AlgoRhythms

Boston Acoustics A70 schematic

11 posts in this topic

Just picked up a decent condition pair of Boston Acoustics A70s for my first restoration project.  I want to develop some chops before tackling something more challenging.

 

Does anyone have a BA A70 crossover schematic?

 

(I had a pair of A40s back in the 1980's and really liked them.  It was the first and only pair of speakers I ever bought without listening to first.  I read the Richard Heyser review in Audio. )

 

Thank you.

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Are you just looking to replace the capacitors?

If so, you could just pull the crossover to verify the capacitor value.  I believe there is just one capacitor in each crossover.

The next question becomes electrolytic vs. film capacitor for the replacements.  Film capacitors are larger, so do not always fit in the existing positions on the crossover.

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Yes, that's exactly what I'll do if I dont fina a schematic.

I most certainly will replace the caps.  I haven't looked inside yet, wanting to see what the paper looks like.  If there are inductor(s) and resistors, I'll replace them as well.

As fare as film vs 'litics..... not sure what way I'm gonna go.

 

Thanks for your reply.

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Looking to know what the value of the inductor is.  I can't tell from seeing it as its covered with hot glue.

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Inductors and resistors generally do not go "bad" like electrolytic caps can over time.

if the intention is a restoration, not sure I would mess with the inductors and resistors, as just the caps would need to be restored (replaced).

 

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16 hours ago, AlgoRhythms said:

Yes, that's exactly what I'll do if I dont fina a schematic.

I most certainly will replace the caps.  I haven't looked inside yet, wanting to see what the paper looks like.  If there are inductor(s) and resistors, I'll replace them as well.

As fare as film vs 'litics..... not sure what way I'm gonna go.

 

Thanks for your reply.

If you're bent on replacing the coils, you'll need to have or find a way to measure what's in there because most coils are not identified. It's most likely an iron or laminate core coil. Those don't go bad over time like lytics do. However, If you want to consider upgrading the coil to a copper foil coil (not sure if this speaker is worth the expense), then It may be worth the effort to determine what the mH and DCR values are of what you have and duplicate them a closely as possible. 

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No, I don't know if i will change anything until i open them up and measure.

I just like having the schematic to look at as its easier to understand than tracing out the circuit from a blob of glue.

I consider these speakers a platform for practicing restoration before attempting an AR or KLH.

 

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I don't think BA's have gotten the attention that AR and Advent has over the years. So, the dearth of xover info. 

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Yeah,  Knowing factory values and layout would be helpful in order to insure that left/right time constants match.

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I don't know if you have completed your recapping job on your A70's, but I worked on mine over the last few months and here are my results.  My speakers are the original series, not Series II.   I use them as rear speakers in a 7.1 system.

I replaced the existing Unicon capacitor with Parts Express NPE's.   I had to use two to achieve the correct value.  (I don't remember what the value was.  I wrote it down on an index card that is taped to one of the speakers, but it is hanging on the back wall and is a PITA to take down.)

I have never considered the stock A70 to be an overly bright sounding speaker, but I did not like the results with the new NPE's at all.   The speaker sounded excessively bright to my ears, a bit "honky."   Perhaps the ESR of the new NPE's, especially two in parallel, was much lower than that of the replaced Unicon even when it was new.   So I added a 1/2 ohm resistor in line with the existing crossover resistor which toned things down a bit.   I still did not consider the performance optimized for stereo reproduction - still a tad ragged with certain material - but not critical considering their limited duty for enhancement of ambience and height my as rear channel speakers.

Of late I have had great success adding Dayton 0.01uF 400V F&F bypass capacitors (74 cents each) to my recapped AR9/AR90/AR91.   So I bought a few more with my last Parts Express order and installed them across the NPE's in the A70's.   I am now extremely happy with the stereo performance of these speakers.   They sound very smooth and balanced.

You may or may like the results...I have found that one often has to find their own way in this and many other hobbies...but it may be worth trying this mod given that it is so cheap and easy to do. 

(BTW, I have no idea why BA mounted the original cap so close to the resistor, but I put the new caps in the same spot.   Standard practice would call for putting that resistor a bit away from everything else in order to dissipate heat.)

 

A70 Recap.jpg

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Nice work. Saved by the F&F once again !

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