I was reading the datasheet for the LM1875
and I noticed that they do have a schematic for a single supply (as opposed to a +/- supply).
If running on a single supply, does the chip amplifer consume significant power during quiet tracks?
Would this be pretty much the same as running the amplifier in Class A mode (vs. Class AB)?
I'm trying to figure out if I really need a +/- power supply, or if I can just go with a single supply (easier to build).
Thanks,
Michael
The datasheet does have a "Typical Single Supply Operation" schematic. Have another look.
Yes, that's what I said. There is a schematic. It's on Page 2. (amused grin)
Michael
Oops! I misread your post.
You shouldn't have any problem with a single supply, but be sure to follow the guidelines on heat sinking. It appears the amp operates in Class B and will idle at about 6W of heat dissipation.
-- Look at "Supply Current" on page 2 and read: "Power Dissipation and Heat Sinking" on page 6. JF
Ah, so apparently the chip does not use more power with a single supply than with a +/- supply.
Is there any reason why I should NOT use a single supply with this chip?
Thanks,
Michael
-- I don\'t think so, but try it and see if it works the way you want it to. If it does you\'ve saved a supply, but if it doesn\'t it\'s only cost you a handful of discretes. JF
Good point. Thanks.
Michael
I would like to point out that the single supply schematic has a 2200 uF capacitor in series with the output speaker. In theory, with an ideal capacitor, this is going to give an 18 Hz cutoff frequency into a 4 ohm speaker (36 Hz into 8 ohms). A real capacitor will have some internal resistance. This is going to give a higher cutoff frequency.
Since the LM1875 is inexpensive, you might try using two of them on a single supply, and putting the speaker between the outputs. You would have to make one amp inverting and the other non-inverting, but you will get twice the output voltage and save the capacitors. A single 24 volt supply will then give +/- 24 volts, or about 17 VRMS, to an 8 ohm speaker, or 36 watts. There are lots of 24 VDC supplies available. The downside of this approach is making sure the DC level of both amps are equal and about 1/2 the supply.
If you check SemiElectric's eBay store you will find PC boards and kits for the LM3875, as well as other single and dual amplifier ICs:
His LM3875s are about $6.50 each. They are larger (56 watts vs 20 watts), but seem to be otherwise quite similar. Most of the additional IC pins are NCs, for heat sinking and mechanical mounting, I suppose. They are only $5.90 each from DigiKey, and the LM1875 is $3.28. Wow: 10 to 15 cents per watt!
Paul
That's a good idea. Thanks.
Michael
Higher? I would have thunk lower.
Bye. Jasen
Yes. My mistake, the series resistance of the capacitor is going to move the break frequency lower. However the series resistance of the capacitor is going to cut the overall gain of the circuit since it will form a voltage divider with the speaker impedance.
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