Tweeter Capacitor Polarity Question

nicoch58

Member
2004-10-06 12:20 pm
"AC power is supplied to the DUT via a Variac set to approx. 10-15 volts AC output, but if a sine wave generator or similar is available with this level of output, this would be fine too. (Generally use lower voltages for larger caps, to avoid too much loading of the generator and DUT, but very small caps may need higher voltages to give a proper reading)

Regrettably, most audio oscillators I have seen don't give a high enough voltage output.

Readings are obtained by attaching a 'sensing' coil to the probe of an oscilloscope and I have found that a miniature 12 volt relay with a coil of 700 ohms impedance (which I just had to hand) was ideal, but this is in no way critical. I would *guess* that almost any small multiturn coil of wire will indicate some kind of reading when one end (only) is attached to a 'scope probe.

Set scope Probe to X1, Time/Div set at 5mS, and Sensitivity set to 20mV. Again, these settings can be varied, but this is a good starting point to see some readings on an 'average' cap.

The coil should be rested at the side of the DUT, but precise location is unimportant since the two 'opposite' readings should be quite positive. Don't deliberately move the coil position between the two readings, though, as this may distort the results.

At rest (i.e. with no signal applied to the DUT) a clipped sinewave should be seen on the scope, and this waveform is the 'datum'.

Connect the AC generator (using both leads) to the cap, and one of two things will happen to the 'scope trace.

1) The 'scope waveform will increase by a significant factor.

2) The 'scope waveform will stay pretty much the same (possible marginal increase).

If the *difference* between the two waveforms is not very well defined, try to reorientate the cap and/or move the coil about relative to the cap until a very positive difference is seen.

When the waveform is as in 1) above (i.e. larger), the 'live' lead of the generator is attached to the 'outer' cap foil.

In reality, it is a lot easier to do than perhaps the above might suggest, but if those instructions are followed carefully, it will give a satisfactory result. There is nothing critical about the procedure, and having achieved a satisfactory reading once, it will seem like 'riding a bicycle' thereafter, and it merely takes seconds for each cap.

All that is being attempted here is to inject a signal, which is then
picked up by the coil through inductance, to either the outer or inner foil. When the signal is sent to the outer foil, a much larger signal will be picked up by the 'proximity' coil than if the signal is sent to the inner foil, which is partially shielded by the then grounded outer foil. "
link
 
Nichoch46..........Thank you very much for your enthusiasm on my question and your research...........I originally read the pdf file you pointed out, however, did not see the 2nd page of it, which is where that information exists............Cool man, I am very grateful to you for your perseverance........Respectfully............Omni.........Frankie thanks you also..........;)
 
My friends...... Tinitus, and Grant..........I am soooo glad to see you here again...........Yes, man, Frankie is coming along well....As you can see, I have FINALLY gotten Frankies' heart re-simmed, through many hours of listening and simulation and tons of computer paper and help from my friends here............Grant, your continued input has been very heart warming..........Frankies new parts have arrived and as we speak, I am in the process of making the changes...........Look for my new reports here shortly.........Omni
 

grantnsw

Member
2006-08-01 1:35 pm
Omni, Hi
This stuff about tweeter capacitor polarity is IMO bunkum!
How can a passive AC device possibly have a preferred polarity?

But , there is a very easy way to find out.. as I'm sure you would know.
Use alligator clips in a test situation....

Simply connect one cap to one speaker, one way, and reverse on the other. Easy. If, in fact you do notice any difference, please post here.
I'll bet you , lol my 2 cents~ that there is no discernible difference at all.
I hope this helps in Frankie's quest. BTW, good choice in expensive caps!

If I'm wrong, well, I'll eat my hat... haha grant
 
For a wound-foil cap with one side gounded, then the connection with the outer side to ground can give less electrical noise coupling to other circuitry than with the 'signal' foil to ground. This is one reason why in the old days caps intended for screen-grid decoupling in valve circuits had a marking to identify which wire was connected to the outer foil.
If the caps are being used as a series connection for the audio, then no difference is to be expected either way round.
 
Thank you all for your input....As was pointed out by nicoch46, the Hovland website listed a preferred direction for the capacitor to be oriented with the inner foil side to the speaker..........I figure this is their method, However, at the Percy Audio site, it is general practice to orient the capacitor with the outer foil side towards the speaker............Again conflicting information..........I believe there is something to be gained, at least for the knowledge, and I greatly appreciate the input you all have given me.............Respectfully...........Omni............
 

grantnsw

Member
2006-08-01 1:35 pm
Hi all,
I hope my post #15 did not sound arrogant. I have an open mind, but this subject perplexes me! lol

Hi Nicoch46... have you reversed the 'polarity' of tweeter caps and found any audible difference? thanks

Tinitus, Hi, glad to 'see' you again, how is your 3-way going?

Omni, mate, I would like to hear of your progress with Frankie. best wishes to all! grant