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I'm rebuilding/upgrading (I'm hoping it'll be an upgrade) a pair of Snell Type E II and a Spica Angelus speakers. The plan is to replace the electrolytic caps, low-cost (also quality?) resistors, and replace wiring (no active or outboard xovers, inductor replacements, etc.). I'm also considering doing the same to a pair of Snell Type Cis. I'm trusting/experimenting that changing these parts, which are quite old now, will yield a better-sounding speaker. It will likely be different, and I'm hoping for the better. I like the way they sound now - with all their really quite unremarkable parts. If I don't with the new bits, I can always put everything back the way it was.All of these speakers use a lot of electrolytic capacitors in their xovers. The Type Cis have a fist-sized lumps of them. I'm trusting that changing some or all of these will create speakers that I like the sound of more than they were stock.
My questions are:
1. Do I need to consider any difference in ESR between the original caps and the ones I'm using as replacements? John Bau, the designer of the Spica speakers, in specifying electrolytic to film cap upgrades to the Angeluses doesn't mention anything about this, but I have heard/read others' comments, some claiming this to be important, others not so much.
2. Does anyone know exactly how Snell "tunes" (or used to tune the speakers of the vintages of the ones I'm working with) their speakers to match the type reference? I know about, and can see, the adjustable/sliding wire wound resistors used for this (not that I know what to do with them), but capacitor and inductor values and trimmed, too? What was their process?
3. Should I measure the exact values of everything I remove and replace with exactly the same values?
Thanks!
- SJ
Edits: 02/10/09Follow Ups:
The Spica's were a time aligned design, so things like ESR are likely more important than conventional designs. In any case, I recommend the Spica user group on Yahoo as the best resource for Spica related issues. There are several heavy engineering types there that have spent a lot of time on the issues surrounding rebuilding Spica crossovers.
see: http://tech.groups.yahoo.com/group/spicaloudspeakers/
...will be of much use because their values likely have shifted with age. Inductors aren't going to change and it's not likely the resistors will either. I'd start with the tweeter circuit first and replace caps and resistors, then re-assemble the speaker and listen for a while. You might even experiment by doing only one speaker and leave the other original. Compare the two my listening while your electronics are switched to mono. Use the balance control to change between the two and see if there's a noticeable difference. Take your time. By doing one branch of the crossover at a time you can spot potential problems much easier.As far as ESR, better quality film caps won't create problems.
Tuning the crossover to a reference, as I understand it, involves changing resistor values or unwinding inductors, which also changes resistance to some degree.
Edits: 02/10/09
Thanks for the reply. The measured approach you suggest is such a good one. Sober, scientific. I just have to find the patience for it (for the Snells, it's more like my brother does; they're his speakers and he's up for some tweaking, but how much is the question).
Okay, so I won't sweat the ESR values.
On the tuning to a reference, I'm good with the top level of the concept, but I'm lost re: the tactical details. I'd like to see this done. What's measured? How are measurements made? What responses are made to the measurements when changes are indicated? For the Spicas, I'd like to know the exact process/methodology used to grade the midwoofers. Inquiring minds want to know ;)
- SJ
For the Snells, the sliding resistor was the tuning device. It's the only adjustment in the xover. I expect it simply balanced the spl outputs of the drivers.
Peter Snell was famous for bundling NPE caps and bypassing the bundle with a small disc shaped nF cap.
Some EE's suggest adding a low value resistor (.25-.5 ohms in series) when replacing NPE's to compensate for NPE's inherent ESR. That's another route you can try with your single speaker upgrade experiments. NPE's are cheap enough. If your old NPE's have drifted significantly (i.e. ESR risen well above 1 ohm) then replacing with film will be quite noticeable. It's like removing a series resistor from the tweeter circuit.
Check the link below for more information on the Spicas.
It's all about the music
I would imagine measurements would include impedance plots and anechoic chamber frequency response measurements.
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