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Mar 1, 07 at 5:31pmhitest

After reading that there was another version of the BA735 with both analog and digital inputs, I looked at the pc board in the speaker and realized the board of the digital-only version has component locations for analog as well. Whether it's worth your time to modify is up to you - I did because I like the speaker system and enjoy the challenge. You will need to add 5 components - 2 surface- mount resistors, 2 surface-mount caps, and a right-angle stereo 3.5mm jack. You may also need to scrape away some of the potting goop they used to secure the jack assembly. I used 33k ohm for R10 and R16. The existing feedback resistors are 75k, so this gives the amp stage a gain of a little more than two - this seemed to produce good results - use smaller values for R10 and R16 if you want more gain. I used 4.7uF non-polarized capacitors for C25 and C38 (Boston probably used electrolytics here). The more challenging work involves the audio input jack. All 3.5mm types I found with this pin placement had the normally-closed and signal-in pins reversed from what this board wants. As such, I had to bend the inner pins outward to reach the holes intended for the outer pins. I also bent to outer pins to the side and soldered small leads from them to the extra holes inboard from the inner pins. Sounds confusing, but if you look at the jack data sheet and the board traces, it makes sense. I used a CUI SJ1-3555 jack (Digi-Key CP-3555NG). Perhaps you can find another jack that has the pins reversed and would make the modification easier. I did not add C20 to the board...looks like there are already a couple 470uF caps in parallel with C20 as it is.
That's it - the circuit is smart enough to detect an analog audio in signal and pipes that to the amplifier. I could find no schematic diagram for this product, so I need to guess at the component values.
Someone that has an actual dual input (analog and digital) version may want to open it up and see what component values Boston really used here, but this seems to work fine.

Thread Recap (last 10 posts from newest to oldest)

Mar 2, 14 at 10:39pm

I found a set of digital 735's at an antique mall for $2. I have really enjoyed the sound of my garage sale analog 745's and thought I had scored another great set of speakers for the wife's computer. However, I could not get the new 735's to work. After a little online research and fiddling around with the new set and the sound card and a quick stereo out to RCA jack hack, it still just would not work. I was so disappointed. Then I found this thread...Yeah! I read every post, decided to the soldering iron out and make 'em analog! After rummaging around it my junk drawer, I found some 1uf 10v standard caps and located a pair of tiny tiny tiny smd 22000 ohm resistors on a junk board instead of the 75000 ohms as required. Couldn't locate and didn't have anything else close. The jack came from an extra old sound card. 5 pins were on the jack, Multimeter test revealed 1 was the common ground. The remaining pins were connected in pairs, pins 2 and 3 were soldered together, then 4 and 5 were soldered together. The dual pin combos were bent to line up with the correct holes in the circuit board. The soldering was done according to hints and tips from all the thread posters. It WORKED !! They sound great. Thanks to all.

Jul 3, 13 at 5:34am

I bought a converter from ebay and attached this to my BA735. It does sound good but am wondering if i do the conversion myself (in case the converter later breaks down) would it still sound good? better?


May 27, 13 at 7:29am
J Scott

I bought all components online from Digi-Key ( as noted in previous posts:
Two 33K 1/2watt resistors (Digi-Key p/n P33KVCT-ND)
(The resistors are: 75KΩ)
Two 4.7uF 35-volt electrolytic capacitors (p/nPCE3011CT-ND)--note polarity when installing
(The caps are: 1uF 10v polarized)
CUI SJ1-3555 jack (/n CP1-3555NG-ND)

However, when I plug in any of my devices, I only get a faint amount of sound when the male jack is about halfway in, and no sound once it is fully plugged in.

Does anyone have an idea where I may have gone wrong?
I mounted the jack to the board and I'm pretty sure all the tabs are in the right position (based on the board leads and the jack schematic), and also made sure the polarity was correct when soldering on the caps.

Could I have possibly overheated the caps and resistors when soldering them on? Maybe have a bad or shorted out connection?


Apr 3, 13 at 3:03pm

Hello ! to All

First Post:

Thanks to all for the valuable information for modifying the BA735 Speakers.

These Digital Speakers were found in the trash and retrieved by a friend of mine.

I hooked them up with a Digital Audio Coax Cable to my computer and worked fine!

So I attempted to modify them with the information found in this forum.

I used 2 4.7 uF caps and 2 33K ohm, 1/2 watt resistors.

Instead of mounting a jack to the board I soldered a 3/c cable to the pads.

They are working and sounding great with the analog input from my Ipod Video.

I meant to take pictures of the finished modification, but I was anxious to
see if they would work properly, which of course they did!

I may still open it back up and take pictures if any one is interested.

Thanks Again!


Mar 27, 13 at 12:51pm

PS. My email is (



Mar 27, 13 at 12:49pm

Hi fellow boston BA735 speakers believers,

My name is Jessie and I really need a copy of the schematic.

Any help you can give me would be recieved with much "love"



Feb 6, 13 at 5:47am

Hello, I have this digital version speaker for about 3 years. could you share the schematics...?
I would like to do some other mods and schematic will be helpfull.


Dec 15, 12 at 3:11pm

REYESFA or MAD MAXX or anyone who has a copy. Thank god for this. It is awesome.

SO happy to not be alone in my endeavors!!!!!!!!!!!!!!!!! Picked up two sets the same day out yard saling and both have this awfull hum. The c59 caps were called out on another forum as being the culprit but replacing them only cut the hum in half.... but that is another story. Of these two there are one dual input and one digital so I am in the middle of the mod. My intent is to use this attached to my phone and computer, two different input amplitudes though so the phone is queiter than the computer. Does anyone suppose that the smt resistor could be replaced with a stereo pot to allow for input gain adjustments????? And does anyone have the ability to share the schematic??? Any input would be appreciated.

Thanks to everyone, including the digikey P/N's

Dec 1, 12 at 5:30pm

Got the schematic from José. Thanks so much. Greatly appreciated.

Dec 1, 12 at 4:23pm


I'm new to this thread but want to mod my BA735s as well. I wonder if reyesfa or Madd Maxx could send me the schematic as well. I'd be very greatful.

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