Hi
I plant to drive my 4Ohm subs with LM3875 non-inv Gainclone. I heard that if I parallel 2 of them through 0.1R resistors on the outputs, they will be happier (less stress)
Would using two in parallel degrade the sound quality? Also would 18-0-18 VAC 2A trafos be sufficient (1 monoblock would have: 1 such trafo, 2 chips in parallel, and will be feeding 1 4Ohm sub, just to be clear)
Thanks
Alex
I plant to drive my 4Ohm subs with LM3875 non-inv Gainclone. I heard that if I parallel 2 of them through 0.1R resistors on the outputs, they will be happier (less stress)
Would using two in parallel degrade the sound quality? Also would 18-0-18 VAC 2A trafos be sufficient (1 monoblock would have: 1 such trafo, 2 chips in parallel, and will be feeding 1 4Ohm sub, just to be clear)
Thanks
Alex
Your 18-0-18 transformer will give you DC rails of about +- 24 V to -+25 V after rectification and filtering.
Look at the graph "Output Power vs. Supply Voltage" in the datasheet. According to that, the LM3875 will deliver about 45 W into 4 ohms at +-25 V. According to Ohm's law, the amount of current required to get 45 W (RMS) out of a 4 ohm load is about 3.3 Amp (RMS). So your transformer may be a little weak. Your power supply will not be able to deliver the increased current that the amp is now capable of. Note that the graph indicates that one 3875 will deliver about 30 W into 8 ohms at +25 V, so two of them in parallel should deliver about 60 W into 4 ohms, but only if the power supply can keep up. Your probably will not, but you can certainly experiement. You will probably see very little if any benefit from paralleling two 3875's with your trafo's.
If you had bigger (higher VA) trafo's giving, say, +-35 to +-40 V supplies, then I would recommend a parallel configuration. But one 3875 is perfectly happy driving 4 ohms with +-25 V supplies (the 3886 is even better with 4 ohms, BTW!).
Look at the graph "Output Power vs. Supply Voltage" in the datasheet. According to that, the LM3875 will deliver about 45 W into 4 ohms at +-25 V. According to Ohm's law, the amount of current required to get 45 W (RMS) out of a 4 ohm load is about 3.3 Amp (RMS). So your transformer may be a little weak. Your power supply will not be able to deliver the increased current that the amp is now capable of. Note that the graph indicates that one 3875 will deliver about 30 W into 8 ohms at +25 V, so two of them in parallel should deliver about 60 W into 4 ohms, but only if the power supply can keep up. Your probably will not, but you can certainly experiement. You will probably see very little if any benefit from paralleling two 3875's with your trafo's.
If you had bigger (higher VA) trafo's giving, say, +-35 to +-40 V supplies, then I would recommend a parallel configuration. But one 3875 is perfectly happy driving 4 ohms with +-25 V supplies (the 3886 is even better with 4 ohms, BTW!).
If you have excel I would recommend searching for and then downloading (or just running in IE) the Overture_Design_Guide13.xls it’s put out by National and it will help you calculate what kind of power you can get with what voltages. And a buch of other things too (gain etc).
You can also add up the output wattage, and the heat wattage (add this twice if using 2 chips), and come up with a close number for power wattage, or amps required. (Just add up watts, and divide by voltage)
The design guide shows that with 25V -/+ you will have about 47.29W into a 4ohm load and that it could use up to about 3amps, and 89.91W parallel would use about 6amps, and 94.58W bridged about that same 6amps.
Hope that helps.
You can also add up the output wattage, and the heat wattage (add this twice if using 2 chips), and come up with a close number for power wattage, or amps required. (Just add up watts, and divide by voltage)
The design guide shows that with 25V -/+ you will have about 47.29W into a 4ohm load and that it could use up to about 3amps, and 89.91W parallel would use about 6amps, and 94.58W bridged about that same 6amps.
Hope that helps.
Static
Just make sure to have as low as possible DC-offset on each amp before you parallel them. Otherwise they can get really hot.
(or make the output resistor bigger, but since you are driving a 4 ohm load I don't think it is a good idear)
Have fun
Thomas
Just make sure to have as low as possible DC-offset on each amp before you parallel them. Otherwise they can get really hot.
(or make the output resistor bigger, but since you are driving a 4 ohm load I don't think it is a good idear)
Have fun
Thomas
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