yes .. the coil shld be ac coupled , if DC component is there , the coil movement will be unbalanced and cause distortion.]I have a speaker that is 8ohm (0.3W) - using v^2/R - will that give me the Maximum RMS voltage i can apply to the speaker? This must be AC coupled to the speaker so that no DC passes to it, because that heats up the coil and may damage the speaker right?
Ive read, from the way the speaker works, that its just a coil moving in the magnetic field of an armature magnet, so is this 8ohm, just the DC resistance of the coil, or.. the AC reactance of the coil? If thats the case, i thought this inductor (because its a coil?) is a function of frequency how can it be fixed at 8ohm? - i read a few posts that suggest that this changes at resonance or something!
well what u said abt class AB has a slight mistake(practicaly), ie it is a class B , for an amp to be a class AB , a sight forward bias is also needed for the push pull o/p stage (remember abt the 0.7 vlt drop at base), else waveform will be chopped at bottom.Lots of the circuits ive seen on the net have mentioned alot about Class A and Class AB operations. So from what ive read, you can tell a circuit is Class A because the output transistors are on all of the time, and Class AB if one transistor is one during the positive of the cycle, and the other is on during the negative - as in a push pull output stage - is this right?
yes u can use sine waveIm trying to simulate amplifier circuits, but im not sure what to use as the input, im not sure how to model 'sound'. I understand its a mixture of frequencies 20hz - 20khz, and will have a maximum voltage depending on the type of input. Can i just use a sinewave input at these frequencies to simulate my circuits, or do i need to take anything else into account?
a power amp needs burst of power , so use a high value capacitor as bufferI know if you pull too much current out of a battery its voltage drops, so how do i get around the problem of supplying power to an output speaker rated at 300mw, can a 9v battery do that? Is there a minimum power i can deliver from a battery and i should choose lowered powered speakers to make sure the rail doesnt drop - or use big capacitors across them to hold them tight?
What kind of Whats? RMS continuous Watts, "maximum" (which is simply double the RMS number) whats or "music-power" momentary whats?Megamox said:I have a speaker that is 8ohm (0.3W) - using v^2/R - will that give me the Maximum RMS voltage i can apply to the speaker?
Correct. DC also causes the coil to be offset in the magnetic gap causing awful distortion (but some guitar players like it).This must be AC coupled to the speaker so that no DC passes to it, because that heats up the coil and may damage the speaker right?
The impedance rating of a speaker is at its lowest, just above its resonant frequency, where the low inductance doesn't raise it much like happens at high audio frequencies where it could be 20 ohms. At resonance its impedance could be 40 ohms or more. Its DC resistance is probably 6 or 7 ohms.is this 8ohm, just the DC resistance of the coil, or.. the AC reactance of the coil? If thats the case, i thought this inductor (because its a coil?) is a function of frequency how can it be fixed at 8ohm? - i read a few posts that suggest that this changes at resonance or something!
Correct.Lots of the circuits ive seen on the net have mentioned alot about Class A and Class AB operations. So from what ive read, you can tell a circuit is Class A because the output transistors are on all of the time, and Class AB if one transistor is one during the positive of the cycle, and the other is on during the negative - as in a push pull output stage - is this right?
You need to use many sinewave frequencies. You also need to combine two frequencies to measure intermodulation distortion. You need to use many levels to see an amplifier's behaviour at clipping and crossover distortion at low level. Frequently squarewaves are used to see if an amplifier is approaching instability which causes ringing on the transitions of the wave. Frequently an amplifier is tested with 2uF across the load to simulate phase-shift caused by speaker resonances or a crossover network.Im trying to simulate amplifier circuits, but im not sure what to use as the input, im not sure how to model 'sound'. I understand its a mixture of frequencies 20hz - 20khz, and will have a maximum voltage depending on the type of input. Can i just use a sinewave input at these frequencies to simulate my circuits, or do i need to take anything else into account?
An audio amplifier isn't used at max output all the time unless you like to play a siren. When it clips the peaks slightly at max output, its average power is probably 1/10th with most music or speech. A little 9V alkaline battery should last a long time (go to www.energizer.com to see) with a drain of only 30mW (maybe only 8mA including idle current). At max output of 300mW a little 9V alkaline battery is fine to provide momentary 33mA. A battery's internal resistance increases as it runs down so a big (1000uF) capacitor across it helps to keep its voltage from flucuating too much.I know if you pull too much current out of a battery its voltage drops, so how do i get around the problem of supplying power to an output speaker rated at 300mw, can a 9v battery do that? Is there a minimum power i can deliver from a battery and i should choose lowered powered speakers to make sure the rail doesnt drop - or use big capacitors across them to hold them tight?
Dean Huster said:The battery's still going to "run down" or "go flat" depending upon which side of the pond you're on.
You don't need 1A power transistors in a simple amplifier producing max power of a puny 250mW. The BC547B and BC557B are rated at 500mA and will be fine by themselves.Megamox said:The output transistors are power transistors, rated at 1A.
High power amps use emitter resistors but your have the diodes to stop thermal runaway.The resistors on the emitter's are there to stop thermal runaway, i wasnt sure what value to use, as ive seen many different values in other designs, so i put in 8ohm, to try and make output impedance = input impedance of the speaker, and have maximum power transfer.
Where are you going to find a transistor with a gain of exactly 300?Originally i calculated to 470k to be 5M
Megamox said:Thanks audioguru! I figured that id need a rating of 1A because at the highest output, the emitter would be at 7.6V across the 8ohm = 950ma? When i said the gain's were 300, i meant on average, id like the circuit to be indepedant of small hfe differences in transistors. Btw what does the bootstrap part of the circuit you added do? It seems like a capacitor on the emitter will reduce the AC impedance of the emitter and have larger swings of current through the transistor - at least thats what happens when a cap is added on the emitter of other common emitter amplifier's ive seen.
No, the speaker is capacitor-coupled to the amp's output which rests at about 4.5V with a brand new alkaline 9V battery. If the transistors don't have any loss (they do) then 4.5V peak into 8 ohms is only 562mA.Megamox said:I figured that id need a rating of 1A because at the highest output, the emitter would be at 7.6V across the 8ohm = 950ma?
Design circuits with DC negative feedback so they work with transistors that have just about any current gain.When i said the gain's were 300, i meant on average, id like the circuit to be indepedant of small hfe differences in transistors.
The emitter-followers have a very low output impedance. They drive the bootstrap cap with about the same signal as their input:Btw what does the bootstrap part of the circuit you added do? It seems like a capacitor on the emitter will reduce the AC impedance of the emitter and have larger swings of current through the transistor - at least thats what happens when a cap is added on the emitter of other common emitter amplifier's ive seen.
In the simple audio amplifier circuit I posted, the input impedance is difficult to calculate because the inverting circuit with negative feedback to its input isn't "perfect" like an opamp. It is a little more than its 27k input resistor.Megamox said:Just wondering, how do you work out the input impedance of these designs?
Megamox said:Just a quick question about the power ratings on the output PUSH-PULL transistors. Maximum current is drawn when they're around 4.5v and that current is 562ma. So the power in the trans is (9-4.5v)*(562ma) = 2.5W.
The 2n3904 is rated at 500mw, wont that be a problem?
Megamox said:Just had a thought, if we added another 2n3904 in parallel with the PUSH 2n3904 and fed the base from the same input, the collector currents would be divided equally between both of them, so each one would have half as much current and so a higher hfe. Would this work and push more power into the speaker at peak?
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