Re: [MLUG] Re: I am famous! Audio question.

[Igor Izyumin <EMAIL:PROTECTED>]

The problem is, you hooked stuff up to the tape head input.  Output from 
the tape head is on the order of millivolts.  It gets amplified quite a 
bit, filtered for de-emphasis, and various other things get done to it.  
By the time it's done, it's at line level (~1V peak to peak) and ready 
to feed into the amplifier.  That's where you want to tap into.   If you 
just feed the line level signal to the tape head amplifier, you will 
overdrive it and get nasty distortion.

I'd say that adding line-level input jacks would be a fairly complex 
project, it can be very difficult to find a good spot inside the 
stereo.  If it has a mechanical volume control, I would suggest hooking 
it up somewhere near it.  Otherwise, find a datasheet for the amplifier 
chip (the chip with 20 pins or so mounted on a heatsink) and use the 
"typical circuit" schematic to locate the input. If you have an 
oscilloscope, use it to find a spot with line-level signal as close to 
the output amplifier as you can.  Don't use diodes (unless you want it 
to sound like death metal distortion), since it would clip the AC; add a 
mechanical switch instead.

In my opinion, this project is a bit more complicated than you initially 
realized.  It might be a better idea to simply build or buy a standalone 
amplifier module, since that is essentially what you are using the 
stereo for.
-- Igor

Christian M. Cepel wrote:

> Since this this topic is dealing with people knowing what they are 
> doing with audio, I thought I'd pose a question that's bothering me.
>
> I pulled a nice sony portable stereo apart last night and tried to 
> solder two RCA jack leads to the same place on the circuit board that 
> the tape 'read' head wires are soldered to.
>
> I'm tired of using a 'car tape adapter' when I want to hook a 
> metronome to it for the bagpipe band (it's gotta be LOUD), or when I 
> want to amp my mp3 player.
>
> I anticipated that line level for the tape read head would be much 
> lower than line level out of my standard 'headphone' devices, and that 
> I'd need to turn those sources down to not send too hot a signal which 
> would be distorted by the amp circuit.
>
> What I did NOT anticipate was that the audio would be horrible, with 
> tremendous distortion (even though it was turned so as to be barely 
> audible), and nasty full-spectrum noise.
>
> I had rather hoped to add Aux inputs in this way to my other portable 
> stereo equipment as well, but this is quite a bump in the road.
>
> My question is this... What is the difference between the AC signal 
> coming from the read head and the AC signal coming from the headphone 
> jacks of my equipment?  My first guess is impedance, and that the 
> distortion and noise are reflections and such, but I've no real 
> knowledge to pursue this guess.  I've been able to find NOTHING online 
> about the particulars for both standard headphone level signals and 
> magnetic tape read heads.  Does anyone know.
>
> A second cause just occurred to me... that perhaps I would need to add 
> a diode between the board and the read head to prevent the input 
> signal from entering the 'read head' circuit.
>
> Anybody have any idea how to make this work?
>
> Many thanks for any help.


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