lm3875 24v dc at speaker outlets ! ? - diyAudio
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Old 29th July 2005, 07:36 AM   #1
stobsy is offline stobsy  United Kingdom
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Default lm3875 24v dc at speaker outlets ! ?

Hi everyone,
I'm having problems getting my 1st gainclone to work,
I am using the "vikash" non inverted diagram.
( I will try to attach it)

R1 =22k (pin 3 to pin 8)
R2 = 680 ohm ( pin 8)
R3 = 220 ohm ( pin 7 to sig in)
R4 = 22k (bridging + & - on sig input)

I am not using any caps on the power in to the chip, as i have a separate 25v psu, ( built for Naim hi-fi) which is suplying an allready smoothed + & - 25v dc. (going to pin 1 & pin 4)

I have assembled the amp with no cicuit board, paying extra care to make sure that nothing was touching anything it shouldn't,
When I powered it up,
(with some old wharfdale diamonds connected)
I got nothing,
I thought mmm this is a nice quiet amp,
but on touching the + sig in, with my finger, still nothing out of the speaker.
Then I thought, connect the - 25v from "Naim" to the groundstar point of the amp, I got a thump from the speakers, and thought,
great, job sorted,
but i was wrong.
I then realized the speaker cone was extended as far as it would go. I switched it off, disconected the speakers and On closer inspection found 24v to be coming out of the speaker terminals !
I dont think that is supposed to happen,
I have stood and looked at it for hours, and can see nothing wrong.
I have even built the second one up and got the same result.
The chips are of the un-insulated type using mica pads, and sleeved washers between them and the heatsink.
I think the problem might lie in the groundstar thingy, I don't think I fully understand it, but I have the neg from the supply going to the groundstar point, for which i am using the heatsink.
The more I think about this the more wrong it seems.

I hope some kind soul could explain to a novice Gainclone virgin, where I am going wrong.
this is the 1st time i've used a forum,
so I hope somebody see's this.

Many thanks
in anticipation.

Stobsy.
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Old 29th July 2005, 08:18 AM   #2
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Count yourself lucky you still have speakers. It is tremendously stupid to hook real speakers to something you've just built provided you obviously cannot follow a simple wiring diagram.
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Old 29th July 2005, 08:25 AM   #3
Nuuk is offline Nuuk  United Kingdom
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Quote:
Then I thought, connect the - 25v from "Naim" to the groundstar point of the amp,
The power supply for your Gainclone must have a positive voltage rail, a negative voltage rail, and a zero volt rail. The ground points are at zero volts, NOT plus or minus 25 volts!

Double check your wiring and as has been said above, always measure the DC output at the speaker terminals before you even connect a test speaker!
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Old 29th July 2005, 08:35 AM   #4
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Default Re: lm3875 24v dc at speaker outlets ! ?

Quote:
Originally posted by stobsy


I then realized the speaker cone was extended as far as it would go. I switched it off, disconected the speakers and On closer inspection found 24v to be coming out of the speaker terminals !
I dont think that is supposed to happen,
I have stood and looked at it for hours, and can see nothing wrong.

Hi STobsy, to test the amp, get 8Ohms 5W resistors across the amp o/p terminals. measure and make sure the DC offset is not too high (not more than 100mV!!).

Measure to see if the tab of the chip is shorted to chassis ground for your case.
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Old 29th July 2005, 08:36 AM   #5
garbage is offline garbage  Singapore
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Default Re: lm3875 24v dc at speaker outlets ! ?

Quote:
Originally posted by stobsy

Then I thought, connect the - 25v from "Naim" to the groundstar point of the amp
like Nuuk has mentioned, +25 goes to pin 1 and -25 goes to pin 4.

your naim supply should have either 3 or 4 wires, ie, either +25/0/-25 or +25/0 and -25/0.

the 0vdc of the naim supply is the one that connects to the starground.

hope that helps.
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Old 29th July 2005, 12:53 PM   #6
stobsy is offline stobsy  United Kingdom
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Smile thanks

Many thanks everyone,
If I haven't damaged the chips I will have another go at it tonight.
if the chips are damaged, does anyone know where i can get replacements, other than farnell, (they have a min order of 20)
which means ordering stuff I don't need.
Again many thanks to all for constructive advice.

Stobsy
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Old 29th July 2005, 04:36 PM   #7
garbage is offline garbage  Singapore
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hi Stobsy

the chips most likely would still be ok.

i guess it might be easier to buy the chips off someone in your area. maybe just post a want to buy ad in your local hifi forums or here.

btw, i feel the lm3875 is best when run at lower rails. i ran mine on 12vdc SLA batts. downside is that the batts require charging after about 8hrs of playtime. i'm using 93db 6ohm speakers, so that voltage is fine for me.

happy DIYing.
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Old 29th July 2005, 06:23 PM   #8
Nuuk is offline Nuuk  United Kingdom
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Quote:
btw, i feel the lm3875 is best when run at lower rails.
Me too garbage but I use SMPS so no worries about recharging"
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Old 8th August 2005, 03:25 PM   #9
garbage is offline garbage  Singapore
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Quote:
Originally posted by Nuuk

Me too garbage but I use SMPS so no worries about recharging"
Nuuk,

just read your website about your SMPS adventures with the GC.
i am in the proess of building an SMPS supply with the LM2588 chip for my VSPS phono, to be powered by one 12v SLA batt.

read that you used up to 4 modules to get +/-24vdc. you might be interested in the LM2588 chip that can be used to do a 24vdc output. you'd need 2 chips, one for + and the other for -.

this is the schematic that i simulated for my 10.8 - 13.8vdc input to give 16vdc output for my VSPS:
Click the image to open in full size.

it has a Vout p-p ripple of 32.94mV. quite respectable figure.

cheers
garbage
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Old 8th August 2005, 04:36 PM   #10
Nuuk is offline Nuuk  United Kingdom
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Sheeshkebabs Garbage - I hadn't thought of making my own SMPS!

But I'm a bit confused. Do you power the SMPS from the battery? If so what is the point? Why not run the phono stage from the battery?
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