would 100k//1500PF sound the same as 500K//500PF?

[jamme61]

looking to drop the resistance for more gain but keep the eq curve? would this work? The 500K // 500pf filter is on my ecstasys plexi mode- just before the last gain stage. 100K parallel with 1500pf sounds right. Don't know the math for this stuff. Thanks

[martin manning]

Is this a case where you have a 470k||470pF and a 470k to ground, and you want to reduce the series 470k to 100k? If so you can't keep the EQ the same by just changing the cap. If you change it to 2n2 the treble boost will begin at the same frequency (720Hz), but it will rise quicker, with the upper -3dB point at 900Hz instead of 1400. If you want to preserve the upper break, use 1n5.

[jamme61]

yes there's another 500k to ground. using 100k with the 1n5 in parallel sounds about right- eq wise and gives me more gain which is what I wanted. Thanks for the help

[tubeswell]

The general rule of thumb to keep the same roll-off point is you double the resistance if you halve the capacitance and vice versa, or you triple the capacitance with 1/3 of the resistance and vice versa etc etc

So dropping 500k to 100k would require increasing the 500pF to 2.5nF to keep the roll-off point the same - all other things being equal.

[roberto]

But the roloff is not the only parameter.

With a 470k-470k the output signal will be half on lowest frequencies, while the 100k-470k gives you 5/6 of the signal on lower frequencies, and there's only 1/6 of gain due to the cap at higher frequencies.

The response of the amp is totally different.

[jamme61]

this is really tough when using your ears. 100K // 1500pf sounds close but still not there. I thought there was some kind of formula that would get me there.

[Jana]

You also have to consider the value of the coupling cap--that is also a part of this network.

[jamme61]

You also have to consider the value of the coupling cap--that is also a part of this network.

4n7

[roberto]

Related to the HPF related to the coupling Cap:

470k//470p + 470k:
f = 1/(2*pi*(940k + 38.5K)*4.7nF) = 36Hz

100k//whatever + 470k:
f = 1/(2*pi*(570k + 38.5K)*4.7nF) = 59.4Hz

[tubeswell]

But the roloff is not the only parameter.

With a 470k-470k the output signal will be half on lowest frequencies, while the 100k-470k gives you 5/6 of the signal on lower frequencies, and there's only 1/6 of gain due to the cap at higher frequencies.

The response of the amp is totally different.

That's why I said 'all other things being equal'

[jamme61]

well I asked and you delivered thanks. getting my daughter to explain the math thanks again

[martin manning]

You might want to try another approach. What is the plate load on the stage before this 470||500pF nw? If it's 100k, you could increase it to 220k, and increase the cathode resistor proportionally. That will get you a few dB of increased gain and not change the frequency response much. You could play with the Ck too; halve it say, or not.

[roberto]

This is an approximation of the differencies between the two solutions.
A cheese paper plot done after dinner on the sofa, but easy enough to be read.

Yea, dB are not logaritmic on this plot, I know.

[roberto]

You might want to try another approach. What is the plate load on the stage before this 470||500pF nw? If it's 100k, you could increase it to 220k

the ecstacy in plexi mode has simply a 100k 820R//680n as a first stage going to 4n7 to 500k gain pot (470p or 4n7 switchable bright cap) going to that 470k//470p to 470k to ground, then the classical 100k 1k (or 820R, depending on the versions), to a CF with 100k as Rk.

A 220k 1k5 totally changes the feel of the amp, and I personally don't like that.

[roberto]

Just to specify, the plot above doesn't take into account the gain pot.
In this case the effect is different, and changes as you change the gain setting.