Vandersteen 2W and RMB-1085

[biekrx]

I have a pair of Vandersteen 2W subs, and recently purchased an RMB-1085. The 2W is a little unusual because it connects in parallel with the speakers via the speaker terminals on the amp. There is a high pass filter added to the line inputs to the amp that rolls off the low frequencies. The gain in the sub power amp then boosts the signal accordingly. The sub amps also have a very high input impedance so that they don't draw very much power from the amp. The theory is that this helps to blend the subs in with the main speakers. From what I have heard in the past, it works really well.

I connected the 2Ws to the RMB-1085 and got a great deal of hum, and very little output. I then found out that many Class D amps have a DC offset on the outputs, and learned from Vandersteen's website that the 2W amp does not like DC offset. The RMB-1085 has around 24V DC offset (No problem for the speakers, but the sub amp doesn't like it).

Has anyone had any experience with this? Is there a way to reduce the DC offset on the 1085? I really like the amp, but if I can't figure this out I will need to trade it for a 1075.

Thanks,
Robert

[Kevin D]

Wow... Just looked up the manual. Very strange indeed. I could think of a few ways of making it work, but you would probably be better off getting another amp.

Kevin D.

[biekrx]

The answer...

So the 2W (now the 2Wq) is a pretty unique sub. I can honestly say that I had sort of a hard time telling that it was working at first because it blends in with the main speakers almost flawlessly. It is not big, booming and bloated. Instead it is tight, deep, and musical. I can't wait to hear them with the 1085.

Not that this is probably a real high demand answer, but I figured I would post what I have learned. I finally broke down and called Richard Vandersteen. He recommended adding coupling capacitors (film type, around 1uF value, at least 63 V rating) in series with both the positive and negative terminals of each sub. (i.e. 2 caps per sub). This will get rid of the DC offset voltage and let the signal pass through.

The one disadvantage he indicated was that I will likely get a big thump when I turn on the power amp because it will send a quick 24V signal to the caps. I will have to see how bad this is...parts are on order to build the adapters.

The other option I considered was an isolation transformer, but I figured that I would try the caps first.

He couldn't understand why I would even be interested in a class D amp when there are so many other great solid state amps around. Hard to say if he has listened to any recently, but I like the 1085. I still have a great deal of respect for his ability to design speakers. Now if only I could afford to buy his speakers NEW.

I'll report on how the coupling caps work.

[biekrx]

No thump!

So I got the parts to put the coupling capacitors in line. I can say that the only thump I got out of the 2W was when I plugged it in to the wall. When the 1085 comes on and the relays switch on, there is not even a small pop. It didn't matter if I went from full off to on or from standby to on. My guess is that Rotel put in a soft start circuit, but I don't know.

The jury is still out on the sound quality. I didn't have time to build up a full set of the coupling caps, so I was only able to listen to one channel, but the sub is definitely working.