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!
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!
.5vp2p would be a pretty high output for an electric guitar. Sometimes they can put out more if turned up to 10 and strummed really hard. Maybe even 2vp2p. But most of the time its much less than that, especially if the guitar volume and tone pots are set sensibly for best sound.
So anyway, the input FET transistor is just supposed to be an impedance converter, from high-Z in to lower-Z out. It doesn't provide any voltage amplification. If you have a scope you should be able to measure the guitar output voltage as a starting point. Otherwise, you could try turning your function generator voltage down quite a bit and then see how the circuit works.
Also, you might notice some loss of low frequencies with the smaller than recommended coupling cap.
Its also possible your amplifier is oscillating if it is acting weird. Poor layout can cause that. Again, a scope can be very useful for taking a look.
So anyway, the input FET transistor is just supposed to be an impedance converter, from high-Z in to lower-Z out. It doesn't provide any voltage amplification. If you have a scope you should be able to measure the guitar output voltage as a starting point. Otherwise, you could try turning your function generator voltage down quite a bit and then see how the circuit works.
Also, you might notice some loss of low frequencies with the smaller than recommended coupling cap.
Its also possible your amplifier is oscillating if it is acting weird. Poor layout can cause that. Again, a scope can be very useful for taking a look.
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I run the guitar dimed and the scope shows about .5vp2p output. My generator also only goes down to .5vp2p (-db button is broken and I havent gotten around to fixing it).
I understand the FET is just a buffer, I am just confused how it works with the generator and not with the guitar.
How would I check for oscillations? A DC coupling cap eliminated any motorboating I heard.
I understand the FET is just a buffer, I am just confused how it works with the generator and not with the guitar.
How would I check for oscillations? A DC coupling cap eliminated any motorboating I heard.
You can always take another external pot and use it to make a voltage divider for the signal generator. Then you can test with lower voltage input signals.
You can measure the guitar output voltage with it disconnected from the amp and with it plugged into the amp. If the guitar output voltage drops drastically, you may have a bad/shorted FET.
You can look at the input to the 386 amplifier and at its output as the gain pot is turned. Does it do anything weird? Break into oscillation at some point? Is there any output when there is no input to the amp?
Do the power supply voltages measure at what they should be? Is there any AC on the DC power supply rails?
IOW, there are lots of tests you can do to help isolate if there is a problem with the guitar, its cable, the amp, etc.
Anyway, most guitars sound best with the tone and volume pots set somewhere around 5. With a working amp, set the guitar pots to 5, then adjust the amp to get the sound you want. Start with gain and tone pots if any at a minimum. Then start turning things up until you get some clean sound, then a little more until it starts to break up if you pluck 2 strings at once. Adjust to taste first with the amp controls, then fine tune with the guitar pots. If that isn't working then you have a problem. Look around and try some measurement techniques until you find out where the problem is, at least in a general sense: in the guitar, the cable, the amp, the speaker cable, the power supply, etc. Once the general problem area is found, then narrow in to find the exact problem.
Also, try to keep in mind the signal generator probably has a 50R output impedance. The guitar will be much higher output impedance, but its still should be DC coupled so you could measure its resistance with an ohmmeter.
You can measure the guitar output voltage with it disconnected from the amp and with it plugged into the amp. If the guitar output voltage drops drastically, you may have a bad/shorted FET.
You can look at the input to the 386 amplifier and at its output as the gain pot is turned. Does it do anything weird? Break into oscillation at some point? Is there any output when there is no input to the amp?
Do the power supply voltages measure at what they should be? Is there any AC on the DC power supply rails?
IOW, there are lots of tests you can do to help isolate if there is a problem with the guitar, its cable, the amp, etc.
Anyway, most guitars sound best with the tone and volume pots set somewhere around 5. With a working amp, set the guitar pots to 5, then adjust the amp to get the sound you want. Start with gain and tone pots if any at a minimum. Then start turning things up until you get some clean sound, then a little more until it starts to break up if you pluck 2 strings at once. Adjust to taste first with the amp controls, then fine tune with the guitar pots. If that isn't working then you have a problem. Look around and try some measurement techniques until you find out where the problem is, at least in a general sense: in the guitar, the cable, the amp, the speaker cable, the power supply, etc. Once the general problem area is found, then narrow in to find the exact problem.
Also, try to keep in mind the signal generator probably has a 50R output impedance. The guitar will be much higher output impedance, but its still should be DC coupled so you could measure its resistance with an ohmmeter.