TL;DR: I recently found a pair of old 2-way Speakers, made by german manufacturer Arcus. As I first listended to them, I thought that they had beautiful bass and highs, but a recessed midrange. I tried to simulate the speakers using speaker simulation software boxsim ( http://www.boxsim.de/ ) and altered the crossover, starting with old parts from the scrap bin. After a few tries i ended up with decent speakers tailored to my listening taste.
Long Report - Intro
Since i was a youth i owned a pair of old, shabby looking Arcus TM 65 3-way speakers, that i considered to sound good at that time. Two decades later i already build a few DIY audio projects, like Salas UFS Phono Preamp and Randys Aleph 30. I felt that my old 3-way speakers where too shouty for my taste. I also want to tinker with active crossovers and bi-amping later on, that’s why i was looking for a pair of 2-way speakers. I found some Arcus TM 55 2-way speakers with beautiful wooden cabinets and bought them. When i first listened to them i was pleased, because the mids seemed to be less aggressive compared to my old TM 65. But soon i realised the mids where lacking completely.
Crossover Design Arcus TM55
Arcus datasheets states that the Arcus TM55 crossover frequency is 4500 Hz. Initially i did not believe that, because regarding their 3-way designs Arcus claimed crossover frequencies of 500Hz / 2000Hz with the same model of woofer and tweeter used.
I checked the crossovers on the TM 55 and i found a fairly simple circuit: one coil acting as a first order low pass filter for the woofer and one HUGE 7uF KP capacitor used as a high pass for the tweeter. Another small coil with a 4,7 ohm resistor in series is connected in parallel to the tweeter, maybe some kind of impedance correction?
So what we have is a very simple crossover, so lets check the values of the coils and simulate the speaker using boxsim for the sake of fun and learning!
Basic Measurements
I started with stripping the crossovers apart to measure the parts values using the simple GM328 Tranistor Tester. The big coil connected to the woofer measured 1,5 mH and the small coil parallel to the tweeter 0,58 mH. So the crossover design looks like this:
I searched the internet for TSP and SPL graphs for the Vifa M21 WN-02 chassis and the Seas H174 1“ dome tweeter. I used REW to measure the impedance vs. Frequency of the chassis . With Vituix CAD i converted the graphs to asci text to make it usable for boxsim. With this data I created a first, approximate model of my 2-way speakers:
First Simulation Results (Original Crossover Design)
The frequency response graph of the first simulation showed me why this speakers sounded quite recessed in the midrange. The graph shows a bass peak between 50 and 200 Hz combined with a huge dip between 300 and 3000 Hz. The highs above 4 kHz are obviously too loud compared to the rest of the frequency spectrum.
The crossover point seems to be at approximate 1500 Hz – one third of the claimed value in the datasheet. This results look quite disappointing regarding that Arcus was a big name in the German Hi-Fi market in the 1980s.
Next Steps
Long Report - Intro
Since i was a youth i owned a pair of old, shabby looking Arcus TM 65 3-way speakers, that i considered to sound good at that time. Two decades later i already build a few DIY audio projects, like Salas UFS Phono Preamp and Randys Aleph 30. I felt that my old 3-way speakers where too shouty for my taste. I also want to tinker with active crossovers and bi-amping later on, that’s why i was looking for a pair of 2-way speakers. I found some Arcus TM 55 2-way speakers with beautiful wooden cabinets and bought them. When i first listened to them i was pleased, because the mids seemed to be less aggressive compared to my old TM 65. But soon i realised the mids where lacking completely.
Crossover Design Arcus TM55
Arcus datasheets states that the Arcus TM55 crossover frequency is 4500 Hz. Initially i did not believe that, because regarding their 3-way designs Arcus claimed crossover frequencies of 500Hz / 2000Hz with the same model of woofer and tweeter used.
I checked the crossovers on the TM 55 and i found a fairly simple circuit: one coil acting as a first order low pass filter for the woofer and one HUGE 7uF KP capacitor used as a high pass for the tweeter. Another small coil with a 4,7 ohm resistor in series is connected in parallel to the tweeter, maybe some kind of impedance correction?
So what we have is a very simple crossover, so lets check the values of the coils and simulate the speaker using boxsim for the sake of fun and learning!
Basic Measurements
I started with stripping the crossovers apart to measure the parts values using the simple GM328 Tranistor Tester. The big coil connected to the woofer measured 1,5 mH and the small coil parallel to the tweeter 0,58 mH. So the crossover design looks like this:
I searched the internet for TSP and SPL graphs for the Vifa M21 WN-02 chassis and the Seas H174 1“ dome tweeter. I used REW to measure the impedance vs. Frequency of the chassis . With Vituix CAD i converted the graphs to asci text to make it usable for boxsim. With this data I created a first, approximate model of my 2-way speakers:
First Simulation Results (Original Crossover Design)
The frequency response graph of the first simulation showed me why this speakers sounded quite recessed in the midrange. The graph shows a bass peak between 50 and 200 Hz combined with a huge dip between 300 and 3000 Hz. The highs above 4 kHz are obviously too loud compared to the rest of the frequency spectrum.
The crossover point seems to be at approximate 1500 Hz – one third of the claimed value in the datasheet. This results look quite disappointing regarding that Arcus was a big name in the German Hi-Fi market in the 1980s.
Next Steps
- simulate alternate crossover designs using boxsim
- buy a measurement microphone to measure SPL vs frequency and response of the speaker in total
- use some old stuff to do a first try in crossover design
- build it and see if something changes
- learn and have fun