Amplifier/speaker question

[DSGarcia]

I am still working on my speaker amplifier problem, but this time my question is regarding output power.

I have selected Visaton K50WP speakers (because they are waterproof) for my application rated 8 ohms, 2 watts, 3 watts maximum power. They also have a 50 ohm version of the speaker available.

For the amplifier, so far, I have selected a Maxim MAX9714 which as a 6 watt output (the closest chip with desired output power and differential inputs). Therefore, I need to somehow limit the output power to protect the speakers. I noticed on the datasheet that a 50 ohm load produces an output in the 2 to 3 watt range. Would my solution be to use the 50 ohm speakers or would that cause problems?

Sound quality is not very important, just want it to be as loud and clear as possible with the limit of using 2" waterproof speakers.
Thanks,
Dale

 

[Nigel Goodwin]

You could simply place a resistor in series with the speaker to limit the power to it.

Why do you require differential inputs on the power amp?.

 

[DSGarcia]

You could simply place a resistor in series with the speaker to limit the power to it.

Why do you require differential inputs on the power amp?.

Nigel,
Would the 50 ohm speakers not work?

I need differential inputs on the amp because the audio outputs from the single-board computer I am using are differential. I plan on using a voltage divider on the SBC speaker outputs to drive the inputs to the amplifier. The SBC speaker outputs only supply 400mW of power.

As Kirk always said: "Scotty, you've got to give me more power."

Thanks,
Dale

 

[Nigel Goodwin]

Nigel,
Would the 50 ohm speakers not work?

Yes, but it won't be very loud - what impedance is the amplifier rated for?.

I need differential inputs on the amp because the audio outputs from the single-board computer I am using are differential. I plan on using a voltage divider on the SBC speaker outputs to drive the inputs to the amplifier. The SBC speaker outputs only supply 400mW of power.

So why do you need a differential input?, it's not required at all - presumably it's a bridged output?, just choose one of the outputs to use as a single ended one.

However, if it's already 400mW, a 50 ohm speaker probably isn't going to provide much more?.

As Kirk always said: "Scotty, you've got to give me more power."

 

[audioguru]

The amp produces 5W into 8 ohms at clipping and about 2.2W into 50 ohms, when it has a 15V supply.

Are you going to run the speakers with a tone continuously at full output?
The 8 ohm speaker will be fine with music or speech if the volume is set for occasional clipping.

It is a crappy little speaker.

 

[DSGarcia]

Nigel and AudioGuru,
The datasheet https://www.electro-tech-online.com/custompdfs/2007/06/MAX9713-MAX9714-2.pdf shows Pout of 6 watts for 8 ohms and 8 watts for 16 ohms. The Output Power vs. Load Resistance chart shows output power dropping off after 16 ohms to between 2 and 3 watts at 50 ohms.

Supply voltage will be 12V producing slightly less than 6 watts into 8 ohms.

Sound output will generally be for Windows event sounds and short voice instructions--no music or CW. Most of the time, the speakers will be quiet (except for system noise). My only concern is to have as much output as possible but not destroy the speakers when the volume is turned all the way up.

Yes, it is not much of a speaker, but 2" x2 is all the room I have. The computer box is only about 4.5" x 7.5" and the speakers will be in a separate box below that. I just need to do the best with what I have.

Nigel, I don't understand the concept of bridged outputs. The SBC audio outputs have Left+, Left-, Right+, and Right-. When I spoke with Maxim about connecting the negative speaker outputs from their chip together to share a common ground (so I could use a 3 pin connector), they said "no" because they are differential outputs (though it seems that they don't know very much more than what can be found in the datasheets). I presumed the same applied to the SBC audio outputs and that is where the differential input requirement came from for the amplifier.

Maxim makes a 1.3 watt (non-differential input) amplifier if I don't need a separate (-) for each channel.

Thanks,
Dale

 

[Nigel Goodwin]

Assuming the output is bridged, which gives a differential type signal, either wire contains the complete information, one is just the inverted version of the other. To feed to a further amplifier you only need to use one of the output pins.

Considering there are hundreds of amplified computer speakers about, at very low costs, why not just use one of those? - if required take the insides out and build it in a smaller case.

 

[DSGarcia]

Assuming the output is bridged, which gives a differential type signal, either wire contains the complete information, one is just the inverted version of the other. To feed to a further amplifier you only need to use one of the output pins.

Considering there are hundreds of amplified computer speakers about, at very low costs, why not just use one of those? - if required take the insides out and build it in a smaller case.

Nigel,
When you say bridged, do you mean the two (-) lines are tied together somewhere? If I can use either L- or R-, can I physically tie these together (L+ and R+ would then be for each channel with a common -)?

Let me make sure I understand this correctly...
For my SBC outputs, I would use 4 resistors (two per channel) to make my voltage dividers. Both would reference the same (-) signal, either left- or right-. This will be the input to the amplifier.

Then, on the output of the amplifier, I can use one of the (-) lines, either Lout- or Rout-, for both speakers (to allow me to use a 3-pin connector).

I can't use off-the-shelf amplified speakers and repackage them because (1) they need to be waterproof, (2) the amplifier and speakers need to be rated for -20C to +70C, and (3), all my interface and support circuitry will be crammed onto one PCB--digital I/O interface, analog inputs, alarms, USB hub, power distribution and E-stop, and last but not least among other things, the speaker amplifier.

Thanks,
Dale

 

[mneary]

A 2" "full-range" speaker with lightweight alnico magnet.... and you intend to protect it? Against what? Sounding bad?

I agree with audioguru. Don't worry about it, just enjoy it (8 ohms or 50, doesn't matter). If it's too loud, you'll know.

If you're building the amplifier for use in a wet location, be sure you plug it into a GFCI.

 

[DSGarcia]

A 2" "full-range" speaker with lightweight alnico magnet.... and you intend to protect it? Against what? Sounding bad?

I agree with audioguru. Don't worry about it, just enjoy it (8 ohms or 50, doesn't matter). If it's too loud, you'll know.

If you're building the amplifier for use in a wet location, be sure you plug it into a GFCI.

mneary,
I do not know much about audio products. If there will be no physical damage to a 2 watt speaker driven by a 6 watt amplifier, I have no problem with it sounding bad at 6 watts. My protection question was in regards to equipment self-destruction and not poor taste.

With regard to your GFCI comment... In addition to circuit protection and a UPS, the entire system runs on a ruggedized external IP66/67/NEMA-4X 24VDC power supply way out of the way and everything after that also has IP67 sealed connectors. Cable are designed for harsh environments and jackets are polyurethane for added protection. Speakers are in a separate enclosure so in case the speaker cone is damaged, the computer and interface electronics stay dry. Reliability is critical, hence the concern about protecting the audio equipment from self-damage.

Thanks,
Dale

 

[audioguru]

The Maxim amplifier produces 5W into 8 ohms at clipping when it has a 15V supply. Clipping is the output level where severe distortion begins if the volume control is turned up higher. The output is 6W into 8 ohms with 10% distortion. The output is 10W into 8 ohms if the volume control is turned up way too high, half the power is nothing but distortion.

The speakers are too small to produce bass sounds. If you filter out the bass with a highpass filter then the speakers will last longer.

 

[mneary]

I'm sorry, I didn't understand that this was industrial. I had visions of a speaker for someone's spa.

If reliability is much more important than quality, then put a pair of antiparallel diodes (e.g. 1N4148) across the inputs, then adjust the gain accordingly. The 1N4148 should clip softly enough, and you can prevent the power from damaging anything.

 

[DSGarcia]

I'm sorry, I didn't understand that this was industrial. I had visions of a speaker for someone's spa.

If reliability is much more important than quality, then put a pair of antiparallel diodes (e.g. 1N4148) across the inputs, then adjust the gain accordingly. The 1N4148 should clip softly enough, and you can prevent the power from damaging anything.

mneary,
I'm not much of an electronics type and need a little more direction. I don't quite follow what you are suggesting. If the speakers will not self-destruct, then I don't have a problem leaving things as is. If I need the diodes, then I will need a little more detail.

As I mentioned in an earlier post, there is a lower output amplifier available, but it does not have differential inputs. Please see earlier post. Do you have any comments for this possibility?

Thanks,
Dale

 

[mneary]

If 1.3 watts is enough, then you don't need to worry at all.

2-3W is really only a tiny bit louder.

As for the + and - inputs, we need a little bit more detail about how they are generated. As Nigel said, you can probably get good results with just one of the SBC signals (e.g. + only).

 

[DSGarcia]

If 1.3 watts is enough, then you don't need to worry at all.

2-3W is really only a tiny bit louder.

As for the + and - inputs, we need a little bit more detail about how they are generated. As Nigel said, you can probably get good results with just one of the SBC signals (e.g. + only).

The SBC drawing is marked Line Out Left+, Line Out Left-, Line Out Right+, and Line Out Right-. It is for 8 ohm head phones or speakers rated for at least 200mW. This is what I want to feed to the amplifier after using a voltage divider on each channel.

I would like to keep both channels, but the 1.3 watt amplifier chip has left and right inputs plus a common signal ground (i.e., does not have left- and right-). I don't know how speaker signals work and whether I can adapt for this situation.

The higher power amplifier has differential inputs that (in my mind) match the SBC audio outputs but has too much power. The lower power amplifier, which as acceptable output power, does not have differential inputs. With my limited knowledge, I don't know where to go from here.

Thanks,
Dale

 

[mneary]

We need to know more about the SBC outputs. Can you find out the chip part number?

 

[DSGarcia]

We need to know more about the SBC outputs. Can you find out the chip part number?

Unfortunately the SBC will not be ready for shipment until (hopefully) next week or the following so I won't know until after it is delivered. It is a new product and has had some delays since it was announced but it appears that it will be in production very soon.

I was trying to get as much R&D done ahead of time as possible to minimize my development cycle. I would like to have prototyping parts on hand so I can start experimenting when it is delivered.

Is the 6W into the 3W speakers still a possibility? If so, I can order an amplifier evaluation board for that particular chip. I already have the lower power evaluation board (with the common input-). Also, it it worthwhile trying the 50 ohm speakers?

Thanks,
Dale

 

[mneary]

I'm worried that the SBC might have a bridged PWM output like your Maxim amplifier. I wouldn't know off hand how to attach another amplifier to it.

 

[DSGarcia]

I'm worried that the SBC might have a bridged PWM output like your Maxim amplifier. I wouldn't know off hand how to attach another amplifier to it.

I'm sorry; I don't understand audio terms such as bridged output. Is there somewhere I can read up on these terms and to understand audio (speaker) signals? I'm a computer guy but out of necessity need to handle the SBC audio output to give it a little more power.

Thanks,
Dale

 

[mneary]

Does **broken link removed** help?