Little Gem MKII with Tone Stack

[krister]

I am having trouble with this. As I turn up the volume it makes noise and when the volume control is turned up all the way it makes a very loud rumbling noise. Similiar problem as in this thread.

http://www.diystompboxes.com/smfforum/index.php?topic=57126.0

Here is the schematic I started with:



I removed the 22uF cap to reduce gain in the gain stage. I also wired the volume the normal way. Not going from the output of the treble pot to the wiper of the volume pot.

Thanks for your help.

[B Tremblay]

How are you powering it?  What is the speaker you're using?  Is it in a shielded enclosure?

[krister]

I am using a regulated power supply running at 12 volts. I am using a 1x12 (8 ohm) for testing. Eventually this will be connected to a 8-inch 8 ohm speaker when I get the bugs out. I makes this noise when connected to this speaker too. It is not in an enclosure yet. I will put it in an aluminum enclosure once I get sounding like it should. Thanks for the reply.

[B Tremblay]

Have you tried it without the feedback caps in the 386s?  Also, what kind of 386 are they?

[krister]

Yep tried that. It sounded worse without them. RS LM386N-1.

[David]

A guy with the handle of Javacody did this about four years ago.  He seemed to have pulled it off.  You might try a bit of searching...

[B Tremblay]

Those RS ones can be a little wiggly.  I like the JRC ones, personally.

[krister]

Well the Little Gem MKII part of the circuit works as expected. The problems start when you add the additional gain stage and tonestack. I've gone over the wiring over and over and can't seem to find the problem. Thanks guys.

[B Tremblay]

OK, that helps.  Does the hum/buzz vary with volume settings?

[krister]

Yes, just touching the volume control makes noise. As you turn the volume control you get a swishing kind of noise. The louding buzzing/rumbling doesn't start until you get towards the top end of the volume control. If you turn it down you get something that sounds normal. If you turn the Volume control off there is no noise.

[Steben]

With "volume normal way" you mean you did not wire it as in the drawing?

[krister]

Yes that is correct.

[krister]

I figured out part of the problem here. The volume pot I was using turned out to be defective. It has been replaced. I still get this rumbling noise when the volume is near its max. I don't get noise anymore as I turn the volume control. I shielded the signal wires which helped also. Any suggestions would be appreciated. Thanks.

[the_random_hero]

What's the output of your power supply? If it's greater than 12V (I've measured some that have come out half a volt or more higher), you could have fried the 386. I'd be building an audio probe and checking the signal path through your amp.

[krister]

I've built an audio probe. The problem is the front end mating with the buffer and power amp. The buffer and power amp sound really, really good. It is like the front end is too powerful and sends the power amp into a tail-spin. I'll try running at 9 volts instead and see what happens. I'll also research ways to back off the kick of the first gain stage. Thanks for the reply.

[krister]

Dropping the voltage to 9v helped. I was looking at ROG's Fetzer valve revisited. It looks like if I use a MPF102 instead of a J201 I will lose some gain which I think just might get me where I want to be. Thanks to everyone for your help.

[krister]

The MPF102 in the first stage did not improve things. The amp did sound better with MPF102 in the buffer stage though. There was still the nasty noise at the top end of the volume control. I decided to try running a wire and resistor (33k sounded the best) from the negative speaker output to the top of the 10k resistor connected to the source of buffer JFET (negative feedback). This got rid of the nasty noise when the volume pot is maxed. The only problem left to fix is the stability of sustained notes, kind of like an octave shift or note doubling. To recap J201 in the first stage, MPF102 in the buffer stage, negative feedback with a 33k resistor. I would say I'm close but not quite there.

Thanks to all.

[krister]

I finally got this amp to a useable place:

I added a 220k resistor between to the wiper of the treble pot and lug 3 of the volume pot. I also added .1uF cap to ground for additional power filtering. Those things made the amp useable. Thanks to Gary at ROG and all who commented here. Now it is time to box it up.

[spud]

Could you draw or annotate a schematic with your changes - I would really like to build this and your mods sound pretty cool to try. 

spud

[krister]

Here you go: