audio amplifier <-- need help

[machll]

hi,

im thinking of building an audio amplifier for my AS level course work, and could use some advice on achieving one to suit my needs.

the amplifier must:
work
if it can be avoided not a push/pull amp

as it is an A level project i also have to specify a problem to assess, therefor the amp i want to build must be temperature controlled by an input transducer(thermistor), but i am not to sure what would be the best option for this type of amp:

1) as temperature increases, resistance in the transducer increases therefore gradually decreasing the volume

or

2) as temperature increases, the increase in resistance through the transducer triggers an 743 op-amp to switch the amplifier off.

also if i cannot integrate a mosfet into this circuit could someone guide me onto how to remove the 0.7v distortion on the push and pull transistor amp? any help (in the form of diagrams or text) would be apprecciated. thanks in advance, Machll

p.s i have only just started electronics for a-level and have no previous experience in the subject so please feel free to correct me if im wrong.

 

[Nigel Goodwin]

Re: a level project <-- need help

hi,

im thinking of building an audio amplifier for my AS level course work, and could use some advice on achieving one to suit my needs.

For a start it would be an idea to fill your location in!, this is a world wide forum, and most members won't know what an AS level is!. For those who don't know, it's a UK school examination - taken at about 17 years old, a newish interim examination between GCSE (16 years old) and A level (18 years old).

the amplifier must:
work
if it can be avoided not a push/pull amp

You need to give far more details of what you want, depending on the power required push/pull is almost definately a requirement. Push/pull isn't hard, it's probably harder not to use it - except for really tiny amplifiers.

What sort of power output do you want?, into what impedance?, and what kind of power supply are you planning using?.

as it is an A level project i also have to specify a problem to assess, therefor the amp i want to build must be temperature controlled by an input transducer(thermistor), but i am not to sure what would be the best option for this type of amp:

1) as temperature increases, resistance in the transducer increases therefore gradually decreasing the volume

or

2) as temperature increases, the increase in resistance through the transducer triggers an 743 op-amp to switch the amplifier off.

Temperature of what?.

also if i cannot integrate a mosfet into this circuit could someone guide me onto how to remove the 0.7v distortion on the push and pull transistor amp? any help (in the form of diagrams or text) would be apprecciated. thanks in advance, Machll

Any amplifier circuit you find on the net will already do this (MOSFET amps do it as well), it's simply done by biasing the output stages in class AB rather than class B.

p.s i have only just started electronics for a-level and have no previous experience in the subject so please feel free to correct me if im wrong.

Does this mean you've not already done GCSE?.

Starting an A level electronics course without doing the previous five years work seems a little rash?.

 

[machll]

Re: a level project <-- need help

sorry for being a bit vague before

im looking for around 10 watts max to drive a standard 8ohm loudspeaker, and most likely a 9v dc variable power supply. the thermistor will measure temp of the MOSFET or PNP/NPN push and pull but im not sure which is best to use(this is where i need guidance).

i want the thermistor to control the volume but i have 2 options i could either:

1)construct a circuit that decreases the volume slowly when a certain temparature is reached (i need to think in further detail how to do this) so the thermistor in this instance acts as a thermal guard and for the mosfet/push and pull to minimize potential damage to the amp

2)construct a circuit whereby once the mosfet/push and pull reachs a certain temperature the thermistor triggers an op-amp comparitor circuit to switch the amplifier off. thus the circuit now has a thermal cutoff point

i prefer the former option but i need advice on which one would be best\easist to make.

i decided to do a level electronics because it is something that im interested in therefore it did not seem irrational to do it.

just give me some pointers :shock:

also:

the circuit will be assembled on breadboard,
what temperature do you reckon would be the max working temp,for my amp?

 

[machll]

also ignore the poll!!! :twisted: :evil:

 

[Nigel Goodwin]

Re: a level project <-- need help

im looking for around 10 watts max to drive a standard 8ohm loudspeaker, and most likely a 9v dc variable power supply. the thermistor will measure temp of the MOSFET or PNP/NPN push and pull but im not sure which is best to use(this is where i need guidance).

For a start you need to do the maths!. Power output is dependent on speaker impedance and supply voltage - where W=V*V/R, with V in volts RMS and R in ohms. To convert the supply to volts RMS (ignoring any losses) you divide by 2.828. This gives 3.18V RMS, so 3.18*3.18/8 gives 1.26W RMS.

If you work the formula backwards you can find the power supply voltage that will give 10W into 8 ohms - then add a few extra volts to allow for losses. About 30V is the right sort of supply!.

i want the thermistor to control the volume but i have 2 options i could either:

1)construct a circuit that decreases the volume slowly when a certain temparature is reached (i need to think in further detail how to do this) so the thermistor in this instance acts as a thermal guard and for the mosfet/push and pull to minimize potential damage to the amp

2)construct a circuit whereby once the mosfet/push and pull reachs a certain temperature the thermistor triggers an op-amp comparitor circuit to switch the amplifier off. thus the circuit now has a thermal cutoff point

i prefer the former option but i need advice on which one would be best\easist to make.

It seems a strange idea, for a start a small amplifier like 10W is easily (and cheaply) designed not to get very hot in the first place - secondly, it's usual practice to have a temperature sensor feeding a fan - so it provides extra cooling if it gets too hot.

The easiest version would be to turn it off at a certain temperature.

i decided to do a level electronics because it is something that im interested in therefore it did not seem irrational to do it.

just give me some pointers :shock:

Try seaching on google, there are LOADS of sites with amplifier designs, for an example try http://www.discovercircuits.com/.

the circuit will be assembled on breadboard,
what temperature do you reckon would be the max working temp,for my amp?

If it's too hot too touch, it's probably getting too hot.

Breadboard isn't a good choice, for a start the output devices need to be mounted on a heatsink - veroboard would be a far better bet, or a PCB - most schools have PCB facilities.

 

[machll]

yeh i forgot to do the maths

that occurred to me after i'd sent the post, but as it goes my school only has breadboard available so im a bit unlucky, but anyway thanks for your help

if you have anymore ideas for me, please enlighten me

 

[machll]

i just remembered, the powerpacks we have at school max out at 17 volts, and thats when its running realaibly, so i'd probably be running it bout 4w-5w so its with the voltage limit of the power supply.

but knowing me ive probs done the maths wrong