Jeff Markwart's 604 filter question

[Naimare]

Hi ,

(maybe this is a question to Jeff but everyone are welcome to answer!)

I'm brand new on this forum and has a couple of questions regarding Jeff Markwart's filter design for Altec 604 (maybe these questions have been asked before but I havent been able to find answers...)

I have 604-8K in 620 cabinets and would like to build Jeff's filter ("Simplified 604-8H EQ Crossover") to replace the stock filters (which I find pretty bad to be honest).

As I am a novice when it comes to filter design, my questions may come across as rather basic but please bear with me...

1. I understand the 2nd and 3rd order LP and HP parts of the filter but have a little problem in understanding the "contour circuit" for the upper mids with the L-pad feature.

The contour circuit have the following components: One capacitor C4 (6 microF) in parallell with two resistors, R1 (3.9 oms) and R2 (3.9 ohms). Parallell to the HF unit, a variable resistor (P1) and a fixed resistor (R4, 2 ohms) is connected. P1 connects to the positive (+) side of the HF element at a point between the R1 and R2 resistors. Finally, another resistor (R3) is placed after the circuit just before the HF element.

a) Is the series resistor of the L-pad R1 only or R1 + R2?
b) Jeff wrote: "If a 25 or 30 ohm potentiometer is hard to find, one-half of an 8 or 16 ohm L-Pad can be substituted by utilizing the 30 ohm element with the center wiper, and leaving the other element un-terminated" - can someone explain this in layman words...
c) If I would like to substitute the parallell variable resistor (P1) with a fixed, what would be a "typical" attenuation value (in dB) to put in a L-pad calculator to figure out the parallell fixed resistor value? 5 dB, 6dB, 10dB?

Finally, a general question regarding inductors. How important is it to get the exact inductor values (0.74mH and 1.7mH). I have found Jantzen copper-foil air core inductors with the value of 0.68mH (-8.1%) and 1.8 mH (+5.9%). They have a tolerance of +/- 2%.

Same question with regards to capacitors. Is +/- 5% from stated values OK or not?

Looking forward to some comments and answers!

Best regards/Anders

[Panomaniac]

My answers (there will be others, maybe better)

The L-Pad. The variable type have three tabs, just like a pot. But they are built differently inside. All you need to do is find the tab that gives you about 30 ohms when measuring from the center tab. You only have 2 choices!
Get the good quality 100 Watt type. The typical smaller ones are junk.

What is a typical value? It's going to be what works for you. Use the L-Pad or variable resistor to find it, then replace with fixed resistors.

Close enough values:
I think you want to be a bit closer than the values you indicate. When a crossover gets very close to "right", tiny differences can be heard. So if you do not plan to tweak by ear, get the right values. For the inductors, get a larger value and unwind. For the caps add smaller values in parallel. You probably want to invest in an LC meter, it will make your life a lot easier. They can be had for not too much $$. Or borrow one, if you can.

The values that Jeff gives may not be perfect for your drivers, cabinets, room, ears. But they will be a very good starting point. And you may be happy enough to go no further. Have fun!

[Naimare]

Hi Panomaniac!

Thanks for your response!

I was able to find Solo CFAC inductors (discountinued) with the precise values at Michael Percy Audio. If I remember correctly, this is the brand Jeff used himself.

I will try your suggestion with parallell capacitors to get more exact values.

For L-pad, I think Fostex has a 100W variant (R80B) which I can source from Germany.

Anders

[Panomaniac]

You're Welcome!
Sounds like you are well on your way.

I just warn against the small L-Pads that are typical here in the USA because they are poorly made. For some reason the 100W types are better. You don't need the power, but the better construction is nice. The Fostex L-Pads should be very nice.

Have fun, keep us posted on how it goes.

[jdiamantis]

Not to be a party pooper, but I though Jeff said on his website that the stock 604-8K (original, not Great Plains) crossover was pretty much spot on, but the componenets could be upgraded.

[Naimare]

jdiamantis,

You're right. And changing caps and inductors on the stock filter would probably be an option too. Mayby that will be another project !

Naimare