Jump to content

Entry for Feb 2018's Guitar Of The Month is now open!
http://www.projectguitar.com/forums/topic/49215-guitar-of-the-month-february-2018

Sign in to follow this  
lingyueqing

Could you help me analysis my schematic for guitar amp

Recommended Posts

I want to build my own guitar amp and I know a little about electronics (handling op-amps and little more) and I would appreciate if anybody could explain me these circuits,

http://mad-science.wonderhowto.com/how-to/build-your-own-mini-altoids-guitar-amp-for-about-5-0135412/

enter image description here

and I found LM386 datasheet as this: http://www.kynix.com/uploadfiles/pdf2286/LM386M-1.pdf

and this one

https://sites.google.com/site/amplificatoare/12w-amplifier-using-741-op-amp enter image description here

I would like every detail that I should know in order to understand those two circuits

Share this post


Link to post
Share on other sites

The first one probably doesn't need much analysis based on what is already detailed in the datasheet. All the information is already there. In broad terms, the LM386 is a single chip, low-wattage power amplifier intended for battery operation where sound quality isn't critical. Maximum output is about 1W. Circuit gain is adjustable between 20x and 200x by varying the impedance between pins 1 and 8. As you have it shown, your LM386 is configured for a gain of 20x, plus has a volume control at the input.

The second one is interesting, but it will never perform well. Essentially it's just an opamp with a current-boosting output stage made up of push-pull darlington transistor pairs T1/T2 and T3/T4, to allow the opamp to drive a speaker. The method used to provide the current boost is really odd though. Rather than simply taking the output of the opamp and boosting it directly, they're measuring the current being drawn from the supply rails via R4 and R5 while the opamp is running and using that the boost the output. Circuit gain is fixed by the ratio of R3 to R1 (270k/10k = 27x). High frequency response has been limited by placing C1 in parallel with R3 to about 11.8kHz (1/(2 x 3.14 x 50pf x 270k) = 11789Hz).

I get the impression that the second circuit has been cooked up as an academic exercise rather than for sensible design. The IC specified (LM741) is a truly awful unit by today's standards. Even the choice of transistors seems haphazard, as none of the types specified come from the same complementary familiies - BC178 is a small signal silicon NPN, AC108 is a small signal germanium PNP. 2N6107 (PNP) already has a complementary unit available in the same series (2N6292 NPN), so why they've specified a random 2N5294 seems weird.

The method employed to develop sufficient current through R4/R5 to drive the push-pull stage is to load the output of the opamp with an extremely low 47ohm resistor (R6). Maximum output voltage swing on the LM741 is to within about 3V of the supply rails, so even assuming you can drive the LM741 hard enough to achieve +/-9V on the indicated +/-12V supply, the LM741 is going to hit the brick wall long before then because of the built-in current limitation of the chip. Current output on the LM741 is limited to +/- 25mA, so max clean voltage swing you can expect is a measly  +/-1.175V. It will never deliver the 12W output that's promised on that page, and will spend a lot of its time distorting like crazy. There are better ways of making something that will move a speaker.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

×