HF resistor change a revelation on 12" Tannoy HPD's

Room acoustics matter a great deal, including between and behind the speakers.

I'd be interested to know exactly what this particular resistor was doing. Doubling or cutting that in half seems like a big deal, but it's also a rather large value, was it in an impedance compensation network?

Hey Elizabeth,
It depends on where the resistor is.


Any time you want to go through a speaker crossover simulation with me I'll be happy to show you why this can be true. 

Best,
E

Congratulations, Dan, not only for the latest improvement, but for the great DIY work you’ve done over the years creating and refining the speakers.

Does the following schematic of the crossover network of an HPD-315 depict the corresponding circuit, at least approximately?

https://pinkfishmedia.net/forum/proxy.php?image=http%3A%2F%2Ffarm4.static.flickr.com%2F3455%2F396944...

And if so, does the 50 ohm resistor it shows correspond to the resistor your post refers to?

If so, as far as I can tell it appears that when the switch the resistor is wired to is in one of the positions that put it into the circuit it would interact with the 4.7 uF cap so as to affect the point below which low frequencies are rolled off, in the path to the high frequency part of the driver. With that point also depending on which tap of the inductor is being used.

Perhaps Erik will comment further, as he is especially knowledgeable about speaker design. But if my supposition is correct I’m not surprised that the change from 27 ohms to 47 ohms had a dramatic effect, as it would affect the frequency at which 3 db of rolloff occurs in that part of the circuit by roughly a factor of two, depending on the inductor tap that is being used. Phase shifts at a range of frequencies, probably in the mid-range or upper mid-range, would also be affected.

Best regards,
-- Al

Thanks for finding that, Al. Unfortunately I could not open the link but I found an alternative crossover image here, including one which foregoes the autoformer entirely:

https://www.hilberink.nl/codehans/tannoy36.htm

I don't have a lot of experience with the use of autoformers in speaker crossovers, as for the most part they predate me, but as far as I can see this is an impedance compensating resistor. It keeps the impedance of the transformer stable as the switch changes.

In this case, it looks like Elizabeth was correct, and the absolute value there is less important than elsewhere, but it's value will probably affect the absolute location of the high pass filter.



Best,

E