Velodyne fsr-15 sub amplifier questions. Stk4040ii

So I got this 15" subwoofer. It works now. But when worms up there is some high pitched noise here and there. It's not the same pattern.
Random low volume noises around 1000hz frequency.

I repaired subs before. But usually it was filter caps and it was 60hz back groundnoise.

The sub plays nice. Hits pretty low.

Want to keep it. Doing some preventative maintenance.
I removed the amp to source capacitors. Notice one transistor is running hot. See pics.

Velodyne customer service phone clerks get very combatative and almost wants to get into fist fight when you ask for service manuals. So I could not find service manual for this amp. But will call velodyne tomorrow to double check.

Attached are some pics.

Transistor that is running hot says "malasya TIP29C" it's labeled q1 on the board. Looks dull brown grey instead of black. It's connected to one of the filter caps

There is another transistor labeled q2, which looks different. Says "8823 TIP30B"

So I'll recap the amp.

1) is it possible to say what could cause q1 transistor to run hot

2) do these all in one power amplifiers (stk4040ii ) go bad like capacitors. Are they wearable components


  • DSC05808.JPG
    945 KB · Views: 195
  • DSC05809.JPG
    963.9 KB · Views: 188
  • DSC05807.JPG
    869.1 KB · Views: 177
  • DSC05810.JPG
    874.2 KB · Views: 174
  • DSC05812.JPG
    943.2 KB · Views: 169
What's it using for a power supply?
One of those SMPS jobs?

Inelco b3 transformer.

The shape of the amp is a little odd. Well the amp works. Sub is 22 years old.
All capacitors, q1 and q2 transistors cost me around 46$ shipped

Maybe a resistor r1 connected to q1 is not powerful enough?


  • DSC05813.JPG
    844.6 KB · Views: 69
  • DSC05814.JPG
    809.2 KB · Views: 52
My first suggestion would be to clean those charred/overheated areas of the PC board.
and all those overheated parts likely have compromised soldering now, which needs re-doing at least.
Those transistors may be weakened from overheating, new ones with a small heatsink fitted would be a good idea.
The diodes D!, D12, small transistors should also be checked, as well as the small caps in that overheated area.
I've read on some threads that
Tip29c and tip30b could be replaced with tip40 tip41
Are there any transistors that i can replace tip29c tip30b with?
I ordered original 29 and 30 from mouser. Will put some heat sinks.
Basically amp works fine. Only after it runs few hours at 9 o'clock volume you can hear weird inconsistent noises
When close to the sub. Not very loud
I'll replace all electrolytic capacitors
Well I replaced all capacitors. Some isues still there . Unfortunately velodyne does not provide schematics..
Too much time to swap things around. Like NMOS suggested better to buy another amp and be done with it.
Sub sound good just hissing noise during first 10mkn of operation.
But the high pitched noise disappeared.

Dc voltage values jump from -400 to + 400mv(measured at subwoofer driver terminals)

When out of the box woofer shows motorboating. Floats sat around 10hz frequency. When the box is sealed you don't notice motorboating.

Probably those stk4040ii power amplifier.i don't know much about these all in one amps. But usually see them on cheap gear. Half of which does not work or about to fail.

Seems like purposeful design to fail right after warranty runs out.
Not sure what I will do now. Probably use some external amplifier.
Last edited:


2017-09-04 8:08 pm
A friend of mine has one of these and we traced out a partial the schematic. To do a full one would require dismantling much of the circuitry as there are two or three daughter boards tightly grouped on the main board.
The chip is under-rated for the power claim for the unit - at least for that you can pull up info on the web and see typical application circuits. I was surprised that the built-in crossover is a simple 6dB slope. His problem was with the signal detection for auto-on/off turning the amp off during quiest parts of the music, then turning it back on with a definite 'click'. This is done through the current source for the diff-input of the STK but we could not trace that part of the circuitry.

Most problems with Velodynes come down to the accelerometer (piezo element) becoming detached from the cone, or the cable to it breaking. Some have two wires and some four. The circuitry related to this is not disclosed.
The concept is a good one but sometimes using tried and true acoustic designs and a higher-slope low-THD crossover and decent PA is more reliable.


2017-09-04 8:08 pm
Hi Richard_James

They key to the success of this unit is having the motion feedback provided by the accelerometer, which can reduce the driver THD from multiple full percents to tenths of a percent. The box size demands a bit of power if you are trying to have modern bass (which I believe is way out of proportion to the rest of the music). The box is also deliberately under-size from an acoustic design view, but it will fit into a room easily.

Still, if you were to try using it as a passive box driven by an external PA fed by an electronic crossover, it could work okay. I always use the Riley-Linkwitz alignment of a 6dB down point and 12- or 24dB slopes - most often 24. Shifting the crossover point from 3dB to 6dB is just a matter of root-2 in the equation, and it flattens the SPL around and through the crossover point.

Note that for a subwoofer, you just need power, so anything will do amp-wise. It should be linear although this is about the only app a class-D is good for - specific load up to 160Hz.

Note also that for correct bass reproduction you need stereo subs positioned with the normal stereo left-right speakers. Anything else is a corruption of the sound and the experience. In a home, you do not want the same set-up as in a theatre; rather, you want proper stereo as this sounds best in a living space optimised for conversation.
I have probably used the same approach in car audio. Good to know the technical term for it

Linkwitz–Riley filter - Wikipedia

Unfortunately I don't have space for 2 subs. Eventually I will but not in the next few years. It's mostly for low end in classical music . I set cross over all the way down so amp plays 50hz or less

And my speakers really can play audible 40hz and above.
Basically speakers are powered by amplifier directly
And sub gets pre-out line from same amplifier.

The driver pretty sure had 4 wire sensor inside the driver. You can see the wires in space under dust cap. Didn't know what it was.

Might be worth checking if it's this feature that is causing the woofer to exhibit motorboating behavior. It's noticeable only when the box is vented(amplifier is not bolted on, or drivers is out of the box)

I am thinking of replacing original amp with Dayton audio spa250.

Dayton Audio SPA250 250 Watt Subwoofer Plate Amplifier


2017-09-04 8:08 pm

If you use an external amp the Velodyne cabinet should be sealed - cover where the plate amplifier used to live if you remove it.

It is unfortunate that the accelerometer is under the dust cap inasmuch as you cannot see if it has become detached. I believe that and broken cables to it are the main weak points of the system.