pocket sized guitar amplifier

[scripted13]

Then the output power will be very low and little transistors can be used.
When the little 9V alkaline battery is brand new the maximum output voltage swing into an 8 ohm speaker will be about 6V p-p. Then the peak current in the output transistors is 3V/8 ohms= 375mA.
6V p-p is 2.1V RMS then the output power is only 0.55W like a cheap clock radio.

You can use 2n4401 NPN and 2N4403 PNP output transistors.

Unclejed's circuit will work very well but its has transistor Q6 connected upside down. It is supposed to be an emitter-follower.
His circuit has a dual polarity supply so it will need biasing at its input and three added coupling capacitors.
Also the battery needs a bypass capacitor.
A high input impedance preamp will be needed as I showed.

cant i alter the resistors so that i can have a better output? how did you know the output without even knowing the resistors used in the circuit? cant i use two npn transistors? lets say we will stick to my original circuit, cant i have at least 10 v p-p out put to supply for the speaker?

 

[scripted13]

Then the output power will be very low and little transistors can be used.
When the little 9V alkaline battery is brand new the maximum output voltage swing into an 8 ohm speaker will be about 6V p-p. Then the peak current in the output transistors is 3V/8 ohms= 375mA.
6V p-p is 2.1V RMS then the output power is only 0.55W like a cheap clock radio.

You can use 2n4401 NPN and 2N4403 PNP output transistors.

Unclejed's circuit will work very well but its has transistor Q6 connected upside down. It is supposed to be an emitter-follower.
His circuit has a dual polarity supply so it will need biasing at its input and three added coupling capacitors.
Also the battery needs a bypass capacitor.
A high input impedance preamp will be needed as I showed.

cant i alter the components in my circuit to have a better output or cant i just alter the components to have a higher gain so that my output would be higher than what you have computed? also how did you compute for the maximum output without knowing the components of the circuit? another question is, when will i use the given currents and voltage in the data sheet of the transistor, for example the Vce and Ic on the data sheets of a transistor?

 

[scripted13]

also what is the typical, minimum and maximum frequency of a single coil guitar pick-up. im confused on what the information on the internet is giving me, they all suggest different values. thanks again in advance.

 

[Nigel Goodwin]

cant i alter the resistors so that i can have a better output? how did you know the output without even knowing the resistors used in the circuit? cant i use two npn transistors? lets say we will stick to my original circuit, cant i have at least 10 v p-p out put to supply for the speaker?

No you can't - the maximum output is entirely dependent on the power supply - for 10V p-p you would need a supply higher than 10V (due to losses in the amp).

10V p-p is only 1.5W anyway.

 

[unclejed613]

in the circuit i posted Q3 and Q4 are constant current sources. Q3's preset current is 700uA which is divided evenly between Q1 and Q2. Q4's preset current is 7mA. Q4 acts as a linear load for Q7 (a resistor would work in place of it, but since the current would no longer be a constant value, it would introduce even harmonic distortion, and wouldn't have the voltage swing it needs to drive the outputs).

you have to realize test circuit conditions are only to establish a baseline for testing for the data sheet. they are not necessarily the conditions you operate the transistors at. otherwise, transistors and other linear devices wouldn't be as useful and flexible as they are. just because the data sheet shows the devices being tested at 10V and 10mA doesn't mean that's where you have to use the device.

the point of this whole exercise is for you to learn how these devices are used in the real world. also, i suspect your teacher wants you to learn how to put theory to use. so, use this opportunity to learn how to put theory to use. it sounds to me like you are looking for a "pre-fab" answer to the problem rather than trying to solve the problem yourself. engineering is about problem solving. this is why your instructor wants you to build from scratch rather than just plugging in a chip.

knowing WHY something works is what separates tinkerers from engineers.

 

[scripted13]

in the circuit i posted Q3 and Q4 are constant current sources. Q3's preset current is 700uA which is divided evenly between Q1 and Q2. Q4's preset current is 7mA. Q4 acts as a linear load for Q7 (a resistor would work in place of it, but since the current would no longer be a constant value, it would introduce even harmonic distortion, and wouldn't have the voltage swing it needs to drive the outputs).

you have to realize test circuit conditions are only to establish a baseline for testing for the data sheet. they are not necessarily the conditions you operate the transistors at. otherwise, transistors and other linear devices wouldn't be as useful and flexible as they are. just because the data sheet shows the devices being tested at 10V and 10mA doesn't mean that's where you have to use the device.

the point of this whole exercise is for you to learn how these devices are used in the real world. also, i suspect your teacher wants you to learn how to put theory to use. so, use this opportunity to learn how to put theory to use. it sounds to me like you are looking for a "pre-fab" answer to the problem rather than trying to solve the problem yourself. engineering is about problem solving. this is why your instructor wants you to build from scratch rather than just plugging in a chip.

knowing WHY something works is what separates tinkerers from engineers.

that was a very nice and inspiring answer, thanks for that, it just made me more enthusiastic about my project, thanks man.

 

[audioguru]

cant i alter the resistors so that i can have a better output?

The power supply voltage sets the maximum output power if the circuit and the resistor values are correct.

how did you know the output without even knowing the resistors used in the circuit?

I know that the emitter resistor values for the output transistors should be very low (about 0.22 ohms to 1 ohm)) so that they do not waste much output power).
Then I used simple arithmatic to calculate the the output voltage swing and output power.

cant i use two npn transistors? lets say we will stick to my original circuit, cant i have at least 10 v p-p out put to supply for the speaker?

No.
Your original circuit is class-A so it wastes A LOT of power ALL THE TIME as I showed you in my post #14. Its battery is HUGE and will not fit in your pocket. Your circuit gets VERY HOT so it will burn you.
Audio power amplifiers use a class-AB circuit. The circuit posted by Unclejed has its output transistors without biasing so it is class-B (then it produces crossover distortion).

 

[alec_t]

it is class-B (then it produces crossover distortion)

But guitarists love distortion . Battery life would be improved over class-AB.

 

[unclejed613]

true, but it's distortion that gets worse with DECREASING signal level (an interesting guitar effect, i made a box once that did this), but can be extreme enough to effectively mute the output below a certain signal level. both of these things are great when you can control them (i.e. the effects box i mentioned and also "noise gate" boxes), but can be a real hassle if you can't turn them "off" or control their thresholds'. i think the OP could get away with fixed bias using 2 diodes between the bases of the output devices. he may end up with a little bit of nonlinearity at zero crossing, but Peavey amps use fixed bias as well, with a little bit of crossover distortion noticeable below a watt (but who runs Peavey amps below a watt?). if this amp is only working at a few watts, the OP can always add another diode drop if the crossover notch is too much. if he glues the diodes to the transistors or the heatsink between the transistors, he will also get some thermal tracking for the bias. Mirage subwoofer amps are another item that uses diodes for bias, and they RTV the diodes to the TO-220 tabs of the output transistors. with proper feedback, most of the crossover notch is slewed out anyway.

 

[audioguru]

But guitarists love distortion.

I think electric guitar players have blasted away the high frequency parts of their hearing so they cannot hear how bad and raspy is their sound.
BUZZZZZZ! BUZZZZZZ! That ain't music, it is noise.

 

[alec_t]

That ain't music, it is noise.

Personally I think most modern music is noise

 

[unclejed613]

i grew up in a time when distortion became part of music, and i kinda happen to like it. i'm a guitarist myself. i guess we all can agree to disagree here. as a "for instance" here in colorado, the classic rock stations like to play a song by Neil Young called "Out of the Blue, Into the Black" they seem to like the "unplugged" (slow, mellow, acoustic guitar) version of the song. i kinda like the "raw" (distortion, "these volume controls go to 11") version better. after all it is a song about a punk rocker (Johnny Rotten). i never liked punk rock much myself. my mother, who thought the music i listened to was noise, absolutely loved Jimi Hendrix's "Star Spangled Banner". my aunt, who was a true blue Beethoven fan, really liked "Hold Your Head Up" by Argent. so we all have our musical tastes, and that's ok. i'm still trying to convince my kids that "Subterranean Homesick Blues" by Bob Dylan was the first popular rap song....

 

[BeerBelly]

With a given voltage, more volume can be obtained by using a lower impedance speaker.
Also by bridging 2 amplifiers.

 

[unclejed613]

With a given voltage, more volume can be obtained by using a lower impedance speaker.

actually that's more wattage because you double the current, but that doesn't always mean you get more volume as some small speakers are terribly inefficient

Also by bridging 2 amplifiers

this doubles wattage by putting two amplifiers in series with one inverted.

with both examples, remember this amp is battery powered so there will be tradeoffs between higher wattage and battery life. the first one has the advantage of using a single amplifier

the OP said "pocket size" so for instance having two 3" speakers might not cut it as far as size, and having a second amplifier doubles circuit board and heat sink real estate. i guess the size of the pocket would matter a lot. i think most of us are assuming pants pocket, which would be about 3"x5"x2" or so. a large coat pocket might allow about 5"x 9"x 4". i've seen a pocket guitar amp (a miniature version of a Marshall Stack) thats just a little bit bigger than the first set of dimensions.
https://www.zzounds.com/item--MSHMS2 and it's 5"x4"x2.5"

 

[scripted13]

a question for all of you gentlemen, can i design a class a amplifier by just using one transistor? can i achieve a gain of 20 by using a 9v battery? if not what volts of battery should i use? thanks all of you in advance.

 

[Nigel Goodwin]

a question for all of you gentlemen, can i design a class a amplifier by just using one transistor? can i achieve a gain of 20 by using a 9v battery?

Yes to all - but a big NO to can it feed a speaker, and will it work as a guitar amp.

Battery voltage has no relationship to the gain of the stage, which would just be a preamp.

 

[scripted13]

so added Vcc to the amplifiers won't affect their gain?

 

[audioguru]

Here is your class-A amplifier.
It is powered from an ordinary 9V battery that lasts about 5 minutes even when the amplifier is not playing.
Its output is 0.35W RMS and is distorted.
Its gain is 19 but can be 20 if R1 is changed to 8.2 ohms.
It gets very hot (and the battery also gets hot) so it will burn you.
Its input impedance is very low so another transistor is needed to feed it. The additional transistor will also get hot and will waste the battery power more quickly.

A class-AB amplifier draws a low current when it is not playing so an ordinary 9V battery will last 24 hours but when the amplifier is playing music fairly loud the battery will last about 16 hours. Its output is about 0.6W with no distortion. When playing, it gets warm, not hot.

 

[unclejed613]

no, the Hfe isn't dependent on the collector voltage. but it does decrease with increasing collector current. this is called "beta droop" and is a cause of distortion in transistors.

there is a very old circuit that uses two transistors in a push-pull configuration that does have some gain, but it's rather crude and would need a few tweaks to get a little more power out of it. it uses two 1.5V batteries for power, i'm sure you could increase these to 3V each side without a lot of trouble. the voltage gain is around 15. the 10 ohm resistor is the speaker load, use an 8 ohm speaker there.

 

[Mr RB]

I think electric guitar players have blasted away the high frequency parts of their hearing so they cannot hear how bad and raspy is their sound.
BUZZZZZZ! BUZZZZZZ! That ain't music, it is noise.

I'll tell you now, if Mozart had had access to electric guitars, all the effects, amps and digital recording etc he would not have wasted another minute plunking away on that silly piano!

The reason all your beloved classical music is full of pianos and violins etc is because the great composers of the day simply did not have any better instruments! The great composers of today DO.