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Old 16th December 2010, 10:21 PM   #1
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Default Need help troubleshooting Ruby amp

Hi,
I assembled on a breadboard a small Little Gem amp from Runofgroove, and everything sounded just fine.
Then I decided to build a ruby and things didn't go so well...
Thing is I get lots of crackling noise when playing at lower volumes, I think the noise is still there when playing louder, but it's less noticeable.

Removing the gain bits (pins 1 and 8) does not help.
If I keep the ruby circuit and connect the guitar input directly to the 386 chip (ie not using the jFet, like the little gem), every thing sounds fine! :P
Because of this I thing the problem is on the Jfet chip.
Is it possible that I got a bad jfet?! its a MPF102

Thanks
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Old 16th December 2010, 10:47 PM   #2
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You didn't link to the schematic. If you have an input or output coupling cap, it can cause this, especially if tantalum electrolytic. Cold solder joints can cause pops and crackles. At higher voltages, rosin flux on the pcb can cause this.
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Old 16th December 2010, 11:10 PM   #3
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links
ruby:
Ruby

little gem:
Little Gems
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Old 16th December 2010, 11:50 PM   #4
sregor is offline sregor  United States
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My 2 cents - you may be overloading the input of the jfet. If your guitar has a level out, try turning it down and increasing the gain of the amp. Jfets are cheap enough that you could try another. Good luck.
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Old 17th December 2010, 12:06 AM   #5
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I tried that, seems the oposite is happening, if I ser the guitar knob louder I think it sounds ok (it's midnight here, cant try louder as the folks are asleep ).
When I play the guitar at lower volumes I get no sound if I play soft, if I play a little harder I get sound but it's terrible, gets a little better the harder I play :P
Will try again tomorrow at higher volumes, might as well try a better breadboard

Regards!
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Old 17th December 2010, 12:24 AM   #6
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I looked at the circuit and at the datasheet of the advanced analog devices AZ386 audio amplifier. The sample circuit on the datasheet seemed very similar to your board. datasheetcatalog.com doesn't have the jrc386 listed. The device has absolutely no noise specification. It is optimized for low current use for battery operation. If a new JFET doesn't help, and there aren't any construction problems listed above, I wouldn't expect much fidelity of it. One tip, check your DC battery voltage when playing, if under 7 get a new one.
I'm getting decent fidelity out of ST33078 and JRC4560 IC's, but they don't have enough power to drive a speaker. See this link for noise & hum wars using a wall supply.Improving a "Disco mixer" to mid-fi performance People doing speakers usually start with a chip amp like an LM3686. Those have their own thread under amplifiers.
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Last edited by indianajo; 17th December 2010 at 12:30 AM.
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Old 17th December 2010, 01:09 AM   #7
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Quote:
Originally Posted by ozzu2000 View Post
Will try again tomorrow at higher volumes, might as well try a better breadboard

Regards!
If you are actually using a push in breadboard instead of soldered connections with a real IC socket, no wonder the connections are ittermittant. Audio is low voltage, low current, and has trouble getting across even solder joints.
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Old 17th December 2010, 01:15 AM   #8
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AVE...

I had similar problems with this amp. I thought that JFET circuit was wrong. So I designed and simulated this little thing below. On diagram there is BF244, but should be BF245C. R4 is for simulation purposes only. Also you should add 220pF capacitor between opamp's input and ground...
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Old 17th December 2010, 11:21 AM   #9
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Neat!
I'm guessing the R4 simulates the 386 input. Right?
Everything remains like the ruby? How did it work out when assembled?
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Old 17th December 2010, 12:39 PM   #10
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AVE...

R4 simulates both 386 input and components before it (like potentiometer). I haven't done it yet (lack of time), but I made some simulations...
My complete design is below. I added some extra parts for normal/bassman mode of operation. And I removed the capacitor connecting JFET's source to drain. It's not necessary, since the capacitor between 386 input and ground works better. R2 is very important - determines the input impedance. It can range from 1.5 Mohm to even 10 Mohms. But be careful - this preamp has pretty big gain, so higher R2 might cause it to be overdriven...
The simulation was done in bassman mode with maximum distortion settings applied - that's why distortion is so high...
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