Raspberry Pi Forums

I have a 1983 vintage boom box, where the cassette stopped working because of a perished drive belt. Because the cassette head was still functional, I will use that connection for the amplifier from a Pi A+. Maybe a B+to take advantage of more USB eventually, but only wireless to start for the challenge.

I have a few questions before I commit the Pi, since I fried one yesterday (long story for another time):

For the cassette amplifier I have 2 each red and white wires left and right, where the whites seem to be signal, and the reds not. Can I tie the red wires together, or is it safer to put a couple of RCA plugs on? Experiments show me it doesn't blow up the stereo either way.

What's the best route to connect sound from the Pi? There are many options, but I'm going to get distortion because the cassette circuit is so sensitive I need the best from the Pi and then I'm going to have to throw away some gain.anyway. I'm expecting the "get a DAC" answer, but isn't there a way of playing with the ordinary audio out, given this is 1980s tech?

I suppose that'll do for now. If I go without the DAC I will be trying to find how to solder directly to the board, avoiding the video feed. There will be a screen involved in the project, and I'm not restricted for space.

Cheers! Looking forward to constructive comments.

If the cassette had the ability to record, then the reds are possibly for the record head, the layout used to be erase head, record head, playback head, although for cassette the record and play were often combined.

The output from the play head will be very small and probably go to a separate preamp before going to the main boom box amp. The output from the Pi audio needs to skip the preamp, which might mean some circuit tracing, depending on the age of the boom box this might be IC based, or still discreet transistors.

It's a boom box, so there is a limit to the quality, but then cassette was pretty limited, I'm not sure an external DAC or USB sound card is really worth the investment, even then they are going to be post preamp level, so you'll still need to trace the main amp. Feeding the Pi into the cassette pre amp is likely to overload horribly and it might also have some EQ shaping that might make things worse. You could reduce the signal with some resistors, but that's not an ideal solution.

There is a chance the preamps would be on a separate board, depending on the model of boom box.
Does the boom box have an aux in anywhere, that would be a more suitable input level.

Joe

This is extremely useful, thanks.

Before I go deeper, if the red wires are as you describe, what might be used as ground? Is it the shield that I failed to mention?

There is an aux input which I'm happy to try next, but it's probably the first thing to be installed, and the hardest component to access from the inside. Right behind the circuit board, which needs to be removed to trace the signal, after taking out the radio tuning mechanism. I want to leave as much original stuff as possible, but I could end up gutting everything.

Acceptable quality in this case would be the equivalent to one of those MP3 cassette adapters, which is no longer a choice since the mechanism is all cut out now.

The Raspberry Pi analog line output should be nicely matched to the AUX input on your boom box.

I think I'm going to have to climb in and find where it connects up - thanks!

The shield will be ground, sometimes the whole erase, rec / play heads mechanism were on a metal plate which was ground. The two red wires will be stereo so left and right to the record head. As they are output wires, then they should only be active if the boom box was in record mode and hopefully won't otherwise contribute to any noise. You could try grounding them with via some small couple hundred ohm resistors if needed, but leaving them unconnected should be OK.

On of my Pis is a model B hooked via the 3.5mm jack into a Roth sound bar and the quality is perfectly fine for Internet radio and mp3, often showing the limits of the source material rather than the fact that the impedance and the levels are not ideal. If I switch to using HMDI via a TV and optical out to the sound bar, it really doesn't sound any different, with the same set of source material.

Oh one other thing, you don't need monster cables or anything with the words "Dr" and "Beats" to make it sound better.

Joe

Thanks again Joe.

I think I have lots of options. I already have a B running through HDMI, and another connected through a 3.5 mm jack into a good stereo. Like you, I'm happy that those work well enough.

The object here is to get everything inside a redundant box with serviceable electronics, looking as original as possible but with a "sonic screwdriver" in the middle.

I can move forward with confidence after your advice. I just didn't want to screw up another Pi so soon.

I'll snip the red wires at least.