SDA phase and subwoofer integration

ambiophonics

I have my SDA-2BTL speakers setup in my main system and would like to fill in the very lowest octave with an old velodyne F-1000B servo sub I have sitting around. The SDAs don't lack in the low bass department but don't reach low enough for some music, most noticeably organ. Anyway, the SDA's obviously require a common ground amp and my only option for hooking up the subwoofer is speaker level as my passive preamp doens't have an any extra line level out. I ran a second set of speaker wires to my sub from the amp so the speakers would still be directly wired to the amp, unfortunately the sub is out of phase with the speakers and now i'm stuck. The sub doesn't have a phase switch, I can't wire the SDA's out of phase with the amp and I don't know that I can open the sub and reverse the woofer connections as I'm uncertain how the servo feedback system operates. Any ideas? I hooked it up to just the left channel to test it and it really helps fill in the very lowest notes. Maybe since most bass is mono anyway I could just use it this way?

F1nut

Interesting question.

Schurkey

1. Why can't you just reverse the speaker wire connections at the subwoofer? What am I not understanding?

2. Wired + to + and - to - at all points, the subwoofer is reverse polarity? Seems suspicious. One wonders how far out-of-phase the passive radiators are at those frequencies.

3. Why can't you provide signal to the subwoofer by using a pair of wyes at the preamp? One side of the wye goes to the power amp, one side goes to the subwoofer. This doesn't fix the out-of-phase issue. I just don't understand why you HAVE to use speaker-level inputs.

4. Could you not just reverse the input to the subwoofer amplifier between the binding posts and the amp? (I'd think it would be easier to just cross the speaker cables at the binding posts.)

ambiophonics

Thanks for the responses so far. Let me try and answer schurkey's questions below, let me know if this seems logical as I may have missed something obvious.

1. Why can't you just reverse the speaker wire connections at the subwoofer? What am I not understanding?

This sub has a servo built into the cone that measures the acceleration of the driver and compares that to the original signal to adjust for distortion. I am not certain of this but I think that reversing the speaker wires going from the internal subwoofer amp to the woofer would put the servo feedback circuit 180 degrees off and cause it to fail in some manner. I opened the unit up to investigate but decided against the "let's see what happens" approach as these woofers with the integrated accelerometers are no longer available.



2. Wired + to + and - to - at all points, the subwoofer is reverse polarity? Seems suspicious. One wonders how far out-of-phase the passive radiators are at those frequencies.

Yes, wired + to - at all points the sub is out of phase. It was my understanding that the SDA's do reverse the phase as the bass gets lower. I might be wrong on this but it's also pretty clear when I tested the sub with just one speaker level input that bass and bass attack was softened greatly with it wired + to +, - to -, and bass was tight and slightly deeper with it wired in reverse.



3. Why can't you provide signal to the subwoofer by using a pair of wyes at the preamp? One side of the wye goes to the power amp, one side goes to the subwoofer. This doesn't fix the out-of-phase issue. I just don't understand why you HAVE to use speaker-level inputs.

You're right, I could buy a set of wyes and hook it up this way but I'd have to run a long set of RCAs to the sub and I hesitate adding a lot of long cables to the passive setup. Also that wouldn't necessarily help with the phase issue as you can't just reverse the wiring on the RCA cables as the grounds are common on the RCA inputs on the sub so you'd just be shorting the two signal wires to each other if you reversed them.



4. Could you not just reverse the input to the subwoofer amplifier between the binding posts and the amp? (I'd think it would be easier to just cross the speaker cables at the binding posts.

I measured the resistance between the terminals on the subwoofer and the speaker level inputs have only 45ohms resistance between the black ground inputs. So if I decided to reverse the speaker wires going into the sub I would be effectively creating a pretty low resistance path between the left and right positive output terminals of the power amp up stream. I imagine it wouldn't like that or at a minimum would degrade the sound quality, separation etc.

westmassguy

The SDAs crossover to the passives around 50Hz (start around 150, but really kick in at around 50). At that point, the passive is out of phase with the mid woofers. Your Velodyne is also out of phase, so the two should be in phase below 50Hz. To be safe, your best bet is to Y off the pre, and run two RCA Subwoofer cables. Set the Velodyne's crossover at it's lowest setting, 40Hz, and adjust the Velodynes volume to fill in the low end.
BTW, I would not advise opening the sub and reversing the wires. The Servo monitors the incoming signal, and adjusts the output signal to the woofer based on the accelerometer readings. Reversing the leads would probably make the servo go ape****.

ambiophonics

Hey Dave,

I tried your suggestion last night and the even with the sub's crossover set to the lowest frequency it seems to be out of phase. I tried it again after this with one speaker wire so I could reverse the phase and it definitely sounds better with the positive from the amp going to the negative input and the negative to the positive.

The sub is sitting just a little to the right of one of the SDAs so it shouldn't be a distance or positioning issue.

I had the same concern about reversing the leads that connect to the actual subwoofer speaker, I've heard of folks opening this subs up with them powered on and having them go crazy.

How do you know the Velodyne is normally out of phase?

westmassguy wrote: »

The SDAs crossover to the passives around 50Hz (start around 150, but really kick in at around 50). At that point, the passive is out of phase with the mid woofers. Your Velodyne is also out of phase, so the two should be in phase below 50Hz. To be safe, your best bet is to Y off the pre, and run two RCA Subwoofer cables. Set the Velodyne's crossover at it's lowest setting, 40Hz, and adjust the Velodynes volume to fill in the low end.
BTW, I would not advise opening the sub and reversing the wires. The Servo monitors the incoming signal, and adjusts the output signal to the woofer based on the accelerometer readings. Reversing the leads would probably make the servo go ape****.

westmassguy

I don't know if it's out of phase. You said it was

ambiophonics

I think I was unclear when I said that, what I meant was the velodyne was out of phase with the SDA's bass. I had it crossed very low so I can't speak to whether it could be in phase with higher bass frequencies. I thought when you said the velodyne was also out of phase you knew something about velodyne subs I didn't know.

westmassguy wrote: »

I don't know if it's out of phase. You said it was

westmassguy

Well you're definitely in a pickle. The sub has speaker and line level inputs, but I have a feeling both have common grounds, which would make swapping the polarity impossible. You could try putting a large value low pass electrolytic in series with the + speaker leads. This should invert polarity.

ambiophonics

Yea, I measured the speaker level input terminals on the sub and the grounds have about 45 ohms resistance between them and the positive terminals have about 1.5k. I opened it up and I didn't see an easy way to reverse things. I'll look into your capacitor suggestion. Currently I'm running just one channel into the sub and it sounds very nice but I'd like it to be hooked up properly.