Hi- & low-pass filtering: how about Bhrngr PEQ2200/T1951

[recnsci]

[quote author="clintrubber"]Hmm, why different for HPF & LPF ?[/quote]
Second order butterworth HPF sound "tighter" for me, compared to more
Bessel response. Butterworth on lowpass always sound piched to me.
Actuall preference would be something like second order buterworth HPF
and first order LPF. This is personal taste only.

I'll post the D&R-circuit tonight, they use TL074 but 'help' the opamp a bit. And they avoid loading below 2k7, which is fine enough IIRIC.

Biggest load is on stage thats driving filter and its roughly equal
resistance of pot that sets filter freq. Load on opamp that forms
filter is roughly three times less.

I still do want to add a SPDT per filter though, smartly placed this can switch between 1st & 2nd order.

Two ways to do that : for HPF SPDT switch can put in parallel to one
of the caps another cap that is 10 times bigger. In LPF put 10 times
smaller cap in series with one of the caps and short it with SPDT.
This will give you roughly first order response.
Other way is to tap off between caps in HPF. This will give you 6dB/oct
with a nice bump around cuttof if filter gain is above 1. And passband
is at unity gain. Equvalent for LPF, you tap between pots.

cheerz
urosh

 

[clintrubber]

[quote author="clintrubber"]I'll post the D&R-circuit tonight, they use TL074 but 'help' the opamp a bit. And they avoid loading below 2k7, which is fine enough IIRIC.[/quote]

OK, FWIW here it is:
D&R hi/lo-pass filter

Simple & straightforward. Circuit on page 4. Actual (simulated) -3dB points from 18 Hz.. 600 Hz (HPF) and 1kHz .. 27kHz (LPF).
This circuit an easy victim for some Q-mods.

 

[clintrubber]

[quote author="recnsci"][quote author="clintrubber"]Hmm, why different for HPF & LPF ?[/quote]
Second order butterworth HPF sound "tighter" for me, compared to more
Bessel response. Butterworth on lowpass always sound piched to me.
Actuall preference would be something like second order buterworth HPF
and first order LPF. This is personal taste only.[/quote]
I see. OK, those 1st/2nd-order switches might be interesting :thumb:

I still do want to add a SPDT per filter though, smartly placed this can switch between 1st & 2nd order.

Two ways to do that : for HPF SPDT switch can put in parallel to one
of the caps another cap that is 10 times bigger. In LPF put 10 times
smaller cap in series with one of the caps and short it with SPDT.
This will give you roughly first order response.
Other way is to tap off between caps in HPF. This will give you 6dB/oct
with a nice bump around cuttof if filter gain is above 1. And passband
is at unity gain. Equvalent for LPF, you tap between pots.

cheerz
urosh

Had the second method in mind yes. Thanks for adding the first one, hadn't considered that one.

BTW, some more on filtering here:
http://www.groupdiy.com/index.php?topic=19824


Bye !

Peter

 

[clintrubber]

[quote author="recnsci"]My suggestions for A would be 1, 1.5 and 2 . If you have dual three
position switch you could have compensation of passband gain
with attenuator on output. Or use another opamp as unity buffer and
feed it from - input of first opamp.[/quote]
I saw that the Alembic LPF-circuit does the feed from the inverting input as you suggested but avoids the need for an unity buffer by using low values for the 'filter-opamp' feedback-resistors. 510, 510 & 620 IIRIC, with a ON-OFF-ON switch across the 510-resistors for a 3-position Q-switch.
This results in gains of 1, 1.45 & 1.82, close to your suggested 1, 1.5 & 2.

So you need to use a filter-opamp that can drive a load as low (heavy) as 620+510 Ohms.
I don't know what the original opamp was they used, but a 5534 could do it and will be OK w.r.t. biascurrent I think/hope. (At least I've seen serious variable-freq circuits using 5534/5532).

Regards,

Peter

 

[clintrubber]

[quote author="recnsci"]Im not Brad but that is butterworth he is sugesting.
Filter math is super easy with equal valued Rs and Cs.
cutoff is at f=1/(2piRC) and Q=(3-A)/2 where A is gain of that
noninverting stage. Note that passband gain is A.
My suggestions for A would be 1, 1.5 and 2 . If you have dual three
position switch you could have compensation of passband gain
with attenuator on output. [/quote]

Hmm, I find that that requires to use C & 2*C.

I don't get the easy Q-variation peaks with the ON-OFF-ON switch when using equal R & equal C values.

The C & 2*C gives up to some 13dB peaking for opamp-feedback of 1/1.85 [-].

The maths will explain this; so far I just did some sims.

Bye,

Peter

 

[KasperNyhus]

Did anyone try to build that calrec filter Jacob posted?

I have made a layout for the LF parametric filter, and it works, but it only reacts on the last few degrees of rotation on the dual pot.

This might indicate that "A-log" isnt standard log, but that doesnt make any sense either because it says "rev log" elsewhere on the schematic.

Does anybody have any idea of what could have gone wrong at my end?

 

[gyraf]

On second thought (8 years later), these pots are probably negative-logarithmic ("C"-type).

I'm pretty sure I have some in a box somewhere if you stop by, would be nice to verify schematic...

Jakob E.

 

[KasperNyhus]

On second thought (8 years later), these pots are probably negative-logarithmic ("C"-type).

I'm pretty sure I have some in a box somewhere if you stop by, would be nice to verify schematic...

Jakob E.

Thanks Jakob!

The pot you gave me works nicely! And it is hereby verified that "A-log" in calrec designs are "C-type" pots.

 

[gyraf]

Thanks for getting back on this - I edited the old post about the pot type

Jakob E.

 

[Kir_For_Pleasure]

Great !