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Considering cheap DIY external amp for netMD

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I am thinking of building an inexpensive (say 5+5W) external amp + loudspeakers for my netMD (an MZ-707, only phones out), sort of a homemade docking station (iPal envy, I guess? :blink: )

I have a bargeload of schematics for audio amps, preamps etc, some new, some positively vintage designs (i.e. 1970-something). The articles on preamps go to great lenghts to explain RIAA equalization (I told you they were vintage, did I not? ;) ), but I can't find anything to couple phones-out to amp-in.

I welcome your suggestions on the best way to get the most mileage out of the phone out plug. Thanks to all in advance

Luca from Italy

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Further musings on the topic. How about a microphone preamp circuit, that might take low-Z input and feed it to high-Z output with acceptable results?

Mics are in the range of 600 ohms, still too much, but modifications may be made to go as low as 150 ohms or so.

Just a thought. Comments welcome

Luca from Italy

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A lot depends on how far along the "hi-fi" line you intend to go.

If you are talking about something which is pleasant to your ears, but wouldn't necessarily satisfy the "audiophile", then you could do worse than look at some of the single-chip IC power amplifiers which are available, and which are fairly cheap.

You need to consider how you are going to power the unit (you might have a spare "wall wart" transformer which can deliver, say 12V at 1 or 2A which should be able to power such an amplifier, and you don't really need to worry about equalisation circuits except perhaps to overcome any shortcomings in your speakers.

When you see "RIAA" equalisation mentioned, it is usually in the context of compensating for the characteristics of moving coil pickups for turntables, you really don't need to go there, as the output of MD players is essentially "flat" and around 1V in amplitude (at "line out" levels), so your amplifier isn't going to need a tremendous amount of voltage gain.

You might already have some active speakers for your computer which has facilities for line input. I have some of those which I use occasionally and the result can be perfectly acceptable, depending on the circumstances.

There is no single simple straighforward answer, but I miriad of solutions which may do what you want, so you are embarking on potentially a very interesting journey ...

Have fun!

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the output of MD players is essentially "flat" and around 1V in amplitude (at "line out" levels), so your amplifier isn't going to need a tremendous amount of voltage gain.

Have fun!

Thank you, Mr. Bass Man. Yes, this is a "6000 channels & nothing on" project, meaning I'd rather be tinkering with resistors than watching TV... and no, I don't need reference class output, just some nice sounding, nice looking, "hey-mom-look-what-I-built" box. I've used powered speakers, the PC kind. Good output, but hey... not homebrewed! :D

BTW, what's the (typical) impedance of an average line-out jack on a soundcard? Just wondering...

About impedance and amplitude: I tried connecting the MD phones out to my A/V receiver, had to crank the volume waaay up, compared to any other sound source I have. I normally listen to CDs at -45dB or so, had to go all the way to -20dB with MD... This made me think I was feeding low-Z into something that expected, say, 100K or more. Which in turn made me look into mic preamps... no rest for the wicked.

Really the MD is putting out 1Vpp? I was thinking more around the lines of 200mVpp...

Thank you for your insight.

Best,

Luca

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  • 3 weeks later...

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