Homemade guitar amp filter

March 17, 2008 9:13 AM

I built this mini guitar amp based on this circuit. But I used a 4 watt speaker rather than the 8 watts specified. What are the consequences of that change?

I'm trying to understand the concepts of the circuit, not just how to assemble it. I know Ohm's law, P=IV, series vs. parallel, etc. But I have no idea how to apply it practically.

So I used a lower resistance speaker. Will this increase my current through the whole circuit? Does that change the power output of the amp? Will that drain the 9V battery quicker? Is it louder than the 8 ohm speaker would be? Does it clip the sound signal at lower volume?

Bonus Question 1: And how about all those capacitors? I understand they are low and high-pass filters, but that's it. Will fiddling with those change the quality of the sound produced by the amp?

I've read a bunch of websites on speaker impedance, but they're generally geared toward home audiophile types and aren't answering this low level questions. And websites dealing with the low level concepts don't seem to step up a level to practical applications.

So any tips on resources for hobbyist level electronics, especially sound related, are also appreciated.

posted by Grundlebug to science & nature (8 comments total)

I meant 4 ohm speaker, not watts. See, I'm new at this...
posted by Grundlebug at 9:20 AM on March 17

I always have a tough time remembering which way the ohm mismatch works between amp and speaker.

Changing values of capacitor will change the point where the rollof begins, so yeah, it will make a change to the tone.

for resources on hobbyist electronics of the musicmaking variety, check out the Tape Op messageboard. There's a section called "DIY Gear" - posting questions there should get some more in-depth responses. I know a couple folks on there have built the Ruby amp, which is similar to the Little Gem.
posted by dubold at 9:49 AM on March 17

I'm not an expert, but my basic understanding is:

Will this increase my current through the whole circuit?Yes (doubled)
Does that change the power output of the amp?Yes (doubled)
Will that drain the 9V battery quicker?Yes
Is it louder than the 8 ohm speaker would be?Yes (twice as loud)
Does it clip the sound signal at lower volume?Unsure, I would imagine so.
posted by sanka at 10:13 AM on March 17

At the same volume level, you get: I=V/R, so half the resistance, twice the current. P=IV, so twice the current, twice the power, and so 6 dB more sound. To convert from Watts to decibels add 6 dB for each time you double the power.

You should check that you are not running too much current through the amp or thought the speaker, or you will fry them. The easy way to check is to put your finger on the amp while it is playing at full volume. If it gets burning hot, your chip is frying and you should turn it off. The proper way to check is to go to national's website and get the datasheet for the chip:

http://www.national.com/mpf/LM/LM386.html#Datasheet

Scroll to page 4, at the bottom right you will see the heat dissipation of the chip in a 4 ohm load. This will tell you how big a heat sink you need to install on the chip to keep it cool. It shows 1.2 Watts of heat at max volume, which is not very much at all. The amp will be fine.


Does that change the power output of the amp? No, unless it was gain limited. The chip can only dissipate so much heat, afterall. The amp also distorts badly when more than 0.2 watts go through it, as you can see from the sixth graph on page 4 of the datasheet. You can tell the amp is gain limited if you put the volume at the maximum and the chip still does not distort or fry.

Will this increase my current through the whole circuit? At a fixed volume setting, yes. But it does not matter, you could always raise the current by turning the volume higher. Heat, gain and distortion are the limiting factors.

Is it louder than the 8 ohm speaker would be? Perhaps, if it was gain limited at 8 ohm.

Will that drain the 9V battery quicker? No, only if you make it play louder than you would have.

Does it clip the sound signal at lower volume? No. There no reason why it would.


You will enjoy reading a book about electronics. When I asked this question, I ended up buying "The Art of Electronics" and "Practical Electronics for Inventor".
posted by gmarceau at 10:52 AM on March 17 [1 favorite]

It will make a difference to the circuit, but ultimately the sound of the final product is what counts. Many amps have a variety of speaker ohm rating possibilities and different speaker loads do produce different tones; experimentation is encouraged.

In my experience, you'll only get trouble if you get below 2 ohms of load. Keep in mind too that solid state amps will be OK with no speaker connected (in general), but tube amps will toast their transformers with no load.
posted by Paid In Full at 11:00 AM on March 17

Umm, the circuit diagram seems to indicate that a 4 ohm speaker is fine- am I missing something?

http://www.runoffgroove.com/littlegem.html , 3d diagram, "4 ohms and up"
posted by jenkinsEar at 2:57 PM on March 17

Yeah, I saw that jenkinsEar. I built it and it works fine. I just wondered what the electrical consequences of switching speakers were.
posted by Grundlebug at 6:55 AM on March 18

For most amplifier circuits, the impedance of the speaker needs to match the output impedance of the amp. If they aren't matched, the amp will be less efficient, and you could damage the amp. The reason is that maximum power transfer occurs when the impedance of the load equals the impedance of the source. If they aren't equal, power is dissipated in the amp itself rather than the speaker, which is bad. The way most amps deal with this is by using a transformer with multiple windings for different speaker resistances.

For your circuit, however, it won't matter all that much, since they aren't trying to match the output impedance anyway. Changing the volume affects the impedance more than the speaker resistance. Its not a very efficient design, but its simple.

As for the caps, yes the values do matter. They control the frequency response and bias points of your amp.
posted by jpdoane at 4:54 PM on March 18