How do I boost the audio output? - Page 1
Forum section: electronics / beginners

author: Epovo
author_rank: Contributor
timestamp: January 20, 2024, 06:43:23 pm
content: I recently bought a wireless microphone. The receiver has a jack and I plugged into the AUX input of my soundbar. The problem I have is, that even with the receiver knob to the max and the soundbar to the max as well, the sound is not too loud.So I thought I needed some amplification; not much, maybe 5X. I went ahead and made a simple one-transistor amplifier following the attached circuit. The amp works fine when driving an 8 ohm speaker. But if I insert it into the line from my receiver to the AUX input, I can't hear anything. Not that I am susprised - the circuit was never designed for that purpose. But I don't know what else to try. Any help would be appreciated-

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 20, 2024, 07:19:17 pm
content: An OP amp would work.

author: BillyO
author_rank: Super Contributor
timestamp: January 20, 2024, 09:25:12 pm
content: That "amplifier" is just about as wrong as you can get. The input impedance will be extremely low and the distortion would be crazy. Look up a simple class AB design and use a properly configured class A pre-amplifier.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 20, 2024, 09:35:34 pm
content: Additionally, the poor speaker would have its cone permanently halfway or more out, due to the DC current flowing thru it.

author: MarkT
author_rank: Frequent Contributor
timestamp: January 20, 2024, 10:32:13 pm
content: My take on a single stage line-level transistor amp:on reflection C1 can be larger like 2µ2 for better bass.

author: Epovo
author_rank: Contributor
timestamp: January 21, 2024, 01:07:28 pm
content: I think I will try the OP amp solution. Thank you all

author: BillyO
author_rank: Super Contributor
timestamp: January 21, 2024, 03:10:12 pm
content: Quote from: Epovo on January 21, 2024, 01:07:28 pmIt won't work. If you drive that transistor of yours with an op-amp you will only end up basically shorting it out. Most op-amps will deliver only a few mA (maybe 39 or so) and it will not do anything to improve the situation as mentioned above.There is a reason amplifiers are designed the way they are and it's because that's what you need to do to get them to work.

author: langwadt
author_rank: Super Contributor
timestamp: January 21, 2024, 03:23:04 pm
content: Quote from: BillyO on January 21, 2024, 03:10:12 pmdid you even read the OP? an opamp would be perfectly fine for ~5x gain for a line input

author: Epovo
author_rank: Contributor
timestamp: January 21, 2024, 03:25:46 pm
content: No, you misunderstood. I'm gonna get rid of that amplifier, that I should have never built. I am going to insert the OP amp between the output of the receiver and the AUX input of my soundbar.

author: BillyO
author_rank: Super Contributor
timestamp: January 21, 2024, 03:27:27 pm
content: Quote from: langwadt on January 21, 2024, 03:23:04 pmYou see this working do you? Did you look at his circuit?

author: BillyO
author_rank: Super Contributor
timestamp: January 21, 2024, 03:28:29 pm
content: Quote from: Epovo on January 21, 2024, 03:25:46 pmOkay, that sounds better. Yes, I did misunderstand.

author: Epovo
author_rank: Contributor
timestamp: January 23, 2024, 05:56:24 pm
content: I used an op amp t741 in the configuration on the image below, but the output does not look good (input is a sine wave) (see attached image). Other than an op amp malfunction, I don't know what else could that be...?

author: BillyO
author_rank: Super Contributor
timestamp: January 23, 2024, 05:58:13 pm
content: Are you running it off a single supply?Your input signal is pure AC so you will either have to use a split supply or create a virtual ground at Vcc/2.

author: Epovo
author_rank: Contributor
timestamp: January 23, 2024, 05:59:35 pm
content: Yes I am...

author: BillyO
author_rank: Super Contributor
timestamp: January 23, 2024, 06:00:38 pm
content: Quote from: Epovo on January 23, 2024, 05:59:35 pmSee my update above.

author: Epovo
author_rank: Contributor
timestamp: January 23, 2024, 06:03:48 pm
content: thank you so much. I'll do that the virtual GND thing

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2024, 07:10:49 pm
content: In addition to the other criticisms of the original schematic, C1 is backwards and will be biased with 0.6V of the wrong polarity. If it doesn't damage the capacitor, it will introduce more distortion.I would suggest using an amplifier IC such as the LM386, if this hasn't been recommended already. I admit, I'm in a rush so haven't had time to read the entire thread.https://www.ti.com/lit/ds/symlink/lm386.pdf

author: langwadt
author_rank: Super Contributor
timestamp: January 23, 2024, 08:31:26 pm
content: Quote from: Zero999 on January 23, 2024, 07:10:49 pmno point in a poweramp and to do ~5x gain of a mic/line signal

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2024, 11:41:19 pm
content: Alright I've had a bit more time to read the thread properly now.There are different types of amplifiers. The type used to drive a speaker, is a power amplifier, whilst the type you need to boost the signal a bit for a sound bar is a small single amplifier.The op-amp solution will work, but it needs to be biased to half the supply voltage and the signal coupled by capacitors.Below are a collection of op-amp circuits showing the dual and single supply configurations. The original poster needs the top right one.Oh I forgot to ask: is it run of a battery, which isn't powering anything else, i.e. a really smooth power supply? If so, the circuit can be simplified a bit, but I'll have to post it later, as I don't have time now.

author: Epovo
author_rank: Contributor
timestamp: January 25, 2024, 04:54:31 pm
content: Thank you all for your help. I am using the simplest one, which I attach. For R1 and R2 I am using a 10K potentiometer.Using a signal generator (the audio ouput of my laptop) I can see the circuit amplifying in the amount I desire by adjusting the potentiomenter. So apparently it works fine. BUT when I connect the circuit into its final destination (between the output of my receiver and the AUX input of my soundbar) I hear nothing.Using the oscilloscope I can see that there is a signal coming out from the receiver when I talk into the mic, *as long as the output of the receiver is not connected*. As soon as I connect it to my circuit, I can't see the signal anymore, even if the power of my amp is off. What could be happening here?PS:If I connect the receiver to the AUX input of my soundbar (thereby bypassing my amp), it still works, but as I said in my opening post, the sound is too low.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2024, 04:59:37 pm
content: How is the op-amp powered? Please post a schematic showing exactly how it's connected to the power supply.Note, the circuit you've posted needs a negative, as well as a positive power supply rail. It will not work properly from a single power supply.

author: Epovo
author_rank: Contributor
timestamp: January 25, 2024, 05:28:24 pm
content: See the circuit attached. I use two resistors to create a virtual ground, as I use a single 9v battery, as suggested by BilliyOQuote from: BillyO on January 23, 2024, 05:58:13 pmAs I mentioned, it seems to work when I connect it to the output of my laptop, but not to the receiver....PS:Apologies for the quality, I will try to get some software do depict schematics

author: alligatorblues
author_rank: Regular Contributor
timestamp: January 25, 2024, 06:29:21 pm
content: This is ridiculous. You can purchase audio amps on eBay for $12. With 24V 3A power supply, you'll get plenty of boost.

author: Epovo
author_rank: Contributor
timestamp: January 25, 2024, 06:36:40 pm
content: Thank you @alligatorblues. I know I can buy it. I am doing this in part to learn something.

author: langwadt
author_rank: Super Contributor
timestamp: January 25, 2024, 06:39:48 pm
content: Quote from: alligatorblues on January 25, 2024, 06:29:21 pmdid you even read the OP?

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2024, 06:53:52 pm
content: Quote from: alligatorblues on January 25, 2024, 06:29:21 pmAnd that would be a ridiculous idea. The original poster requires a signal, not a power amplifier.If it's a ready built solution the OP is looking for, then he should search for a pre-amplifier, not a power amplifier.Quote from: Epovo on January 25, 2024, 05:28:24 pmIt sounds like an issue with loading. The rail splitter has an equivalent resistance equal to both of the resistors connected in parallel, which is 1k. This isn't a problem, at DC, because they only carry the tiny bias currents, but at audio frequencies, the voltage will bounce around, when the circuit drives a small load. Adding bypass capacitors will reduce the impedance of the ground node considerably, by charging and discharging as the op-amp's output swings between positive and negative.A couple of other notes:I would recommend adding another resistor to limit the gain to a maximum level, otherwise it'll make the potentiometer difficult to adjust and it very easy for the op-amp to be driven into saturation.I've added labels to denote the voltages in the circuit, with the all important 0V node, the point from where all voltages are referenced to.EDIT:Is this a stereo amplifier?Why use the old LM741? It's pretty noisy and doesn't work well off such low voltages.It would be better to use the TL074. One op-amp for each channel, with the other as a rail splitter. The spare op-amp could be used to boost the rail splitter circuit and reduce the impedance, without big capacitors.

author: Epovo
author_rank: Contributor
timestamp: January 25, 2024, 07:02:55 pm
content: Thank you all very much. I found out what was going wrong - It was a rookie mistake (the kind I do) with the connectors

author: tooki
author_rank: Super Contributor
timestamp: January 26, 2024, 10:34:01 am
content: Quote from: Epovo on January 25, 2024, 07:02:55 pmNamely…?

author: EPAIII
author_rank: Super Contributor
timestamp: January 26, 2024, 11:02:51 am
content: If you want to learn about OP Amp circuits and how to design them, tryThe OP Amp Cookbook, by Walter G. Jung.It is somewhat old, but the basics do not change, just the IC part numbers. And even the classic 741 still works today. I have made line amplifiers like you want with them many times. It can have a high impedance input and can supply enough current at the output to drive almost any audio input, which should be high impedance.And if your 741 circuit lacks performance in some area, there are many, many OP Amps with the same pin-outs and better specifications that can be plugged into the same socket as the 741 to provide better performance. I have started with a 741 many times and pulled it and just substituted a better one. That works quite well for audio circuit design.But you gotta have a dual (+ and -) supply or make a virtual ground in your circuit. And if you use a single supply, it (the supply) can't share a ground with either your input device or your output device.https://nortonsafe.search.ask.com/web?q=op%20amp%20cookbook&ssdcat=321&lang=en&source=nag&year=2015&locale=en_us&geo=us&version=22.23.10.10&plang=sym:en&buildname=retail&heartbeatid=4ae3d98c-56e4-4df2-adf1-00b1d085e47e&eapenabled=false&env=prod&vendorid=1014930&plid=866&plgid=41&skup=21389648&skum=21376863&skuf=90001202-fa&endpointid=4ae3d98c-56e4-4df2-adf1-00b1d085e47e&lic_type=2&lic_attr=17059858&psn=7jjxx287wjvq&templatecat=sbu_w_1000_5039_n360dsp_retail&schemacat=sbu_w&schemaver=1.0.0.0&olpchannel=retail&osvers=10.0&oslocale=iso:usa&oslang=iso:eng&os=windows&showuninstallsurvey=1&installstatus=updated&vendorsrc=firefox&machinelocation=us&sw=0&3in1=0&npw=0&hp=0&dsp=0&cdest=nag&annot=false&vendorConfigured=ask&o=APN12178&prt=ngc&ver=3.21.0.6&tpr=111&chn=1014930&guid=4ae3d98c-56e4-4df2-adf1-00b1d085e47e&doi=2023-11-26&browser=FireFox&prod=DS&doi=2023-11-26&installSource=nag&cmpgn=oct23&darkMode=false&sameTabLaunch=falseQuote from: Epovo on January 25, 2024, 06:36:40 pm

author: alligatorblues
author_rank: Regular Contributor
timestamp: February 11, 2024, 02:07:32 am
content: Quote from: Zero999 on January 25, 2024, 06:53:52 pm

author: Zero999
author_rank: Super Contributor
timestamp: February 11, 2024, 09:56:45 am
content: Quote from: alligatorblues on February 11, 2024, 02:07:32 amWhat a bizarre rant.Exactly how does that help the original poster, with boosting the signal level of a wireless microphone, to the level required for the auxiliary input of a soundbar?