How to amplify Stereo Headphone Outputs

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Dave Dixon
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Post by Dave Dixon » Thu Nov 20, 2008 9:46 am

I built a nice little headphone amp with a Maxim MAX4410 and a couple of discrete parts. The two headpohone jacks took up the majority of the PCB layout!!! It came out about the size of a bic lighter. Those Candy tin amps on fleabay work really well. I bought one last year for a steal. Good luck.

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GoingFastTurningLeft
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Post by GoingFastTurningLeft » Thu Nov 20, 2008 11:17 am

Bigglez wrote:
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?
I have some PC speakers (Altec-Lansing?) that I figured out that the volume control is a gain control, because when I turn the volume all the way down with headphones plugged into the aux jack, i hear noise/hissing.

IMO this is a bad design, since you will have a varying level of "music" and a constant level of hiss. I think it is much better to have a constant gain that ramps the audio up where you need it and overpowers the intrinsic hiss then drops it down with an audio taper pot.

Or even better: Gain and output controls :D

Bigglez
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Post by Bigglez » Thu Nov 20, 2008 11:52 am

GoingFastTurningLeft wrote: I have some PC speakers (Altec-Lansing?) that I figured out that the volume control is a gain control, because when I turn the volume all the way down with headphones plugged into the aux jack, i hear noise/hissing.
I doubt the control changes the gain of the amplifier -
it attenuates the signal. The argument is whether
the attenuator is better at the input or the output
side of the fixed gain amplifier.
GoingFastTurningLeft wrote: IMO this is a bad design, since you will have a varying level of "music" and a constant level of hiss.
Assuming that the hiss is being generated in the
amplifier and not part of the input signal or a ground
loop or power supply noise, or detected RF noise from
a nearby digital computer box.

A negative feedback amplifier (such as an op amp)
will have no control of the input referred internal noise,
which is amplified by the open-loop gain. Luckily, very
low noise op amps are readily and cheaply available.
GoingFastTurningLeft wrote:I think it is much better to have a constant gain that ramps the audio up where you need it and overpowers the intrinsic hiss then drops it down with an audio taper pot.
If the attenuator is on the input side of the amplifier
it protects the amplifier from distortion caused by
input overload.

An attenuator on the output has two problems, it
increases the 'sending' impedance of the amplifier,
and is non-constant across the range of the control.
Secondly, it forms a low pass filter with the capacitance
of the output cable and whatever is being driven.

A lot of audable noise in amplifiers is caused by a
lack of bandwidth limiting. The circuit I presented
earlier has a 20dB gain, rolling off at 50kHz. It
would be quite easy to roll that off earlier to allow
an upper -6dB point of, say, 20kHz. Thus reducing
any HF noise that can't be heard but may cause
distortion in the audio passband.

One final point, the link to a simple bipolar transistor
circuit has a couple of limitations. The gain is set
by the hFE of the transistor, not the circuit components
(I doubt two of these will work exactly the same).

Also, the output side volume controls are in parallel,
turning down one will cut off the other.

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CeaSaR
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Post by CeaSaR » Thu Nov 20, 2008 9:21 pm

Bigglez,

Thanks for the observations, but my post and links were meant as sort of
a James Burke type of thing; to show how something unrelated led me
through twists and turns to come to a bit of information unrelated to the
first circumstance.

ie: need a microphone preamp<- search web for schematics that use
what I have on hand, test assumption in software, find correct input
and revise assumption -> Hey! The output is 300 mV, a spec I read back
in the 70's/80's when I devoured HiFi literature. That's the number
brodave38 should have been looking for.

Similar to how James Burke showed how a grain of wheat, thousands of
years ago, led to the Atomic age.

brodave38 should go with your design, it is much more stable and
perfectly suited for stereo (2 channel) use.

As for me, I will try the microphone section only, of the scehmatic I posted,
mainly because I have the parts on hand, no need to buy anything.

Again, thanks for your observations.

CeaSaR

Addendum:
Bob Scott wrote: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.

See above. If my son is interested in better later on, then I will move on to opamps.
Hey, what do I know?

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Post by Bigglez » Fri Nov 21, 2008 12:18 am

CeaSaR wrote: Thanks for the observations, but my post and links were meant as sort of
a James Burke type of thing
That's a relief! I though we might have been following
the work of AmericanRube Goldberg...
CeaSaR wrote: As for me, I will try the microphone section only, of the scehmatic I posted,
mainly because I have the parts on hand.
Post your results (and PIX if possible)!

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CeaSaR
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Post by CeaSaR » Fri Nov 21, 2008 8:52 am

Bigglez wrote:...I though we might have been following
the work of American Rube Goldberg...
Why thank you. It is a compliment to be noted for thinking outside the
box. Rube was actually quite the genius to think of all the different ways
to accomplish such mundane tasks.

For those that don't know who James Burke is, check out this:

http://en.wikipedia.org/wiki/Connections_(TV_series)

(don't click if whole line isn't underlined/shown as a link)

URL tag doesn't like this for some reason, so I just gave the whole addy
to copy/paste into your browser. If you get the chance, watch his shows.
They will leave you either impressed or puzzled, maybe even both at the
same time, yet still entertained.

I'll be finishing up the preamp soon. I'll let you know about it when it is
done and tested. I hope brodave38 does so as well.

CeaSaR
Hey, what do I know?

brodave38
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Revised Schematic

Post by brodave38 » Fri Nov 21, 2008 1:26 pm

To Bigglez:
Please provide a revised schematic for adjustable gain preamp.
Thanx a whole bunch!
BroDave38

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Post by brodave38 » Fri Nov 21, 2008 1:53 pm

[quote="CeaSaR"]brodave38,
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.

From BroDave38
You are right-on with the info! Now I have a good idea about Aux inputs. When I drive rental cars, I want to be sure that I don't blow up the radios when I plug a device into the Aux Jack!! I have always assumed that I can insert a Line Level output into most audio device's "Aux In". Thanx So Much!
BroDave38

Bigglez
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Post by Bigglez » Fri Nov 21, 2008 2:35 pm

CeaSaR wrote:
Bigglez wrote:...I though we might have been following
the work of American Rube Goldberg...
Why thank you. It is a compliment to be noted for thinking outside the
box.
I knew you'd smile! BTW, while Rube created
drawings of his concepts, Heath Robinson actually
built his similar 'artwork' as scuptures.
CeaSaR wrote: URL tag doesn't like this for some reason, so I just gave the whole addy
to copy/paste into your browser.
Yes, that's a problem for all of us. I have a very
solid work-around, only takes a few more steps.
Open a new browser window and point to:
www.tinyurl.com

Cut n'paste the problem URL and let tinyURL
create a shortcut URL. Finally, paste the new
URL into your message. I like to use the BBCode
tags for embedded URLs.
CeaSaR wrote: I'll be finishing up the preamp soon. I'll let you know about it when it is
done and tested. I hope brodave38 does so as well.
Great!

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Re: Revised Schematic

Post by Bigglez » Fri Nov 21, 2008 3:27 pm

brodave38 wrote: Please provide a revised schematic for adjustable gain preamp.
Here's what I had in mind:
Image

The input signal is attenuated by a stereo (ganged)
pot for volume. The amplifiers are configured for
non-inverting gain of 9 (standard value resistors).
The input and output impedances remain the same,
but as "The Hoff" pointed out earlier the output will
always have the most amplifier generated noise.

Some nit picking. The correct description is
"fixed gain with input attenuator". A variable gain
amplifier would have the volume control in the
op amp's feedback path. Doing so would not change
the noise floor of the project, but would make the
volume control non-linear.

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CeaSaR
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Post by CeaSaR » Sat Nov 22, 2008 7:31 am

Bigglez,

What is the component that is missing between C6 and voltage divider
R1/R2 in the battery section? Possibly a 0.1 or 0.01 uF bypass cap? It
looks like the label for C6 obscured the symbol/label.

Just curious.

CeaSaR
Hey, what do I know?

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Externet
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Post by Externet » Sat Nov 22, 2008 9:32 am

It's IC1 :smile:
- Abolish the deciBel ! -

Bigglez
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Post by Bigglez » Sat Nov 22, 2008 10:56 am

CeaSaR wrote: What is the component that is missing between C6 and voltage divider
R1/R2 in the battery section? Possibly a 0.1 or 0.01 uF bypass cap? It
looks like the label for C6 obscured the symbol/label.
Nothing is missing, you're just not familiar with the CAD
tool used for this diagram.

The symbol you have seen is actually the power pins for
the IC. Usually these are hidden (to avoid clutter) and
would automatically appear in the board layout. I made
them visible here so that they are not forgotten
when a hand-wired prototype is constructed.

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CeaSaR
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Post by CeaSaR » Sat Nov 22, 2008 12:24 pm

Gotcha. I wasn't sure what they were. I should have deduced from the
numbers on the lines that that is what they were for. You learn something
new every day.

Thanks,

CeaSaR
Hey, what do I know?

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CeaSaR
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Post by CeaSaR » Sun Nov 23, 2008 7:18 pm

Bigglez wrote:Post your results (and PIX if possible)!
Single channel works quite well, enabling the available dynamic mic to be
used through the line input of the music computer. While the condenser
mic worked well enough for the first few recordings, the ease of use just
wasn't there, and it picked up too much. The project is built with my old
stash of TH parts and bits of salvage. Now to solder up a second channel
and make an enclosure. Looks like son #1 is going to get a new
microphone for the holidays. Pics to come later.

CeaSaR
Hey, what do I know?

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