Velodyne Servo Amp Mod Help

[JonL]

I have a Velodyne F1200 subwoofer that uses an accelerometer and a servo amp to reduce distortion. Mine suffers from what is apparently a very common problem: the accelerometer fails and the amp occasionally gives the driver a terrible "thump," sending the woofer cone through its full excursion. Sometimes it starts oscillating like that. The accelerometer is unobtainable.

My thoughts... I can buy a new universal plate amp for not too much money, but money is tight right now. Or... I wonder if it is possible to bypass the feedback loop on the existing amp and run it without the servo function. Yes, I know the distortion will be higher, but I'm willing to give it a try.

I'm posting the schematic for the amp and a block diagram, and I'd be grateful if one of you smart people could tell me how to bypass the feedback loop, if it's indeed possible. I've got nothing to lose by trying...

Thanks!

 

[timblade68]

I've no advice for the bypass, but.....if you decide to go with a plate amp, take a look at the 'Oaudio' line. I've a velodyne va1012 that is now housing the Oaudio BASH 300W amp, just took a few cuts to enlarge the opening slightly and it dropped right in. These amps sound nice and have all the little adjustments to dial them in seamlessly.

 

[BinaryMike]

I would be happy to assist with the bypass surgery, but I'll need a more legible schematic.

 

[JonL]

Thanks, Mike. I was hoping you'd see this thread

I'll PM you with the schematic file. I don't know why it attached the way it did.

 

[Rick Vestal]

Common problem indeed. I have an SPL1200 II that suffered the same fate. Full power oscillation nearly disassembled my house. After a very heated discussion with Velodyne, I had someone at the machine shop at work build me a blank plate and I run mine passive now. I'm crossing over using the LFE crossover in my preamp into a JRDG mono power amp. It's made quite a difference. I've never run mine even close to threshold of distortion and it is certainly punchier now...

 

[roger2]

I have a Velodyne FSR10BV with what I believe to be a similar issue. The problem manifests differently though. When turned on, and with no input signal present, this sub will output brief, loud bursts of what sounds like full range, unintelligible, audio signal. It is not white noise, not pink noise.

So yeah, this will be a very interesting thread to follow Looks like a few people have similar issues with Velodyne servo subs. Would it be possible to get the schematic that will be sent to BinaryMike attached to the thread so we can follow along with his suggestions?

 

[JonL]

Schematic is attached to the first post.

When I first power mine on, the woofer cone s l o w l y sucks in, then BOOM! Out it pops! Then, it slowly sucks in again, and BOOM! If I damp it with my hand, I can get it to stop and then it'll sometime even play ok for a while, but it always resumes these big excursions... either as a slow oscillation or at random intervals.

 

[JonL]

Here it is as a PDF which might enlarge a bit better. My original JPG enlarges much better than the file that's linked here. Also, I've put up the original on Photobucket, here:

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Maybe that can be downloaded and zoomed on your computer.

 

[BinaryMike]

The design is difficult to analyze because of connections that apparently go off-sheet and aren't well labeled, but it appears that motional feedback is summed into the main audio path via C23 and R37. I suggest lifting a lead on either one of these components in order to break the loop.

 

[JonL]

Thanks Mike! I'll try it tomorrow (or maybe even late tonight) and report back.

Jon

 

[JonL]

Mike, you might be a genius!
Turns out the schematic I have is not correct for my unit, but I was able to figure out the corresponding capacitor to C23 on my amp and I simply lifted one leg. The thumping and oscillation seems to be gone, and the unit seems to working properly otherwise. I'm going to let it play for a while before declaring victory... but if this works, what a simple fix for all those Velodynes out there with failed accelerometers.

 

[BinaryMike]

I enjoy working on problems like this. Has anyone produced a set of specifications for the original cone acceleration sensor?

 

[JonL]

Small fly in the ointment, but it's livable for me. I have to keep the sub's volume control VERY low after this. I guess without the negative feedback from the accelerometer circuit, the gain of the amp is tremendously increased. This could be a problem for others who do this more than for me.... I have very sensitive speakers and I run them with way more power than needed, so my preamp volume control is always really, really low. To get a realistic match from the subwoofer I would normally have to have the volume on the subwoofer amp turned up pretty high (since it's getting a low signal from the preamp yet the main speakers are plenty loud). I now have the subwoofer amp volume down just below "1." Before it was up around 5. Someone who has a more normal match of speaker sensitivity and amp power might not be able to turn down the sub's amp enough.

I'm not aware of anyone digging into the accelerometer itself. I thought about it, figuring it was probably an off the shelf item at one time, and there ought to be a workable replacement out there. I think it may require major surgery on the speaker itself to gain access to the accelerometer.

Despite the volume situation, I'm very pleased to have a working subwoofer and I am very, very grateful to Mike for providing the solution!

 

[BinaryMike]

Well, you could reduce gain in the summing stage by increasing the value of R20. Raising it to 120K would reduce gain by a factor of five (14dB), for instance.

 

[JonL]

Update: My amp is not the same as the one in the schematic, and finding the equivalent resistor to R20 was beyond my capabilities. I could probably have found it if the board was more accessible. I'd have to desolder too many connections to remove the board and didn't relish the hassle. Instead I made up a couple of attenuators (voltage dividers built on a couple of RCA jacks) for the input. Probably not as good from a distortion point of view as reducing the gain inside the unit, but it's doing the job. Before I had to have the sub's volume control set right at the threshold between "off" and "too loud," which happened somewhere just below "1" on the dial. Now it seems to be fairly happy at around 2 or 2 1/2, and it no longer has hair-trigger sensitivity. So all is good, and once again a huge "thank you" to Mike.

 

[HiFiDude]

Thanks for sharing the schematics ...,
just brought a Velodyne F1200B home from a local thrift store ..., my spending was 15$
This sub will be a very nice addition for my pair of Magnepan SMGa
Might even add my pair of Technics EAS-10TH400 aswell ..., even they really are reserved for a pair of Magnepan Tympani IA i am working on

 

[JonL]

Hopefully yours works without the failed accelerometer issue. From reading around the 'net, it seems to be an extremely common problem. As I said in one of my posts, that schematic was incorrect for my unit, pretty major differences. Perhaps it's the right one for yours. Good luck with it!

 

[hoggwild89]

i think

I am pretty sure the feedback is simply plugged into the circuit board and just unplugging it will work.

 

[JonL]

Maybe that would work, but I wouldn't think so. It's a feedback loop, so the servo circuitry in the amp is looking to see the difference between what the amp is sending to the speaker and what the accelerometer is reporting that the speaker is actually doing. The amp then attempts to cancel out any differences. I'd think that if you simply remove the accelerometer signal, the amp will think that the speaker isn't putting out enough of what is being asked, and it will try to keep increasing the signal to the speaker until the amp can't put out any more. I may be wrong about all this, hopefully one of the experts here will chime in.

 

[HiFiDude]

First thing i did after i had posted in the tread, was to inspect the electronics .., noticed the connector circuit board was used for aswell the accelerometer signal and power amp out for the woofer .., so i did reattached the board and the plugs ..., have been testing my F1200B for many hours .., no problem with the electronics.., i did alone by choice of music manage make a priority for a new foam surround .., small investment since i did really like the performance combined with my maggies