[Teknofossil58]

Hello all,
              I have looked around for diagrams of the RM 40. I also noticed a number of people requesting the same schematic. Anyone know whether one was ever posted?

Thx, -T

[HAL]

You might want to post what version of the RM40 you have.  There have been a few configurations over the years.

Seen both dual circular planar and single ribbon tweeters.

[GeorgeAb]

Hello all,
              I have looked around for diagrams of the RM 40. I also noticed a number of people requesting the same schematic. Anyone know whether one was ever posted?

Thx, -T

No, there has never been one posted that I know of. How quick do you need? Just wondering if this is a "what if" exercise, or you have a xo issue. I have been meaning to reverse engineer and post the RM-40, just haven't got around to it. If you need sooner, I will move it up the "to do" list. 

It is going to be just like the RM-V60 Shaukat posted or the RM-30 I posted just with different values for the capacitors and inductors. Brian was on to something with his minumum phase philosopy (simple crossovers, which to work well meant doing non-simple things in choosing his drivers and cutoff frequencies) and he stuck with in the Ribbon Monitor series; a capaitor in series going to L pad with inductor in parrallel to input for tweets, the same with an addition inductor in series going to the mids. Woofer as a low pass is just the opposite with inductor in series and capacitor in parrallel to input. This is the RM-V60 and RM-30. The RM-30 looks convuluted (not drawn as efficiently as Shaukat's), but if you look at it you will see it is the same as RM-V60 just slightly different component values, tweets and woofers are same values, slight difference in cap value for mids inductors are same. 

[GeorgeAb]

I have been meaning to reverse engineering the RM-40's crossover and post for the last couple of years. Easy to procrastinate as not an insignificant effort. Decided to just go for it. Will post all the schematics for ribbon monitor series in seperate post to make it easier for folks to find. 

[Teknofossil58]

Wow... thank you very much for the responses. I was asked by an acquantence if I could check out and determine the correct components in the crossovers, returning them to their original factory configuration. We all know the fun changing things can be... But....

Thanks, -T

[GeorgeAb]

T,

You are most welcome. Required some motivation as been meaning to do this for years.

If you check the component values, you will need to keep a couple things in mind. Most importantly is to remove the lead going to the ribbon tweeter. If you do not, you will fry the ribbon! Been there, done that; the first time I did this. The meter that measure capacitance puts power in the system to determine capacitance value, enough to fry the ribbon. You will have to desolder the lead of the midrange large inductor (3.6mH) going to the large midrange capacitor along with the leads going to the L pad high side for both the tweeter and midrange. This will isolate the components to measure with your capacitance meter. If you do not remove leads there are parrallel paths and you will not get incorrect reading.

Another consideration is when Brian built these he used high tolerance and tested and calibrated the capacitance value to get the desired value. From his website: "We build passive crossovers to very tight tolerances. For us that means four decimal places, or 1/2000th of 1%. Each crossover is trimmed to exact value, no tolerances. This means, for example, 1.600uF, not 1.6uF." Not sure if this is that critical.

However, with this in mind, since the cutoff frequency is determine by both the value of the inductor (typical tolerance 5, 10, or even 20%) and capacitor (Fc=1/(2*pi*the square root of (L*C))), to get the exact cuttoff frequency you would have to first measure the value of the inductor and then calculate the value of the capacitor needed to get the exact cutoff frequency. From there you would add capacitors in parrallel to get the desired capacitance to get exact cutoff frequency.

Hope this makes sense. With all that said, if you check the value and expect a value of 55.6uF and get that you are totally fine.   

[ST86]

George,

Thanks for your efforts in tracing and posting everything and for the warning regarding frying the tweeter.

Looking at the RM40 schematic the total panel loading works out to about three ohms.  The series resistor brings the total load to about 5 ohms until the inductor starts to kick in at the higher frequencies.  A general question, and George I am not questioning or doubting your work:  Five ohm load with eight ohm L-pad?  It is my understanding this type of mismatch should be avoided but Brian made it work.

Thanks,
Ed

[ctviggen]

Supposedly, he really did trim everything very well, and I think even did listening tests to ensure everything met his satisfaction.  I talked to him a few times and always thought he was engaging. 

[GeorgeAb]

Supposedly, he really did trim everything very well, and I think even did listening tests to ensure everything met his satisfaction.  I talked to him a few times and always thought he was engaging.

I probably should be more clear in making a technical point. If you are replacing/upgrading your capacitors, recommend measuring the caps value and put in same value. Each speaker and channel will likely have a slightly different capacitor value based on the value of the inductor used. The critcal frequency (-3dB at bandpass) say 280Hz for bandpass of the mids is determined equally by the value of the inductor and capacitor. Since the inductor value is fixed and can vary by as much as 10% or even 20%, it is likely that Brian measured the inductor's value and then solved for the capacitance value to get the exact desired cutoff frequency. From there he tailored the capacitor to that value.

I can see how that point could get lost when I originally wrote as follows:

Another consideration is when Brian built these he used high tolerance and tested and calibrated the capacitance value to get the desired value. From his website: "We build passive crossovers to very tight tolerances. For us that means four decimal places, or 1/2000th of 1%. Each crossover is trimmed to exact value, no tolerances. This means, for example, 1.600uF, not 1.6uF." Not sure if this is marketing hoopla or that critical. However, with this in mind, since the cutoff frequency is determine by both the value of the inductor (typical tolerance 5, 10, or even 20%) and capacitor (Fc=1/(2*pi*the square root of (L*C))), to get the exact cuttoff frequency you would have to first measure the value of the inductor and then calculate the value of the capacitor needed to get the exact cutoff frequency. From there you would add capacitors in parrallel to get the desired capacitance to get exact cutoff frequency. Hope this makes sense. With all that said, if you check the value and expect a value of 55.6uF and get that you are totally fine.   

[GeorgeAb]

George,

Thanks for your efforts in tracing and posting everything and for the warning regarding frying the tweeter.

Looking at the RM40 schematic the total panel loading works out to about three ohms.  The series resistor brings the total load to about 5 ohms until the inductor starts to kick in at the higher frequencies.  A general question, and George I am not questioning or doubting your work:  Five ohm load with eight ohm L-pad?  It is my understanding this type of mismatch should be avoided but Brian made it work.

Thanks,
Ed

Yes, the mid panel impedence in the midband is 5 ohms for the RM-40 and 626R, 9 ohms for the RM-30, and 4.5 ohms for the RM-V60. I find the use of an 8 ohm L pad to be elegant. It provides an 8 ohm load to the amplifier, and gives us the V for varialble in (V)MPS. Yes, the L pad does have to dissapate some power, but Brian used 50W L pads for the midrange and either 25W or 50W L pads in the tweeters. I do not see it as an issue.  https://www.bcae1.com/lpad.htm