HI All,
I posted this test methodology as part of another thread. After writing it, I thought it might carry more gereral interest among the more technicaly minded members. I decided to expand it a bit and post here. It might prove useful to anyone who has a system capable of powerful, fully extended, low bass.
Many audiophiles have claimed that the sound vibrations in the air could couple into the equipment and act as a vibration source. I agree. After an aggresive vibration reduction program, I designed the following test to see if I could determine the effectiveness of the program.
My main system (not my solid state HT system) is based on tube equipment. My preamp (Audio Research SP-11 Mk2), the main speaker amp (Audio Research D-250 Servo Mk 2) and DAC (Anodyne) all are tube based. Tubes, as you may know, are infamous for microphonic problems excited by external vibrations. Besides this, I was VERY interested in my turntable system. Turntables are very sensitive to vibration and can produce acoustic feedback that is not obvious but detrimental to all aspects of playback.
I'm not going to go into detail about the various vibration reduction techniques I used. Suffice it to say that they are extensive.
The subwoofer amp is solid state Levinson No. 23. My subs are dual Kinnergetics SW-800's which are rated at 4 ohms and go to 15Hz with power. The Levinson can output an honest, continous, 400W per channel into that load. This performance is important because the subs function as an important part of the test equipment used in this test.
I disconnected the Levinson's input from the crossover. I ran a Tektronix pulse/function generator, the kind where you can program the number of sine wave cycles and frequency in a burst and control the burst duration and repetition rate, into my sub amp. I monitored the output of the main tube amp with a very sensitive Tektronix storage oscilloscope. I could measure micro volts. The preamp was switched to the Moving Coil photo input, the most vibration sensitive mode because of its very high gain. I placed the stylus on the turntable platter. The turntable was not turning. Preamp gain was adjusted to about 3dB higher than my highest normal listening setting. With these settings the entire amplification chain was engaged in the signal path.
I then triggered the generator to produce a series of very low frequency, high power sine wave bursts through the subs. Believe me, every loose thing in the room was rattling. All, that is, but the tubes and turntable. If the amps or the turntable were vibration sensitive I would be able to see it indicated by a varying voltage on the scope. I saw NOTHING. I tried different frequencies of sine wave bursts to identify resonances but found NOTHING. I even tried a continuous frequency sweep. NOTHING! I was pretty happy with my vibration reduction program.
Anyone with a subwoofer and access to the required test equipment can duplicate my test. There are many variations of test equipment that can be used. Prior to my tests my gut feeling informed me that I had no significant vibration problems. Now I have confirmation. I only wish that I had run a test series before I started the vibration reduction program so I could compare the before and after performance.
I do know the system now sounds better. More solid, better focus, a precise and stable soundstage and bass that is a clean as a whistle with no overhang no matter which program source is in use. It's almost as though a subtle fog has been lifted from the sound. The fog was not something I identified as a problem. I only noticed it after it was gone. My system sounded good before. Now, ........ well, it's hard to be humble. :T
Sparky
I posted this test methodology as part of another thread. After writing it, I thought it might carry more gereral interest among the more technicaly minded members. I decided to expand it a bit and post here. It might prove useful to anyone who has a system capable of powerful, fully extended, low bass.
Many audiophiles have claimed that the sound vibrations in the air could couple into the equipment and act as a vibration source. I agree. After an aggresive vibration reduction program, I designed the following test to see if I could determine the effectiveness of the program.
My main system (not my solid state HT system) is based on tube equipment. My preamp (Audio Research SP-11 Mk2), the main speaker amp (Audio Research D-250 Servo Mk 2) and DAC (Anodyne) all are tube based. Tubes, as you may know, are infamous for microphonic problems excited by external vibrations. Besides this, I was VERY interested in my turntable system. Turntables are very sensitive to vibration and can produce acoustic feedback that is not obvious but detrimental to all aspects of playback.
I'm not going to go into detail about the various vibration reduction techniques I used. Suffice it to say that they are extensive.
The subwoofer amp is solid state Levinson No. 23. My subs are dual Kinnergetics SW-800's which are rated at 4 ohms and go to 15Hz with power. The Levinson can output an honest, continous, 400W per channel into that load. This performance is important because the subs function as an important part of the test equipment used in this test.
I disconnected the Levinson's input from the crossover. I ran a Tektronix pulse/function generator, the kind where you can program the number of sine wave cycles and frequency in a burst and control the burst duration and repetition rate, into my sub amp. I monitored the output of the main tube amp with a very sensitive Tektronix storage oscilloscope. I could measure micro volts. The preamp was switched to the Moving Coil photo input, the most vibration sensitive mode because of its very high gain. I placed the stylus on the turntable platter. The turntable was not turning. Preamp gain was adjusted to about 3dB higher than my highest normal listening setting. With these settings the entire amplification chain was engaged in the signal path.
I then triggered the generator to produce a series of very low frequency, high power sine wave bursts through the subs. Believe me, every loose thing in the room was rattling. All, that is, but the tubes and turntable. If the amps or the turntable were vibration sensitive I would be able to see it indicated by a varying voltage on the scope. I saw NOTHING. I tried different frequencies of sine wave bursts to identify resonances but found NOTHING. I even tried a continuous frequency sweep. NOTHING! I was pretty happy with my vibration reduction program.
Anyone with a subwoofer and access to the required test equipment can duplicate my test. There are many variations of test equipment that can be used. Prior to my tests my gut feeling informed me that I had no significant vibration problems. Now I have confirmation. I only wish that I had run a test series before I started the vibration reduction program so I could compare the before and after performance.
I do know the system now sounds better. More solid, better focus, a precise and stable soundstage and bass that is a clean as a whistle with no overhang no matter which program source is in use. It's almost as though a subtle fog has been lifted from the sound. The fog was not something I identified as a problem. I only noticed it after it was gone. My system sounded good before. Now, ........ well, it's hard to be humble. :T
Sparky
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