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 Post subject: modding for tape input on amp
PostPosted: Dec Fri 28, 2018 4:26 pm 
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I'm not sure if this is the correct way to express this but I have a friend who is trying to run an old 8 track tape player through a Magnavox stereo amp that came out of a console. The console likely had a phono and that's it. The tape player has no volume or level controls and according to him sounds too " hot" coming into the amp and results in distorted sound.

Seeing as how a great many of the older stereos I've worked on seem to have different resistors/components attached to the various inputs... as in what appears to be a means to compensate for the signals, I'm wondering if there's a means in which to measure the incoming signal from the tape player and to "quieten" it down? Sorry if this makes zero sense...


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 Post subject: Re: modding for tape input on amp
PostPosted: Dec Fri 28, 2018 5:48 pm 
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Using an O'scope to get an idea of the voltage output of the tape player would be a start... then
make a pair of voltage dividers to knock down that voltage to a level which is more or less on par with whatever the phono pickup puts out. Could use fixed resistors and calculate the relative values needed to get the voltage into the desired range. A lazy way to to it would be to use a new or used stereo (dual, ganged) volume control with the wiper taps connected to the stereo phono inputs.... that way you can adjust the output voltage to whatever level sounds best. No need to measure or calculate anything- just set the pot to where the sound quality is best.


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 Post subject: Re: modding for tape input on amp
PostPosted: Dec Fri 28, 2018 9:31 pm 
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lorenz200w wrote:
Us A lazy way to to it would be to use a new or used stereo (dual, ganged) volume control with the wiper taps connected to the stereo phono inputs.... that way you can adjust the output voltage to whatever level sounds best. No need to measure or calculate anything- just set the pot to where the sound quality is best.


this above

i did it once for an unknown mismatch as well by installing a "volume control". then, i installed two fixed resistors for each channel in place of the stereo volume control like an L pad.

it worked great.

steve

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 Post subject: Re: modding for tape input on amp
PostPosted: Dec Fri 28, 2018 9:47 pm 
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Location: Utica, NY 13502 (USA)
Do you know if the phono input is for a magnetic cartridge? If so, a compensation network needs to be added to reverse the effects of the high gain, RIAA compensated preamp stage or just bypass it.

Dave


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 Post subject: Re: modding for tape input on amp
PostPosted: Dec Fri 28, 2018 11:36 pm 
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Dave Doughty wrote:
Do you know if the phono input is for a magnetic cartridge? If so, a compensation network needs to be added to reverse the effects of the high gain, RIAA compensated preamp stage or just bypass it.

Dave


I agree, it would be far easier and work much better to tap into the console line amplifier section than to undo the RIAA curve and reduce the voltage to be compatible with the phono input. Even if you get it to work, you are then taking a very nice, clean, line voltage signal and putting out a very low and probably noisy signal with, unless you are very careful, lots of hum, just to boost it back up again.

One might be tempted to just build a high-pass filter followed by a passive voltage divider. It would likely be pretty noisy and have poor frequency response, but it depends on how good it needs to be. A proper method would be to build an active amplifier with a proper "anti-RIAA" filter in it, if you were going to attempt it without digging into the line amp section of the console itself.

Brett


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 Post subject: Re: modding for tape input on amp
PostPosted: Dec Sat 29, 2018 5:02 am 
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This is a Maggie, it uses a ceramic cartridge.

-David


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 Post subject: Re: modding for tape input on amp
PostPosted: Dec Sat 29, 2018 12:56 pm 
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Location: Utica, NY 13502 (USA)
dberman51 wrote:
This is a Maggie, it uses a ceramic cartridge.
Then I would use the voltage divider approach that Steve posted above.

Dave


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