Lets look at some numbers, ok? I just looked up the oldest (and lowest performance) production Op Amp - - - the LM 741A in my "National Operational Amplifiers Databook - - - 1995 Edition".
It shows that the LM-741A (which goes back to when I was a kid in college) has an input resistance of 6 Meg Ohms, typical. Pray tell me how that input resistance is going to interfere with the output resistance of a phono cartridge having a few hundred ohms and being terminated with 47 KOhms?
Moving on - - - this op amp has a gain x bandwidth product of 1 MHz. That means that if one used it in a situation covering up to 20 KHz, it would be useful with a Gm (Voltage Gain) up to 50. If 20 KHz it not enough for you, use 4 of them in cascade, yielding a bandwidth of 80 KHz, with each stage having a gain of only 12.5. That solution would cost you about 75 cents.
As for slew rate, this op amp only has 0.5 Volts per microsecond. Please ask one of the 12AX7 tube pre-amplifier companies what their slew rates are. You will find that most tube equipment ignores specs like distortion, slew rate, noise figure, s/n ratio, etc.
So, now take a modern op amp and see what you get!
Craig
It shows that the LM-741A (which goes back to when I was a kid in college) has an input resistance of 6 Meg Ohms, typical. Pray tell me how that input resistance is going to interfere with the output resistance of a phono cartridge having a few hundred ohms and being terminated with 47 KOhms?
Moving on - - - this op amp has a gain x bandwidth product of 1 MHz. That means that if one used it in a situation covering up to 20 KHz, it would be useful with a Gm (Voltage Gain) up to 50. If 20 KHz it not enough for you, use 4 of them in cascade, yielding a bandwidth of 80 KHz, with each stage having a gain of only 12.5. That solution would cost you about 75 cents.
As for slew rate, this op amp only has 0.5 Volts per microsecond. Please ask one of the 12AX7 tube pre-amplifier companies what their slew rates are. You will find that most tube equipment ignores specs like distortion, slew rate, noise figure, s/n ratio, etc.
So, now take a modern op amp and see what you get!
Craig
Comment