Audio amp help  - Page 1
Forum section: electronics / beginners

author: jeffheath
author_rank: Regular Contributor
timestamp: February 11, 2019, 10:55:34 pm
content: Looking for the simplest solution to take stereo composite audio (red and white connectors) and output to a headphone jack (stereo) with a volume control. I know there are tons of amp boards on amazon, but apparently something like a small class D amp board isn't a "linear" amp and would not sound good with headphones. The closest thing I could find was a phono preamp that happened to have a volume control and phono output, but cost way too much. Suggestions?

author: graeme.c.payne
author_rank: Newbie
timestamp: February 11, 2019, 11:35:05 pm
content: Not sure if you are looking for a finished product or a DIY project. If you want a finished product, there are a lot available with a wide range of price and performance. (I searched for "stereo headphone amplifier" on Amazon. Keep in mind that headphones demand the same high quality audio but at much lower power levels than needed by speakers.A "phono" amp is definitely not what you need. A phono amp is designed to equalize the output of a phonograph and provide it to the line-level input of an audio amplifier.Sent using Tapatalk

author: jeffheath
author_rank: Regular Contributor
timestamp: February 12, 2019, 12:17:38 am
content: Quote from: graeme.c.payne on February 11, 2019, 11:35:05 pmDefinitely DIY, although I thought if I could find something cheap enough that did this I would just use it altogether. The only thing I can find that does what I need is this:https://www.amazon.com/Sabaj-Audio-Portable-Headphone-Amplifier/dp/B01EUFQAQS/ref=sr_1_29?keywords=stereo+headphone+amplifier&qid=1549928508&s=gateway&sr=8-29and it's just plain too expensive. I don't need uber audio quality, I was just looking for something like a DIY schematic of a solid composite audio to (stereo) headphone jack setup.

author: oPossum
author_rank: Super Contributor
timestamp: February 12, 2019, 02:21:01 am
content: You can use a 'high' current op-amp such as the NJM4556 for a headphone amp.Google 'cmoy headphone amp' for some circuit ideas.https://en.wikipedia.org/wiki/CMoy

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2019, 12:09:26 pm
content: Quote from: jeffheath on February 11, 2019, 10:55:34 pmModern class D amplifiers are fairly linear can and have very respectable THD figures. They're commonly used in smart phones and portable music players. It's quite likely you won't be able to hear any difference between a modern class D amplifier and a class AB amplifier, if you don't know which type you're listening to i.e. a blind test.There are other reasons not to want class D such as RFI emissions, although they can be minimised by proper filtering.What's your budget?How about the MAX97220? It's class AB, not D and modules are available off ebay.https://datasheets.maximintegrated.com/en/ds/MAX97220A-MAX97220E.pdfhttps://www.ebay.co.uk/i/382694778166?chn=ps

author: GregDunn
author_rank: Frequent Contributor
timestamp: February 12, 2019, 06:41:18 pm
content: What impedance do your headphones have? If they're low impedance then you'll want an amp which can deliver more current and still keep distortion below audible levels. That's the real specification you should be looking at, not whether it's Class D, linear, or what have you. Some headphone amps will work OK with the high impedance phones but not the lower ones.Since your budget is so restricted, you'll need to check the chip specs carefully before building, to make sure you get a suitable device.

author: jeffheath
author_rank: Regular Contributor
timestamp: February 12, 2019, 09:23:26 pm
content: Quote from: GregDunn on February 12, 2019, 06:41:18 pmThe impedance is about 30 ohms, so I think something like this lm386 amp might work, but my concern is that this, as well as the cmoy amp, are meant to be preamplified, (like from a smartphone) and won't amplify from "line level" audio.

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2019, 10:36:15 pm
content: Quote from: jeffheath on February 12, 2019, 09:23:26 pmLine level audio is already a fairly high level, so you don't need much amplification to drive headphones. The LM386 set to a gain of 20 will probably be too much gain for headphones if they're very sensitive. There should be a dual ganged potentiometer for volume control.https://en.wikipedia.org/wiki/Line_level

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 12, 2019, 10:46:37 pm
content: An LM386 will easily drive low-impedance headphones supplied from common consumer line-level. The LM386 has a minimum gain of 20 (26dB) and you can set it for gains up to 200 (46dB) using simple capacitor and resistor.Your aversion to modern digital amplifiers seems unfounded. If you examine the data sheets, you will likely find that a modern digital amplifier chips have lower distortion than that decades-old LM386.

author: jeffheath
author_rank: Regular Contributor
timestamp: February 13, 2019, 12:59:36 am
content: Quote from: Richard Crowley on February 12, 2019, 10:46:37 pmThe funny thing is, I don't have any aversion to digital amplifiers, It's just that I'm new to audio and didn't even know that "linear" just means analog.I looked at the data sheet for the MAX97220 and was wondering if I could just add a ganged potentiometer as shown, (for volume control) or if there's "more to it than that"

author: oPossum
author_rank: Super Contributor
timestamp: February 13, 2019, 01:15:07 am
content: Quote from: jeffheath on February 12, 2019, 09:23:26 pmThey would probably work just fine as designed for headphone amp use. If you need more gain it is usually just a matter of changing the feedback resistor. (LM386 is an exception to this, it has 2 selectable fixed gain settings.)Quote from: jeffheathThat is all that is needed. Use an audio taper pot rather than linear taper.If you choose to use a Class D amp, be sure to carefully read and follow the PCB layout guidelines. They can be sensitive to layout and not work properly with a bad PCB layout.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 13, 2019, 02:33:17 am
content: Here is a nice stereo (dual) 10K audio-taper pot from a North American source. $1https://www.allelectronics.com/item/apt-10kd/10k-x-2-audio-taper-potentiometer/1.htmlAnd here is a complete headphone amp board all assembled and tested for $2.https://www.amazon.com/TDA1308-Headphone-Amplifier-Headset-Preamplifier/dp/B07KMBZDYT

author: L_Euler
author_rank: Regular Contributor
timestamp: February 13, 2019, 11:47:11 am
content: You can use one of these: TPA 6021A4Typically used for speakers integrated in computer monitors and can also drive headphones. The full data sheet offers example circuits for speaker and headphone outputs.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 13, 2019, 03:40:59 pm
content: Quote from: oPossum on February 13, 2019, 01:15:07 amThe LM386 can be set for any gain between 20 and 200 using a capacitor and resistor. See the data sheet for details.QuoteYes this can be a "trap for young players" and not necessarily a good beginner project. But there are assembled and tested PC boards with the chip and all the passive components for barely more than the cost of the bare chip.

author: tooki
author_rank: Super Contributor
timestamp: February 13, 2019, 07:06:01 pm
content: With class-D amps, buy one made specifically as a headphone amp. The ones for speakers generally do not allow you to bridge the two negative outputs for the L and R, as is normally needed in headphones. (To do it, you have to, at minimum, add a resistor network.) So it's better to just get a purpose-built one.

author: Audioguru
author_rank: Super Contributor
timestamp: February 13, 2019, 07:26:38 pm
content: The Headbanger circuit above adds positive feedback to cancel most of the negative feedback for lots of gain at low frequencies, a loss of high frequencies and lots of distortion and hiss.

author: Zero999
author_rank: Super Contributor
timestamp: February 13, 2019, 07:31:46 pm
content: Quote from: jeffheath on February 13, 2019, 12:59:36 amThere's really no such thing as a digital amplifier. It's a marketing term. The output is analogue, because it's PWM and there's an infinite range of duty cycle. Some class D amplifier accept a digital input, but that makes them a type of digital to analogue converter, as the output is still a continuously variable duty cycle.Anyway, a class D amplifier generally makes little sense for something so low powered as a headphone amplifier. Yes, the MAX97220 just needs a dual ganged potentiometer for a volume control and it should be taper, rather than linear. This is because the human ear has a logarithmic response. Doubling the power will not make it twice as loud. To double the volume we need something like 8 times the power, but it's dependant on the sound being plated.

author: jeffheath
author_rank: Regular Contributor
timestamp: February 13, 2019, 08:14:53 pm
content: I looked up the tda1308, and it's actually eol'd by the manufacturer, which is weird. I found an equivalent, the ts482, and drew this based off of the datasheet. I don't want to wait for anything to come in from china, so I'll just buy a breakout board and the get the components on mouser. I'm thinking it's class AB, so I shouldn't have to worry about board layout. I think i'd have to tinker with the complete board to get the volume pots connected anyway. Would I have to worry about the feedback resistors conflicting with the volume pot values?

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 13, 2019, 10:32:38 pm
content: You have left out a couple of important components:Rin1 and Rin2 are critical to the proper operation of the op-amp circuit.For unity gain they should be the same value as Rfeed1 and Rfeed2If you provide Rin1 and Rin2 properly, then there will be no significant interaction between the value of the pot and the rest of the circuit.

author: jeffheath
author_rank: Regular Contributor
timestamp: February 13, 2019, 11:38:39 pm
content: Quote from: Richard Crowley on February 13, 2019, 10:32:38 pmI'm not sure whether or not I should have the capacitors before or after the pots, but I can change that pretty quickly, so I'll go ahead and get everything and put it together.

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2019, 09:21:34 am
content: Quote from: jeffheath on February 13, 2019, 11:38:39 pmThe capacitors should be after the potentiometers, otherwise the bias point of the circuit will shift, as the volume is adjusted. A capacitor is needed to block the DC voltage to the potentiometer.

author: paulca
author_rank: Super Contributor
timestamp: February 15, 2019, 05:08:11 pm
content: I recently built an audio/headphone amp with help from folks on here.https://www.eevblog.com/forum/beginners/decoupling-dilema/msg1508254/#msg1508254If you want I can send you a bill of materials, KiCAD files, gerbers and a few already made PCB boards (for postage only). It does require some surface mount soldering though.Works perfectly, very high quality and very loud. Designed to fit into a box available online which I can dig out the part code for too.

author: jeffheath
author_rank: Regular Contributor
timestamp: February 15, 2019, 09:38:50 pm
content: Quote from: paulca on February 15, 2019, 05:08:11 pmI appreciate that; but this really just stemmed from a project I was doing and made me think; hmmm... what IS the simplest way for me to hook my headphones up to say, a dvd player, or game console or something, (and have volume control) so I want to see how this "one chip operation" does. So.... thanks but no thanks; but that is a very nice amp. Have you thought about posting the project on github?

author: jeffheath
author_rank: Regular Contributor
timestamp: February 17, 2019, 04:15:48 am
content: So I got the parts in from Mouser. I put it together, and it works great! The ts482 chip was all of 60 cents, and it took minimal circuitry to add composite audio input and a volume control. I can't say I hear any noise, even with my cheap bench power supply. I updated the schematic to put the capacitors in the correct location.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 17, 2019, 05:03:16 am
content: Quote from: jeffheath on February 17, 2019, 04:15:48 amThe correct location for the capacitors is between the pot and the input resistors.Quote from: Zero999 on February 14, 2019, 09:21:34 amYou may not notice it immediately but the DC from the op-amp will cause premature deterioration of the pot with increased noise over time.

author: jeffheath
author_rank: Regular Contributor
timestamp: February 17, 2019, 06:28:16 am
content: Quote from: Richard Crowley on February 17, 2019, 05:03:16 amOops.. I thought he was saying I had it the wrong way and didn't even process the rest of his post.. Fixed for (hopefully) the last time... Will see what it sounds like.

author: bson
author_rank: Unknown
timestamp: February 17, 2019, 09:34:50 pm
content: If you attenuate the output and reduce the gain accordingly instead of the input, you 1) reduce the amount of gain needed, 2) provide a fixed input impedance, 3) make the load more resistive (although HPs are already mostly resistive), and 4) makes the input offset voltage non critical: little gain is applied, and it's divided down on the output by the volume control, so output can be DC direct. It's only power amplifiers where output attenuation isn't possible, but for µW type loads like HPs it's the go-to. If you attenuate first you push the signal down further in the noise floor, only to pull it back up again with gain which invariably pulls up noise with it. (See Friis' formula.)Here's a super simple HP amplifier, but you'd be surprised how good it sounds with a monitor-like headphone like the AKG K102 and a quality source. I sell a slight variation on it, purely based on word of mouth, to some very picky people.Thumb:Full PNG:http://www.rockgarden.net/download/eevblog/hpa.pngKiCad v5 schematic:http://www.rockgarden.net/download/eevblog/hpa.schEnjoy!

author: paulca
author_rank: Super Contributor
timestamp: March 16, 2019, 10:57:00 am
content: You could also consider a Baxandall configuration which produces a stable logarithmic active volume control. ie. the potentiometer is in the gain loop.https://istem.engineering.osu.edu/sites/istem.engineering.osu.edu/files/uploads/5557volumecontrolsproject.pdf

author: Zero999
author_rank: Super Contributor
timestamp: March 16, 2019, 02:00:20 pm
content: Quote from: Richard Crowley on February 17, 2019, 05:03:16 amThat's what I said:Quote from: Zero999 on February 14, 2019, 09:21:34 amQuoteTh normal issue is wiper noise, when the potentiometer is adjusted, but in this case it's worse, because the DC bias point of the amplifier will change, as the potentiometer is adjusted.