E Gadgets 3: DIY Preamp with LM386

Last Updated on May 14, 2020

First Published on May 7, 2020

Making Guitar Preamp with LM386 and under testing. Hum noise is still easy, and 6000HZ noise with around -60dB on -12dBV peak recording. Checking with Audacity's spectrum analysis.

Seems to be from PC internal? > No Windows Laptop PC and RasPi 3 have the same noise.

Reconnection of USB reduces the noise. Just Grounding?

LM386 is the well known audio amplifier.

Data Sheet of LM386

The IC is biased internally through PNP transistors' emitter-base leak current. The input impedance has 50K ohms. Comparing 1M/68K in Fender's tube amps, it seems to be lower, so high inductance picks may not do good work for high tone with LM386. However, as long as 2.7 henry picks, it's good in the theory. Several vendors are manufacturing. I have only UTC's one so far.

Note that the similar IC is TA7368PG, however TA's input impedance is 27K ohms and the gain is up to 40dBV. TA's low input impedance tends to emphasis low-mid pitch comparing to LM as a guitar preamp. I tested its module as the preamp which is preferred jazzy.

Data Sheet of TA7368

Remember the work of oscilloscopes or voltmeter. The voltage of some load is observed by high impedance probes of these and the work is also in LM386 and Fender's amps as well. If you set the load at the side of the output, the difference of input impedance can be omitted as long as the load is much lower impedance than input. the load should be lower resistance and the current should be higher to obtain enough voltage for treading on noise. HOWEVER, this idea is not useful for the guitar-to-amp connection. It's useful for the preamp/effector-to-amp connection. Note that the high impedance input suppresses tone chracters of pickups come from pickups' inductance.

Download PDF File: First Version of Electric Guitar Preamp (V.1.0.x)

So, it's still making and considering of the odd in the theory. I like the sound when overdriven. Voltage followers at In/Out are not applied.

You may wonder why grounding through a 0.05 uF capacitor and a 10 ohms resistor before C4 doesn't exist. It exists in typical applications. The grounding makes a Zobel network with a loud speaker. This preamp isn't assumed to connect with a loud speaker, but with a resistive load. The Zobel network is not needed so far.

By the way, LM386 can boost bass sound by connecting Pin No. 5 (output) and No. 1 (Gain) through a 0.033 uF capacitor and a 10K ohms resistor according to the datasheet. It may be fun with the preamp for bass guitar. Also wrote that the load impedance 4 to 32 ohms. Hmm.

The issue is the input impedance. High power pickups for high speed "LA style" players to use hammering and pullng with "scratching" may reduce high pitch tone. However, I'm a ukulele player though.

Issue1: B10K for RV1 is unbalance to change the parameter. B2K is a candidate.

Issue2: B100 for RV2 is a little small. It's difficult to distort sound with diodes. B500 is a candidate.

Issue3: Electromagnetic shield. The big hostile is the guitar to aura the emission.

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