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Can I tap into these audio circuits by desoldering Op-Amps and putting wires in their place? Answered

In my continuing quest to make an under-cabinet kitchen radio (see Chapter 1, Chapter 2, and Chapter 3), I'm looking at the original PCB for the boombox I'm hacking.

Based on the work of three predecessors (Convert a radio tape player..., How To Make A Audio... and Play and Recharge Ipod...) I'm looking at ways to bring in audio from two sources.  I'm putting in a CD-ROM drive to be my CD player, and I'm adding a headphone jack for my BlackBerry so I can listen to MP3s, Pandora, and streaming radio.

It appears that there are two ICs that act as preamps in the boombox, one for the CD portion, and one for the cassette.  See the pictures for where they are located on the board.  Photos 3 and 4 are pinouts for these ICs as found online.

I want to remove the BA4558 and solder in a connecting cable for the audio out from the back of the CD-ROM: Left to OUT1 on the board, Right to OUT2, and ground to VEE.

Likewise, I want to remove the BA3308 and solder in a cable with a male 1/8" headphone jack to plug into the BlackBerry.  Left to OUTPUT1, Right to OUTPUT2, and ground to GND.

Then I'll be able to switch the selector from the boombox to CD and play CDs, and to Cassette and listen to the 'Berry.

Will this work?  What are the likely pitfalls?


Exactly! Though you could do it this way if you feel confident enough:

For the Blackberry, use the headphone out and set the volume as low as possible and connect to the "outs". While under power, slowly turn up the Blackberry volume until it matches a previous known volume at the boombox. If you cannot get it to come up to volume, connect it to the input and again slowly turn up the volume to match the volume. Try the same type of thing with the CD drive, but you might need to construct an attenuator before trying this out. You wouldn't want to fry the boombox.

Let me know what you are thinking of doing. I'll help you from whichever way you choose. I'll check back tomorrow.


The project is on hold at the moment.  Need to get some other things taken care of before I come back to it, and I also need to consider placement and mounting of some components before I proceed with any wiring.

The other option I've thought about, though, for connecting the audio components, is to bring them directly into the switch that controls the audio source. Having trouble figuring out which contacts go where, though.  When I have a moment, I'll post some more pics of that part of the board and see if you have any advice, if you don't mind.  :-)

Thank you very much for all your help with this.  I'll be sure to reward you with a mention in the (eventual) instructable this is all leading up to!

Here are a couple of pictures of the switch in situ.

The three positions are (from bottom to top in these photos) Tape, Radio, CD. I can see that the outer four pins are grounded, but how can I tell what everything else is?

UnderCabinetRadio 052_crop.jpgUnderCabinetRadio 053_crop.jpg

I've been pondering this one for a couple of days. It is very hard to tell without following the traces to the power amp and/or the preamps for the respective inputs. What I can deduce (hopefully correctly) is that the 2 center sets of pins (left and right), denoted by the left ones being connected to the large(ish) ground plane coming from the lower left outside pin, are probably the (on the right) left and right positive and (on the left) respective grounds (negatives) for the lines feeding the amp section. That's a combo between intuition and a WAG as to their actual purpose. Trace them and see if they end up at the amp. That would be my starting point on that switch. The other pins for the "tape, radio and cd" probably include power lines for their respective circuitry along with audio paths. They will have to be traced back to verify just what they are.

I would definitely do some sleuthing before hooking up an input directly to the switch. You don't want to hook up to a power pin or ground pin. Not good for your Blackberry, etal.

Actually, if you have the outputs located from the tape and CD preamp sections, apply a signal to them, 1 at a time, and see which pin corresponds to the signal when the switch is in the correct position. This would be faster than trying to trace the lines around the board.


While it is possible, you need to find out what the levels are that are coming from the respective outputs. Do you have a multimeter with low level AC RMS readings, probably in the 200mV-20 volt range? Starting with the highest range, and only moving down in order to get a satisfactory reading, check to see what they are putting out and see if what you want to inject there is of the appropriate level. Too much and you risk blowing circuitry, too little and you won't be able to drive the amp sufficiently. While you are at it, read what is at the input. If "your" inputs are similar, you may be able to leave them in there and just cut off the input traces before the chips.

"Gotta know the details..."


I think I understand, but let me restate for clarity.

I should use a multimeter to check for low-level AC voltage on each side of each IC and compare to the output of the CD-ROM drive and a bared headphone cable plugged into the BlackBerry.  Then hook it up based on what seems to make the most sense.  I would need to do this while the whole thing is running, right?

I get that (I think!).  Trouble is, the boombox is in pieces.  I might be able to cobble it back together to the point where I can get a cassette to play through it, but the original CD player was shot to begin with.

I'm thinking I might be testing this in a trial-and-error manner.  I know this risks producing the sweet smell of burnt components, but as long as the BlackBerry isn't in danger, everything else I'm playing around with is disposable.