Author Topic: Audio amp help  (Read 2041 times)

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Offline jeffheath

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Audio amp help
« on: February 11, 2019, 10:55:34 pm »
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?
 

Offline graeme.c.payne

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Re: Audio amp help
« Reply #1 on: February 11, 2019, 11:35:05 pm »
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.
 

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Offline jeffheath

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Re: Audio amp help
« Reply #2 on: February 12, 2019, 12:17:38 am »
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.
 

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Definitely 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=8-29 and 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.
 

Online oPossum

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Re: Audio amp help
« Reply #3 on: February 12, 2019, 02:21:01 am »
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
 
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Online Zero999

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Re: Audio amp help
« Reply #4 on: February 12, 2019, 12:09:26 pm »
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?
Modern 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.pdf
https://www.ebay.co.uk/i/382694778166?chn=ps
 
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Offline GregDunn

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Re: Audio amp help
« Reply #5 on: February 12, 2019, 06:41:18 pm »
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.
 

Offline jeffheath

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Re: Audio amp help
« Reply #6 on: February 12, 2019, 09:23:26 pm »
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.
The 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.
 

Online Zero999

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Re: Audio amp help
« Reply #7 on: February 12, 2019, 10:36:15 pm »
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.
The 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.
Line 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
 

Offline Richard Crowley

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Re: Audio amp help
« Reply #8 on: February 12, 2019, 10:46:37 pm »
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.
 

Offline jeffheath

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Re: Audio amp help
« Reply #9 on: February 13, 2019, 12:59:36 am »
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.
The 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.  :palm: 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"
« Last Edit: February 13, 2019, 01:11:02 am by jeffheath »
 

Online oPossum

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Re: Audio amp help
« Reply #10 on: February 13, 2019, 01:15:07 am »
The 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.

They 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: jeffheath
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"
That 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.
 

Offline Richard Crowley

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Re: Audio amp help
« Reply #11 on: February 13, 2019, 02:33:17 am »
Here is a nice stereo (dual) 10K audio-taper pot from a North American source.  $1 
https://www.allelectronics.com/item/apt-10kd/10k-x-2-audio-taper-potentiometer/1.html

And here is a complete headphone amp board all assembled and tested for $2.
https://www.amazon.com/TDA1308-Headphone-Amplifier-Headset-Preamplifier/dp/B07KMBZDYT
 

Offline L_Euler

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Re: Audio amp help
« Reply #12 on: February 13, 2019, 11:47:11 am »
You can use one of these: TPA 6021A4
Typically used for speakers integrated in computer monitors and can also drive headphones.  The full data sheet offers example circuits for speaker and headphone outputs.
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Offline Richard Crowley

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Re: Audio amp help
« Reply #13 on: February 13, 2019, 03:40:59 pm »
LM386 is an exception to this, it has 2 selectable fixed gain settings.
The LM386 can be set for any gain between 20 and 200 using a capacitor and resistor. See the data sheet for details.

Quote
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.
Yes 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.
 

Offline tooki

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Re: Audio amp help
« Reply #14 on: February 13, 2019, 07:06:01 pm »
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.
 

Offline Audioguru

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Re: Audio amp help
« Reply #15 on: February 13, 2019, 07:26:38 pm »
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.
 

Online Zero999

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Re: Audio amp help
« Reply #16 on: February 13, 2019, 07:31:46 pm »
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.
The 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.  :palm: 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"
There'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.
 

Offline jeffheath

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Re: Audio amp help
« Reply #17 on: February 13, 2019, 08:14:53 pm »
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?
« Last Edit: February 13, 2019, 08:28:00 pm by jeffheath »
 

Offline Richard Crowley

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Re: Audio amp help
« Reply #18 on: February 13, 2019, 10:32:38 pm »
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 Rfeed2

If 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.
« Last Edit: February 13, 2019, 10:44:46 pm by Richard Crowley »
 

Offline jeffheath

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Re: Audio amp help
« Reply #19 on: February 13, 2019, 11:38:39 pm »
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 Rfeed2

If 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.
I'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.
 

Online Zero999

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Re: Audio amp help
« Reply #20 on: February 14, 2019, 09:21:34 am »
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 Rfeed2

If 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.
I'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.
The 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.
 

Offline paulca

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Re: Audio amp help
« Reply #21 on: February 15, 2019, 05:08:11 pm »
I recently built an audio/headphone amp with help from folks on here.

https://www.eevblog.com/forum/beginners/decoupling-dilema/msg1508254/#msg1508254

If 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.
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Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline jeffheath

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Re: Audio amp help
« Reply #22 on: February 15, 2019, 09:38:50 pm »
I recently built an audio/headphone amp with help from folks on here.

https://www.eevblog.com/forum/beginners/decoupling-dilema/msg1508254/#msg1508254

If 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.
I 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?
 

Offline jeffheath

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Re: Audio amp help
« Reply #23 on: February 17, 2019, 04:15:48 am »
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.
 

Offline Richard Crowley

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Re: Audio amp help
« Reply #24 on: February 17, 2019, 05:03:16 am »
...I updated the schematic to put the capacitors in the correct location. 

The correct location for the capacitors is between the pot and the input resistors.

The 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.

You may not notice it immediately but the DC from the op-amp will cause premature deterioration of the pot with increased noise over time.
« Last Edit: February 17, 2019, 05:05:01 am by Richard Crowley »
 


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