Ruby Questions

[ED]

My Ruby I built the other day is working well except for a couple of things.

a) The note sustains for a while, then breaks up into this poppy blur.

Could this have something to do with my speakers? I have 4 speakers linked in parellel. 2 of them .5watts @ 8ohms, one .3watts @ 8ohms and the other .25watts @ 8ohms. They are all the same diameter (about 5cm)

Or maybe the MPF102?Or maybe some of my connections are dodgy? I dunno i'm just guessing
   
b) After playing the amp for a couple of hours the signal volume drops dramatically and after that a weird noise steadily gets louder and louder and eventually takes over the guitar signal. Quite annoying. When I replaced the battery though it plays fine again so i assume its something to do with the battery when it starts to run low.

The main problem is the note breakup though i'd appreciate if you guys can explain what is going on.

Thank you runoffgroove for all the awesome articles you have!

:lol:

[puretube]

it`s not the speakers themselves;
but the combination: 2ohm load (4x8ohm par.)
might be too low (dunno the schem....)
-for the circuit & battery-

[Peter Snowberg]

I would try using the speakers in a parallel series combo. Make two pairs of speakers and put them in series. The result will be 8 ohms which the 386 will appreciate.

From your description of the sound it sounds like you might have a bias issue with the JFET. That could give you a "poppy blur".

Take care,
-Peter

[Travis]

Two ohm load is pretty low for the 386.  Try using two sets of paralleled speakers in series (8ohm load).
http://cache.national.com/ds/LM/LM386.pdf
Page four.

For the note breakup, if you don't have an appropriate trimmer to bias the FET, try a bunch of them.  Finding the right one can really make a difference.  And/or, make sure that you have a heat sink on your 386.  It is pretty easy to exceed the max temp ratings.

[spongebob]

You could try adding a so called Zobel circuit to the output of the 386, that's a resistor (10 Ohm) and capacitor (47nF) in series connected to ground and usually helps to cure instabilities.

The JFET should cause no problems in this circuit, it's used as a source-follower (no biasing needed).

And don't expect a 9V battery to last forever with this little amp, this thing eats batteries rather quickly! I'm now using a rechargeable 12V lead battery as a power supply, but if you already own a 9V wall-wart you should consider adding a DC jack to your amp.

[ED]

thanks guys ill try the series parallel wiring now :lol:

[PB Wilson]

My Ruby was acting up as I plyed it and the background noise combined with a pulsing/buzzing sound kept getting louder until it stopped altogether. I thought it was a problem with the circuit, but a fresh battery fixed it right up. I'm thinking of using a 12v battery pack I recently got at Rat Shack for more headroom and longer battery life.