Wanted a small subwoofer for my home office. Have had my eyes on the small 6.5 inch Rechorn D-165 for some time. Only 30 Euro, and a really good build quality.
There is no official data on the passive units, but from trials with 3 different weight adding, and comparing a measured impedance with simulation in WinISD, I find the parameters for the passives to be something like this:
- Vas = 70 l
- Qms = 2,8
- Fs =15 Hz (without any added weight)
- Sd = 150 cm2
This is not exact at all, but I think near enough to do some more simulations.
It takes nothing less than 190 g of added weight to each passive to get the fs down to 26 Hz!
And the passive units are not really build for that. They sag/tilt quite a lot, due to only having the rubber surround and no spider!
I’m thinking of making a kind of thread suspension (a la Duelund) to level it out.
It is clear that one passive is not enough at all, no surprise and why I bought 2 in the first place.
If you did anyway, using a box of 10 liter, you would get a very good response down to 25 Hz, but the passive would bottom out at something like 90 db @ 25 Hz.
In comparrison, the D-165 in a vented box of 15 liter, tuned to 25 Hz will do almost 96 db @ 25 Hz.
But you will also have to find room for a good sized port adding to the volume!
Construction
Simulation with WinISD
Simulation with WinISD is straight forward. Volume = 9.5 l:
Adding a 2. Order highpass with a 3 db peak at 30 Hz gives a very decent result
Preamp
For the sub I created a small preamp including a 2. order high pass 25 Hz slightly peaking with ca +3 db, Variable 2. order lowpass filter 37,5 Hz – 154,5 Hz, and Volume control.
One dual opamp TL072 used and a simple regulator based on BD139/BD140
All with through hole components, so that anybody can solder it. More information here.
Simple spider
Made a very simple spider of a nylon string. Made two smale holes in chassis top and bottom through which I mounted the string. made a single loop arounf the center screew and ajusted it so that it will keep the diaphragm from sagging.
Now mounted with 190 g extra weight on each passive, which is of course a lot for such a small unit, but now it is tuned to 26 Hz.
Putting it together
Powered by one of my Class D amps, and a small SMPS from Connex Electronics SMPS240R with +-40Vdc, which in 4 ohm is around 200W.
The sub works really well, goes super deep, and play resonably loud. The Rechorn D-165 is a really good subwoofer unit, with very good T/S parameters for small subs. It has a high build quality and is super cheap. What’s not to like? š …… the passives …. well, too flimpsy. Made them work, but would go for something ore substantial, llike an Earthquake 8 slaps, fo almost the same amount. Think it will do better …. or alltogether go for a vented design … will end up a little larger though, to make room for a goo sized vent.