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View Poll Results: Was a good idea to use two lm1875 or a big stk chip
Use stk 8 27.59%
use lm1875 21 72.41%
Voters: 29. You may not vote on this poll

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Old 10th January 2013, 11:03 AM   #71
PetruV is online now PetruV  Romania
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Quote:
Originally Posted by blu_glo View Post
55K instead of 50K is not a problem just changes gain slightly.
- what value is R3?
100k
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Old 10th January 2013, 11:31 AM   #72
blu_glo is offline blu_glo  England
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For least DC offset on the output R3 should be the same value as the feedback resistor in your case 55K. The datasheet suggests a 22K resistor in this position on P.2 for the dual supply application. So 100K may be a bit high.

http://www.ti.com/lit/ds/symlink/lm1875.pdf

Last edited by blu_glo; 10th January 2013 at 11:34 AM.
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Old 10th January 2013, 12:01 PM   #73
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Can you show me a pic of your power supply and wires? - The actual project as you have it on your bench, not the schematic!
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Old 10th January 2013, 01:55 PM   #74
PetruV is online now PetruV  Romania
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Quote:
Originally Posted by blu_glo View Post
For least DC offset on the output R3 should be the same value as the feedback resistor in your case 55K. The datasheet suggests a 22K resistor in this position on P.2 for the dual supply application. So 100K may be a bit high.

http://www.ti.com/lit/ds/symlink/lm1875.pdf
Ok i thought it for something else but i will ad another 33k+22k resistor
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Old 10th January 2013, 01:57 PM   #75
PetruV is online now PetruV  Romania
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Quote:
Originally Posted by blu_glo View Post
Can you show me a pic of your power supply and wires? - The actual project as you have it on your bench, not the schematic!
Sure!
As for psu just an computer psu with 12 0 -12v outs usen on amp
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Old 10th January 2013, 02:46 PM   #76
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Is it too difficult to find a 15+15V ( 30 V center tapped)50/60 W transformer ?
You said that you have a 18+18 V one...is it a too high voltage for the application?
When I told you not to mix Pc & audio things , there are some reasons why:
a clean linear supply....is a clean linear supply !!
When you have it done, you know that if something's not going to work, it's not
its fault.
Second, what is written on the decal on the PC psu ? + 12V @ how many amperes max ?
and what about the -12 V line ? is it the same ? I guess no
and EKKKK! what and where are those ceramic caps ?
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Old 10th January 2013, 02:55 PM   #77
PetruV is online now PetruV  Romania
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Quote:
Originally Posted by picowallspeaker View Post
Is it too difficult to find a 15+15V ( 30 V center tapped)50/60 W transformer ?
You said that you have a 18+18 V one...is it a too high voltage for the application?
When I told you not to mix Pc & audio things , there are some reasons why:
a clean linear supply....is a clean linear supply !!
When you have it done, you know that if something's not going to work, it's not
its fault.
Second, what is written on the decal on the PC psu ? + 12V @ how many amperes max ?
and what about the -12 V line ? is it the same ? I guess no
and EKKKK! what and where are those ceramic caps ?
Its too high voltage for testing and could damage something,but once the amp is out of beta testing i will use it,the psu is 50a 12v and just 0.8a for the minus voltage so that might be a problem
And yes it didicult to find one,i thried so hard for 3 years to find one for a tda7294 and they were very expesive,util recently my mom accidentaly therw out the tda7294

Last edited by PetruV; 10th January 2013 at 02:57 PM.
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Old 10th January 2013, 03:13 PM   #78
PetruV is online now PetruV  Romania
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Tried it and there is no buzz when i touch the input,otherwise there is pop on on/off and the chip gets quite but not very hot
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Old 10th January 2013, 04:07 PM   #79
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Quote:
Originally Posted by PetruV View Post
Its too high voltage for testing and could damage something,but once the amp is out of beta testing i will use it,the psu is 50a 12v and just 0.8a for the minus voltage so that might be a problem
And yes it didicult to find one,i thried so hard for 3 years to find one for a tda7294 and they were very expesive,util recently my mom accidentaly therw out the tda7294
The max transformer voltage I'd recommend is 22-0-22Vac too much more and you'll risk going over the max. You'll need a bridge rectifier rated 5A 200PIV and a couple of off-board electrolytics at least 4700uF and at 35V or more. You should also tidy up the wiring on that board. You should also have a couple of 100uF (gain 35V) on the board wired in close to the IC pins, which, I assume, is also where your ceramic disk ones are. The ceramics are fine for supply rail decoupling so no problems there. You should follow the datasheet I posted earlier as I notice other components missing.
And your power supply is OK for testing but not so to use in anger!

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Old 10th January 2013, 04:26 PM   #80
PetruV is online now PetruV  Romania
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Quote:
Originally Posted by blu_glo View Post
The max transformer voltage I'd recommend is 22-0-22Vac too much more and you'll risk going over the max. You'll need a bridge rectifier rated 5A 200PIV and a couple of off-board electrolytics at least 4700uF and at 35V or more. You should also tidy up the wiring on that board. You should also have a couple of 100uF (gain 35V) on the board wired in close to the IC pins, which, I assume, is also where your ceramic disk ones are. The ceramics are fine for supply rail decoupling so no problems there. You should follow the datasheet I posted earlier as I notice other components missing.
And your power supply is OK for testing but not so to use in anger!

Thanks to you it now works ,im so happy i will write your name on the pcb so everybody who takes a peek in the casing will see your name and ask why is something written on the pcb,i will say to threm that you helped me the most
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