I have just built this circuit to regulate the signal of my guitar, that is about 600mV, because my pedals distort with it. Inspired by the circuit of a Marshall 8080 amplifier that cuts the signal to 50% before the effect loop, and the input buffer circuit of a Boss MT pedal, this is what I made that I attached the photos of.

This small amp does the job, the output signal is 300mV now. And it makes the sound of my guitar better, sounds more interesting. However, now it is this amp that is distorting. The signal of the guitar is too much for this as well. If I play louder, the distortion appears.

Do you have any suggestions what I should modify to avoid the amp to distort, and be able to handle the 600mV input signal or maybe a little more (I only have a multimeter to measure the signal, when I strum the guitar strings heavily I can read about 600mV value).

 

Add a volume control:

 

Without any input signal, measure the DC voltage between Source to Ground, Vcc to Collector, and the actual Vcc to Ground voltage.

My suspicion would be an improperly biased transistor.

 

1. Adjust the drain resistor so the quiescent voltage is 1/2 supply.

2. Change the ratio of the source and drain resistors to adjust the gain.

 

Can't see any reason why that should distort, unless the DC output has some impact downstream. Biassing, such as it is, is ok until input >8v p-p, then output clips to 0v. AC gain is approx 0.5. Output essentially flat above 100Hz or so, as you'd expect from this device.

 

I agree Irving, it should not distort.

But something is going on, and that is the reason I requested the VDC quiescent voltages.

 

1. Adjust the drain resistor so the quiescent voltage is 1/2 supply.

2. Change the ratio of the source and drain resistors to adjust the gain.

I am not sure if this is related or not, but someone I talked to pointed out that in spite of that I took the idea from the Boss MT, half of the essential part was left out from my design.

So this is my circuit:


The full details of the buffer part in the Boss MT:



So as I was told, this thing works on half power (not sure is this is the correct word for this). Did you refer to this problem with your suggestion?

The person who pointed this out suggested a different design, so that I can avoid the AC part, and keep the design simple for beginners like me:



The part encircled with blue is the J112. What do you think of this version? Canit solve the problem that you wrote about, or that is not related to this? Sorry but I am a total beginner, I have very little knowledge about this.

 

If the amplifier circuit involved has previously amplified without adding distortion, the probably cause of distortion is operation outside the linear range. Often that is caused by higher input voltage level, BUT it can also be caused by a reduced power supply voltage. This often happens with attempts to operate amplifiers at 3.3 volts instead of 5 volts or 12 volts.

 

MrBill, I also agree that the cause for the distortion is that the DC quiescent bias is incorrect. I have asked for the DC bias voltages twice already, but the TS has so far ignored the question.
I will find other threads instead.

 

MrBill, I also agree that the cause for the distortion is that the DC quiescent bias is incorrect. I have asked for the DC bias voltages twice already, but the TS has so far ignored the question.
I will find other threads instead.

The reason why I ignored that request is because I have no clue what it means that you referred to. I only have basic knowledge about circuits an components. If you mean the voltage going through the transistor legs, in the order of D G S, they are 8.65V, 0V, 0.80V.

This small amplifier has never sounded clean at full signal strength. I get the clean signal when the link is: guitar->USB Interface. In case this amplifier is in between, there is distortion. I need to turn down the volume knob on the guitar to 1/2 position for the distortion on the amplifier to disappear. In that case this small amp sounds great anyway. However, it has to be considered that I have Treble Bleed mod on my guitar volume pot, so in normal situation the amp would output a different sound when turned to 1/2. That's why it would be better if I could feed the amp at full signal strenght from the guitar. Because if I connect this amp to other guitars, without the treble bleed mod they may not sound fine.