ruby speaker squeeling? question...

[waky]

so guys i dumped my old ruby for a fetzer ruby, everything is ok when i plug in the headphones (i puted 20ohms sleeve resistance to lower the volume) but as soon as i switch to my speaker the thing squeels like crazy, i dont really know why since on headphones its dead quiet.

any thoughs? i wired it as in the rog faq

http://runoffgroove.com/faq.html

[newfish]

Ah, mine also squeals depending on what I use as a speaker.

With Headphones (unsure of impedance, but it's at least 16 Ohms), there's very little 'piggy'.

With an old hi-fi speaker (my test speaker for everything), it squeals at the drop of a ham.  Impedance of this speaker is much lower (6 Ohms or so).

One thing I tried last night was to change the 100nF cap going between the 386 and ground (at work, so am unsure which pin it goes to) for a 47uF.
I built one of Tim Escoban's excellent Smokey Clones, which doesn't squeal, and this design has the 47uF, not the 100nF - so that's my thinking.

It seemed to work too.

Maybe it's time for a small output tranny to put a fixed, minimum impedance on the output - or have a 'Cab / Headphones' swith to monkey around with the impedance.

[Lurco]

do a "zobel"-search in this forum 

[newfish]

Wow.  Didn't know such a thing ever existed.

Thanks Lurco.

I'll put some of the original components back in now I sort of understand why they were there in the first place...

More book learning is required.   

[newfish]

Further update if anyone's interested...

Rebuilt my Ruby, but with the input to the 386 going to the inverting input (pin 3), as per the examples given on the National Semi datasheet.
Also used the original 47nF instead of the 0.1uF just before the 'Volume' pot - as I suspected the larger signal passed through the 0.1uF was overloading the 386.  Either way, it's a lovely 'sparkly' sound.

I'm running this at 12v using one of the small sized key fob batteries (23A).

Data sheet I used can be found here...

http://www.datasheetcatalog.com/datasheets_pdf/L/M/3/8/LM386N-1.shtml

I've built the 'Gain 200' version, but with a pot controlling the amount of gain on pins 1 and 8.

I missed off the 'Bypass' cap (pin 7) this time - not that that should have any influence on squealing, right?

I'm very very pleased with this little beast - so thankyou to ROG for posting the schematic.