Questions about pads

[rotation]

Hi,

In most of my tube mic pres i am using combinations of 1k and 220r resistors for -20dB pads.
But Jensen schematic for their standard -20dB pad shows combination of 2 619r resistors and 1 169r. This values are different from 1k and 220r that NYD, IOAUDIO and many others are using. Can someone please explain me difference between this two?
Another question> i built several HJFP preamps on Mnats pcb with external 1k and 220r resistors for -20dB pads. This time i would like to make it simpler and use pad that is one pcb already, so i can use spst switches i already have.
But the problem is that Mnats is using voltage divider made of 162k and 1,82k resistors, while original Hamptone preamps have combination of 162k and 40,2k resistors. Both should be -14dB attenuators. To me it looks like there must be mistake one of schematics. Am i right?

Miha

 

[emrr]

1k/220r and 619r/169r 'U' pads arrive at same gain result.

1k/220r = crudely 2k22 load to mic and 220r source to preamp.

619r/169r = crudely 1k407 load to mic and 169r source to preamp.

Any mic and preamp combination may like one or the other better.    One loads the mic a bit more heavily, both are reasonably speaking correct bridging relationships.   With a transformer coupled preamp, the source presented to the input may be the more critical aspect with some transformers.  

 

[rotation]

Hey Doug,

what coincidence, i just went through several pages of your older posts
searching ideas for some American style tube pre i want to build

I will try to find the answer myself about the second question. It looks like Mnats made mistake. On his checkplot R12 (it should make part of pad) is marked the same as 1k82 which is normally attached to source of jfet...

Miha

 

[skipwave]

Yes, that's a mistake. With L-pads the loss is simply related to the ratio of the sections to the whole.

So, if you pick the signal voltage off the top of the lower resistor (other end to ground), the voltage division is equal to the ratio of that resistor to the sum of the two resistors:

Incorrect gives you 1820/(1820+162000)=0.01

Whereas, correct is 40200/(40200+162000)=0.2

In practice your microphone outputs 100mV into the transformer primary, 1V at the secondary (ignoring core losses), and the incorrect pad gives you 10mV. The correct pad gives you 200mV or 6dB higher than your microphone output or the net result of 20dB gain in the transformer and 14dB loss in the pad.