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

["Christian M. Cepel" <EMAIL:PROTECTED>]

Turns out yes, I'm gonna give up, and no, I didn't get away clean.

Everything worked fine for a while until the thing had been hooked to 
the metronome via the car/cassette adapter (like I'd been doing before 
this little project), at pipe band practice for about a half an hour.

After about a half an hour the thing began distorting and cutting out, 
getting worse and worse.  I took that metronome off and put on one with 
a lot lower line level, but the damage had been done.  The preamp, while 
not completely fried is at least crispy around the edges.

Oh well.  I never played tapes on the thing, which was the reason for 
trying to make it a dedicated AUX.  I knew about the differences in line 
levels vaguely, and knew I should have searched for a place to put the 
aux in just behind the amp/volume portion of things, but I didn't have 
the courage to look or to mess with anything on that side and hoped I 
could make a decent hack out of 'retasking' the r/w head input on the 
tape side of things.  I should have went the other way just for the 
benefit of not having to have the tape spindles on and rotating and 
wasting juice.  Oh well.  I've now got rca plugs nicely built into the 
case that lead nowhere *sigh*.

Thanks for everyones advice.

 //Christian M. Cepel

Harris, Michael C. wrote:
> At a dollar per hour for your time, the task you have chosen will be a loosing proposition.  You can probably buy a replacement with a line in for what you are spending in time to bugger up what you have.  If you put any large signal in at the low level input of a tape head you can/will cook the preamp, (think over clocking to 350ghz, or 5 volts where .5 belong) bad idea, so don't do that. 
>
> If you are dead set on using what you have, Schematics are available for most electronics from Sams photo facts http://www.samswebsite.com/photofacts.html but a schematic will cost more than the tape player!
>
> If I have not talked you out of this project yet, the source selection from tape to radio is usually done at line level, you can probably pick an input point close to that switch.
>
>
>
> Mike
> kcØpah
>
> -----Original Message-----
> From: EMAIL:PROTECTED [mailto:EMAIL:PROTECTED] On Behalf Of Igor Izyumin
> Sent: Monday, October 31, 2005 1:48 PM
> To: MLUG Members
> Subject: Re: [MLUG] Re: I am famous! Audio question.
>
> 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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--
  //Christian

Christian Marcus Cepel            | And the wrens have returned &
EMAIL:PROTECTED icq:12384980  | are nesting; In the hollow of
371 Crown Point, Columbia, MO     | that oak where his heart once
65203-2202 573.999.2370           | had been; And he lifts up his
Computer Support Specialist, Sr.  | arms in a blessing; For being
University of Missouri - Columbia | born again.    --Rich Mullins

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