In our Drum Dept Inventory, we have a number of Porter & Davies BC Classic Drum Stools & companion BC-2 Amplifiers to drive them. For those who don’t know them, they are a vibrating drum stool, fed usually by the kick drum, either locally using it’s mic preamp where you loop the drum mic thru the preamp and on out to the house console, and can dial in the appropriate level where you can feel the kick drum coming up out of the drum stool. Or, it can take a monitor feed from the kick or anything else that has the LF timing of a song.
We had been using Buttkicker BKA-1000’s and their companion vibration motor-driven stools, though the BC-2 system seems a lot more user-friendly and reliable, though like everything that goes out on tour, they too break down.
I have one in the shop that had to be put back into working order, and it seemed to work ok…for about 20 seconds. Sitting on it, feeding it pink noise passed thru a 50hz 1/3 Octave filter yields random amplitude sinewave, with wide min/max dynamics….14dB or so. Then, the degree of vibration just drops off next to nothing.
Internally, there’s a series-parallel lamp array that has PTC thermistor/switches across them between the driving amp and the shaker motor that’s mounted to the wooden base inside the drum stool. I’ve replaced the PTC thermistors with fresh ones from the factory, but still have the same issue….works for about 20 seconds, something must be heating up, and the seat isn’t vibrating anymore. The drive signal is still the same magnitude, but watching the AC mains power analyzer, the current draw has dropped way down, reflecting the absence of the higher drive current that had been there before.
After confirming the amp & the protection lamp/PTC array is all working, by first driving a 4 ohm bass cabinet, and never loosing drive current, then driving a dummy load with the same test signal, watching the waveform, feeling the heat sink temperature continue to rise until it was too hot to leave my hand on it, I began to believe the problem is NOT with the amp system. Gotta be something with the drum stool transducer.
I put a 1 ohm sense resistor in series with the drum stool transducer, monitored that current with an RMS meter, then monitored the voltage across the transducer with a second RMS meter and manually maintained 10mV across the sense resistor for a 10mA constant current source. The resultant voltage read across the transducer would be the impedance, read in mV / 10. I swept the frequency range from 10hz to 10khz to see what it was. It was a very linear impedance, unlike a loudspeaker where there is the free-air resonance, followed by a wide minimum impedance trough above the free-air resonance, and then continues to climb in impedance in it’s inductive range. This started at 4.7 ohms @ 10hz, 5.5 ohms @ 100Hz, 9 ohms @ 1kHz and 34 ohms @ 10kHz, nearly a straight line curve, similar to a shaker table motor impedance.
I tried driving it at 1A 50hz, monitoring the voltage across the sense resistor, using a Philips PM2535 system voltmeter, which has a FAST RMS reading mode with a min/max storage system. I only got from 1A down to 970mA over the course of 10 minutes, so that didn’t reveal much. I returned to using the 50hz 1/3 Oct pink noise, again using the min/max reading mode, and again, found the drive current dropped off after around 20 seconds. The max voltage out of the amp was over 12V rms, and I got a max current reading of 2.96A, then after it had dropped off, no longer yielding strong vibration, the max reading I got was around 391mA.
Finally something being revealed that makes sense. The AC mains current sourcing the BC-2 amp likewise dropped way down from what it was reading when I had full drive current. Seems like there is some internal protection on the seat transducer that isn’t working normal. Now waiting to hear back from Tim Porter at Porter & Davies on my findings. We thought for sure it was the PTC thermistors in the lamp array being worn out. Or, something on the power amp or power supply.
We had been using Buttkicker BKA-1000’s and their companion vibration motor-driven stools, though the BC-2 system seems a lot more user-friendly and reliable, though like everything that goes out on tour, they too break down.
I have one in the shop that had to be put back into working order, and it seemed to work ok…for about 20 seconds. Sitting on it, feeding it pink noise passed thru a 50hz 1/3 Octave filter yields random amplitude sinewave, with wide min/max dynamics….14dB or so. Then, the degree of vibration just drops off next to nothing.
Internally, there’s a series-parallel lamp array that has PTC thermistor/switches across them between the driving amp and the shaker motor that’s mounted to the wooden base inside the drum stool. I’ve replaced the PTC thermistors with fresh ones from the factory, but still have the same issue….works for about 20 seconds, something must be heating up, and the seat isn’t vibrating anymore. The drive signal is still the same magnitude, but watching the AC mains power analyzer, the current draw has dropped way down, reflecting the absence of the higher drive current that had been there before.
After confirming the amp & the protection lamp/PTC array is all working, by first driving a 4 ohm bass cabinet, and never loosing drive current, then driving a dummy load with the same test signal, watching the waveform, feeling the heat sink temperature continue to rise until it was too hot to leave my hand on it, I began to believe the problem is NOT with the amp system. Gotta be something with the drum stool transducer.
I put a 1 ohm sense resistor in series with the drum stool transducer, monitored that current with an RMS meter, then monitored the voltage across the transducer with a second RMS meter and manually maintained 10mV across the sense resistor for a 10mA constant current source. The resultant voltage read across the transducer would be the impedance, read in mV / 10. I swept the frequency range from 10hz to 10khz to see what it was. It was a very linear impedance, unlike a loudspeaker where there is the free-air resonance, followed by a wide minimum impedance trough above the free-air resonance, and then continues to climb in impedance in it’s inductive range. This started at 4.7 ohms @ 10hz, 5.5 ohms @ 100Hz, 9 ohms @ 1kHz and 34 ohms @ 10kHz, nearly a straight line curve, similar to a shaker table motor impedance.
I tried driving it at 1A 50hz, monitoring the voltage across the sense resistor, using a Philips PM2535 system voltmeter, which has a FAST RMS reading mode with a min/max storage system. I only got from 1A down to 970mA over the course of 10 minutes, so that didn’t reveal much. I returned to using the 50hz 1/3 Oct pink noise, again using the min/max reading mode, and again, found the drive current dropped off after around 20 seconds. The max voltage out of the amp was over 12V rms, and I got a max current reading of 2.96A, then after it had dropped off, no longer yielding strong vibration, the max reading I got was around 391mA.
Finally something being revealed that makes sense. The AC mains current sourcing the BC-2 amp likewise dropped way down from what it was reading when I had full drive current. Seems like there is some internal protection on the seat transducer that isn’t working normal. Now waiting to hear back from Tim Porter at Porter & Davies on my findings. We thought for sure it was the PTC thermistors in the lamp array being worn out. Or, something on the power amp or power supply.
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