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.

 

[mech986]

I think this thread could use a reactivation.

IMO, the use of passive equalization to extend the frequency response at the expense of a very low ohmage and difficult amplifier load is one that the Infinity designers seemed to like and sort of brought with them a quirky eccentricity that some reviewers viewed as a positive trait - like having a finicky Ferrari, needing to be highly tuned, run on only the best fuel, and armed with a cadre of mechanics at your beck and call. Similarly, since you needed "the best" amps that could source a 1.5 ohm load, and "ordinary and regular" amps not designed to push loads like that would literally die and flame out, people went off and decided that this was a positive discriminating factor for the speakers, and some high end equipment. Never mind that it left dead amps in their wake, IMO.

Today, I suspect that appropriate active equalization below 60hz and removing the crossover components that inflict the low impedance load is an easier and more effective way of dealing with this, albeit making the speakers essentially conventional and "ordinary" in design, but much easier to drive with a majority of vintage and modern amps.

Other options would be just to let the speakers roll off naturally without the offending crossover components and add in a good subwoofer/crossover and not stress the RS series double woofers beyond reason. I suspect the cabinets in this series may have been a bit undersized as well for cosmetic reasons.

 

[fhawk618]

I have two pairs of Kappa 9's which employ that signature Infinity crossover trickery to extend the bass response of a relatively small (volumetrically speaking) cabinet loaded with two 12" IMG woofers. They are perhaps the most evil of the Infinity line... dropping to 0.7 ohms at 33 Hz and pretty much everything below 100 Hz is 2 ohms or lower. Removing the inductor or applying a resistor in the woofer circuit will kill the bass response... the Kappa 9's even came equipped with a switch to add a 1 ohm resistor to the woofer circuit to attempt to make them more friendly to lesser amps, but there is a serious penalty in the bottom end. I'm just "stuck" with owning Krells and Aragons to drive them... not a bad thing to get stuck with, but the purchase cost can be a bit brutal...

 

[AKE13]

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

Hi Jim, I just come across your article and findings about Infinity RS4b. I know your article is old and you may not even get a chance to see my reply now. I was searching to get information about the issue I have with my EMIT tweeter. I am not expert in electronics and have a basic general understanding. My tweeter will cuts off, if volume is kind of high, but midrange & woofers are working. When I lower volume level, after few seconds it comes back to normal. Seems like tweeter has a protection device. What do you suggest me to do. Appreciate your feedback.
Regards, Kevin

 

[markuus3889]

Hi Kevin,

What amplification are you using? Sounds like clipping which will activate the self resetting fuse. Are both tweeters cutting out? I have only experienced tweeter cutout on high sound levels with poorer amplification. With 150 Watt minimum High Current amplifiers I can play to unhealthy levels with no cutout. Have the crossovers been worked on? Might need a rebuild there.

The tweeter protect fuse can be faulty and can be replaced. It is cheaper to replace the fuse than it would be to replace EMIT diaphragms if you were to eliminate the fuse altogether.

Mark K.