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

Thanks, Erik. Not sure why my link didn’t work for you, although strangely when I tried the link using Google Chrome on a different computer than the one I used to submit my previous post I had to refresh the page to get the image, after initially going to the link and getting only a small symbol with an "X" in it. That issue did not arise with Chrome on the computer I used to submit my previous post, and it did not arise with Firefox or Edge on either computer (using Windows 10).

In any event the schematic that link depicts is the same as the first of the three schematics shown at your link.

... it’s value will probably affect the absolute location of the high pass filter.

That is what I was getting at, of course, and given the interaction with the 4.7 uF cap that will occur I would expect the effect of a change between 27 and 47 ohms to most likely be considerable. Although it’s hard to be certain without having some idea of the turns ratios of the inductor/autoformer taps.

Best regards,
-- Al

Hello Eric, Al and Elizabeth (all three of you are some of my favorite people in all of Audiogon).I was actually trying to find my crossover schematic, and somehow it got lost in my computer (I'm not on the best of terms with it, as it continually makes me feel quite stupid). Anyway, the values as shown on the schematic Eric posted is similar to how mine are, though the pots are removed, autoformer taps are replaced with Alpha Core ribbon inductors, Mundorf S-I-O caps (the two 6.8uf caps in the LF circuit is a single Mundorf 22uf cap, there are 3.9mh, 2.2mh, and 3.9mh inductors, and the 50ohm resistor shown in the diagram near the autoformer is the one that was changed to 27ohm, and then back to 47ohm as discussed. The original factory crossovers are housed in a pocket on the rear of the speakers, and can be easily used by just changing a few wires on a terminal block.

Al, I thank you for your nice comments regarding my system. It's a pity we can't exchange visits, I've got to think we would both enjoy ourselves.

Regards to all,
Dan

Thanks for the nice words, Dan. I do have a couple of relatives in the Seattle area, so if and when my wife and I decide to make a cross-country trip I’ll be sure to keep you in mind.

To provide some perspective on the effects of a change between 27 ohms and 47 ohms, if in the interests of simplicity we make the very crude approximation that the low frequency rolloff of the combination of that resistor and the input capacitor of the high frequency section (which I assume is 4.7 uF) is unaffected by the circuitry that is further downstream (even though that circuitry is essentially in parallel with the resistor), we have the following:

The corner frequency of the RC combination, i.e., the frequency at which the rolloff is 3 db and the phase shift is 45 degrees, would be:

F(3db) = 1/(2piRC)

For 4.7 uF and 47 ohms F(3db) = 721 Hz

For 4.7 uF and 27 ohms F(3db) = 1255 Hz

The rest of the circuitry will have its own effects on those numbers, of course, as well as affecting the rolloff of the RC combination itself. In any event, though, that’s quite a difference!

As to why the results with the other 47 ohm resistor that was used a few years ago were so different, I can’t say of course. But I would guess that while the better quality resistor is probably helping, unrelated things that have changed during that time (perhaps even additional breakin of various components) are probably more significant contributors.

Best regards,
-- Al