Low Power Audio Amplifier

[indecided]

HI guys,

I'm looking for a circut for a small, say 5-10W audio amplifier for the line out of my PDA-phone to interface with my car stereo. This is a seperate output from the headset output, but after testing, is still marginally sufficient to play back through a pair of earphones.

The original design was just a 1:1 audio transformer with a couple of resistors and caps - i assume just to isolate DC and convert the impedence into something matched? I am essentially stuck here because audio transformers are no easier to obtain then coded rotary switches here, they are hard to obtain..

The main purpose of the amplifier is just to boost the line-out signal to an acceptable level to make sure i do not always have to fiddle with my car stereo's volume control in order to get a decent volume (lets say switching from the CD player or something).

Would appreciate if someone could point me in the right distance. Perhaps a circut with 5-10W of power. (or less, my car amplifiers would do most of the work) Requirements are less part count, easy to obtain parts.

Also, I could use some advice on audio amplifier design on stripboards - ground planes and such..

Thanks!

 

[Russlk]

If I understand correctly, you are using this amplifier to drive a pair of earphones. Mono phone? A few hundres milliwatts should be enough power, what kind of earphones do you have? A matching transformer should work, the line out is usually 600 ohms, what is the impedance of the earphones?

 

[indecided]

I'm looking for a circut for a small, say 5-10W audio amplifier for the line out of my PDA-phone to interface with my car stereo

Nope..i want to boost the output of the line-out from my pda-phone so that it matches more with my auxillary input of my car stereo..

 

[Russlk]

Ok, the problem is that the PDA output is low impedance and low voltage while the input to the amp is high impedance. A transformer will solve that, look at the Xicon transformer, 600 ohms to 10K ohms, Mouser part number 42TL019, $1.46US (www.mouser.com).

 

[audioguru]

Why use a transformer? The PDA has a line output (you said so) and the aux input of the car stereo is line level and maybe 10K or more. Most line outputs are designed for a 10K load, not the dead short of a 600 ohm transformer. Your 1st transformer probably loaded it down too much.

The line output of the PDA should already have a coupling capacitor to isolate DC. Just use a shielded cable with appropriate connectors on each end. You'll probably need two connectors at the stereo for both channel inputs. If it's too loud, add a pot ahead of the two connectors. :lol:

 

[indecided]

audioguru :

i think i used wrong terminology.. the PDA is LOW level output AFAIK. how about SOFT headphone level... better?

Either way, i'll test it out and come back with the results 'cos i already have the wiring up. If it's too soft at my nominal cd-listening volume, i'm going to have to do something about it...

 

[audioguru]

Hi Indecided,
Many line outputs can't drive low impedance headphones without an enormous loss. They are designed to connect to the 10K or so impedance of an amplifier like what you already have. I wish you have good luck that the levels match. :lol:

 

[indecided]

Okay..let me just recap here.

At 'volume level 24', my car stereo blasts music from the tuner/compact disc.

when i wire up the output from my phone to the aux, at volume level 24, the volume level is 'normal speakerphone listening level'. Bascially, I just want the output it to be louder at 24 so i dont have to crank up the volume on the stereo..

Surely it can't be that complicated... I just have a HARD time getting audio transformers here, so is there a digital alternative?

Speaking of which, does anyone have a good mono electret mic amp design using a 5572? I have a reference here

 

[indecided]

Hmm.. I managed to locate a small audio transformer in a telephone set..
or actually, i caniballized one..

either way, i prefer to use a new one...going to be using it for a while.

So...the assumption is that i'm to upconvert the low impedance to a 10k impedance for different levelling?

Also, managed to get Farnell who does have something.. BTW i'm not in UK i'm in Malaysia.

Someone check this one out - https://uk.farnell.com/jsp/product-details/text/CD121/6013.html

*edit* this is unaffordable - turns out I was looking at the part for the shield, this is RM90 which translates into 23.70 US$!! Scrap the transformer... can somebody just point me at a simple amplifier?

I assume that 10k Primary and 2k Secondary is what i want.

There's also a 1:1 600 to 600, which should be a good bet?

Here are the original schematics for the line-out part, which a 1:1 was used for isolation purposes (i guess)

**broken link removed**

If I convert it to 10k, will it still need amplification?

 

[audioguru]

Hi Indecided,
If your 1:1 transformer was any good in the circuit it should have passed the signal like a piece of wire. I don't know why it has a 1K resistor in series with its output.
You have many unknowns:
1) The output level and impedance of the PDA line out.
2) The input sensitivity and impedance of your car stereo' aux input, impedance divided by 2 for input to both channels.
Using a transformer to step-up the voltage without knowing these important spec's is asking for failure.

You showed a good electret mic preamp that uses a dual NE5532 or TL072. Why not use its unused opamp as an amplifier? Delete the input's 10K resistor and connect the negative end of the 2.2uF cap to the PDA's line output. Connect the 1.5K resistor directly to the 10uF cap without using the switched 27K resistor. Don't use a pot on the output and connect the output 2.2uF cap to the aux inputs of both channels of your stereo. Use a 20K or 25K pot wired as a rheostat gain control instead of the 33K resistor and don't bother using the 2 puffs cap. It will have a gain adjustable from 1 to 14.3 or 17.7 which is plenty.
I recommend using a rheostat gain control pot that replaces the 33K resistor in the mic preamp circuit too, so that you can turn down its gain if it overdrives. Don't use an output pot. :lol:

 

[indecided]

Audioguru,

Thanks for your ever-useful comments.

I'm going to build a whole preamp stero output, because i'm planning on using my PDA to playback music as well... Got some BB OPA2134 samples so i'd just use them.

Just a question.. if you notice the mic preamp has the 10k,and two 27k resistors for impedance setting purposes. Given that you said to remove the 10k resistor (and turn it into a line-out amp) what value of impedance does it now expect?

I know my PDA's line output impedance is unknown, but my judgement tells me that since the electronic volume control still works, it's likely the same as the headphone output, which should be 16/32 ohms. (which means that it's not a line-out, just a tap from the headphone output). Either way, I still need the gain to be increased - wired up directly to AUX, it sounds terribly soft on my car stero.

Should the circut be modified in any way to accomodate this assumption?

Hope that I don't get feedback from the speakers given that the microphone is going to be mounted in the car..with a preamp.

 

[audioguru]

Hi Indecided,
Sure, make a stereo preamp similar to the circuit that you posted. Don't use the 10k input resistor nor output pot and use a gain adjusting pot instead of the 33k resistor. It will be able to be driven by any circuit that can drive an impedance of 10k or much less.

With the gain turned up, you're darn right you'll have feedback! Without filtering for its bias, it will also have a good amount of alternator whine if you power the preamp from the car's 12V. I have added filters to eliminate alternator whine and here is how it should be wired with your dual opamp. :lol:

 

[indecided]

Thanks audioguru! What do you do for a living, by the way?

Just a bit.. I was planning on regulating the voltage supplied either way. With a 78L09/78L12. Is that recommended and what are the benefits associated with it and will it help with alternator whine?

Also.. in the schematic it was mentioned to tie the 10k resistor to the other channel's 10k. What is the purpose of this link?

 

[audioguru]

Hi Indecided,
I'm just an early-retired bum! :lol: I worked with audio equipment most of my long career and do some consulting now.

Most opamps won't work with a supply voltage as low as 9V, and a regulator needs an input voltage 2 or 3V higher than their output so a 12V regulator won't work in a car. The 47 ohm resistor and 1000uF cap and 100uF cap should take care of noises.

The 10k resistor establishes the reference DC voltage for the opamps. It connects to the two 27k resistors that set a half-supply voltage. It allows the output of the opamp to swing equally positive and negative with respect to the half-supply voltage. :lol:

 

[indecided]

Hmm.. they won't?

That's strange considering the references were built to be used with a battery box, and you know how 9V batteries supply power... What 9V?

Or is there something i'm missing out here? something to do with supply-rail logic?

 

[audioguru]

Hi Indecided,
You are correct, your OPA2134 opamps work with a supply voltage down to 5V, less than the 6V from a dead 9V battery. The TL072 doesn't like only 6V.

Try the circuit powered by the car battery without a 9V regulator and add it if the circuit produces alternator whine. :lol:

 

[indecided]

Yup.. I think the 5532 works down to 5v as well. Besides, it's not a battery i'm using

BTW audioguru, given the dualie here, when I make the microphone preamp, how do I go about with the 10k "to the other channel" part?

 

[Nigel Goodwin]

Yup.. I think the 5532 works down to 5v as well. Besides, it's not a battery i'm using

BTW audioguru, given the dualie here, when I make the microphone preamp, how do I go about with the 10k "to the other channel" part?

You simply connect a 10K from the existing pair of 27K's to the inverting input of the other opamp - you only need the single pair of 27K's.

 

[audioguru]

You simply connect a 10K from the existing pair of 27K's to the inverting input of the other opamp - you only need the single pair of 27K's.

Hi Nigel and Indecided,
Both channels have a 10k resistor connected to the non-inverting (+) input of the opamp, and the other end of both 10k resistors connects to the junction of the two 27k resistors. :lol:

 

[indecided]

Okay.. So I don't duplicate the 27k pair BUT I do duplicate the 100uf cap, right?