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Speaker amplifier

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DSGarcia

New Member
I have a small single-board computer that has 200mW audio outputs to drive 8 ohm speakers. I would like to build an amplifier circuit to drive 15 watt 8 ohm speakers with the SBC audio outputs as input to the amplifier circuit.

Specifications:
I would like it to run on 5 volts if possible (but I can go as high as 10VDC)

It will be used in an industrial environment so the quality does not have to be very good--no better than a cheap notebook.

Because these will be external speakers, I would also like to know how to add short-circuit protection to the circuit.

Thanks,
Dale
 

DSGarcia

New Member
I found some audio amplifier devices that will do most everything I need on the Maxim-ic.com web site--MAX9704, MAX9708, & MAX9709 are the devices of interest.

The remaining problem is how do I convert the 200 mA 8 ohm speaker signal from the SBC into a signal the audio amplifier is expecting. I really don't need the signal amplified; all I really need is the current amplified.

Thanks,
Dale
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Just an attenuator is required (two resistors), to drop the voltage to be suitable for the amplifier. However, you going to need a LOT more than a 10V supply to get 15W in 8 ohms - 12V will only give you about 2W.
 

DSGarcia

New Member
Nigel,
Thanks. I still have to download and check the datasheets to see what I will need. There are a couple of things I can do. I actually have a 24 VDC supply, but wanted to allow the circuit to run on 13 VDC from a car battery. I can also purchase 4 ohm speakers if necessary. I happen to have 5VDC handy from an existing regulator.

Thanks,
Dale
 

Nigel Goodwin

Super Moderator
Most Helpful Member
With a car 12V (nominally 13.8V) supply you can get only 4W in to 4 ohms, or 16W in to 4 ohms if bridged - for more than that you need either speaker transformers (rare and expensive) or a switchmode power supply to generate a higher supply rail.
 

Speakerguy

Active Member
Is cost any issue? A DC/DC converter is going to be your best bet. TI makes some nice ones for $15, Astrodyne makes some real nice ones for more.
 

DSGarcia

New Member
The Maxim datasheets all claim that the parts will run from a 10 VDC supply. For example, the MAX9704 (the 10 watt stereo part) will run from 10-24 VDC. Oddly, the higher output wattage part, the MAX9708 (21 watts per channel at 8 ohms) has a maximum of 18VDC for the supply.

Assuming this is correct, I can run the 10W MAX9704 part directly from my input voltage of 12-24VDC. The others, I would use a 12 volt regulator because of the upper limit is lower than 24 VDC.

Is there much perceived difference ("loudness") between 10, 15, and 20 watts? Other than the audio amplifier, is there any difference between an 8 ohm 15 watt speaker and a 4 ohm 15 watt speaker?

Thanks,
Dale
 

Speakerguy

Active Member
20 watts will be 3 dB louder than 10 watts. Minor difference really.

SPL diff = 10 log (p1/p2)

As to the next question, you need to know the efficiency of the speaker (in db/W). The watt rating of the speaker gives no indication of how loud it will be, just how much power the voice coil can dissipate as heat.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
DSGarcia said:
Nigel,
I forgot to ask....what value resistors and are they used as a voltage divider?

Yes, an attenuator is simply a voltage divider - the values depend on the reduction you require, and the relative impedances. Probably the easiest solution if to use a potentiometer and adjust it until it's how you want.
 

stevez

Active Member
Regarding short circuit protection - a fuse might be a good choice. Some ICs are internally protected and will shut down.
 

DSGarcia

New Member
Nigel Goodwin said:
Yes, an attenuator is simply a voltage divider - the values depend on the reduction you require, and the relative impedances. Probably the easiest solution if to use a potentiometer and adjust it until it's how you want.

Thanks everyone for the information. At this point, I still need to pin down the input signal problem and would like to address Nigel's comment.

I think that the best fit for my application is the MAX9708 which will output 21 watts per channel at 8 ohms. The speakers are rated at 15 watts with a 25 watt maximum.

The gain is selectable at +22, +25, +29.5, and +36dB. (The MAX9704--10 watts--gain is also selectable, but the only value in common with the MAX9708 is 29.6dB of gain).

Because these are surface mount parts and I have no skills in building surface mount prototype circuits, I would really rather not have a trim pot that needs to be adjusted on every board. (I will have the PCB manufacturer stuff the pilot production boards.) Is there a way to calculate the value of the resistors that should be used?

Thanks,
Dale
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I didn't mean permanently add a pot, use one during your testing phase and adjust it to what you want, then measure the values of the two halves - replace by similar fixed values, or at least ones in the same ratio.

So what supply are you using for 21W?. You're looking about 40V single ended, or 20V bridged.
 

DSGarcia

New Member
Nigel Goodwin said:
I didn't mean permanently add a pot, use one during your testing phase and adjust it to what you want, then measure the values of the two halves - replace by similar fixed values, or at least ones in the same ratio.

So what supply are you using for 21W?. You're looking about 40V single ended, or 20V bridged.


Nigel,
I will be using 10VDC for the circuit (to allow me a system supply voltage of 12-24VDC). The Maxim parts have a charge pump to increase the voltage internally with very few external support parts.

I suppose I can place pads (in parallel) for a surface mount pot as well as the fixed resistors on my circuit board.

What value pot should I use and is there anything special I should do to make the SBC 8 ohm 200mW audio output happy?

Thanks,
Dale
 

Nigel Goodwin

Super Moderator
Most Helpful Member
DSGarcia said:
Nigel,
I will be using 10VDC for the circuit (to allow me a system supply voltage of 12-24VDC). The Maxim parts have a charge pump to increase the voltage internally with very few external support parts.

Right, got you!.

I suppose I can place pads (in parallel) for a surface mount pot as well as the fixed resistors on my circuit board.

Just solder wires to a pot, and temporarily solder the wires to the board.

What value pot should I use and is there anything special I should do to make the SBC 8 ohm 200mW audio output happy?

A 4K7 or 10K should be fine, and you don't need to do anything special for the audio output.

If you want to work out the total gain you need, simply calculate the voltage level for 200mW at 8 ohm, and for 21W at 8 ohm - this then easily gives you the total gain required - you can then calculate the attenuator to reduce the signal to match the gain of the new power amp.
 

DSGarcia

New Member
Nigel Goodwin said:
If you want to work out the total gain you need, simply calculate the voltage level for 200mW at 8 ohm, and for 21W at 8 ohm - this then easily gives you the total gain required - you can then calculate the attenuator to reduce the signal to match the gain of the new power amp.

Nigel,
Thanks for the information. However, I am not sure how to calculate gain and somewhere in the back of my mind, I think that with audio circuits and speakers, calculating voltage level is not as simple as applying ohms law.
Thanks,
Dale
 

audioguru

Well-Known Member
Most Helpful Member
The MAX9708 has an output power per channel of 21W into an 8 ohm speaker when it has an 18V supply and when it is turned up too loud so the 10% distortion sounds terrible.

With only 10V for its supply the power per channel into an 8 ohm speaker is only 6W at clipping.

The output power is nearly doubled when its two channels are paralleled and drive a 4 ohm speaker.

It is tiny and has 32 pins. How are you going to solder it?
 

DSGarcia

New Member
audioguru said:
The MAX9708 has an output power per channel of 21W into an 8 ohm speaker when it has an 18V supply and when it is turned up too loud so the 10% distortion sounds terrible.

With only 10V for its supply the power per channel into an 8 ohm speaker is only 6W at clipping.

The output power is nearly doubled when its two channels are paralleled and drive a 4 ohm speaker.

It is tiny and has 32 pins. How are you going to solder it?

AudioGuru,
Thanks for the info. I went to the datasheets and noticed the output power vs input voltage chart. Normally I will have 24VDC, but sometimes I will have only 12VDC. I had been planning on using a 10 volt regulator which would have handled both cases until you mentioned the reduced output. Is there a circuit I can use to clip the voltage at 18 VDC when my input voltage is above 18 VDC, but supply the input voltage when the voltage is between 12 and 18 (and still provide the protections that a regulator does)? I suppose I could use the 9708 part (10 watts per channel) because it will take the full 24 VDC.

I can live with the distortion at the higher output levels. The speakers are rated for 15 watts each and if they turn it up full blast, let them live with the distortion. This is for an industrial application for windows event sounds, recorded voice messages, etc. and not for music or anything that requires much quality.

As far as soldering, I will have the PCB manufacturer stuff my boards.

Thanks,
Dale
 
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