.

Due to space limitations (new furniture), I have no room for the pre. I now need to turn the amp into a stereo integrated amp. To save me buying another transformer, could I get away with either using 1x 120VA transformer (low power I know, but I don’t listen loud anyway), or could I safely parallel the 2 transformers to give more power? What are the grounding options?
 
#211
zebra100 said:
To save me buying another transformer, could I get away with either using 1x 120VA transformer (low power I know, but I don’t listen loud anyway), or could I safely parallel the 2 transformers to give more power? What are the grounding options?
Click to expand...

I understand you want to convert dual mono amp into stereo unit with a potentiometer at the input, hence the integraterd version.

If you connect two transformers in parallel, you will be getting more power only into low impedance loads, when single transformer couldn't before provide more power.

Otherwise there is not much gain in using transformers in parallel, it may actually degrade the sound.

If one 120VA transformer worked fine with a single channel, it might as well work fine with both channels.

Besides, if you are considering using 2 transformers in parallel, why not use them as previously, one per channel?
 
#216
Transformers (UK & Europe)

As per Peter's recommendation for Polish transformers I bought two 22-0-22 300va transformers from Toroidy (http://www.toroidy.pl/index.php?option=com_content&task=view&id=6&Itemid=13) with which I have been very happy. I am just about to order some 25-0-25 300va from them. They are not cheap but they are well made - please see for both the details of the transformers (in Polish!!) and Peter's use of them - http://www.diyaudio.com/forums/showthread.php?s=&postid=930263&highlight=#post930263
 
#217
I've been reading around and it seems people recommend increased capacitance on the power rails. I've got some 2200uf caps that i was thinking of trying in the LM4780 kit, would this have any negative effect ?

also how do i rig the inputs for bridge mode ? i.e 2 chips in bridged mode left and right. The diagram provided with the kit shows two inputs for bridge mode. I'm abit confused.

cheers
 
#218
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