LM386 power output and speaker ratings for guitar amplifier application

Thread Starter

pistolplc

Joined Oct 3, 2022
5
Hi, I I have a question regarding speaker ratings when using an LM386-1 with a 9 volt power source (battery) and old/vintage speakers.

Specifically, I have numerous speakers rated at 1/4 watt or .3 watts (many of them speakers from old transistor AM and/or AM/FM radios). Ohm ratings on them typically range from 3.2-8. I want to hook these up to an LM386 based guitar amplifier (basically the "Smoky Amp" circuit, or something similar.

My question is, can these speakers handle the LM386 output? The data sheet for the LM386-1 lists the power output as "VS = 6 V, RL = 8 Ω; Min: 250 mW, Typ: 325 mW."

Do you think these speakers will generally handle the power here? Or am I treading on thin ice when I'm running at 9v and, say, a 4 ohm, .3 watt speaker? Is there anything I'm missing here, or a good way to ensure I don't blow a bunch of speakers? Thanks in advance.
 

dl324

Joined Mar 30, 2015
15,113
Welcome to AAC!
LM386-1 with a 9 volt power source (battery)
If you're talking about this type of battery:
1664813543703.png
I don't think you need to worry much about blowing up speakers. Just listen to the audio and turn down the volume when they start distorting. If you plan on having a wide dynamic range, use a higher power speaker.
 

MrChips

Joined Oct 2, 2009
27,122
Welcome to AAC!

What kind of guitar and guitar music are you playing?
LM386 amplifier is rated for 125mW output. It will sound like trash for electric guitar amplification.

If you want decent sound from an electric guitar, think loudspeaker+cabinet+amp all in combo.

You want to aim for 8" speaker minimum, 10" is better in a properly built cabinet.
10W minimum amp, 15-30W even better.
 

Thread Starter

pistolplc

Joined Oct 3, 2022
5
I'm actually planning on building these into the radios, so it's more like a cool little gritty practice amp. It's no Deluxe Reverb, that's for sure, but it's kinda neat for something you can just keep around as a vintage decoration but also crank out some AC/DC when the mood strikes you. But if I manage to sell or give these away to friends, I want to avoid them blowing up if at all possible.
 

Papabravo

Joined Feb 24, 2006
19,288
Hi, I I have a question regarding speaker ratings when using an LM386-1 with a 9 volt power source (battery) and old/vintage speakers.

Specifically, I have numerous speakers rated at 1/4 watt or .3 watts (many of them speakers from old transistor AM and/or AM/FM radios). Ohm ratings on them typically range from 3.2-8. I want to hook these up to an LM386 based guitar amplifier (basically the "Smoky Amp" circuit, or something similar.

My question is, can these speakers handle the LM386 output? The data sheet for the LM386-1 lists the power output as "VS = 6 V, RL = 8 Ω; Min: 250 mW, Typ: 325 mW."

Do you think these speakers will generally handle the power here? Or am I treading on thin ice when I'm running at 9v and, say, a 4 ohm, .3 watt speaker? Is there anything I'm missing here, or a good way to ensure I don't blow a bunch of speakers? Thanks in advance.
An audio amplifier and a speaker are AC devices and you should analyze them with respect to AC signal levels and impedances.
  1. 9V is a DC source and it has almost nothing to do with the AC characteristics of an amplifier or a speaker. It does limit the voltage swing to something smaller than 9V P-P.
  2. 4Ω is an average AC impedance over some subset of the audio range of 20Hz-20kHz. In particular you don't get a uniform impedance over the entire range.
  3. 3 watts is the maximum RMS power that the speaker can withstand without failing. It is highly probable that the sound at these power levels will be painful to listen to.
 

MrChips

Joined Oct 2, 2009
27,122
It would be hard for an LM386 to blow any loudspeaker.
Also PP3 9V battery will not last very long. They are designed to deliver no more that 50mA of any length of time.

If you want to be creative and inventive, you can take four identical small radio speakers and wire them in 2S2P, i.e. two in series and then two pairs in parallel. Mount them on a speak baffle and they will sound a lot better than one individual loudspeaker.

For power amp, look for an off-the-shelf class-D module.
 

Thread Starter

pistolplc

Joined Oct 3, 2022
5
For some reason, I thought it was a 10W amplifier...
Yeah, pretty small wattage on this. I've not seen that older spec sheet with the 9v rating - that gives me pause because many of these speakers are rated for less than 1/2 watt. I've basically been thinking that the LM386-1 was basically a 1/4 watt output device, but may be I'm wrong.

What tool/device would I need in order to measure actual RMS power output to a speaker?
 

Thread Starter

pistolplc

Joined Oct 3, 2022
5
It would be hard for an LM386 to blow any loudspeaker.
Also PP3 9V battery will not last very long. They are designed to deliver no more that 50mA of any length of time.

If you want to be creative and inventive, you can take four identical small radio speakers and wire them in 2S2P, i.e. two in series and then two pairs in parallel. Mount them on a speaker baffle and they will sound a lot better than one individual loudspeaker.
Yeah, hopefully these don't blow out all the time. Fun idea on the "wall of sound"! I'll have to give that a shot.
 

Papabravo

Joined Feb 24, 2006
19,288
Yeah, pretty small wattage on this. I've not seen that older spec sheet with the 9v rating - that gives me pause because many of these speakers are rated for less than 1/2 watt. I've basically been thinking that the LM386-1 was basically a 1/4 watt output device, but may be I'm wrong.

What tool/device would I need in order to measure actual RMS power output to a speaker?
You need instruments that measure true RMS values, like the following:
https://www.fluke-direct.com/product/fluke-115-digital-multimeter-with-true-rms
 

Audioguru again

Joined Oct 21, 2019
5,166
The datasheet of the LM386 shows a graph of peak-to-peak output voltage into speakers that are 4, 8, 16 ohms and infinity at supply voltages from 4V to 12V.
With a 6V supply and an 8 ohm speaker, the p-p output is 4Vp-p and a simple calculation gives output power at 250mW. The graph of distortion shows horrible 2% distortion and another graph shows low distortion at 125mW.

With an 8 ohm speaker and 9V supply the max output is 6Vp-p which is 563mw with bad distortion. 282mW produces low distortion with an output that is 4.3Vp-p.

A 4 ohm speaker produces lots more heating and less output power than with an 8 ohm speaker.
With a 9V supply the max output into 4 ohms is 3.5Vp-p which is 383mW. A graph of heating shows 820mW of heat.
With a 9V supply the max output into 8 ohms is 6Vp-p which is 563mW and 520mW of heat.
 

Jerry-Hat-Trick

Joined Aug 31, 2022
162
Maybe 50 years ago I built what was called a “sweet 16” combination of 16 small speakers in a series parallel arrangement which produced a surprisingly good sound and punched above its weight in terms of perceived volume for the power of the amplifier. If you are thinking of driving a number of speakers together, something along these lines might be good?
 

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MrChips

Joined Oct 2, 2009
27,122
The rationale for multiple small speakers makes sense as follows.

A small diameter loudspeaker (under 6") has good high frequency response but does not have enough surface area in order to produce the acoustic power required at low frequencies.

On the other hand, a large diameter loudspeaker (above 8") has poor high frequency response. The large size and weight of the speaker cone causes the cone to "break up" at higher frequencies. Hence high quality speaker systems use cross-over networks to split the audio content into low, mid, and high frequencies to drive woofers, midrange speakers and tweeters. No single loudspeaker can handle the full audio frequency range.

By using multiple small loudspeakers you are able produce the full audio range without cone breakup.
 

Thread Starter

pistolplc

Joined Oct 3, 2022
5
The datasheet of the LM386 shows a graph of peak-to-peak output voltage into speakers that are 4, 8, 16 ohms and infinity at supply voltages from 4V to 12V.
With a 6V supply and an 8 ohm speaker, the p-p output is 4Vp-p and a simple calculation gives output power at 250mW. The graph of distortion shows horrible 2% distortion and another graph shows low distortion at 125mW.

With an 8 ohm speaker and 9V supply the max output is 6Vp-p which is 563mw with bad distortion. 282mW produces low distortion with an output that is 4.3Vp-p.

A 4 ohm speaker produces lots more heating and less output power than with an 8 ohm speaker.
With a 9V supply the max output into 4 ohms is 3.5Vp-p which is 383mW. A graph of heating shows 820mW of heat.
With a 9V supply the max output into 8 ohms is 6Vp-p which is 563mW and 520mW of heat.
Pardon my ignorance, but can you share the calculation/formula you're using to get the power from the p-p voltage output and speaker impedance?
 

Audioguru again

Joined Oct 21, 2019
5,166
RMS power is from a sinewave that has no harmonics. Peak power is simply double the real RMS power because the power in the harmonics is the same as the power in the fundamental sinewave.
 
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