Here are the steps:
1. Connect the center pin of the output RCA jack to XLR pin 2.
2. Connect pin 2 to the chassis grounding tab provided by the jack.
3. Connect a 0.47uF cap and 1M resistor to pin 1. The 1M resistor connects to ground. The 0.47uF cap needs to go to ground through a resistor that's equal to the output impedance of the preamp (measure it if you aren't sure).
The cathode follower is pretty crappy as it is, I wouldn't make it into a split load PI!
1. Connect the center pin of the output RCA jack to XLR pin 2.
2. Connect pin 2 to the chassis grounding tab provided by the jack.
3. Connect a 0.47uF cap and 1M resistor to pin 1. The 1M resistor connects to ground. The 0.47uF cap needs to go to ground through a resistor that's equal to the output impedance of the preamp (measure it if you aren't sure).
The cathode follower is pretty crappy as it is, I wouldn't make it into a split load PI!
yes, desiderata for cathode followers, high mu and high transconductance...i use the 6kn8 or the Japanese 6RHH8, 6N23, 6N30, ecc88, etc...
the 12ax7 while having a mu of 100 has a transconductance of 1,250mmhos at 0.5mA, and ecc88 otoh, has a transconductance of 12,500mmhos at 15mA, 90v, mu of 33...
this topology has an almost cult following, i do get a lot of request to build a marantz 7 line amp clone....
the 12ax7 while having a mu of 100 has a transconductance of 1,250mmhos at 0.5mA, and ecc88 otoh, has a transconductance of 12,500mmhos at 15mA, 90v, mu of 33...
this topology has an almost cult following, i do get a lot of request to build a marantz 7 line amp clone....
Because my power amplifier is MC-275 with XLR input, and I would like to use XLR for comparison to RCA
No, if the output impedance of your preamp was 128K, you wouldn't be using it!
With a couple of clip leads, a multimeter, and a 10K pot, you can measure this yourself.
Ugh, it's really too bad that you have the new MC-275 with the XLR inputs. The unbalanced inputs go to both the first stage and to what they call a "unity gain buffer", which is a voltage divider feeding a grounded cathode amplifier to invert the phase. The problem is that the voltage divider will never be a particularly wonderful way to cancel out the gain provided by the tube (looks like a 12AX7), as there will be a bit of a difference in gain from one tube to another.
This wouldn't be that big of a deal if there was a well balanced differential stage at the input of the amp. For unbalanced operation, one grid could be grounded, and for unbalanced operation, you'd just tie the other grid to Ve- on the XLR jack.
For the XLR input on this particular amp, there's also an assumption that both legs are driven with equal and opposite signals (not a responsible assumption to make). The ideal thing to do in this particular case for you is to buy a pair of output transformers for your preamp if you want to try the XLR inputs, but this further complicates things since you need to drive the miller capacitance sitting at the input and your preamp isn't going to be too happy driving a low impedance load.
In the end, just stick to your unbalanced connections.
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