How to amplify Stereo Headphone Outputs

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brodave38
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How to amplify Stereo Headphone Outputs

Post by brodave38 » Mon Nov 17, 2008 2:03 pm

Is there a low-cost way to amplify stereo headphone outputs (50mV?) to
Line Level Inputs (500mV?) with a portable device powered by 3-14V DC?
It doesn't need to have gain controls, but should be a good impedance match. Thanx,
Bro. Dave38

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Externet
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Post by Externet » Mon Nov 17, 2008 2:12 pm

Welcome, Dave.
If the earphone output level is lower than the line level, is because the volume knob is low. Crank it up.
A LM386 chip per channel is the simple way to do it, if you insist in amplifying. The data sheet and application notes are on the net, and no need to match impedances..
Miguel
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brodave38
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Stereo Headphone Output Levels

Post by brodave38 » Mon Nov 17, 2008 2:31 pm

Thanx for the fast reply, Externet! Please bear with me, but The output device is a cell phone with no gain controls. I believe that the outputs are
50 mV and aren't Line Level Inputs 500 mV minimum? And isn't the
characteristic impedance of earphone outputs a complete mismatch with
Line Level impedance? Is the earphone jack on a Laptop able to connect
directly to line inputs on an audio amplifier?
Thanx Again,
BroDave38

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Externet
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Post by Externet » Mon Nov 17, 2008 2:45 pm

If an earphone output has no volume control, it is new to me, but I do not have a cell phone :sad:

Line level is as you say, about 500mV RMS depending what the manufacturer wanted to do in the design.

The impedance of earphones outputs could be somewhere from 8 to 200 Ohms, whatever the manufacturer wanted to do and the earphones recommended for/supplied with the gadget are.

The line input impedance is usually 10K Ohms if not more in modern equipment. The mismatch of feeding a line input with a earphone output is equivalent to pull a bicycle with a bulldozer. Not meant to, but hell it can ! Effortlessly.

Yes, a laptop earphone jack is also capable to feed a line level input on an audio amplifier. And several too. Just set the volume control to the correct level.
The lower the impedance of the audio source, the greater the number of line inputs in parallel it can feed to.
Like the bulldozer pulling now half a dozen bicycles, the bulldozer did not even notice you added more...

Miguel
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Re: How to amplify Stereo Headphone Outputs

Post by Bigglez » Mon Nov 17, 2008 4:43 pm

brodave38 wrote:Is there a low-cost way to amplify stereo headphone outputs (50mV?) to
Line Level Inputs (500mV?) with a portable device powered by 3-14V DC?
It doesn't need to have gain controls, but should be a good impedance match.
Sure! This is a simple request and you have many
choices. Musicians need all sorts of "boxes" and
there are vendors that sell these if you want to buy
a "box" and plug it in. Here's one.

For much less money you can build the same thing
from a kit, Here's one.
Basically you provide the labour! (if you have the time,
some tools, and a little guidance).

Finally, you can roll your own if you have a schematic
and would like to construct a one-of-a-kind project.
(We can certainly help you with a schematic).

The low impedance of the earphone (your source)
does not have to be matched to your line input.
(Going the other way would be an issue).

brodave38
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Post by brodave38 » Mon Nov 17, 2008 5:37 pm

Thanx for the info, Bigglez!
If you would be so kind as to show me a schematic, I would"roll my own"!
The kit you showed me appears to have an 8 ohm output. What I think
I'm looking for is a device that has a linear gain of 10, matched to a
standard(?) earphone input of about (50?)mV with an output at (500?) mV
which I've been told is what "Line Level" is. I need to get this clarified, as
I've heard that line level cannot exceed 2V P-P. What is the standard for
Line Level?
Thanx Again,
BroDave38

Bigglez
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Post by Bigglez » Mon Nov 17, 2008 6:54 pm

brodave38 wrote:The kit you showed me appears to have an 8 ohm output. What I think
I'm looking for is a device that has a linear gain of 10, matched to a
standard(?) earphone input of about (50?)mV with an output at (500?) mV
which I've been told is what "Line Level" is. I need to get this clarified, as
I've heard that line level cannot exceed 2V P-P. What is the standard for
Line Level?
I agree. For most pro audio the line level is taken to
be "zero dBm 600 ohms" and "plus four dBu".

Specs are hard to nail down for the non-pro
audio market. Some equipment vendors use the
"zero dBm 1k ohms" level, while others use 10k
impedance at -10dBV. If you are not familiar with
these terms try here.

Here's a handy calculator

The headphone amplifier kit is all that you would need,
although it can drive headphones or small speakers the
outputs can equally drive the line level inputs of other
audio gear.
brodave38 wrote: If you would be so kind as to show me a schematic, I would"roll my own"!
Instead of the dual audio amplifier you could use a
dual op amp, with external resistors to set the gain
to 10 (as you suggest). There are tons of low noise
audio op amps, and most will run from a single 9V
battery.

Here's a "straw man" design if anyone wants to take
shots at it. The quad op amp is the popular LM324.
Layout etc. in not critical. I didn't bother with an
on/off switch or adding an LED power on indicator.

Image

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frhrwa
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Post by frhrwa » Tue Nov 18, 2008 7:43 am

what about those little amps that people used on their turntables to boost the needle output levels? used to be able to buy them at Radio Junk..?

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CeaSaR
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Post by CeaSaR » Tue Nov 18, 2008 8:43 pm

I have a Line In connection set up on 2 stereos for just such purposes
(computer headphone and mp3 player headphone), and they drive the
Line Input just fine. I can switch from Line In to either Radio or CD with
no apparent drop in audio level. I use the volume control on either the
computer or mp3 player to set the Line In level.

You state you have no gain on your phone. No volume control at all? You
have to be able to adjust it somewhere in your phone's settings.

If you cannot find the volume setting, and cannot drive the Line In
sufficiently, by all means use Bigglez suggestion. Don't try to use a Phono
preamp, as they (ususally) have RIAA equalization (which one depends
on the age and manufacturer's design) that would change the way your
music sounds.

The only thing I would add to Bigglez's schematic would be a pair of 10k
or so pots (could be stereo) at the output to adjust so you don't overdrive
the Line Inputs.

Good luck, and let us know what worked.
CeaSaR
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Bigglez
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Post by Bigglez » Tue Nov 18, 2008 9:16 pm

CeaSaR wrote: The only thing I would add to Bigglez's schematic would be a pair of 10k
or so pots (could be stereo) at the output to adjust so you don't overdrive
the Line Inputs.
Good idea! Volume controls were not requested.
I assume that the audio amp accepting a line input
has a volume and a balance control.

Let's wait to hear back from the OP.

brodave38
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Post by brodave38 » Wed Nov 19, 2008 5:13 pm

Thanx to you all for the GREAT info! I will build the device you showed me,
Bigglez, with 10K pots (audio taper?) as suggested by CeaSaR, because one thing I am concerned about is over-driving any device I plug the unit into! I have the Radio Shack Phono preamp referred to by frhrwa and I was concerned about the RIAA curve distorting the sound.
Since it has no gain controls, the pre-amp seems to over-drive my PC line input, but I can still use it for converting LP's to CD's. Thanx for the Wikipedia reference!! I hope this doesn't sound weird, but I want to build a device that causes an adjustable delay of a few (milliseconds?) between a
mixer line level output and the amplifier input so that a person can hear the headphone output of the music before it is heard coming out of the loudspeakers. Is this possible? Thanx Again to You All!!
BroDave38

Bigglez
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Post by Bigglez » Wed Nov 19, 2008 5:56 pm

brodave38 wrote:I want to build a device that causes an adjustable delay of a few (milliseconds?) between a
mixer line level output and the amplifier input so that a person can hear the headphone output of the music before it is heard coming out of the loudspeakers. Is this possible?
Yes, an audio delay has many uses, and singers and
musicians typically have problems in live performances
due to room echo and PA delays. (I'm neither, but I do
notice live performers cupping their ear, or removing
their radio ear bud monitors while singing).

Another application is lip sync in movies and digital
TV sets. Here's a componentthat would help you understand
the technology used.

An example of a long delay is in live TV and radio to
block airing of inappropriate language, often called a
dump button it spools the live audio for five seconds
or so to allow studio staff to edit the broadcast in
real time to stop a word or phase from being aired.

These devices are digital, and a shorter digital delay
(fractions of a second) is quite possible. Older analog
techniques used a mechanical delay-line that is
physically large and often fragile.(If knocked or tapped
it causes a glitch in the output signal).

Another electronic circuit for such a task is the CCD
delay line (called a bucket brigade), but digital electronics
has pushed this technique away.

Finally, here's a trade magazine article about a digital
audio delay circuit. This may be just what you need!

Bigglez
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Post by Bigglez » Wed Nov 19, 2008 7:02 pm

brodave38 wrote:I will build the device you showed me,
Bigglez, with 10K pots (audio taper?) as suggested by CeaSaR, because one thing I am concerned about is over-driving any device I plug the unit into!
Its a moot point but for my two cents worth I would
place the gain controls ar the input of this circuit.

It has two advantages; Firstly the op amps drive
the ouput directly, with lower impedance than via
the new gain control pots. Secondly, the input
signal to the pre-amp may overload it directly if
the input signal is too 'hot'.

Would you like a revised schematic?

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CeaSaR
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Post by CeaSaR » Wed Nov 19, 2008 8:26 pm

brodave38,

This thread comes at a serendipitous time. My son is in need of a mic
preamp for his burgeoning recording hobby and I found this schematic.
Unsure of what a mic would output, I threw the mic section into CircuitMaker
and used 50 mV for the input. I found that if you changed C1 to 0.01 uF
and C2/C3 to 4.7 uF, the output would be 2.2 volts p-p (+1.1 v to - 1.1 v).
Thinking further, I decided to look up some general mic specs here, and
found that I should probably be using between 2 to 5 mV. Plugging in the
new value and setting C1-C3 back, I saw 300 mV at the output. !! :idea: !!
I remembered that most consumer gear has an Aux Input rated at 300 mV.
So, shoot for the magic number (600 mV total, +300 to -300). I'm not
sure about the input impedance. 10k? 50k? 100k? It has been a long time
since I've read component specs, and most don't come with them printed
on the box anymore.

Bigglez,

I agree that there should be a level control on the input to prevent
overdriving the preamp. A secondary level control at the output wouldn't
hurt if the preamp is designed with extra headroom. That way you could
tailor both the input and output for the best sounding match to the main
amplifier.

CeaSaR
Hey, what do I know?

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Bob Scott
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Post by Bob Scott » Thu Nov 20, 2008 1:48 am

CeaSaR wrote:brodave38,

This thread comes at a serendipitous time. My son is in need of a mic
preamp for his burgeoning recording hobby and I found this schematic.
I think you should consider using op-amps instead of the old obsolete discrete transistor design. The op-amps have stability and power supply noise rejection instrinsically built into their design.

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