[JN] Input circuit question

Philip Yates toobman57 at gmail.com
Thu Feb 11 14:13:34 CST 2021

If you take the output from the 2nd half of the 12AX7 instead of the 1st
half, you could raise the high frequency rolloff by using a smaller plate
resistor in the 1st half (the 1st half's Miller capacitance, which is set
by its plate resistor Rp, sets the rolloff). This may or may not sound
better, you could use a 0.5 Mohm pot (in the 1st half), dial it in for the
best sound, and then replace with a "real resistor." I THINK the amp's gain
is about 40 now, low values of plate resistance in the 1st half will
increase the gain (perhaps to 90 at very low Rp, not sure). What's the
value of the 12AX7's cathode resistor? I can't make it out in the photo.
The design is so simple, that alone makes it interesting, and those
low-noise "micro SMPSs" make it even more appealing.


On Thu, Feb 11, 2021 at 9:21 AM BBands via Sound <sound at soundlist.org>

> Thank you Michael, down the rabbit hole I go.
>      John
> On Thu, Feb 11, 2021 at 2:41 AM Michael Ulbrich via Sound <
> sound at soundlist.org> wrote:
>> Basically it's a differential amplifier ("long tailed pair") with only
>> one input driven and the output taken from only one side as well.
>> It's called "cathode coupled amplifier" or "single ended differential".
>> The AX7 on the right is a grounded grid stage driven by the left cathode.
>> For the signal it can be seen as a bypass to ground and effectively
>> decouples the common cathode resistor (= replaces the cathode bypass
>> capacitor).
>> Oder so ähnlich ...
>> Best ... Mi
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