I'm a newb to electronics, especially analog electronics, and have run into this problem: I'm trying to amplify an audio signal (output of a ISD1932 chip) and put together a basic non-inverting amplifier using an LM358 OpAmp - exactly the circuit shown as the very first "Typical Application" on page 11 of this datasheet: http://www.national.com/ds/LM/LM158.pdf
When hooked to the audio recorder, I get quiter, rather than louder volume, and my oscilloscope shows that while the input waveform (straight out of the recorder) is about 5Vpp, the output was around 2.5Vpp.
To try to simplify the issue, I then built the amp circuit on a separate breadboard and used a sine wave from a function generator (set to about 1KHz f, 5Vpp) to feed into the amp. The output was a flat line with a little bit of "wiggle" - less than .2Vpp. I tried 2 difference LM358 chips (on the off chance that the 1st one was bad), with the same results.
I suspect I'm missing something obvious, would someone please clue me in?
Thank,
Juliean.
PS: one more question - for audio signals like this, does it matter if the amp is set up as inverting or non-inverting?
As its an audio signal, what you should be using is
AC Coupled Inverting Amplifier
on the top of page 16, or
AC Coupled Non-Inverting Amplifier
on page 17.
Gain is easy to set, the capacitors keep any DC out of your recorder.
You could follow it with
DC Coupled Low-Pass RC Active Filter
on page 17 if you have high frequency noise to filter out as well.
I didn't read in depth, but I believe these designs all intend for you to have +/- power supplies as well, not just single sided +5V.
Wouldn't a gain of 1 just be non-amplified? What would that get me?
Right now, the output from the voice chip fed into a speaker is very quiet. I'd like to make it louder (the intent is to use this as a door-bell/annunciator). I am open to the possibility that I'm doing this all wrong...
Most op-amps are mainly voltage amplifiers and make poor POWER amplifiers by themselves. Op-amps are just not normally designed to drive standard 8 ohm speaker loads.
There are many audio amplifier chips that contain both internal op-amp and power stages to offer a more complete solution for driving speakers. Things you have to know or define before selecting components is the impedance of the speaker, power rating of the speaker, efficiency of the speaker, actual SPL sound level you require, source voltage and current capacity avalible for your application.
An op amp by itself is quite capable of generating a audio line level output that is suitable for plugging into standard computer speakers that contain the audio amplification needed to drive their internal speaker(s) to useful levels. Computer amp/speakers can be found quite cheap or even used in thrift stores for a couple of bucks.
Ah! Now that you've drawn my attention to it, I see that I'm using a DC circuit. D'oh.
I will try the AC circuits.
3 questions on the AC versions:
Which to use - inverting or non-inverting? Does it matter?
Both circuits show a Cin and Co capacitors, but don't give a value. What should it be?
The circuit shows a 3Vpp @ +5V supply - which is less than what I currently have. Presumably going to a higher supply voltage would increase this?
I am looking to drive a 2-3" speaker with this, and want it to be about as loud as a typical doorbell. Would one of these amplifiers me enough? Or do I need something more? How big a power supply would I need to drive this?
If there's a good book/tutorial/etc. on these topics (preferably an on-line one) that someone can point me to, that'd would be great.
Lefty - that actually answers a bunch of my questions.
The PC speaker idea is interesting - I have a bunch floating around, maybe I can make space inside one to fit my various other components.
Still, I'd prefer to just design/build the whole thing from scratch. Can you recommend a specific chip that'd be capable of driving a 2-3" 8 Ohm speaker? My concern isn't for great fidelity, all the system will say is phrases like "back door" or "front door".
impedance of the speaker, power rating of the speaker, efficiency of the speaker, actual SPL sound level you require, source voltage and current capacity avalible for your application.
KE7GKP:
An LM386 is the most common chip used for the application you described. It is designed to take a relatively low line-level voltage and drive a small speaker, while operating from a relatively low supply voltage.
From your link, it looks like with 6V this give me about 1/3 Watt, and with 9V I can do about 1/2 Watt. So together with the 1/2 W speaker from SFE (above), would that get me the volume level I'm looking for?
Also, If I go with the 9V version, can I just feed it directly from a 9V wall wart, or would I need a regulated power supply for it?
From your link, it looks like with 6V this give me about 1/3 Watt, and with 9V I can do about 1/2 Watt. So together with the 1/2 W speaker from SFE (above), would that get me the volume level I'm looking for?
Most likely, however speaker efficiency and proper speaker baffling/mounting enter the picture for actual SPL sound level output Vs audio electrical power consumed.
Also, If I go with the 9V version, can I just feed it directly from a 9V wall wart, or would I need a regulated power supply for it?
Yes you can power it directly from the a 9vdc wall wart. However you may have to add additional filtering caps if there is too much 60hz ripple voltage coming from the wall wart. I've used LM386 chip in some projects in the past and they do work well, however hum and other low level noise can be a problem if circuit layout, signal routing and power quality are not up to snuff. It's all about learning to solve the problems as you come across them if you want to learn and build this kind of stuff yourself. Otherwise buy pre made solutions.
retrolefty:
From your link, it looks like with 6V this give me about 1/3 Watt, and with 9V I can do about 1/2 Watt. So together with the 1/2 W speaker from SFE (above), would that get me the volume level I'm looking for?
Most likely, however speaker efficiency and proper speaker baffling/mounting enter the picture for actual SPL sound level output Vs audio electrical power consumed.
Also, If I go with the 9V version, can I just feed it directly from a 9V wall wart, or would I need a regulated power supply for it?
Yes you can power it directly from the a 9vdc wall wart. However you may have to add additional filtering caps if there is too much 60/120hz ripple voltage coming from the wall wart. I've used LM386 chip in some projects in the past and they do work well, however hum and other low level noise can be a problem if circuit layout, signal routing and power quality are not up to snuff. It's all about learning to solve the problems as you come across them if you want to learn and build this kind of stuff yourself. Otherwise buy pre made solutions.
retrolefty:
Yes you can power it directly from the a 9vdc wall wart. However you may have to add additional filtering caps if there is too much 60hz ripple voltage coming from the wall wart. I've used LM386 chip in some projects in the past and they do work well, however hum and other low level noise can be a problem if circuit layout, signal routing and power quality are not up to snuff. It's all about learning to solve the problems as you come across them if you want to learn and build this kind of stuff yourself. Otherwise buy pre made solutions.
Well, it worked quite well, but as you said, there are hums. I ran the LM386 off 5V and it definitely amplified the sound. However, the amount of noise generated was impressive. I guess it amplified not just the signal but all the noise, too. I guess I will now have to explore filters, but I'm not really sure whether I need high-pass or low-pass. There's no specific frequency for the noise that I can find - when I hook up the scope and hit FFT, there's no noticeable spike at any frequency - just hum all across the spectrum (assuming I'm interpreting the scope results correctly, which is a big if...).
