Altec 19 lpad bypass

[artist]

Hello,

I'm new to this forum, but I am a huge Altec/JBL fan. I recently refurbished the crossovers on my Altec 19's (new caps, new resistors) and I am in the process of bypassing the lpads as I never use them.

I'm confused as to what has to be done. Some people are telling me that I must use two resistors per lpad (one resistor in series and one in paralell so that the amp sees a constant 8 ohm load and the drivers are protected). Others are telling me that I should set the lpads to the "sounds best position" and measure the two pins (there are three pins on the lpad, the third appears unused) where the crossover connects, to get the resistance. When I do it this way I get 3 ohms for the horn and 10 ohms for the bass. Does this make sense given that the bass is an 8 ohm driver?

According to the schematic below a flat response in an ideal room would be 22 ohms on the bass and 3 ohms on the horn. Do these numbers represent resitance in paralell/series or are these numbers I can simply "plug in" to the crossover?

Thanks in advance for the help.


Altec19 Xovr for 416B,802G on 811

+Input----.---L1---.----L2-----.-------------->416+Terminal
| | |
|---P1---. |
| C1
| |
-Input-.-----------|-----------.-------------->416-Terminal
| |
| C2 |-----C4------|
| | | |
| .--C3-.--R1--.--R2--.--R3-->802+Terminal
| | |
| L3 P2
| | |
| | R4
|-----------.------------.------------->802-Terminal

"." denotes junction, "-|-" denotes crossing

L1 0.5mH 0.5 R
L2 1.2 mh 0.5 R
L3 1.0 mh 0.5 R
C1 21 uf
C2 8 uF
C3 16 uF
C4 6 uf
R1 3.9 R
R2 3.9 R
R3 3.0 R
R4 2.0 R
P1 HF-Level 8.0 R variable Resistor default Setting 3 R for max linearity
P2 MF-Level 33 R variable Resistor default Setting 22 R for max linearity

all L wound on E-Core,
L-values measured with Metrix LCR-Bridge,L-R with Beckman3090
all C are 100V
all R 9W/WW
all P 10W/WW

[Claus K]

Hi artist,

those comments from other people you mention refer to real "L-Pads", a special type of potentiometer where you actually have two resistors the values of which are changed when you turn the axis (and they have all three pins connected).

In this crossover configuration however P1 and P2 are not really L-Pads but simply variable resistors.

As a result, If you are happy with the settings you mentioned you can simply replace each one with a fixed resistor of the value you have set them to (3 Ohms for P1 and 22 Ohms for P2).

Regards

Claus K

[artist]

Hi Klaus,

Thanks for your help. I think I will try the 3 ohm/ 22 ohm combination.

Regards,

Pat McDermott

[Gary L]

Hey Artist, Now that you have the info to get this done I think it only fair that you tell us WHY?
I had 19s for almost thirty years and you are correct about the dials rarely getting used but what is the bennefit of bypassing them?
I might add that I moved a few times durring my love affair with the 19s and the setting did change from one room to another.
I also found the speakers sounded much better when the room was full of bodies if the MF was tweeked a bit higher.
Why bypass?????
Thanks

[artist]

Hi Gary,

I tried the bypass in response to a post on the Altec Users board. The person reported that the 19's became more like electrostatics while retaining all the great characteristics of horns. As far as I can tell he was right. My 19's image like mini-monitors and have more presence than stock. Part of this is due to the fact that my amp (a gainclone) has a very short signal path (7mm) and is very transparent. I also completely redid my crossovers with the exception of the inductors, rewired the crossover and braced the cabs (top and bottom) with 2x2 maple.

It is easy to to reverse the resistor mod if you don't like it. I ended up going with 5 ohms on the horn and 22 ohms on the mid/bass. 3 ohms (on the horn) sounded good on perfectly recorded material, however, the majority of what I listen to is far from perfect.

Regards,

Pat McDermott

[Gary L]

Artist, Very interesting and the search for perfection is alive and well in your neck of the woods.
It would be great to have two pairs side by side to really hear the difference from the same amp.
I have a few choices in amps to run mine with but don't have the tech knowledge to know which one would work best for the 19s.
Yamaha MX 1000, Yamaha MX 800 and two Crown D150A in mono mode one per speaker.
I have to get it all together and see which one sounds best.
I am sure someone here will have some input on this when I get the 19s built.
Thanks. Gary L