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Old 20th February 2005, 05:30 PM   #11
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Sorry about the room acoustics thing. I wasn't trying to get anybody bent out of shape. It was more as a 'keep in mind' and maybe tweak later idea.

Merlin, your xover comes out just over 72Hz. Looks good.

I would also suggest lowering the summing resistors, R1 and R25, as cocolino suggests.
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Old 20th February 2005, 06:16 PM   #12
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Summing amp fixed. See attachment

Should I take out the resistors I put in to compensate for the input bias current wherever I was able to (R20,R27,R28)? I see most circuits don't have them. Can they be detrimental in some way?

Also as far as component selection... What kind of caps should I use in the signal path and what opamps should I get without getting too exotic and expensive?

What about the caps on the power leads of the opamps (no shown on schematic)How do I determine how big they should be and what type of caps to use here? Would mylar caps be ok here since I already have a bunch?
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Old 20th February 2005, 06:58 PM   #13
MBK is offline MBK  Singapore
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About the bias resistors - nobody puts them , true - but I do think ppl should pay attention to detail, so, good thinking. In your case though don't worry because:

- you don't need R20 and R27 , C1 and C10 will very quickly drop in their Z above DC - so you don't have any significant series input impedance to compensate for.

- you don't need R28 because you have an inverting configuration and you keep the noninverting input at ground level. The op amp will keep the inverting input at virtual ground.

More to the point, I'd rather scale down your resistor values for lower noise and lower potential HF pickup. You use up to 85 k. You could keep the values between 5k and 10k for best results (rescale caps of course, too).

And, do you need all those DC blockers anyway? The HP has a much higher cutoff than DC. If it does its job at all... no DC. Besides your power amp may even have another input DC filter.

The LP needs at most one DC filter, best at output. That is, IF your power amp doesn't have one at input anyway...

MBK
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Old 20th February 2005, 09:51 PM   #14
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Quote:
originally posted by merlinx76
Also as far as component selection... What kind of caps should I use in the signal path and what opamps should I get without getting too exotic and expensive?
I`d go for MKP, KP (both Polypropylen) or KS (=Polystyrene respectively Styroflex as those things are called here in Europe) film caps. KS-film-caps might be a bit difficult to source (especially the bigger values, where 47nF is mostly already the biggest You can get) but MKP`s are readily avaiable and cheap.

About the opamps I barely dare to give a recommendation as there are so many different opinions.
Anyway, IŽd say an OP604 (single version) or OP2604 (dual-version) isn`t neither exotic nor expensive or hard to get.
Certainly there may be better op-amps meanwhile but for something "unexotic" they really should work fine (at least they do for me).
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Old 20th February 2005, 09:55 PM   #15
Bricolo is offline Bricolo  France
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how big is a 47nF styroflex?
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Old 20th February 2005, 10:09 PM   #16
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Quote:
how big is a 47nF styroflex?
A Styroflex film cap is bigger than an equivalent value/voltage MKP film cap.
The one`s I have are cubic parts 12,5x12,5x12,5mm (Siemens, 63V) but there are axial leaded versions also.
As I said 47nF is mostly the biggest value avaiable, also 100V seem to be the upper voltage limit in that they are to get.
Anything above 47nF or 100V IŽd consider as quite "exotic".
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Old 20th February 2005, 10:24 PM   #17
Bricolo is offline Bricolo  France
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the 47nF is 12.5*12.5*12.5?
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Old 20th February 2005, 10:31 PM   #18
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Quote:
the 47nF is 12.5*12.5*12.5?
mm
Yes, that`s right.
Most MKP`s of that value & voltage would only have about 1/4 of that volume.
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Old 20th February 2005, 10:37 PM   #19
Bricolo is offline Bricolo  France
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Yes styroflex is big, but 12.5mm for such a high (for a polystyrene cap) isn't big at all

At my local shop they have some axial ones, I have a few 680pF from them, and they are appros 10mm long (you must have 4 holes between the legs on a veroboard)
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Old 21st February 2005, 02:56 AM   #20
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OK, I took out whatever I thought wasn't necessary. Less caps in the signal path. Can I take the outupts directly from the opamp like I have on the highpass section? Do I need a series resistor for the output or a resistor from output to ground?

I kept a cap on the output of the LPF for a bit of protection from subsonic freqs. (my sub amp has an input impedance of 5kohm)

I also reduced the value of most of the resistors throughout the circuit and changed all the caps accordingly.

Heres what I have now.
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