A question about attenuation (input jacks)

[Pedro Vecino]

My understanding is that the classic voltage divider on the low sensitivity input jacks (classic Marshalls, Fender, etc.) has an attenuation of 6dB (division by four). What attenuation would the upper voltage divider have on the same low sensitivity input?
The musical effect is good for my purpose but I am curious to know what attenuation it produces.

Thanks!

[Helmholtz]

The classic input voltage divider attenuates by a factor 68/136 = 0.5 corresponding to - 6 dB.

The upper voltage divider in you picture attenuates by 120/167 = 0.72 corresponding to - 2.9 dB.

Note that 6dB means 2 for voltage ratio but 4 for power ratio.

[Pedro Vecino]

About half the original, then. Very grateful, Helmholtz!

[g1]

The upper voltage divider in you picture attenuates by 120/167 = 0.72 corresponding to - 2.9 dB.

Wouldn't that be 47/167 for the upper 'low' jack?

[Helmholtz]

Wouldn't that be 47/167 for the upper 'low' jack?

No, the 120k goes to ground.

[eschertron]

This input is too high.. this input is too low.. ah, this input is just right!

-goldilocks

[Helmholtz]

-goldilocks

???????????

[Pedro Vecino]

Definition of Goldilocks (Entry 2 of 2)
: having or producing an optimal balance usually between two extremes.
...

The high input sounds perfect, but:

[eschertron]

I like the "Mr Bean" example... kind of fussy, hard to please in a non-rigorous way. Goldilocks IMHO was a spoiled, self-congratulatory brat.*






oh! I think that putdown (*) should be in the other thread.

[loudthud]

The formula for the attenuation in dB for a Voltage ratio is 20*log(V/V) . Use base 10 log.

[J M Fahey]

That top pair schematic is wrong and even if they wired it that way, it still is wrong, because it *always* pads down signal and to boot shows too low impedance to guitar pickup.

Starting with the correctly wired lower pair, which is what fender and 99% of manufacturers do, including Marshall, plugging a Guitar into the High (sensitivity) input, signal gets no resistive attenuation, is loaded by 1M resistor, and gets a combined "grid stopper" value of 34k ... what´s not to like?

Plugging into the Low (sensitivity) input gives you 2:1 or 6dB attenuation you get 136k//1M impedance and "grid stopper" becomes about same as before.
So far so good.

BUT

on top input pair:

1) you NEVER get unattenuated signal.

2) you NEVER get the benefit of high (1M) impedance

3) High (sensitivity) input is labelled WRONG because it now has the MOST ATTENUATION
47/167=0.28x=-11dB

4) Low (sensitivity) input is also WRONG because it has the LEAST ATTENUATION
120/167=0.72x=-2.9dB
Not THAT bad but unnecessary and input impedance is 100kish instead of desirable 1M

Now if they had not felt the URGE to use that top jack leaf switch, things would have been much better.

[Helmholtz]

That top pair schematic is wrong and even if they wired it that way, it still is wrong, because it *always* pads down signal and to boot shows too low impedance to guitar pickup...

How this?

The upper input pair seems to be wired exactly as the lower pair, except that one of the 68k is replaced by 47k, the other by 120k.
Upper high input has an input impedance of 1M, no attenuation and grid stopper is 47k//120k.

And FWIW, the input impedance of the "standard" low input is 2x68k = 136k as the 1M is shorted.