Alternative to tapped inductor in 3110a or 3115a?

[4343]

Hi Robert

You cannot use multiple standard inductors simulate a tapped inductor. A tapped inductor acts like a transformer. And since, there are not separate primary and secondary winding, these devices are often referred to as Autoformers or Autotransformers. I don’t know of anyone that is manufacturing these devices, so unless you can track down some JBL crossovers. You might need to wind your own coils, which is not something that should be attempted without test equipment to measure the inductance.
And due to the necessary inductive interaction between taps, I would recommend using an iron core rather than an air core design. Rewinding an Erse Super Q coil would be a good starting point for such a project.

Here is a link to the inductance ratios that would be needed:

http://www.audioheritage.org/vbullet...hread.php?5376

...

Baron030

I'm not seeing the values you got.

From the chart, -6dB is supposed to be 25.1% of the original inductance or 0.8283mH. That would certainly be closer to half the wire than what I made, there's many layers on that 2.5mH I started with, and only one layer on top to get to 3.3mH. It looks like you subtracted 25.1% to get 74.9% of 3.3mH, or 2.4717mH.

I read "% of original inductance" to mean the end goal, not the amount to remove from the original inductance.

By my reading:

-8dB is 15.8% or 0.5214mH.
-10dB is 10% or 0.33mH.

>EDIT

If you look at the notes under the model, the -10dB value is in fact listed. (-10dB is 330 micro Henries = 0.33 milli Henries)

END EDIT<

Using that chart, I think it would be fun to build a 3110A with a few different steps. -6, -8, -10 could be -1, -2, -3, -6, -9, or maybe for testing, all of the them!

-1dB=79.4%=2.6202mH
-2dB=63.1%=2.0823mH
-3dB=50.1%=1.6533mH
-4dB=39.8%=1.3134mH
-5dB=31.6%=1.0428mH
-6dB=25.1%=0.8283mH
-7dB=20.0%=0.66mH
-8dB=15.8%=0.5214mH
-9dB=12.6%=0.4158mH
-10dB=10.0%=0.33mH

I'll be looking for some more wire now.

[Baron030]

Quote by 4343:
From the chart, -6dB is supposed to be 25.1% of the original inductance or 0.8283mH. That would certainly be closer to half the wire than what I made, there's many layers on that 2.5mH I started with, and only one layer on top to get to 3.3mH. It looks like you subtracted 25.1% to get 74.9% of 3.3mH, or 2.4717mH.

Hi 4343
Yes, I did simply subtract the 25.1% to get the 2.4717mH value for the -6 tap. I thought the idea would be worth trying. And in a way I am actually surprised how quickly you have been running with it. The next big question is are you getting 0.8283mH of inductance when you measure it between pins #2 and #5? If not then my logic is really flawed. And the chart’s turn ratio should be followed instead. This would mean that a 3.3mH I-Bar coil would need to be completely unwound and the total number of turns counted. And then use the bare core and a lot of fresh of wire to rewind it completely with taps. And with there is the issue of can Pin #1 be the lead closest to the iron core Or does Pin #5 needs to be closest to the iron core? The point being that does the secondary winding need to be closest to the core? Or can secondary winding be wound on top of the primary winding? If the secondary can be on the outside then a 3.3mH I-Bar when it would mean that a lot less of the coils’ original wire would need to be removed.
Baron030

[Baron030]

For those of you following this thread, you can tell I have more questions than I have answers right now.

Hi 4343, Are you getting 0.8283mH of inductance when you measure it between pins #2 and #5?

I have been digging through some boxes tonight and I have turned up almost all of the parts necessary to construct the HF section of the 3110a crossover network. I have a 16 ohm load resistor to stand in for the HF driver. A 0.02mH coil and a 3uf cap for the “Max Boost” section. I have 2-8 ohm resistors that will have to stand in for a 3.9 ohm resistor. And 2-4.3uf caps that will have to stand in for the 8uf cap. As well as 2-40 ohm resistors that will stand in for the 2-39 ohms resistors. And I have one more interesting find, a pair of Jantzen 15 gauge air core inductors. I know I am not hitting all of the part values spot on. But, it might be close enough for a good test.

I am beginning to suspect that turn-ratios might be more actuate and important then relative inductances. Now, I really don’t want to damage a 3.3mH coil for this test, because I may have some uses for them later. But, if I nick the insolation in just a few outside edges, I should be able to create some taps at those points. And then I should be to test the voltage drives to see they the match mathematical formulas or not. Unfortunately, I have to make some bathroom repairs this weekend and other projects as well. So, it might take me quite a while before I can post some CLIO results and then have some real answers.

Robfive, please post the 3110 equivalent network schematic that gets around the tapped inductor. Maybe the focus can shift to just adding the 2380 CD compensation to it.

Baron030

[robfive]

Baron030, thanks for your help in this. I know what it's like to have several projects going on at once. Please fix your bathroom before messing with this inquiry.

Here is a link to the 3110 equivalent HP x-over posted by 4313B back in 2005:

http://www.audioheritage.org/vbullet...ll=1#post52084

It looks like a straight-forward second-order filter and L-pad.

Thanks,
Robert

[4343]

Decided to try the coil again, this time with 8 taps. I tried to start at -10 dB, but ran across a measurement error at that tap. (I left the wire I unwound on the floor and measured from the unwound end to the end of the wire.) It turns out my meter adds a lot of inductance to the reading with the straight wire in the circuit. I ended up winding a little past the reading I wanted and then cutting and stripping the wire to take a more accurate reading, while unwinding a little to get to the exact number required, then cutting and stripping again, before tying in a long piece and starting that whole process again for the next tap. In the end I ended up with 7 taps on my 8 position switch: -9, -6, -5, -4, -3, -2, and -1.

Later I realized that the top of the inductor would yield -0 dB, so I used it as the last position on my switch. Today I dragged out my HP Distortion Measurement set (generator and meter in one). It measured precise 1 dB steps from 0 to -6dB and exactly 3db to -9.

Just need another core and I'll wind another after I get done swapping the drivers around to put the 16 ohm 2445's on the flat fronts. I'm also thinking that the cheap wafer switch should get swapped to become a terminal strip so it does not get fried. I did that for the "HF Boost" using a 3 position strip and a wire with a lug on it to select Max, Med, or Min. For now, the "gain" switch works great, and I will be able to determine exactly which tap I want very quickly. The switch might stand up to home use, but these are SR tops, so failure is not an option once they are in service.

The 0dB setting is actually quite useful if you want to use the filter to passively bi-amp using an amp with level controls. Set to 0dB, and adjust the level with the amp. Passive bi-amp means paralleling the inputs of two amps with a full-range signal and letting the high level filters in the box do the frequency splitting. (If you do that you don't actually need the taps on the inductor at all, and as the post on the 3110 equivalent circuit says, put a 16 ohm L-pad after the output and you won't miss them, even without bi-amping...) Passive Bi-amping is not very common in SR work, but I learned long ago that having a passive in the box can sometimes save your bacon...

[4343]

Has anyone reworked the 3110a to have a fixed HF gain of -10db and not use L3 and S2?

That's easy if you just build one with a standard (non-tapped) 3.3mH inductor. I just measured mine set to 0dB, i.e. all the taps floating open and the signal drawn from the top of the 3.3 mH inductor ((the junction of R2&R3, C5 and L3 connected to the junction of S1 pin 3 and R4). Using a 16 ohm L-Pad connected to the output pins I adjusted it to read -10dB (compared to the output level when it was not there). I then measured it's resistance out of circuit. From ground to the wiper, I measured 4.5 ohms, and from the wiper to the plus output I measured 40 ohms.

With a 150W driver connected, I can see not using a 100W L-Pad! Using one to adjust the level to match the LF, then putting fixed resistors in that match the values measured on the L-Pad is a common sense way to dial in any level you might need. If you use high enough wattage resistors, it could survive...

[robfive]

4343,

So are you saying that we could get around the tapped inductor by using a standard 3.3mH inductor in its place (creating -0dB) and placing an L-pad resistor pair infront of the midrange? That sounds good to me.

[4313B]

we could get around the tapped inductor by using a standard 3.3mH inductor in its place (creating -0dB) and placing an L-pad resistor pair infront of the midrange? That sounds good to me.

Nope. You cannot treat a tapped autotransformer like an inductor.

If I have time tomorrow I'll work up the 3110A equivalent network. I suspect it will be similar to what I already have along with the compensation network placed in parallel with the resistor pad and L-pad.

[4343]

Nope. You cannot treat a tapped autotransformer like an inductor.

If I have time tomorrow I'll work up the 3110A equivalent network. I suspect it will be similar to what I already have along with the compensation network placed in parallel with the resistor pad and L-pad.

Thanks!

I don't have any way to measure anything but the levels right now, so having a definitive answer would be great.

[4313B]

Has anyone reworked the 3110a to have a fixed HF gain of -10db and not use L3 and S2?

Here you go.

Don't fret about the 316.2m value shown on the schematic for the tapped autotransformer T1. It is indeed 3.3 mH with a turns ratio of 0.3162 to 1 for the -10 dB voltage drop as shown in figure 1.


The DCR of L3 in the equivalent is 0.5 ohms.

[4313B]

Here is the standard value equivalent. I forgot to force it in the original.

[Baron030]

Wow, 4313B !!!
You sure make it look easy. I bet that Leap Crossover shop software didn’t come cheap.

In researching this, I did turn up a 3310A variant. The high pass section is identical to the standard 3110A version.
But, the low pass section is a little different. It has no zobel network and different L1 and cap values.
Here is the schematic below. I am not sure which 3110A network version would be better for which drivers?

Baron030

[4313B]

In researching this, I did turn up a 3310A variant. The high pass section is identical to the standard 3110A version.
But, the low pass section is a little different. It has no zobel network and different L1 and cap values.
Here is the schematic below. I am not sure which 3110A network version would be better for which drivers?
Baron030

[Baron030]

Thanks again 4313B

It looks like N4671 network is nearly identical to the standard 3110A. But without the impedance dip at 400 Hz.
From a practical point of view, low pass section of the N4671 variant might be easier for people to build because the 3.3mH (L1) and a single 30uF cap can be substituted for (C2 & C3). Both of which are stock off the shelf values. A stock 2.6mH coil might be harder to find. I did notice that the smallest coil that parts express stocks is a Jantzen 0.025mH 18ga coil (255-198). So, it is the only item that will need some custom fabrication or trimming. If you own an inductance meter and you are building the 3110A network, then please record and post here the number of turns which will need to be removed to change a stock (255-198) coil to 0.020mH.

Baron030

[GrooveControl]

Wow! Just what I was looking for! Thanks!

One Q. How much impact will the ab switch have on the top end? Can you add to the chart?