[JN] Input circuit question

Philip Yates toobman57 at gmail.com
Thu Feb 11 15:00:43 CST 2021


It definitely has a cool factor! Kind of annoying how much time I spent
looking at the thing. ;-)

Easily Amused Phil

On Thu, Feb 11, 2021 at 2:20 PM BBands <bbands at gmail.com> wrote:

> 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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