Rubicon Monitors - Further Work
Checking out the Crossovers
I am well aware that, in changing the tweeters I will have altered the frequency response significantly. I was also aware that I did not have any detail of the crossovers as installed, only that there were a great many operational amplifiers on the circuit board; 14 in fact and I had no idea what they all did.
I had one of the monitors open for yet another fault so, after fixing it I took the opportunity to measure the crossover frequency response. Here it is and quite a surprise. Ignoring the various measurement artifices, it shows a crossover frequency of around 3 to 4 khz, a hump at 7khz on the treble and various wobbles and slopes on the LF drive. I thought there might be something wrong with the crossover, so I took the other monitor apart and tested it. The response was the same. Whatever the reason for the various contours, it was abvious that they wer not appropriate for the new tweeters.
Samson were unresponsive to my requests for a circuit diagram so I decided that I would build a new crossover and I ordered some parts for a Linkwitz-Rliey 4th-order active crossover. Meanwhile, I spotted an old post on Group DIY a member had circuit diagrams for an R6 model and he kindly sent me a copy. He did point out the anomaly that the files were called R* but the drawings labelled R6. Here they are R8 Amp R8 preamp. I worked through the crossover circuit against my R5 crossover and discovered that, apart from socketry and IC numbering, they were the same. So, no need to build a new one. I could modify the existing one.
The Samson Crossover Circuit
The circuit is very interesting and parts of it, new to me - in particular, the use of a 4-pole state-space filter for the main crossover element. I have only seen 2-pole versions previously There is not much on the Internet about this circuit. I found some mention but no analysis, not that it matters; printed on the circuit diagram is the text "4 pole Linkwitz-Riley Xover", so I am happy to go with that.
There are numerous gain/frequency shaping stages, all nicely labelled on the circuit. Before the State-Space filter there is a section that gives boosts at 6.5 khz and 22.1 khz and cuts at 2.8 khz. After the Low-Pass output of the 4-pole filter, there is a passive pole zero pair labelled "LF Tilt Network" and after that, an active 3-pole under damped HPF with an cut-off frequency of 45 hz.
On the High-Pass side there is a gain adjustment stage controlled by a 4-position switch. It is not a straightforward gain switch but a variable filter that alters the gain above 10 khz. A tone control of sorts.
This graph shows the gain of the H.F. stage at three different settings out of the four. (This is after the change of crossover frequency)
Here is a plot derived from a simulation performed in LTspice. The broken lines indicate phaase. it shows the gain of the ribbon adjustment stage on each of the four settings. It does indeed look much like the treble section of a Baxandall tone control. No harm done.
I had not considered it previously, but this gain adjustment satge gives a 180 deg. phase inversion. Should I have changed the polarity of the tweeter?
I set about changing the response. Firstly, I changed the crossover frequency from 3.19 khz to around 2.2 khz by doubling the capacitors in the Linkwitz filter. Then I removed the various boosts and cuts that would now be in the treble region, by removing resistors R21, R24, R26 and R40. The response of the treble signal still looks to be lacking at the high end but it is changeable by the ribbon control. I did not make a note but the treble response looks similar to the 0dB setting shown above. I figured at this point I would modify both monitors and have a listen.
Listening to music, one thing was immediately apparent, the stereo image was much sharper than before. Apart from that it is hard to make any comparisons of before and after. I have listened to some KRK monitors in another room and they always sound "better", more impressive in some way than my speakers. It may be the speakers or it may be the room, or a combination. I have, repeated listenings on the two systems after all of these mods and I am somewhat disappointed to say that the KRK's in their room still sound "better" than my modded R5's.
I made no mention in my original Modification article, but I spent some time adjusting the gain of the treble amplifier so as to come up with the correct relationship between treble and bass speakers. I ended up reverting to the original gain. After these additional mods, I performed a frequency sweep with a microphone at 600 mm from a speaker, positioned on-axis and at a horizontal level judged to be equidistant between the bass and treble speakers. Here is the result, for what it is worth, it will be greatly affected by the room response. I guess it shows that the tweeter level is set in the right ball park.