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Need help with an audio amp

AidanKing

New Member
Making a class A audio amp . Was wondering what's the reason for putting the output in parallel to the collector instead of in series? Tried both and got better results with the speaker in series . Pls help .
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Post the schematic, however you should be aware that pretty well every class A amp posted here has been pretty useless. There seems a strange 'mystique' around class A amplifiers, and the idea that they are 'simple' - they aren't generally simple at all, and it's difficult (and expensive) to make a decent one.
 

audioguru

Well-Known Member
Most Helpful Member
A load in series with a transistor has DC current in it.
A speaker should never have DC current in it because the current causes the cone to be forced away from the central magnetic field and maybe hit the magnet structure, and the DC causes unwanted heating in the speaker. A speaker uses AC not DC.
 

Fusseltier

New Member
It's also for matching impedance.
Class A are generally for preamps, and operates on the linear part of the output curve. AB, Or push pull for power amps. I really never cared for class D audio amps but they are useful in some applications. Personally I'd never use a class D amp for hi fidelity music. But you don't need discreet components anymore, you can get complete power amps in one module that are very good quality sound.
They have the preamp and power amp all in one block and you just supply power and the audio in.
I do still make discreet component amps for myself and make my own custom speakers, which some people actually buy from me because they are hi fidelity and quality.
Right now I use an 830 watt per channel (2 channel) push pull power amp for my music into my custom speakers and if I'm remembering it has 0.1% THD at full power.
Its been years since I built everything and calculated the speakers total impedance. I'm actually running a 12 channel multitrack analog mixer into the amp for all of my audio sources for convenience to set levels independently. Maybe I'm old fashioned, but I prefer analog for everything. And yes, the amp is a really heavy piece of equipment, takes two people to carry it, and i mounted it flush in my wall.
 

Fusseltier

New Member
Except matching impedances is something you rarely ever want, it's mostly a VERY bad idea, particularly for audio.

You usually want a low impedance output feeding a high impedance input.
matching impedance is for maximum power transfer.
Before the 90s we always did it, but some equipment started having low output impedance and high input impedance.
It depends on the application you're trying to do, but I'm from the matching impedance way of doing things, especially having worked in communications for 18 years and having a fcc license which was required to work on transmitters., and working at an am-fm radio station and a uhf TV station.
Actually, the only equipment we used with low output impedance were the distribution amplifiers because they weren't balanced linrs, everything else was matched impedance and balanced lines.
And for transmitters, they had to have matched impedance output to the antennas or things could be damaged.
Audio was generally balanced 600 ohm lines.
The difference between the two types were balanced was matching impedance and unbalanced low output to high input. We were always taught to match impedance for maximum power transfer.
 

audioguru

Well-Known Member
Most Helpful Member
A speaker has a resonant frequency. At resonance it has a much higher impedance than at other frequencies. Then if you match its impedance it will still resonate and sound boomy.
 

Fusseltier

New Member
And you definitely want matching impedance for your audio power amplifier and speakers. You could damage your amp if your speakers impedance is too far off from the amps output impedance, or damage the voice coil.
But low power, and preamps you can get away with low output and high input impedance.
 

Fusseltier

New Member
An 8 ohm speaker means it's impedance is 8 ohms at 1khz. Not resonant frequency.
Speaker impedance is how many ohms at 1khz. It's impedance because it's a reactive resistance measurement because it is an ac signal on a coil, which is an inductor. For inductors it's called XL or X sub L is the way it's pronounced.
Its not the resonant frequency.
XL=2πfl inductive reactance formula
And just for your reference
XC=1/2πfC capacitive reactance formula
 

crutschow

Well-Known Member
Most Helpful Member
And you definitely want matching impedance for your audio power amplifier and speakers.
If you mean matching the speaker to the amp's rated output speaker impedance, yes.
But that is definitely not the same as matching impedances for maximum power transfer as you referred to in post #7.
The output impedance of a typical solid-state amp is a fraction of an ohm, much less than the speaker.
For maximum power transfer with a fixed load, you want the source resistance to be as low as possible.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
matching impedance is for maximum power transfer.
Which isn't something you often want, and certainly NOT in audio.

Before the 90s we always did it, but some equipment started having low output impedance and high input impedance.
No you didn't - you are completely mistaken.

If you go back a LOT further, to valve amplifiers, then the speaker WAS matched to the output of the amplifier - which was why valve amplifiers were so appallingly inefficient.

It depends on the application you're trying to do, but I'm from the matching impedance way of doing things, especially having worked in communications for 18 years and having a fcc license which was required to work on transmitters., and working at an am-fm radio station and a uhf TV station.
Actually, the only equipment we used with low output impedance were the distribution amplifiers because they weren't balanced linrs, everything else was matched impedance and balanced lines.
And for transmitters, they had to have matched impedance output to the antennas or things could be damaged.
RF is a different thing entirely, and needs to be matched to prevent destructive reflections.

Audio was generally balanced 600 ohm lines.
The difference between the two types were balanced was matching impedance and unbalanced low output to high input. We were always taught to match impedance for maximum power transfer.
Again, you don't want maximum power transfer - it's FAR too inefficient - it's theoretical best is only 50%.

For audio you want voltage transfer, not power transfer.

I'm quite horrified that you've worked at a radio station and a TV station yet don't have even a rudimentary knowledge of electronics? - this is pretty basic stuff.
 

audioguru

Well-Known Member
Most Helpful Member
A modern amplifier has negative feedback so that its distortion and output impedance are very low. If you have an amplifier with an 8 ohm output impedance driving an 8 ohm speaker then you will hear boomy resonances and smeared sounds of the speaker. The negative feedback causes very good damping of the resonances and allows the amplifier to properly control movement of the speaker cone. A good amplifier has a "damping factor" of over 200 then for driving an 8 ohm speaker its output impedance is 8/200= 0.04 ohms.

Of course the amplifier must "be able" to drive the impedance of your speaker which is not its output impedance.

Here is the damping factor and output impedance of an excellent amplifier:
 

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unclejed613

Well-Known Member
Most Helpful Member
And you definitely want matching impedance for your audio power amplifier and speakers. You could damage your amp if your speakers impedance is too far off from the amps output impedance, or damage the voice coil.
only in vacuum tube amps with output transformers.

with solid state amps you want as low an output impedance as possible in order to have proper control over voice coil movement through the high damping factor. this very effectively flattens the frequency response curve of the speaker because driver and box resonances are damped. a properly designed solid state class AB amp also has very low distortion, very low noise floor, a very flat frequency response, and a good slew rate.
a good rule of thumb for solid state amps and speakers is the speaker's continuous power rating should be about twice the rated output power of the amp... this reduces the likelihood of frying the speaker if somebody bumps the amp up into hard clipping. other than that most solid state amps are ok with whatever reasonable load you connect to them, but most are not very stable below 2 or 3 ohms.
 

unclejed613

Well-Known Member
Most Helpful Member
Here is the damping factor and output impedance of an excellent amplifier:
they must have a huge gain margin in their amps, but i must agree they are very nice amplifiers...
 

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