Samson Rubicon Studio Monitor Repair

The Monitors

I own a pair of these monitors. They have 5 inch bass speakers and s ribbon tweeter. The crossover frequency is about 3.2 khz (revised from 2khz since receiving circuit diagram). They are well regarded, having received good reviews, see Sound on Sound  and they have given me good service and over perhaps 7 years. However, I have had a couple of faults which I suspect are related. The first one was caused D.C. to appear on a bass speaker, threatening to throw the cone across the room. It was under guarantee so I sent it back. It was returned, "No Fault Found". The fault reoccurred within a short time. The second, a tweeter open circuit. I suspect this may also have been caused by D.C. but I have little proof. The first fault righted itself after removing and refitting the power amplifier board and has reoccurred. The tweeter required replacement.

Update Feb 2019, 2-years later

The fault described above reoccurred with increasing frequency. It tried on numerous occasions to trace the source but it always cleared when I opened the box. Until one day it didn't. I measured the supply voltages to the pre-amp board and the -ve was missing. A tap with a screwdriver handle on the power-amp board would restore it an another would see it go off again. So, I had an inkling of its source. I took the main amp board out (again) and looked at the soldering of the components in the -ve regulator through a magnifying glass and I spotted one that was suspect. It was on the series resistor to the emitter follower regulator transistor. I re-soldered it refitted the board. I tested it using a hit from the screwdriver and it was OK. It was fixed.

DIY Reflection Filter for Voice Recording

 Finished 2

I have been interested to try a Reflection filter since reading an article in Sound on Sound 2006 I got around to making something of the sort in 2015. Here it is

I chose to mount acoustic foam on a semicircular frame made of my favourite material, wood. At the top and bottom would be hoops made of laminated hardwood, five stretchers between the two and a wooden cross-frame to support the thing. The microphone would attach the the frame and it in turn would attach to a microphone stand. The acoustic foam I had was 420 mm square. Two together would give 820 mm and allow a radius of 261mm so that would set the size of the thing.

Rubicon Monitors - Further Work

Checking out the Crossovers

Crossover Output high levelI 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.

Making a Ribbon Microphone Using a Commercial Element

 

 

Mic Body with Homemade Transformer

 I purchased a ribbon microphone element branded "Alcatron". I made a simple wooden body from some reclaimed English Oak and mounted the ribbon assembly inside a rectangular routed pocket.

The transformer is home made and looks it. The laminations are taken from a small valve output transformer, I sure they will be plain iron, though I don't know. They measure 2 inch * 1.5 inch and I used a stack height of 0.3 inches. This a large core for a ribbon transformer but it was to hand. The voltages generated by the ribbon are tiny so there is no possibility of the core saturating. To give some perspective, I calculated at what voltage saturation would begin. For a 0.25 inch square core with 20 turns, 64 millivolts would give a peak flux density of 1 Tesla at 20 Hz. The ribbon output will be one thousandth of that.