DIY Speaker Upgrade Help

[newcharliepuch]

Hey everyone, this is my first post to the forum.

The diagrams I talk about in this post are enclosed in the zip file
Speaker Diagrams.zip (sorry about the bad quality but i had to compress the hell out of them to get thme to fit)

I am posting concerning a pair of Sony SS-MF750H Floor standing speakers I own. I was recently comparing my speakers to a pair of my roommates Polk bookshelf speakers. Upon listening I noticed everything wrong about my speakers, mainly the mids and highs of the speakers did not sound good at all. I became interested in what types of crossovers were used in my speakers and this is what I found (see picture one).

After analyzing the values for the capacitors and the resistance for each of the drivers I became confused. The 4 drivers in the speaker unit are all wired in parallel. The two 8 inch drivers have a resistance of 15.2 ohms the midrange has a resistance of 7.8 ohms and the tweeter has a resistance of 4.7 ohms. Using the formula (1/((1/R1)+(1/R2)+…+(1/Rn))) to calculate the total resistance I obtained a value of around 2 ohms which doesn’t make sense because measuring the terminals at the back of the speaker I got a value of 7.8 ohms.

However there was another feature that really confused me about these speakers, which was the value for the capacitors on the tweeter and midrange. The midrange had a 2.7uf cap and the tweeter had a 1.3uf cap. To calculate the crossover frequencies I used the equation C=(0.159/R*F) where C is the capacitance (in farads) R is the resistance (in ohms) and F is the crossover frequency (in Hz). I solved for F=(0.159/C*R). (for the R I wasn’t sure weather to use the resistance of the driver, or the resistance of the entire speaker so I tried both and I will list both: driver then entire speaker) for the crossover frequency of the tweeter I obtained 28.2 kHz or 17kHz (depending on if I used the 4.7 ohms for the driver or the 7.8 for the entire speaker) for the midrange I obtained a value of 7.5 kHz. My problem with these values is that they seem to be way to high to be the proper crossover points for my speakers.

I wasn’t convinced so I tested the response of each driver using true RTA and a microphone I had around. I set each driver on the floor so that they would fire upwards and suspended the microphone about 6 inches from the driver. I turned my amplifier to a mid volume and connected the speaker wires directly to the terminals of the speaker, so that I would bypass any sort of capacitor. I used TrueRTA and performed a quick sweep where it sweeps all the frequencies and records the peak value for each 1/24 octave. I took screen shots of the graph for each driver, and I’m including them here. Because I live in a dorm it is not always the quietest environment. I’m not saying that these graphs are anywhere close to being absolutely accurate; however I do think they are able to tell you something about the speakers.

After looking at the graphs I concluded that proper crossover frequencies should be around 1400 Hz for the low to mid cutoff point and 4800 Hz for the mid to high cut off point.

I guess my whole point in writing this message would be to ask if anyone could give me any insightful information of whether It would be wise to attempt to build a new crossover for these speakers based on the information and resources I have (I cant find any information about the parameters for the drivers from Sony) Or if it would just be better to upgrade the capacitors to better ones (I’m pretty sure the ones being used now are cheap electrolytic ones) or would It be best just to leave them alone. (buying new speakers is not an option right now)

Thanks to anyone who can point me in the right direction!

Charlie

[Cal Weldon]

Spelling corrected

[newcharliepuch]

Bump

[planet10]

You have to consider the XO when calculating the total impedance. Because of the caps the mid & tweeter will have very high impedance below their cutoff.

I'd like to see pictures of the drivers.

This looks to be a speaker designed by the marketing/accounting department. The mid XO point is choosen because that is where the midwoofers roll off. The tweeter is just tacked on so they can call it a 3-way.

On 1st blush (i'd like to see a pic (& impedance curves)) of the drivers, i'd say toss the midrange completely and make it a 2.5-way system.

You might be better off starting from scratch thou -- i bet an FE127e or FR125s would be quite a bit better. I suspect the boxes aren't anything hot either.

dave

[sdclc126]

I honestly think you'll be wasting your time building new crossovers for these speakers - the improvement would be marginal at best. You're better off replacing the drivers AND the crossover, and yes ditch the mid & go for a 2-way.

The drivers Sony uses for most of its speakers IMHO are in the "junk" category - very high distortion etc. I recently rebuilt a similar pair for my roommate, basically gutting the whole things except for the cabinets, which got rib bracing, damping panels, foam panels & polyfill, then Peerless & SEAS drivers, and a 12 db crossover at 3000 hz.

The difference is night and day, especially given the non-ideal box alignment and non-optimized crossover. There wasn't even a real crossover in the original - just a single cap on everything, and the drivers were just basement stuff.

If you don't want to build from scratch but just use the cabs as a starting point this would be the recommended way to go.

[newcharliepuch]

Thanks for the advice to everyone who posted! However scraping the speakers and buying new ones, or even buying new drives is out of the question for me at the moment. With school eating up most of my money and free time it’ll be a few years before I can afford/design a nice pair of speakers.

I was mainly asking the question to see if there was anything I could do that might increase the quality of the speakers a little but without costing me a whole lot. I’m not looking for superb quality audio. But, maybe just a little better than what I have now. A friend of mine recommended upgrading the capacitors in the cabinets to some higher quality ones. I’m considering doing it if it will make a difference since it will only cost me about $20.

Next time I take my speakers apart I’ll take some pictures of the drivers and post them. But for now I need to finish my school work. Maybe tomorrow.

P.S. can anyone tell me how to post multiple pictures?

Thanks again

Charlie

[planet10]

Originally posted by newcharliepuch I’m considering doing it if it will make a difference since it will only cost me about $20.

puzzlecoat (paper cones?), ductseal, better wire, brace the box. try disconnecting the midrange and hook the tweeter up to the midrange cap. That's less than $20 and done well can transform a speaker

P.S. can anyone tell me how to post multiple pictures?

either multiple posts or put the pictures up in web space & link to them.

dave

[newcharliepuch]

ok I got some pictures of the speakers hope these help

[newcharliepuch]

another

[newcharliepuch]

and the last one