Ruby (runoffgroove) circuit problems

[fossilshark45]

Hi all, I have been trying to get this LM386 circuit working for a while now and its giving me a headache!

When I plug in my function generator set to .5vp2p 1khz I get a very loud output (into a small ~1w speaker) and the circuit seems to function normally. When I plug in my guitar I get a very farty output with the O'scope showing much lower output then with the function generator as an input.

Instead of an MPF102 I am using a 2N5484, and a 4.2K instead of the 3.9K resistor. The O'scope shows no clipping and 1:1 output off the buffer circuit as it should (with both guitar and Fgen input). This holds true both when the buffer is connected to the 386 circuit and when it is not.

I do not have the 1K gain pots yet but I have ran the tests both without a gain resistor and with a 1.2K gain resistor with the same results (just louder/more distorted with the 1.2K gain resistor).

The only other change I made is I'm using a .022uF cap as the coupling cap between the buffer and the 386 instead of the 47nF.

It seems to me like the issue is the buffer can't drive the 386, when I install the 10K pot I get 0 output with my guitar plugged in (even with the pot dimed) but with the Fgen on the input the pot acts perfectly fine as a volume and again gives proper output off the 386 into the speaker. This is very confusing to me as I have seen 386 circuits that work perfectly fine with guitar directly (through a coupling cap) on the input.

If the issue is the improper Jfet, does anyone have a transistor buffer circuit that would work? The MPF102 and J201 are hard to source these days.

Side note: I plan on having this amp be able to work both with small external speakers AND with headphones, In the past I have seen the 386 drive both with no issues I just wanted to mention it in case anyone had any suggestions regarding that.

Any advice appreciated!

https://www.runoffgroove.com/ruby.html

[Loudthud]

The 2N5484 should work fine in this application, and the pin configuration is the same.

Did you check the resistance of all the resistors before you installed them ? If that 1M5 resistor is 1K5, the generator will drive it fine but a guitar will barely be heard. You can probably measure it without removing it from the build, just reverse the meter leads and take the highest reading.

[fossilshark45]

yes I did double check that with a meter.

[Tassieviking]

If you are talking about the capacitor between the FET and the volume pot then that might be your problem.
When you use a capacitor and a resistor together you end up with a filter, in this case it is a High Pass filter that only lets frequency's above the set filter frequency through.
The 47nF cap and the 10k pot makes a high pass filter at 339Hz
The 22nF cap and the 10k pot makes a high pass filter at 724Hz
Nothing below those frequencies will be heard, you eliminate them.
If I ever made that amp I would use at least 220nF to allow bass frequencies through, but that is just me, it is set a such a silly high HP filter setting to play through a crap speaker.

Have a play with this calculator to see what I mean:
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php#answer1

[fossilshark45]

Thanks for all your help guys, turns out the issue was the crappy speaker I was testing with lol.

Tass, yes the 22nf cap was a little low. I really liked the sound for guitar but this is intended to be used for bass. Switched that out for a 4.7uF and it sounds great.

[Tassieviking]

Sometimes these tiny amps surprise you when you hook them up to a good speaker, many people think that they cant drive a 4 x 12" cab but "surprise"