L56 crossover error

[ivica]

Hello All

I haven't given up on this yet. I am learning as I go...

Since the last post, I learned that I may have been chasing a red herring. The snubber circuit I was complaining about is probably to reduce a self resonant peak in the tweeter response and as already pointed out is below the cutoff frequency anyway. So I will be reconnecting L3 to the tweeter terminal.

I am still trying to extend the high frequency end of the woofer and have obtained some 0.6mH inductors to replace L1. I think I want to build a microphone circuit and use something like TrueRTA to chart the stock performance of the speaker before I continue.

Ed

Hi wa3drc,

There is one possible solution to 'amplify' the response of your bass-driver, in the way that you use L-C low-pass filter but in stead of using driver impedance to be about 8 Ohms, recalculate the network L, C elements as the driver impedance is about 3 Ohms, and then recalculate the response (using SPICE) using L, C but in the simulation put that the bass impedance is 8 Ohms. Let try with these:
C=37.5 uF, and L=0.675 mH, and Bass R=8 Ohms.....
You will get almost +6dB at about 930Hz, while -3dB would be at about 1480Hz, and -6dB at 1670Hz ( all the data are not exact values).

regards
ivica

[fpitas]

Hello All

I haven't given up on this yet. I am learning as I go...

Since the last post, I learned that I may have been chasing a red herring. The snubber circuit I was complaining about is probably to reduce a self resonant peak in the tweeter response and as already pointed out is below the cutoff frequency anyway. So I will be reconnecting L3 to the tweeter terminal.

I am still trying to extend the high frequency end of the woofer and have obtained some 0.6mH inductors to replace L1. I think I want to build a microphone circuit and use something like TrueRTA to chart the stock performance of the speaker before I continue.

Ed

I have and use TrueRTA, but it doesn't feature time windowing. You may want to get a program that does, so you can perform time windowed measurements to take the room out of the equation. Otherwise you'll need to measure outdoors away from reflecting surfaces, or in an anechoic chamber. I use HolmImpulse, which is free and works well other than a tendency to crash occasionally, but there are others around that work fine.

Francis

[4313B]

Hello All,

I recently acquired some L166 and L56 speakers. Both needed re-foaming, but the L56's were good enough to try first. (I already have another working pair of L166's that I have had since ~1976) I thought they sounded dead and then after some internet searching see a few complaints about this issue.

While I was re-foaming I did some searching and found schematics of the crossover networks for both speakers. I put the schematics into LTSpiceIV. The response of the L166 crossover looked as I would have expected, BUT boy was I surprised with what I saw for the L56 network! The L56 circuit has a huge notch (-40 dB) at 1.34 kHz. This is due to a series LC (L3 and C5) at the tweeter terminals of the crossover. Along with the notch comes a large jump in phase at the notch frequency as the circuit changes from inductive to capacitive with increasing frequency.

I didn't find any discussion of this behavior when searching. I am wondering if anyone knows what JBL engineering might have been thinking when they put this network there??? Is it possible the speaker response has a huge spike they were trying to counter?

Anyway, I decided to remove this network and listen... I cut the L3 inductor wire at the connection to the positive tweeter terminal. (You can always solder it back) The response in LtSpice looks far more normal when modelling this. I listened for hours after this, and feel they no longer have a void in the middle. Please let me know if anyone else agrees. I did not notice any "peaking" that JBL may have been trying to notch either. I am leaving my speakers with the LC removed.

Please let me know if anyone knows a reason for this series LC being there. I can send frequency response plots and/or LTSpice schematic files to anyone interested.

thanks,
Ed

The conjugate is there to mitigate the resonant frequency of the dome... without it the high pass filter wouldn't behave as intended. If that 1.0 mH coil is a small square coil then it also has the standard 7.5 ohm DCR value that JBL used on many of their domes.

And yes, if you are modeling a filter using a standard resistor load instead of the impedance of a transducer you leave out the conjugate filter(s) that are intended to directly affect specific transducer impedance.

Photo of the two conjugate coils JBL liked to use. Both have a DCR of ~ 7.5 ohms. 1.0 mH for the domes and 3.5 mH for the midranges.

[SEAWOLF97]

One can easily hear the result of the low crossover in the L56, and its not good. The harder you push it the worse it sounds. The L46 comes pretty close to eliminating that issue at 3000. The L86 sounds great even when pushed very hard.

I mostly agree with RD on this one.

Have had 2 pairs of L56's . LF had poor power handling , bottomed out when being pushed a bit.
Had L46 , much better.

L86's are gems. Found no issues. Should have kept them.

[4313B]

Ditto

G.T. did the best he could with what he had to work with though.

I just couldn't hang with anything less than the L96 at that time. L96's on top of B380's, you know, just for fun.

I'm glad people liked the L86. I thought it was a very nice offering using very affordable components.

I replaced too many 034's in the 2-ways.