HI All,
Have you ever wondered if your turntable was really free of acoustic feedback even if you don't think any is present? I have. Sometimes these things are very subtle. This is also true of tube microphonics which I place in the same basic category of feedback. I decided to design a test that would definitively tell me if there were any problems with feedback in my system, even minutely. Here is how I did it.
My system is composed of tube equipment, all Audio Research. This means there are a lot of tubes. I also have a good turntable system on a heavy lead weighted stand. I had no prior indications of feedback either from tube microphonics or from the turntable. I have very good biamped subwoofers that are capable of sub 20 Hz performance.
I designed my test around the subwoofers. First, I disconnected from the inputs to the sub amplifier (Levinson No. 23 @ 400W/channel into the subs 4 ohm load) and drove the Levinson from a rather flexible Tektronix function/sweep signal generator. This gave me a signal source that could run from less than 1 Hz to a megahertz. I was interested only in the frequency range of 10 Hz to 1Khz. I could run either steady frequencies or I could sweep the generator. I ran it in the burst mode.
I triggered my Tektronix 7000 series scope from the function generator. The scopes input plug in was a high gain Tektronix preamp that is also very quiet and had both high and low pass filters so I could limit out af band distrubances. With the scope I monitored the output of of my main system amplifier, a ARC D250 (250W/chan).
Then I qued my cartridge down on a stationary record and selected the phono input on my ARC SP-11 preamp. The cartridge was a low output MC mounted on an Eminent Technology ET-2 air bearing arm.
I adjusted the frequency generator to give a series of sine waves in a burst sequence. I varied the number of waves in the burst but generally used 10. Different sine frequencies were used so I could detect any resonances in the system. I then turned the signal level up such that he subs were reproducing the signal burst. Obviously, I started with low frequency sine waves. The burst mode was used so that sequential sine waves did not hide feedback behavior.
Thump, thump, thump as the ashtrays and doors rattled with the bass energy. I monitored the scope looking for ANY output from the main amp. Remember, the main amps only input was from the cartridge resting on a stationary record. A huge amount of acoustic energy was being pumped into the room. If any tube was microphonic or if the turntable was sensitive to vibration, I would have seen it on the scope.
The happy news is I found exactly zero feedback problems. This verified my initial thoughts before I started this test.
Sparky
Have you ever wondered if your turntable was really free of acoustic feedback even if you don't think any is present? I have. Sometimes these things are very subtle. This is also true of tube microphonics which I place in the same basic category of feedback. I decided to design a test that would definitively tell me if there were any problems with feedback in my system, even minutely. Here is how I did it.
My system is composed of tube equipment, all Audio Research. This means there are a lot of tubes. I also have a good turntable system on a heavy lead weighted stand. I had no prior indications of feedback either from tube microphonics or from the turntable. I have very good biamped subwoofers that are capable of sub 20 Hz performance.
I designed my test around the subwoofers. First, I disconnected from the inputs to the sub amplifier (Levinson No. 23 @ 400W/channel into the subs 4 ohm load) and drove the Levinson from a rather flexible Tektronix function/sweep signal generator. This gave me a signal source that could run from less than 1 Hz to a megahertz. I was interested only in the frequency range of 10 Hz to 1Khz. I could run either steady frequencies or I could sweep the generator. I ran it in the burst mode.
I triggered my Tektronix 7000 series scope from the function generator. The scopes input plug in was a high gain Tektronix preamp that is also very quiet and had both high and low pass filters so I could limit out af band distrubances. With the scope I monitored the output of of my main system amplifier, a ARC D250 (250W/chan).
Then I qued my cartridge down on a stationary record and selected the phono input on my ARC SP-11 preamp. The cartridge was a low output MC mounted on an Eminent Technology ET-2 air bearing arm.
I adjusted the frequency generator to give a series of sine waves in a burst sequence. I varied the number of waves in the burst but generally used 10. Different sine frequencies were used so I could detect any resonances in the system. I then turned the signal level up such that he subs were reproducing the signal burst. Obviously, I started with low frequency sine waves. The burst mode was used so that sequential sine waves did not hide feedback behavior.
Thump, thump, thump as the ashtrays and doors rattled with the bass energy. I monitored the scope looking for ANY output from the main amp. Remember, the main amps only input was from the cartridge resting on a stationary record. A huge amount of acoustic energy was being pumped into the room. If any tube was microphonic or if the turntable was sensitive to vibration, I would have seen it on the scope.
The happy news is I found exactly zero feedback problems. This verified my initial thoughts before I started this test.
Sparky
