RS4b crossover question

[ji4m]

Long ago my first Infinity speakers were a pair of Monitor IIa (using the Walsh tweeter). Although they made me smile, a great deal came along, and I traded those in toward a pair of Quantum 2. These stayed with me for many years until I was forced to sell them to finish off a long college career.

After a long absence, I have made it back into the Infinity fold with a newly acquired pair of Infinity RS4b speakers. The woofers are recently refoamed and the cabs are in fair shape. I made a pedestal for the foot of each speaker because the originals were removed long ago. Overall, the speakers sound fine and I'm pretty happy with them.

Having read about the amplifier torture test of the big Kappas, I ran an impedance curve of my little RS4b. Turns out these aren't so friendly either, with a 1.6ohm dip at 35hz and a 1.7ohm dip at 1.8kHz (midrange pot centered). This surprised me, so I verified both low readings the old-fashioned way with a signal generator, amp, series resistance, and voltmeter. Sure enough, those are real.

These lows seem almost irresponsible, so I moved on to the crossover. My speaks differ from the RS4b Technical Sheet found at the Infinity Classics website as follows:

Woofer section - 450uF changed to 1000uF, 50mH changed to 40mH, 5mH changed to 2.5mH, zoebel network removed.
Midrange section - 19uF changed to 16.5uF, both inductors changed to 1mH, 6.8uF changed to 5uF.
Tweeter section - 4.5uF changed to 6uF, tweeter polarity reversed on schematic but wired normally in the speaker.

Any parts not mentioned are unchanged from the schematic. I unsoldered a few joints to measure some parts directly. Their measured values match their labels as listed above.

A simple analysis of the crossover circuits reveal a reasonable agreement with the measured dips. It makes me wonder why Infinity chose such a difficult load. I have many questions, but for now two stand out.

Can any other RS4b owner verify the polarity of their tweeter wiring? The schematic shows phase reversal, but my tweeters are wired normally.

Why does the woofer crossover include a high-pass filter section (series 1000uF cap)? Checking other Infinity schematics, this is a trend with dual-woofer speakers. I am no expert but this was unexpected.

Thanks,
Jim

[geoff727]

Why does the woofer crossover include a high-pass filter section (series 1000uF cap)? Checking other Infinity schematics, this is a trend with dual-woofer speakers.

Subsonic filter to get rid of the really low frequencies, I think. Why this would matter on anything other than the low frequencies you get from warped records, I don't know. Even then, with a sealed box enclosure, it doesn't make a great deal of sense. Some of us have had some discussions here about removing those large value woofer series caps. When the day comes that I dig into my RS-IIIA's, the 1200uf's are coming out.

I am no expert....

I wish I was.

[geoff727]

Woofer section - 450uF changed to 1000uF, 50mH changed to 40mH,

I believe that large shunt inductor (your 40mH) is the second part of the subsonic filter. The low frequencies that make it past the large cap will have a path of least resistance back to the negative terminal through that big inductor. My -IIIA's have exactly the same thing, and they'll come out too. My RS-II's, though, don't have that large shunt inductor, only the large series caps.

Can any other RS4b owner verify the polarity of their tweeter wiring? The schematic shows phase reversal, but my tweeters are wired normally.

Eventually, I'm going to do some listening tests with the -IIIA's, with the EMIT wired for normal polarity, and then out of phase.

Do yours have the zener diodes across the EMIT like the schematic shows? Now I wish someone would explain what those are in there for.

[ji4m]

Yes, my tweeter circuit includes the 1n5343 7.5V zener diodes. I believe these are protection diodes that limit the maximum voltage swing to the EMIT. They clamp the peak voltage to about 8.2V (7.5 + 0.7) in either direction wrt ground (16.4Vpp, 5.8Vrms).

Thanks,
Jim

[ji4m]

I wired a switch into the crossover to investigate the purpose of the leading 1000uF cap and 40mH inductor in the woofer section. After some investigation, it appears that the C and L combination is used to extend the bass response lower in frequency.

The Fs of the woofer (#902-5030) measures 34.7Hz, and in the RS4b's sealed box of about 2.3 cu ft, the response would drop to -3dB at 55Hz. The additional components lower the response to 35Hz.

Attached is a screen shot comparing the frequency response of both crossovers. I didn't normalize the curves to 0dB, but it is the difference I am trying to show.

The extended bass comes at a price, pulling the speaker impedance down to 1.6ohms. Adding a 2ohm resistor in series with the 40mH inductor raises the impedance dip to 2.5ohms without a large change in the bass output. I plan to alter the crossover to find a practical compromise in bass versus impedance dip.

Thanks,
Jim

[rabbit]

I wired a switch into the crossover to investigate the purpose of the leading 1000uF cap and 40mH inductor in the woofer section. After some investigation, it appears that the C and L combination is used to extend the bass response lower in frequency.

The Fs of the woofer (#902-5030) measures 34.7Hz, and in the RS4b's sealed box of about 2.3 cu ft, the response would drop to -3dB at 55Hz. The additional components lower the response to 35Hz.

Attached is a screen shot comparing the frequency response of both crossovers. I didn't normalize the curves to 0dB, but it is the difference I am trying to show.

The extended bass comes at a price, pulling the speaker impedance down to 1.6ohms. Adding a 2ohm resistor in series with the 40mH inductor raises the impedance dip to 2.5ohms without a large change in the bass output. I plan to alter the crossover to find a practical compromise in bass versus impedance dip.

Thanks,
Jim

A lot of the Infinities used passive bass equalization in order to extend the bass response of the speaker. To do this some speakers used dual voice coil Watkins woofers with dual series-resonant circuits tuned to different frequencies whilst others such as the Rs4b's just used a single voice coil winding driven by an LC high pass filter.

To equalize the low end response the speaker needs to draw larger amounts of current from the amplifier at the lower frequencies in order to compensate for the natural roll off of the speaker in the sealed box. This is why the impedance dips to very low levels at low frequencies. If you put your meter across the voice coil terminals you will see the voltage across it will increase the lower the frequency and then drop down below the cutoff frequency of the LC filter.

Unless you use active equalization I doubt if there is much you can do to tame the impedance of these speakers. You just need to use suitable amplification equipment that is up to the task of driving low impedance loads.