[JN] Input circuit question

BBands bbands at gmail.com
Thu Feb 11 14:19:49 CST 2021


Thanks for the notes. I had in mind to run it off a 12 volt battery.
Portable, don'cha know?! Ordered the SMPSs last night.

      John

On Thu, Feb 11, 2021 at 12:14 PM Philip Yates via Sound <sound at soundlist.org>
wrote:

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