Audio amplifier project - help with speaker ratings/types

Thread Starter

jameselder232

Joined Oct 31, 2010
40
Thanks for the advice, i can't use the TL074 if their min supply is 7V. So it looks like i might have to try and get hold of a decent opamp myself for use in my project then, if i want a decent output.

Well with the bridging i was thinking about doing two separate circuits, one for the left channel and one for the right channel, and inverting one of these and bridging them together to then power two speakers connected in parallel. Does this sound like a stupid idea?

As for the impedance matching, I'm sure i read that somewhere its just our lab coordinator said in the advice section of the spec to do impedance matching. I still need to do input impedance matching i suppose though.

I'm definitely going to look into using a transistor or two instead of the diodes, thanks for the advice.

The department said they will not spend more than about £1-1.50 on a speaker so it looks like I'll just have to make do with what i have, and they'll have to consider that when marking us.

I really do appreciate all the help, thank you.
 

Thread Starter

jameselder232

Joined Oct 31, 2010
40
I've got a circuit using a transistor to replace the diodes, but I'm not sure about biasing so I'm going to have to read up on that. In particular what do i want to set the emitter voltage to in relation to the collector voltage? Is it half? so if the collector voltage is 3V say, the emitter should be 1.5V? so adjust the pot on the BJT base to provide this voltage?

Thanks, in the mean time I'll see if i can figure it out.
 
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Audioguru

Joined Dec 20, 2007
11,248
Well with the bridging i was thinking about doing two separate circuits, one for the left channel and one for the right channel, and inverting one of these and bridging them together to then power two speakers connected in parallel. Does this sound like a stupid idea?
For stereo you need two separate amplifiers each driving its own speaker.
A bridged amplifier has two amplifiers with one of them inverted. Their current is double the current of a single-ended amplifier.
If you connect 2 speakers in parallel then their total current is double.

As for the impedance matching, I'm sure i read that somewhere its just our lab coordinator said in the advice section of the spec to do impedance matching. I still need to do input impedance matching i suppose though.
Impedance matching is never done for audio circuits. If the amplifier's input impedance matches the signal source impedance then you are throwing away half the signal level.
 
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Thread Starter

jameselder232

Joined Oct 31, 2010
40
Yeah i understand about the stereo audio, when i wrote the above post i was planning to 'cheat' with mono audio output, but using both channels to increase the power output. But to be honest i think i might as well do it properly.

As for the impedance matching, well i don't know why we are advised to do it, but I'll just write the reasoning behind not doing it as my explanation, as when i researched on the net i got the same answer about it never being done for audio.

I've got the idea about biasing the transistors now (I think), and have found a good compromise between output voltage and crossover distortion in my simulation. However i get a lower power output than i did in my previous circuit? But the distortion is reduced i suppose.

I replaced the two diodes with an npn transistor, and a pot to control the base voltage/current.

I'm spending so much time on this but it seems i cannot increase the power output. Am i right in thinking the output amp has a set gain based on the transistor specs (Hfe)? And so there is a maximum current gain possible unless i cascade amplifiers together, or using darlington transistor pairs?

If this is the case maybe i have tweaked my output amp as far as i can, i have reduced the distortion greatly also. And so to increase output power i need to bridge an inverted output (although from what you said earlier i might not have enough current - I don't understand this point fully yet) or cascade amplifiers/transistors together.

I feel like I'm asking too many questions now :p

Thanks
 

R!f@@

Joined Apr 2, 2009
9,919
Impedance matching is never done for audio circuits. If the amplifier's input impedance matches the signal source impedance then you are throwing away half the signal level.
Why do u say tht guru. Can u please explain why the signal is lost.
 

Thread Starter

jameselder232

Joined Oct 31, 2010
40
OP
Wud u prefer to use another amp like LM386
We are not allowed to use any IC amps. This counts towards my degree course and the specification is to create an audio amplifier consisting of:

preamp
tone control
output amp
2 speakers (doesn't have to be stereo though)

We are advised to use 3 AA batteries although we can use more but will be marked down. And also we are limited to using the components in the labs unless we purchase our own, or have a good reason and can persuade them otherwise.

We are given advice which is to do impedance matching (but this isn't relevant for audio amps) and also to have an inverted output.

Thanks anyway.
 

Audioguru

Joined Dec 20, 2007
11,248
"Impedance matching is never done for audio circuits. If the amplifier's input impedance matches the signal source impedance then you are throwing away half the signal level."

Why do u say tht guru. Can u please explain why the signal is lost.
If the amplifier's input impedance is the same as the source impedance then you have a voltage divider that reduces the signal level to only half. The other half of the signal is wasted in the voltage divider.
 

R!f@@

Joined Apr 2, 2009
9,919
We are not allowed to use any IC amps. This counts towards my degree course and the specification is to create an audio amplifier consisting of:

preamp
tone control
output amp
2 speakers (doesn't have to be stereo though)

We are advised to use 3 AA batteries although we can use more but will be marked down. And also we are limited to using the components in the labs unless we purchase our own, or have a good reason and can persuade them otherwise.

We are given advice which is to do impedance matching (but this isn't relevant for audio amps) and also to have an inverted output.

Thanks anyway.
Then U can make pretty descent sounding amp using Darlington pairs. It can drive 2 8 ohm speakers. One can be a little woofer and the second one can be a piezo tweeter.
Make a small cabinet to mount the two speakers.
U can run it off using 3 AA batteries for around a minute , just to show it and test. Better yet u cud use rechargeable AA cells that can supply 2A.
 

Audioguru

Joined Dec 20, 2007
11,248
If you are not allowed to use IC amps then you probably must use only transistors for the preamp and tone controls and not use opamps. Besides, few opamps work from a supply as low as 3V to 4V.

Most Baxandall tone controls circuits use bass and treble controls with a single opamp or transistor. Look it up in Google.
 

Thread Starter

jameselder232

Joined Oct 31, 2010
40
If you are not allowed to use IC amps then you probably must use only transistors for the preamp and tone controls and not use opamps. Besides, few opamps work from a supply as low as 3V to 4V.

Most Baxandall tone controls circuits use bass and treble controls with a single opamp or transistor. Look it up in Google.
We are allowed to use op-amps, just not any ready made audio amp ICs. The advice is to use op-amps for the preamp, high and low pass filters for the tone control and transistors for the output amp.

I don't understand how they can give us a spec with advice to use op-amps, but we must use a 4.5V battery. Especially since the only op-amps they provide are the 741, 358 and the TL074.

I was looking into using a buck boost converter last night to increase the supply voltage, and maybe provide the negative rail.

I can't use the 358 as i think it has a max amplification of 1.5V less than the rail, and so if i used it with single supply and virtual ground, i wouldn't be able to get the amplification required.

I will check out the tone controls, but i do like the circuits i have found already, they provide bass/treble cut and boost, and i can understand how they work too.
 

Thread Starter

jameselder232

Joined Oct 31, 2010
40
Then U can make pretty descent sounding amp using Darlington pairs. It can drive 2 8 ohm speakers. One can be a little woofer and the second one can be a piezo tweeter.
Make a small cabinet to mount the two speakers.
U can run it off using 3 AA batteries for around a minute , just to show it and test. Better yet u cud use rechargeable AA cells that can supply 2A.
We are given two really cheap speakers to use...

Also you say it can run for a minute, do you mean before the batteries need replacing? Why is it the case if so? Because that wouldn't really be acceptable, i think they'd spot that when they test the circuit.
 

Audioguru

Joined Dec 20, 2007
11,248
The LM358 has many problems for audio but it works with a supply as low as 3V. It doesn't matter that its output cannot go higher than 1.5V less than the positive supply because your power amplifier will have some voltage gain.

Darlington transistors have a voltage loss of about 2V each so the output of an amp will be nothing when using a supply as low as 4.0V (your three AA cells used a little).

Bass has nothing to do with how long the battery lasts. Name-brand AA cells (in North America) from Energizer or Duracell have their specs on their datasheets shown on the internet. The voltage of three cells begins at 4.5V then quickly drops to 3.6V. You are making only a single amplifier so you can cheat a little and select high gain transistors for making two (stereo) bridged amplifiers. Each amplifier will have an output at clipping into 8 ohms of 0.25W when the battery voltage averages 4.0V. Each amplifier will have an efficiency of about 60% so each amplifier takes 400mW from the battery or a total of 800mW for stereo. Then the three AA cells will last only about 1 hour before the voltage drops lower than 4.0V then the volume must be reduced to avoid clipping.
But, you never play music at full blast all the time so the battery will last about 10 times longer.
 

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Audioguru

Joined Dec 20, 2007
11,248
Your tone controls circuit uses one opamp for the bass and a second opamp for the treble. But real tone controls circuits use a single opamp to do both like this (my component values are a little different from yours so use yours if you want. Bias the opamp a little lower than half the supply voltage since the max output of an LM358 is 1.5V less than the supply voltage).
 

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R!f@@

Joined Apr 2, 2009
9,919
Bass has nothing to do with how long the battery lasts.
Please Audioguru, do not make such silly mistakes concerning Audio. I though u knew what u were talking abt. I have doubts on how well u know audio and it's power consuming capabilities.

It's a known fact that low range which is known as bass requires all the power that it's supply can produce.

Here are some pointers tht I want u to think abt and tell me why bass has nothing to do with how long a supply can last.

1. Why does a woofer amp produces more heat compared to it's high frequency counterpart?

2. Why does supply voltage sags when powerful bass is played?

3. Why does a car amp shut's down when high gauge wires are used to power it from the battery?

4. Why does a car battery drain fast when a high end car setup is played with engine shutdown?

5. Why does car audiophiles recommend to use a high capacity alternator if one has to beef up a car audio setup.

I can go on & on.

Taking all these into account,

I want everyone or anyone to prove tht this has nothing to do with current demand.

And prove me wrong this has nothing to do with battery capacity.

And ones again, prove me wrong tht a battery is not rated for it's current capacity.

And last but not the least, prove me wrong tht a battery drains slower if it's load current is at a minimum.

So, tell me, does bass has nothing to do with how long a battery last?
 
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Audioguru

Joined Dec 20, 2007
11,248
Extremely loud and continuous bass? It is RAP, not music.
Some people crank up the volume of the woofer much too high so of course its amplifier uses a lot of current and gets hot.
Some woofers are inefficient so they need a lot of power to be heard.

But here we have a cheap little speaker that cannot produce any bass. It produces only mid-range frequencies, something like a telephone. The amplifier has such a low output power (0.25W or less) that you won't hear any bass even if the speaker could produce bass due to the Fletcher-Munson curves (we don't hear low-level bass frequencies well).
 

Kermit2

Joined Feb 5, 2010
4,162
75% (or more)of the wattage will be used reproducing the lowest 10% of the audio spectrum. *( In normal acoustic environments with normal audio equipment and tonal settings)

There is no wool left on this sweater for anyone to 'nitpick' over. Really
 

R!f@@

Joined Apr 2, 2009
9,919
This is not nitpicking. We must be sure of the things we say, and I am obliged to know the truth and to show the proper way if one is misled.

Point is tht even if bass is not heard tht does not mean amp is not trying to reproduce it. Unless they actually cut off at the desired frequency.

Of course we cannot hear it due to speaker quality and it's low performance but this does not mean amp in not trying to reproduce them. The amp will be dumping bass to the speaker and will waste power.
Only way for the battery to last long is to cutoff bass from ever injecting into the amp.

Tell me this is not true.
 

Audioguru

Joined Dec 20, 2007
11,248
Point is tht even if bass is not heard tht does not mean amp is not trying to reproduce it. Unless they actually cut off at the desired frequency.

Of course we cannot hear it due to speaker quality and it's low performance but this does not mean amp in not trying to reproduce them. The amp will be dumping bass to the speaker and will waste power.
Only way for the battery to last long is to cutoff bass from ever injecting into the amp.
The OP asked about his project, not a high power car sound system. His project does not have any bass.

The OP already said that his little speaker sounds bad when it tries to produce bass so he used his tone control to reduce the bass and it sounds much better. Therefore the bass is cut off.
 
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