Audio Amplifier - Page 1
Forum section: electronics / beginners

author: staticshock
author_rank: Contributor
timestamp: May 10, 2019, 01:16:39 am
content: It's been awhile and I may have forgotten some information, but any information provided should sort of point me in the right direction. I need some help on designing an audio amplifier for voice. I'm going to have a 7Vrms input that needs to output one 10Vrms and three 3Vrms. I'm assuming I'll need to amplify the 7V to 10V and perhaps a voltage divider to get the 3V. I've attached a diagram of what the circuit should look like roughly. I'm a bit confused or just may be thinking too hard on how to achieve my objective.

author: soldar
author_rank: Super Contributor
timestamp: May 10, 2019, 09:23:18 am
content: When dealing with voice you are better off specifying peak to peak because RMS is not a fixed value like it is for a sine wave signal.

author: Zero999
author_rank: Super Contributor
timestamp: May 10, 2019, 12:26:52 pm
content: You haven't provided enough information: what are the input and output impedances?Does it need to be Hi-Fi or just telephone quality?

author: staticshock
author_rank: Contributor
timestamp: May 13, 2019, 12:22:07 pm
content: The input and output impedance aren't specified. There is really no restrictions to the design. I believe the voice audio is over a radio system so I guess telephone quality.

author: Zero999
author_rank: Super Contributor
timestamp: May 13, 2019, 06:42:44 pm
content: A potential divider can be used to reduce the voltage. If the output impedance is too high, simply connect a unity gain op-amp circuit to the output.The calculator, linked below, can be used to calculate the resistor values. Use the E24 resistor sequence if you just want cheap, commonly available resistor values.http://www.ti.com/download/kbase/volt/volt_div3.htmA non-inverting op-amp configuration can be used to amplify 7V to 10V.https://www.google.com/search?client=firefox-b-d&q=on-inverting+op-ampThe calculator linked to previously can be also used to calculate the resistor values for the non-inverting op-amp, but where it says "output" enter the input voltage and where it says "input" enter the output voltage. This is because a non-inverting amplifier is just an op-amp connected to a potential divider, so it works in reverse. This is the magic of negative feedback: the op-amp does what it can to keep its inputs at the same voltage.

author: staticshock
author_rank: Contributor
timestamp: May 22, 2019, 05:36:27 pm
content: I've converted all Vrms to Vpp and Vamp, but it seems to affect the simulation calculations in PSpice. They are slightly lower than the actual values. Another thing I wanted to add to the circuit is adding potentiometer in the feedback of the op amps to have the ability to amplify the voltage slightly. One potentiometer seems to work, but one is causing an op amp output to clip. Any help? I can screenshot the schematic and output graph if needed.

author: strawberry
author_rank: Super Contributor
timestamp: May 22, 2019, 06:03:03 pm
content: OP go into cliping when it reaches maximum specified output voltage amplitude ( some OP are rail to rail ..input/output)In simulation could be some bug or value/setting error...

author: AlanS
author_rank: Unknown
timestamp: May 22, 2019, 10:32:53 pm
content: Quote from: radiolistener on May 22, 2019, 07:45:15 pmRadioListener - You just made my day. I'll stop laughing sometime around lunch. Good stuff.

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2019, 08:31:24 am
content: Quote from: staticshock on May 22, 2019, 05:36:27 pmThat doesn't make any sense without schematics, simulation results etc.

author: staticshock
author_rank: Contributor
timestamp: May 24, 2019, 03:34:57 pm
content: Well I've figured out the potentiometer. But now I need to add some noise filtering. I've always had trouble understanding where to place capacitors in the circuit. Attached is a drawing of the schematic.

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2019, 04:22:09 pm
content: Quote from: staticshock on May 24, 2019, 03:34:57 pmIt looks sensible, although normally potentiometers aren't used in the feedback loop. The usual way is to use more gain than necessary and attenuate the input with the potentiometers, although it will work.The only capacitors you really need are on the power supply rails. You can AC couple the signals around the potentiometers, so there's no DC flowing through them, which can cause noise, when they're adjusted.

author: staticshock
author_rank: Contributor
timestamp: June 05, 2019, 05:54:23 pm
content: From my thinking, I set the pots in the feedback because the requirement for the amplifier should output 7V to 11V for the first op-amp and 2.5V to 3.5V for the second op-amp. This was the only thing I could thing of to meet that requirement. Other things that I've been thinking of including or modifying in my circuit is a voltage regulator and an automatic gain control. The voltage source available is 28V and I'll need to step it down to 16-16.5V. I don't think I've been exposed to using voltage regulators except voltage dividers..... so not too sure what I can do. As for the AGC, again not a lot of knowledge either than what its purpose is. Could it be something that I can add into my circuit? For my simulations, it's all been done with PSpice, but I'm not sure if PSpice can simulate noise.

author: Audioguru
author_rank: Super Contributor
timestamp: June 05, 2019, 06:47:01 pm
content: Why are you amplifying 7V to 10V or 11V? The difference is only a little louder.Why are you reducing 7V to 3V? The difference is only a little less loud.I wonder why Radiolistener posted a 70 years old vacuum tubes amplifier?

author: staticshock
author_rank: Contributor
timestamp: June 05, 2019, 07:24:34 pm
content: The input 7Vrms signal is apparently too low and it's just been written down that it needs to be amplified to 10Vrms for intercom. The 3Vrms will be going into three different channels for intercoms as well.I think he may have posted to troll me

author: Audioguru
author_rank: Super Contributor
timestamp: June 05, 2019, 08:34:06 pm
content: Whoever told you that 7V is not loud enough and it must be amplified to 10V never tried it and does not understand that our hearing 's sensitivity to loudness is logarithmic. 10 times the power sounds only twice as loud.Maybe the 7V must be reduced to 3V to avoid clipping in the other intercom amplifiers.

author: staticshock
author_rank: Contributor
timestamp: June 10, 2019, 01:29:21 pm
content: Well I've just been updated with more information and the input is NOT 7Vrms, but it is 0.7Vrms. I don't think it poses any major issues other than changing resistors?Also is it better to keep the op amps as non-inverting or change to inverting? I'm also having trouble finding an Automatic Gain Control (AGC) chip that will fit with the existing circuit.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 10, 2019, 03:21:18 pm
content: Well 7Vrms or 0,7Vrms is some difference; for the OPA is just a matter of the dividers, you're right there.If inverting or non-inverting might depend on the output impedance of your 0.7Vrms; if it's low-Z it's possible to use an inverting opamp, if it's hi-Z you rather go for the non-inverting since it has high impedance input.Why do you need an AGC?

author: staticshock
author_rank: Contributor
timestamp: June 10, 2019, 05:48:36 pm
content: My senior acquaintance suggested it. I'm assuming for the same reasons as in Radio applications: want the output to be constant no matter what the input is. (It's not a requirement, but could it be beneficial to my circuit?)

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 10, 2019, 05:58:09 pm
content: hmm, well an AGC is on the receiver side of a transmission, its existance is owed to the fact of signal fading and other influences on the signal in the transmission path.on the sender side signal conditioning is normally done with a compressor; I wouldn't bother to build that discrete but rather go for a ready product like the SSM2166

author: Audioguru
author_rank: Super Contributor
timestamp: June 10, 2019, 07:45:53 pm
content: Now that you reduced the input to 0.7V does the output still need to be amplified to 10V?10V RMS is a very high signal level, maybe it must drive a loudspeaker? Then you need a power audio amplifier, not an opamp.Where does the noise come from, is the intercom used in a noisy factory? If you filter away high noise frequencies then you also filter away the important high frequency consonants sounds of speech.Background noise jumps up and down during speech when AGC is used.

author: staticshock
author_rank: Contributor
timestamp: June 10, 2019, 07:52:31 pm
content: I will find out in the next few days about all of that.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 10, 2019, 10:32:31 pm
content: Have a look at simple single chip audio amplifiers:TDA2030 or TDA2050.Nowaday's small class D amplifier boards from China / Ebay are also common.

author: staticshock
author_rank: Contributor
timestamp: June 12, 2019, 07:48:37 pm
content: Well I have been updated with new information, but my design is pretty much going back to a blank slate. I'm now required to replicate existing circuits, but modify them for a variable gain. The existing circuits used AD840 op amps, but they are now obsolete. From TI I've found OPA846 as the cross reference, but if anyone knows of another substitute for that op amp, please let me know. Also most circuitry use transformer coupling, which I've never been introduced to. Is there a certain type of transformer I need to use?

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 12, 2019, 08:05:07 pm
content: maybe you should fill the blanks for us.normally in audio circuits you don't work with variable gain, but you adjust the signal level before the amp by a simple pot.also I don't understand why to take such rf opamp for an intercom; why aren't some normal TL082/084 not enough? why transformer coupling?if you want some serious advise you should tell more what we are talking about

author: staticshock
author_rank: Contributor
timestamp: June 21, 2019, 06:10:07 pm
content: The circuit consists of a differential input with transformer coupling and then goes through a network of resistors and capacitors. The differential input then goes through a TLE2144 Op-Amp to become one signal that gets amplified by another TLE2144. (For now, only one output is needed for testing.)Transformer coupling has always been used so it was preferred for consistency as well as impedance matching of 600.I've read that for audio circuits, polystyrene and polypropylene capacitors are recommended()