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Old 29th March 2002, 03:30 PM   #1
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Default newbie question: headphon amp alternate use

hey all, brand new to the board, iv'e been doing a lot of work on an mp3 player system for my car, and i've recently discovered a fascination with this DIY stuff... so i decided to do some audio stuff.

anyway, ive been looking around for a project to get myself started, and i think im gonna do a headphone amp. yeah i know, every newbie starts with that!! but im not planning on using if to drive headphones, rather, for a custom installation of a playstation in the backseat of my car. i want to be able to mount some speakers for the playstation sound and have a volume control and a headphone bypass for the backseat psx console.

i found a couple projects on headwize that looked like possibilities, guitar amp Chu Moy amp and meier amp.

now the questions i have are this:
1. if i have a small set of 3 1/2 inch speakers, can i run them off of one of these headphone amps, with the input being the signal from the psx. im not looking for ear splitting volume just something to control the sound.

2. i want to be able to plug in headphones to the audio output and have it cut off the signal to the speakers and only be heard in the phones. i know it is super common, but i dont know what the audio jack is called.

3. i know the meier amp would be overkill for what im trying to do, but im thinking of getting some phones and making an amp for my minidisc player. on the meier amp it shows one signal, for the crossfeed network, if i use something like this on another design do i need to do that twice (one for each channel) or is that just installed on the one channel? im a little confused.

sorry about the newbie questions, but i have to start somewhere.
~mike
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Old 29th March 2002, 05:19 PM   #2
paulb is offline paulb  Canada
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Hi,
A headphone amp generally won't have enough current capability to drive speakers directly. If you build a small power amplifier, you can also use it to drive headphones - just put a resistor in series (see the Headwize site).
The easiest solution for a small power amplifier is to use a chip amp like those from National (www.national.com). Check out the LM1877, for instance. Also look at Texas Instruments (www.ti.com) - they have amps to do exactly what you want (headphones / speakers). Generally the datasheet or app notes will give you a circuit, usually fairly simple.
The jack you want is just called a switched or closed-circuit jack.
I haven't looked at the crossfeed circuit, but it just mixes the two channels together a bit (reduces the stereo effect somewhat) for headphone listening. I would guess that the circuit is probably the part that connects the two channels. Some people don't like the effect; you may not want to bother.
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Old 29th March 2002, 06:20 PM   #3
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paulb....
thanks for that info...

what sort of power do headphone amps usually put out. what should look for when selecting the power amp at national?

also, i know the concept behind the crossfeed, to blend the right and left channels with a time delay, to mimic the natural listening situation (without headphones). i'm just unsure if the crossfeed switch is only used on one channel, or is it needed to be installed on both. looking at the meier amp design... switch 1a and its following network, is that only wired on the left channel, or is an identical network wired in the right channel. as for the bass enhancement, only one switch is needed for that component...

i still have a lot to learn!!

thanks
~mike
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Old 29th March 2002, 06:50 PM   #4
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Not to discourage anyone from DIY'ing, but the simplest solution that I can think of for the original problem would be an el cheapo car poweramp with a pot at the input. I think I've seen an 88W (yeah right, its like 11wpc into 8Ohms) little jensen power amp at Fred Meyers for like $20. Looking online or at RS might get you something even cheaper. Now, I know that this isnt exactly HiFi, but neither is a PS.

If you really do want to build something yourself, then checkout ESP projects 19 or 72. They are both single IC approaches like paulb mentioned.

Non
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Old 30th March 2002, 12:03 AM   #5
paulb is offline paulb  Canada
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Headphone amps put out on the order of tens of milliwatts, driving an impedance of somewhere between 32 and 600 ohms. You'll probably need at least a few watts to hear anything from your speakers in a noisy car environment, and their impedance is likely 4 or 8 ohms.
At National (or TI), primarily check the distortion specs. While some here argue that distortion specs are not the most important thing, it is a good general guideline.
Also make sure that the chip is suitable for running from a single 13.8V supply; some need a higher voltage. The two projects mentioned by Nonlethal unfortunately won't run from such a supply. The LM1877 make be a suitable choice.
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Old 30th March 2002, 01:02 AM   #6
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thanks again for the info.

i was going to ask about the voltage requirements. but you answered my question that there are ones out there that will run on car-powered dc.

others....still wondering about the crossfeed circuitry....

~mike
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Old 30th March 2002, 02:46 AM   #7
paulb is offline paulb  Canada
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Okay, I looked at the Meier schematic. The 2.2K resistor and 200nF cap need to be repeated on the other channel, the other 2.2K resistor (connecting to the X) connects both channels and you only need the one.
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