Amp Source current

[3lectrokid]

Hey,

I have brought some **broken link removed**'s and i am going to use 6 for a 5.1 speaker system. I read the datasheet but could not work out how much total current current will be used if the transformer is at 30v output which is regulated at 24v DC.

Also i need to work out how big my heatsinks will need to be if i have an 8Ω speaker at 24v single supply in and the ambient temp will be 25 deg c. i dont undertand the datasheet much

 

[MikeMl]

The power dissipation in the regulators is a function of the loads (presumably some sort of audio amplifier chip) which the regulator feeds. If that is the load, the current will low at zero or low volume, and will increase as the volume is turned up. You will have to measure the current being drawn by the amplifier. Temporarily feed it from an adjustable lab power supply set to 24V, and check the current draw at various listening volumes. When you know that, you will be in a position to design the heatsink requirements for the regulator(s).

 

[crutschow]

From the data sheet "Power Output vs Supply Voltage" graph, your maximum speaker power output for a 24V DC single supply would be about 5W (using the 12V dual supply voltage point on the graph).

(Is 5W per speaker sufficient for your purposes?)

From the "Power Dissipation vs Power Output" graph for an 8Ω load, the maximum chip power dissipation would also be about 5W. Thus your heatsink would need to dissipate 5W maximum per chip. From the "Device Dissipation vs Ambient Temperature" it looks like a 10C/W heatsink per chip would be more than sufficient.

The current draw from the 24V DC supply would be 10W/24V = 0.4A per amplifier maximum. But under normal listening levels is likely significantly less.

 

[3lectrokid]

looking at the graph i though that i would look at 24v and find that it runs on 20w

 

[audioguru]

A sub-woofer will be useless with only 5W driving it.
The entire 5.1 amplifier will be almost useless with only 5W for each speaker.

Power amplifiers don't need the supply voltage to be regulated. With a 30V supply, the output power is about 9W per amplifier into 8 ohms.

 

[audioguru]

The output of the LM1875 is 20W into 8 ohms when it has a supply that is plus and minus 17.5V (35V total).

 

[3lectrokid]

ok i get that now. so if i got a +35v transformer what heatsinnk would i require

 

[crutschow]

The output of the LM1875 is 20W into 8 ohms when it has a supply that is plus and minus 17.5V (35V total).

According to the "Power Output vs Supply Voltage" graph in the LM1875 data sheet, it requires a supply voltage of about ±22V for 20W output @ 1% distortion. I assume your power value is for the output at clipping with zero amplifier saturation voltage?

 

[3lectrokid]

sorry i do not understand what "zero amplifier saturation voltage" is. wold u mind explaining

 

[crutschow]

sorry i do not understand what "zero amplifier saturation voltage" is. wold u mind explaining

Zero saturation voltage would mean that the amplifier can drive the speaker to the maximum supply voltage with no voltage drop in the amplifier. In reality there usually is a minimum voltage drop in the amplifier so the maximum speaker voltage will be a volt or two less than the supply voltage.

 

[audioguru]

A 35VAC transformer produces rectified and filtered 47.5VDC. Each amplifier will have an output of 24W into 8 ohms.
If you want 35VDC then use a 26VAC transformer.

 

[audioguru]

According to the "Power Output vs Supply Voltage" graph in the LM1875 data sheet, it requires a supply voltage of about ±22V for 20W output @ 1% distortion. I assume your power value is for the output at clipping with zero amplifier saturation voltage?

Sorry, I mixed up the numbers. 20W into 8 ohms is 35.8V p-p so the loss in the amplifier is 8.2V.

 

[crutschow]

looking at the graph i though that i would look at 24v and find that it runs on 20w

If you look closely at the bottom graph label it states (±V) which means a dual-supply with each supply being equal to that voltage (or a single supply of double that voltage).

 

[3lectrokid]

ok so if i got a 35 volt transformer what voltamperage on the transformer would i need to power 6 LM1875T's

 

[3lectrokid]

If you look closely at the bottom graph label it states (±V) which means a dual-supply with each supply being equal to that voltage (or a single supply of double that voltage).

Ahh. ok i have seen that symbol before but have never understood what it meant. thanks

 

[crutschow]

Assuming a nominal listening level of 10W per speaker, the speaker current would be √(10/8) = 1.12A rms. Adjusting for a single supply instead of a dual gives a power supply current of 0.56A per amp. The total power supply current for 6 amplifiers is thus 3.35A .

But as Audioguru noted you need more power for a good subwoofer. I would suggest at least 50W with 100W and up being more typical.

 

[audioguru]

ok so if i got a 35 volt transformer what voltamperage on the transformer would i need to power 6 LM1875T's

If the filter capacitor is big enough then the supply will be 47.5VDC. The output power from each amplifier will be 24W into 8 ohms and the heating of each amplifier is 18W. So the supply power into each amplifier is 42W. Six will need a total supply of 252W. You can cheat a little because all 6 amplifiers will not play at full blast at the same time continuously. Maybe 200W so the transformer will be 35V at 5.7A.

 

[3lectrokid]

so if 1 watt = 1 volt amp should it be this:

5.7*35=199 watts
199 watts = 199 volt amps.
so is this the size i would need for my transformer. what happens if i do say bring more than 199volt amps through the transformer. will the transformer overheat and the coils start melting and cause a fire

 

[mneary]

The transformer would overheat if you exceed its rating. But as audioguru said, you're unlikely to ever use more than 200VA. Running music at an average anywhere near the amp's rating will be clipping the peaks and will sound really horrible.

Even then, you're even less likely to use the maximum power for an extended period of time (thermal time constant of a 200VA transformer is in the tens of minutes).

A fuse or breaker is an absolute must, regardless of how close to the ratings you are running.