snitch56 wrote:Quad wrote:
It would be good to replace the old Jensen bipolar 'lytic
in the crossover with a newer film cap.
Will replacing the electrolytic capacitor with film alter the sound any from the original design?
The short answer is, "Almost certainly yes."
That said... There is a "But..." Several of them, actually...
Contrary to a lot of commonly believed voodoo and hokum, there is nothing inherently evil about electrolytic caps. But the dynaco A-25's, good as they were, were built to a price. Like nearly all manufacturers of anything, SEAS' or Scan-Speak's (Most A-25's were built by SEAS, but some were built by Scan-Speak) bean counters demanded the use of the very cheapest capacitors that their engineers and product guys would consent to using (and still be able to live with themselves). That meant electrolytics that were not "total crap," but not the best, either.
The dynaco A-25 crossover employed electrolytics with a somewhat higher ESR, or "Effective Series Resistance" compared to the modern, high-quality replacement film caps most people these days look to for replacements. Without going into the detailed and arcane technical jibber-jabber, this basically means that with the replacement poly caps of the same value, the tweeter is going to sound louder, altering the octave-to-octave balance - the "voicing" - of the speaker system vis-?-vis the A-25's as they left the factory in Denmark some 4 decades ago. The difference is not huge, but is enough to alter to sonic character of the system as a whole, which is either a good thing or a bad thing, depending on how one felt about the original A-25's when they were new.
But unless you are old - like me (age 60) - you have never heard a factory-fresh dynaco A-25. If you own clean, well-maintained and unmolested ones, the drivers will still be OK, but the voicing of the speakers will have subtly shifted over the past 40+ years. This is because the aforementioned ESR of the crossovers' original capacitors has shifted ever upward over time, lowering the levels of the tweeters.
So, when you replace the aged old caps with sweet new polypropylene caps of the same value, the perceived sonic difference will be even greater, because you are in effect taking a speaker with a duller midrange and top end than intended by its engineers, and turning it into a speaker with a hotter midrange and top end than intended by its original engineers.
If you really want A-25's that could stand up to scrutiny as museum accurate "period pieces," the obvious approach is to replace the leaky old electrolytics with brand new electrolytics with the same electrical characteristics. This might be hard - or even impossible - to do. So you may have to resort to some trial and error experimentation. The aim would be to duplicate not only the capacitance values of the original devices, but the Effective Series Resistance, impedance and mechanical resonance (microphony) properties, too. You might have to resort to playing around with values, including adding some resistance in the form of series resistors in order to duplicate that "neutral" sonic balance that dynaco A-25's were famous - and valued - for in their day.
How does one do this? Well, if you think you are golden-eared enough to recognize perfect "sonic neutrality" when you hear it, I say, "Not a chance." Double-blinded tests have proven again and again that the claims to possession of super powers made by self-appointed Name-Your-Audio-Society "golden ears" NEVER stand up to rigorous scientific scrutiny. The best way - the ONLY way, in my opinion - is to dig up old test reports from 1968-69, when reviews of the then new A-25's were first hitting the street. These old test reports, from periodicals - some still with us, some since defunct, others unrecognizably mutated - like "The Stereophile," "High Fidelity," (defunct) "Stereo Review" (now a weak and puny shadow of its former self as 'Sound+Vision'), "Audio" (defunct) and "Hi-Fi News & Record Review", contained not just reviews and subjective opinion, but solid and complete technical measurements that could be of great value in duplicating the sonic characteristics of the original A-25's when they were the newest, hottest audiophile bargain-of-the-century. Of course, this would require you to get up off your Barcalounger-calloused butt and do some old-fashioned library research (Google won't cut it if what you want is only available on microfiche!). Also, you will need to buy some goodies like calibrated microphones, real-time analyzers, et al., as well as maybe convert that unused study into an anechoic chamber.
But unless you are a dedicated - and well-heeled - enthusiast, the above approach is impractical. Yet, if you just replace the old caps with the so-called "better" modern polys, you might like the result more when you are done, but the speakers you have created in the upgrade process will NOT sound like A-25's did when they were new. They will - FOR SURE - sound like something else that has never existed before!
As a practical matter, I think a conservative approach is best for most people. Listen... Change devices... Listen some more... Add or subtract values... Listen.... Change device manufacturer.... Listen..... Until you you think you have a pretty good idea about what is "right." Write down what you have arrived at for brand, values, etc. Then... pull it all out and go back to the original "un-improved" version. If there is a big, night-and-day difference between the sound of the original and the sound you arrived at - the sound you "liked," it is time to BE SKEPTICAL! People are often attracted to - seduced by - rising midrange characteristics, a bit extra "ping," "sparkle" or "sizzle" in the top end. These tend to make a speaker sound louder and/or more "efficient." But it is NOT accuracy! This path leads to THE DARK SIDE!! As millions of Bose owners will tell you, it is not against the law or immoral to like crappy speakers. But it's a terrible shame to not recognize coloration for what it is - a departure from neutrality - from fidelity - usually deliberately and cynically engineered into products to appeal to a company's target suckers - uh... customers....
The difference between otherwise good A-25's with capacitors past their prime, and properly restored and carefully "re-capped" ones, is certainly noticeable, but it is not radical. The basic voicing of the speaker should still come through, but with maybe a touch more airiness. But if it now sounds like a JBL studio monitor, YOU'VE DONE SOMETHING WRONG!!! From here, you have three options: Back to Square One and this time DON'T SCREW IT UP! Or... Send them off to someone who is expert at restoring tired-sounding A-25's and PAY HIM to do it right. Or.... Revert the A-25's back to their original "un-improved" state, put them on eBay, and sell them to someone who will love and respect them for what they are. Then, do a search on eBay and apply the money you got from selling your dynacos toward purchasing the JBL studio monitors you really wanted in the first place.
Lastly, there is much about "capacitor voodoo" thrown about that is just that. People buy expensive "upgrade" parts based on recommendations they read or hear, they install them, and then they claim to see the whole Meaning of Life Itself revealed and the Universe laid at their feet! Yet most of the time, instead of real improvements we are dealing with experimenter bias, Hawthorne effect and placebo effect.
For more scientific and objective treatments of "Cap-ology", check out the links below:
http://sound.westhost.com/articles/capacitors.htm#ref
http://www.madisound.com/services/upgrading_speakers.php/#components
http://www.hificritic.com/downloads/APassiveRole.pdf
http://www.faradnet.com/
http://members.cox.net/pjay99/captest.htm
The reading can get a bit dry, and not all them agree, but if you slog through these, you will know how capacitors really work, what differences they actually make in loudspeaker performance and finally, perhaps saner approaches to rejuvenating speakers with tired caps.
Cheers!! '(lol)'