386 amp speaker impedance

[slashandburn]

Right to the point here:  Turns out almost all of those 45mm 8-ohm Mylar speakers I've been making little novelty guitar amps from are not 8ohm after all.  It would appear I've been given 32ohm speakers and it's taken me an embarrassingly long time to notice.   In my defence, they all function, and the small speaker is no more than a novelty on most builds I'm not beating myself up too much about it. The silver lining is that I could've sworn my earliest build (with the actual 8 ohm speaker) sounded better, so I'm slightly reassured that I'm not completely losing it.

In trying to take something from this heinous error (and perhaps get use out of the 32ohm speakers I have left) I was wondering how best to alter the Ruby amp circuit  to make them play a bit nicer with the different speaker load.

In other words, given that the ruby isn't all that fussy about speaker impedance (though arguably sounds best paired with an 8ohm speaker), what tweaks, if any, should be made if one was to build something specifically for a 32ohm load?

[Elijah-Baley]

Maybe you are clever than me, but I know that using two 8 Ohm speakers in series the load is 16 Ohm, when those are in parallel the load is 4 Ohm. Waiting for other opinions maybe you can modify the final impedence using a resistor, I think it should be at least 1w.

[slashandburn]

Thanks Elijah, the thought had crossed my mind, a resistor in parallel.  Then I quickly remembered I'm in over my head and needed some advice.   

The output cap value should probably be tweaked one way or another also, but I'm not entirely certain and I'm fighting a slight hangover today.  Figured it was better to post on here and experiment once my head has cleared and I've taken some of you lots thoughts on board.

Feel free to mock me for taking nearly a year to notice I had the wrong speakers.

[GibsonGM]

Why not just use FOUR of the 32 ohm speakers in "double parallel", make a mini cab??
2 in ||, then those in || with another 2...

You'd have to use something like a 10R resistor to play the 'resistor trick', and as mentioned, it will take most of the current so much have some power rating. Won't be the same as other speakers either, as it's purely resistive.

No mocking - it happens!  lol

[Digital Larry]

What exactly are you trying to accomplish?

I know everyone has heard about "maximum power transfer" when impedances are matched, but if you add a resistor in order to do this, I don't see what benefit you're getting except a space heater for a mouse.  I really doubt the amp will sound "better" as a result of doing this, unless you want it to distort at a lower volume.

If you want more volume, I'd suggest putting two of these speakers in parallel as someone else suggested - or more, until you hit 4 ohms.  I'm thinking that the LM386 4 ohm rating is a minimum, to keep its output current and total power dissipation from frying the chip.  My suspicion is that its output impedance is probably a lot lower.  I briefly read through the TI spec http://www.ti.com/lit/ds/symlink/lm386.pdf and it says it can drive loads from 4 to 32 ohms, but they don't quantify the output impedance.  You can check the charts - at a certain point it starts to distort a lot.  This being a guitar effects forum, can't tell if that's what you want, or not.

Using a higher impedance speaker results in less power (and probably lower volume, depending on the speaker efficiency).  As usual, take my comments with a grain of salt - I've never messed with the LM386 and for all I know there is some magic mojo going on.

[slashandburn]

Great idea Mike! A cabinet of tiny speakers is pretty appealing and would let me use up most of what I've got left.

I'm not sure what I'm trying to accomplish. If I had any equivalent 8ohm speakers left I could do a simple A/B test and try to hear the difference. I guess these things are okay with the 32ohm speaker.  The tonal response of the 45mm isn't exactly a deal-breaker here, and I don't mind how they sound as it is. I'm just a little frustrated not knowing for sure the extent that using the "wrong" speaker alters the character of these builds, and wonder if and how to compensate  should I decide to intentionally use the 32ohm speakers in future.

As an aside, regarding the 386 and it's magic mojo - I do love this chip, it was basically my route into this pedal building thing and I'd encourage anyone (particularly metalheads) to have a play around (ideally with a real guitar speaker, not so much with the tiny tweeter!)

[amptramp]

You could always run the amp at a higher voltage.  For each volt on the speaker, you get 31.25 mA current drain.  More volts would bring you closer to what the speaker needs.  Double the voltage would get you double the current, so four times the wattage.

But as suggested above, the best option is additional speakers in parallel.  Four of them would give you 8 ohms.

[bool]

Ok so Amptramp was the one to explain it scientifficaly; my first impulse when reading your OP was to say 'so build a 4x2" minicab then!'

What else, imho ....

[slashandburn]

Not sure where I got "Mike" from, I obviously meant Ron. Hangover is easing somewhat, the world looks clearer now and filled with miniature 4 speaker cabinets!

[GibsonGM]

Not sure where I got "Mike" from, I obviously meant Ron. Hangover is easing somewhat, the world looks clearer now and filled with miniature 4 speaker cabinets!

Maybe you are clever than me, but I know that using two 8 Ohm speakers in series the load is 16 Ohm, when those are in parallel the load is 4 Ohm. Waiting for other opinions maybe you can modify the final impedence using a resistor, I think it should be at least 1w.

Why not just use FOUR of the 32 ohm speakers in "double parallel", make a mini cab??
2 in ||, then those in || with another 2...

You'd have to use something like a 10R resistor to play the 'resistor trick', and as mentioned, it will take most of the current so much have some power rating. Won't be the same as other speakers either, as it's purely resistive.

*cough*

[Cozybuilder]

Why not make a run of novelty mini-Marshalls, with 4 of these 32 ohm speakers in parallel?

[slashandburn]

Ha! Sorry Mike, my apologies. Clearly my heads still a little cloudy! Two different profile pictures of your respective pooches was too much for my little brain, it would seem!

Why not make a run of novelty mini-Marshalls, with 4 of these 32 ohm speakers in parallel?

You know, if this works out well I might just need to accidentally order another few dozen of these godforsaken 32ohm speakers.

[GibsonGM]

LOL, no trouble.  Been there    I'd like to hear what it sounds like when you get a 'cab' done!!  Recording is necessary....

[PRR]

Parallel resistor just wastes power and makes heat. The "sound" of the '386 does not change a lot with load. The power goes with load, the 32 taking about 1/4 as much power as the 8, which indeed makes a small half-watt amp into a puny one-eighth-watt wimp.

On paper, doubling the supply voltage makes the same power in 4X load. And yes selected '386 now run on 18V. That is one way to use-up excess 32r speakers. I do suspect that proper 8r 45mm speakers are cheaper than new power supplies.

Aside from getting 8r speakers, the idea to mini-stack four has great merit. Half the problem is getting electric power into the speaker(s). The other half (often a bigger half) is getting sound OUT of the speaker. 45mm 1.75" is MUCH smaller than most sound waves. It is a poor "air paddle". A quad gives four times as much coupling from cone motion to acoustic energy. As good as a 3.5" speaker!

Of course when you get serious you run the '386 into a Full Stack, eight 10" speakers (design-built for 8 ohms). As big as a half-wave of any guitar tone. Makes a mighty sound.

[slashandburn]

Thanks for the informative responses gentlemen! I'll hopefully get something thrown together over the next few days. I'll throw together a demo video as requested. In the meantime you can all place bets on how many minutes of playtime I'll manage to record before you hear the wife yelling at me to turn it down?

I realise that with a 4 inch "cab" the logical thing to do would be to get a 3.5" full range speaker. Or at least just something with more low to mid range than these things but I'm too curious now not to give it a go.