Problem with basic amplifier

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
I want to build an amplifier from spare parts, and built one exactly like this one:

my speaker is a bose 273488004, the transistor is d2375, resistor is 2.7k, since it's the closest i have. i'm using 6x1.5V batteries in series, so 9V total.

now, the problem is, i do not understand the input. i want to use a mono jack for a guitar, it has a positive lead, and a ground. when i connected both input wires to each of the jack lugs, no signal came from the guitar. when i connected a headphone jack (1 green 1 blue and 1 red to the input leading to the capacitor, and the ground to the input leading to ground), nothing happened, no sound was comming from my phone.

Then, i connected the blue and green to bottom input wire, the red to the top - and ti can hear Jimi playing, it works. BUT. i have a spare ground wire, from the headphone jack, no idea where it connects. if i connect the mono guitar jack input, and connect both the input wires to the positive lead, since it's mono, what do i do with the ground? i also noticed the transistor is very hot, is this normal?

Second problem, i'm using batteries. i have no ground. i'm using the negative side of the first battery as the ground. not sure if this is a problem. I'm only experience in guitar wiring electronics, so sorry for a general lack of knowledge, but it's really hard to find answers to such questions online.

All input is appreciated, thanks.
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
connecting the two input wires to the positive mono jack lug, produced no sound when the guitar is plugged in. I understand that the signal is very weak, as the string is distorting a magnetic field and phisically moving electrons back and forth to create a frequency, but i thought that was the point of the amplifier. to amplify the signal. or is it too small to amplify, and therefore i need to amplify it before i can amplify it?:D i'm confused.

Not going to ask anything else, since i know i'm missing something very basic.
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
Hello,

The shown amplifier is a bad design.
There is a DC on the speaker and the efficiency is low.

On this page you will find more complex, but much better designs:
https://sound-au.com/projects-0.htm#pwr

Bertus
Thank you, but the point is to build the simplest possible design amplifier, from scrap electronics, without ordering chips and waiting for them to arrive. i have caps, resistors, transistors, diodes and 500k pots.

If you can point to a design you think i could build, then please do, i would realy appreciate it, since i do not have any idea which of these designs to choose from.
 

danadak

Joined Mar 10, 2018
3,891
This is what simulation shows (not using full model for speaker, just an
R ) -

upload_2019-8-27_12-59-1.png

Note the current and power exceed the capabilities of your transistor,
you will probably burn it out.


Regards, Dana.
 

to3metalcan

Joined Jul 20, 2014
261
The input impedance is TINY in that schematic. A guitar pickup is an inductor on the order of 1-3 Henries with a few Kohms of DC resistance. Its output impedance is HIGH. Apart from how bad the amplifier is (it's bad) almost no signal is going to make it into your input and even if you added a ton of gain the frequency response would be heavily lowpassed (all the bright details of the notes would be lost.) A guitar amplifier needs an input impedance of at least 100Kohms and preferably 1Mohm or more to sound good.
 

Audioguru

Joined Dec 20, 2007
11,251
It is a horrible "amplifier" design:
1) The DC in the speaker will kill the speaker and quickly kill the little battery. A speaker uses AC, not DC.
2) The polarity of the 1uF input capacitor is backwards which might turn off the transistor.
3) The guitar pickup needs a Jfet preamp with an input of at least 1 million ohms.
 

ci139

Joined Jul 11, 2016
1,179
the transistor is very hot, is this normal?
src : http://www.farnell.com/datasheets/59764.pdf → relevant data ::
V.cc = 9V , R.L = 8Ω , R.B = 2kΩ ,
NPN BC547 (TO-92) :
T.J.max = 150°C
P.max = 625mW (de-rate above 25°C = 5mW/°C) ,
R.ΘJC = 83.3°C/W ,
R.ΘJA = 200°C/W ,
h.fe ?guessed.mean( 125 , 900 )? = 299 = ß(!) (derating at higher base currents ≈ 0.5*** = 50%)
V.be ? looked up using LT.spice ≈ 950mV
. . . (the following is simplified !!! and not exact)
V.c = 9V - R.c·(*I.c + **I.b) . . . = 9V - R.c·(*ß / 2*** + 1)·I.b ... = 9V - R.c·(*ß / 2 + 1)·**(V.c - V.be) / R.b
/!\ *I.c = ß(!)·Ib /// 9V - V.c = R.c·(ß/2+1)·(V.c - V.be) / R.b = R.c·(ß/2+1)·V.c / R.b - R.c·(ß/2+1)·V.be / R.b
/!\ **I.b ≈ (V.c - V.be) / R.b /// V.c = ( 9V + 8Ω·(299/2+1)·950mV / 2kΩ ) / ( 8Ω·(ß/2+1) / 2kΩ + 1 ) ≈ 5.95V
I.b = ( 5.95V - 0.95V) / 2kΩ ≈ 2.5mA → I.c = 299 / 2 · 2.5mA ≈ 374mA
P.TOT = 5.95V·374mA + 0.95·2.5mA = 2.2W = 2200mW ( / 625mW) = 3.56x allowed max (+250°C goes here , 16Ω is BC547 's approximate total resistance)
....
max I.c ← (150°C - 25°C)·5mW/°C → 625mW - 25mW = 600mW max @ 150°C (i thought something's wrong here ...)
max I.c ← (150°C - 25°C)·5mW/°C → 625mW - 625mW = 0mW max @ 150°C (... either a keyo or a keyo on a calculator)
→ max power at 25°C is 312.5mW the junction temp is then 87.5°C and apropriate I.c = 36.1mA I.b = *139µA R.b = 56k73Ω
about ::

__________________
→ if you replace the R.b with 16kΩ(57k) you run at the edge of the blade e.g. at maximum values with I.c 71mA(36m) and I.b *500(139)µA ≈ median *72(36m)mA to 8Ω speaker is ±36(±18)mA dynamic power → P = I²R → 10(5)mW Peak → 7.3(3.5)mW RMS ←← it's the dynamic energy range of the tot energy sinking into the speaker of which some 5% is (transferred to air = ) converted to actual sound pressure .e.g 10(5)mW / 20 = 500(250)µW is radiated away (which at the 1m distance ? has 40(20)µPa → 20*log10({40/20} (20/20)) = 6(0)dB loudness ??? "Light leaf rustling" (10dB Wiki))("hard/impossible to hear at meter distance")
_________
about :: https://www.analog.com/en/design-center/interactive-design-tools/power-dissipation-vs-die-temp.html
PD-1.gif
_________
about pickups : http://tomsguitarprojects.blogspot.com/2014/12/electric-guitar-output-voltage-levels.html
 
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BobTPH

Joined Jun 5, 2013
2,255
The bose "speaker" you linked to is not a speaker, it is an amplified speaker with the amplifier built in. What you need to attach a guitar to it is a preamp. Look for "guitar preamp" on google, there are tons of them out there.

Bob
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
Thank's everybody, now i know i'm way more clear on what to do.

I know the design is bad, I delibirately chose the most simple design I could build, to learn through experience. Obviously i'm going to end up a completely different design, i just want to understand the basics.

But are the speakers realy already amplified? the website says "This power is driving the two 2.25″ paper cone speakers (code 273488004), which Bose calls Twiddlers.They are the same speakers used....", it refers to them as speakers twice, and before it talks about an amplifier board that drives the speakers.
I disasembled the broken bose sounddock, and now have the speakers, so do they realy only need to be connected to a pre-amp circuit, and then will work?
 

Audioguru

Joined Dec 20, 2007
11,251
You need to know the resistance and impedance of the raw speaker. Measure the resistance of your leads on your multimeter then measure the resistance of the speaker, then subtract the resistance of the leads on the meter. An 8 ohms AC speaker will measure about 6 or 7 ohms DC.

The speaker is tiny at only 2.25" in diameter so it will fry with the DC in it from that horrible "amplifier" circuit.

The guitar magnetic pickup feeds a preamp circuit that has a high 1M input resistance but not much gain. The preamp drives an audio power amplifier that is designed properly.
 

MrChips

Joined Oct 2, 2009
20,215
Getting reasonable sound from an electric guitar is a lot more complicated than what you think.

1) The electrical signal from a guitar pickup is very weak, about 100mV.
The impedance of the pickup is generally very high, about 100kΩ. The purpose of the preamp is to present a high input impedance, about 1MΩ and output a much lower impedance of about 1kΩ. Also the preamp will boost the signal to about 1V.

2) The amplifier needs to take the 1V signal and boost both the voltage and current in order to drive a speaker with 8Ω impedance. For this the amplifier must deliver power.

The circuit you have shown does none of the above.
Don't believe everything you find on the internet.
 

MrChips

Joined Oct 2, 2009
20,215
If you still want to experiment with a single transistor amplifier, you need to increase the impedance that the transistor sees as a load. In order to do this, add an impedance matching audio transformer to match the impedance of the 8Ω load, i.e. going from about 2kΩ to 8Ω.

amplifier with output transformer.gif
 

ci139

Joined Jul 11, 2016
1,179
what they got @ http://audio-schematic.blogspot.com/2010/09/gambar-skema-rangkain-audio-amplifier.html -- selected because it can be modified for a suitable power for the 6LF22 +9V battery

how i got it to do something at 9V
(it requires a preamp for pickup - and likely some modifications before it works as it simulates . . .)
BC547_TEST-2b.png +revised BC547_TEST-3b.png
(i'm not designing audio too often -- last v. has not yet been passed to the frequency analyzer)
__________________
ready made parts : for pre-amp : MC34071 or other low voltage hi-slew-rate opAmp ... ; power amps : TDA2822M , TBA820M , KA2201/N , ...
using such -- ensures the likelihood of supply voltage independence
 
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