Amplifier for speakers

[pball_inuyasha]

I would like to build an amp for some old stereo speakers I have. I have tried looking up different amp schematics but don't know what kind to use. If I could get some simple direction toward a particular amp type or schematic that would be of great help.

I would be using this to amplify a computer or mp3 player source for use with un-amplified speakers. I don't know how many watts those output but a 20 or more watt amp would be good. I have a 24 V .5 A power supply laying around I could use. So if anyone has any good advice that would help.

I have experience soldering and working with electronics, just no experience in making circuits and really doing more than assembling them.

 

[Mikebits]

I have a 24 V .5 A power supply laying around I could use. So if anyone has any good advice that would help.

That supply is too small for 20W.

 

[Hero999]

That PSU can only output 12W so largest output your amplifier can give is just 8W.

 

[audioguru]

The little 24VAC transformer will make a rectified and filtered 32VDC. A single power amp with a 32V supply will have a max output of about 28V p-p. 28V p-p is 12.3V RMS which is 18.9W in an 8 ohm speaker.

Get a much more powerful transformer.

 

[rjvh]

8 till 10 Watt is already a lot of noise

i asume that you dont run a disco so for home use it would already be fine

Robert-Jan

 

[Speakerguy]

Use an LM1875. And get a beefier power supply.

 

[Mikebits]

Go for a real man supply like this one.
**broken link removed**

Intended as humor only

 

[Mikebits]

Just to give you an idea of a part you might use (Available at Digi-key)
**broken link removed**

Guru or someone else can probably suggest a better part. I don't do much audio.

 

[pball_inuyasha]

Ok disregard any thing related to output power I said. I am not familiar with how that is calculated and amplifiers in general.

I don't really know what to say, If someone could just refer me to a good schematic of a stereo amplifier that can run off of anything 0-20V or so and has some volume control. I could work with that.

I can always get another power supply if needed. I'm just trying to do this as a project and hopefully cheaper than buying something.

Thanks for the input so far.

 

[audioguru]

If someone could just refer me to a good schematic of a stereo amplifier that can run off of anything 0-20V or so and has some volume control.

An amplifier with a supply of only 20VDC will produce a max output of about 17V p-p. 17V p-p is 6V RMS which makes only 4.5W into an 8 ohm speaker. A little louder than a clock radio.

All amplifiers have a volume control.

 

[Speakerguy]

I don't really know what to say, If someone could just refer me to a good schematic of a stereo amplifier that can run off of anything 0-20V or so and has some volume control. I could work with that.

Read the LM1875 data sheet. Everything you need to know (schematic, PCB layout, etc) is in there.

 

[audioguru]

The datasheet for the LM1875 shows that its output power is only about 4.5W into 8 ohms when its supply voltage is only 20V.
That is flea power for people with rice-paper walls.

The LM1875 is designed for a 50V supply. Then its power is 20W for people with cardboard walls.

 

[pball_inuyasha]

ok thanks guys I'll take a look at that then

 

[El Kit]

**broken link removed**

It´s possible to use a lower Voltage supply. The formular for the output power is:

P(out) = Vcc^2 / 8*R

The output voltage of the amplifier always is at least 3V lower then the input. With the formular you get the peak-power output. The continious output is lower because the supply voltage will sink if the current will get higher.

Example:
Transformer 24VAC
Voltage at condenser = Veff * SQR(2) = 24V * 1.41 = 33.8V
Rectifieing needs 1.2V
33.8V - 1.2V = 32.6V
Amplifier needs 3V or more
32.6V - 3V = 29.6V
29.6^2 / (8*8 Ohms) = 13, 7 W maximum peak power @ 8 Ohm
The minimum supply voltage (for continiuos power) will be at minimum 27V.

 

[Hero999]

That formula gives peak power, doesn't account for the voltage drop across the transistors and I don't know where you got 8*R from.

 

[El Kit]

P = U^2 / R
This is the main formular as you shurely know. It uses the AC - output voltage at the speaker.

I just convertet it so you can use the DC voltage supply at the condenser now, but at first you have to subtract the drop voltage of amp because it´s level is not 100%.


(I´m sorry, because it´s still difficult for me to write in english, however I´ll try to explain tomorrow a bit more details)

 

[Hero999]

Here's the formula to calculate the maximum peak power at the speaker:
[latex]P_{PEAK}=\frac{(V_{CC}-V_{DROP})^2}{R}[/latex]

It takes into account the voltage drop across the driver transistors.

 

[audioguru]

"Maximum Peak power" is confusing because it is simply double the RMS power number.

When the volume control of an amplifier is turned up way too high then it -produces "maximum peak power" because the output is extremely distorted square-waves and the usually unwanted harmonics are the same power as the RMS power of the amplifier.

 

[pball_inuyasha]

Ok if we could get back to making an amp. I looked up the specs of a LM1875 since that was suggested earlier. There are two schematics in the specs page. First off which one would be better. Second off I have some questions about the diagram.

I am familiar with to wire up something using a diagram but I am confused about the five different inputs that are used. This chip has the following pins.

+ IN
- IN
- Vee
Output
Vcc

Could I get a simple explanation of what is what. Such as power supply and the audio source.

I know this can use a 16-60 V power supply but I'm unsure of the amp rating that should be used for the power supply.

If I can figure this stuff out I should be much closer to building an amp

 

[audioguru]

The datasheet shows a schematic of the amplifier with a dual supply and another amplifier with a single supply. A pcb layout is shown and it has wiring for either type of amplifier.

The input to the amplifier is Vin and ground.
+VCC is the positive supply.
-VEE is the negative supply if you are ussing it.
Ground is the common of the supply.
Output and ground connect to a speaker.

If you use a 50VDC supply then the max low distortion output power is 25W. Then the input signal level is 673mV which is pretty high. You probably need a preamp to amplify the 150mV from a line-level source.

When the output is 25W then the LM1875 heats with 15W. So the total power from the power supply is 40W. You won't play music or speech at full output all the time so a transformer with a power rating of 30W will be fine. For stereo the transformer should be 60W.

To make 50VDC the transformer needs to be 36.5VAC. For 60W then the current from the transformer will be 1.64A.