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SIT 1 and crossovers
SIT 1 and crossovers
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Old 10th October 2017, 10:25 PM   #1
rickmcinnis is offline rickmcinnis  United States
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Default SIT 1 and crossovers

I am using my SIT 1s to drive JBL 2441s with the TRUEXTENT diaphragms.

The last couple of years have been in search of the best crossover arrangement.

I have been using the input and output caps of the SIT1 as part of my crossover. 0.32uf at the input and 25 uf at the output. These are placed near the loudspeakers for ease of changing the value. There is a 4/5 mH choke in parallel along with a shunt resistor to set level and 5 uF of capacitance to roll off the 2441s which make the blending with the FOSTEX T500s MUCH more homogeneous. The T500s input is taken from the junction of the 25 uF cap and the choke, followed by a cap and a choke giving what is a very slow slope fourth order. I can say the FOSTEX sound much better with as little lower frequencies directed toward them. I used to insert them at the amplifier's output.

Looking at measurements made with REW looks like I am getting a 3rd order crossover slope at the nominal 500 Hz.

I have been using Mr. Waslo's Xsim and have found there to be a good correlation between what I see there and what I measure but I am wondering what to do with the spec'ed 4 ohms of output impedance. I have tried using a four ohms resistor in series with the model which seems to improve my correlation but wonder if this is real or coincidence.

Xsim is a great tool, especially for those wanting to learn how crossovers work.

My question is how do you include the output impedance in the crossover model? Is the four ohms in series correct or should it be four ohms in parallel? From the model the four ohms in series really affects what happens down the line where the four ohms in series simply attenuates which makes me think that is not what is happening with output impedance.

Am I on the right track or is it impossible to use the output impedance within the Xsim model?

Any thoughts or counsel, please?
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Last edited by rickmcinnis; 10th October 2017 at 10:29 PM. Reason: more detail
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Old 11th October 2017, 05:31 PM   #2
Nelson Pass is offline Nelson Pass  United States
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SIT 1 and crossovers
You can treat the SIT-1 as a voltage controlled voltage source in series with
at 4 ohm resistor at the output.

I am not familiar with Xsim, but you can use Spice (LTSpice or MicroCap etc.)
on this after you come up with a model for the loudspeaker driver.

Examples of modeling that from the impedance curve can be found in:

http://www.firstwatt.com/pdf/art_cs_amps.pdf

Or you can just plug parts in and measure the frequency response across
the driver terminals and/or the acoustic output.
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Old 11th October 2017, 06:34 PM   #3
rickmcinnis is offline rickmcinnis  United States
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THANKS for your note.

I assumed the 4 ohms in series was right since it correlated with what I was measuring but nothing beats being sure.

Being a master of SPICE would make XSim superfluous for you but for those of us on the mortal plain XSim is a great tool. In conjunction with REW you can see where you are going and make good predictions of what can happen.

I had read over that article (again) but still felt unsure.

I am going to sneak in another one. I am wanting no components in series with the driver so i am using only a shunt resistor to set level. All seems to be fine. The amps do not seem to run any hotter - but then they always run HOT.

Would you be concerned about me using a 3.8 ohms resistor in parallel with a (measured) 12 ohms impedance of the driver? Sure seems to sound better! I was using the carbon DUELUND resistors which did sound better than regular ones but there is no resistor like no resistor!

I have done plenty of plugging in parts in both the system and into Xsim. There is so much to be learned from just trying things! But it is still good getting guidance from someone who is a few lifetimes ahead.

THANKS,
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Old 11th October 2017, 06:41 PM   #4
Zen Mod is offline Zen Mod  Serbia
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SIT 1 and crossovers
be sure that Pa is having plenty of experience in shunting drivers

so , my comment (while expecting his) is - why not , if you like it more
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Old 11th October 2017, 08:09 PM   #5
Nelson Pass is offline Nelson Pass  United States
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SIT 1 and crossovers
Quote:
Originally Posted by rickmcinnis View Post
Would you be concerned about me using a 3.8 ohms resistor in parallel with a (measured) 12 ohms impedance of the driver?
Nice thing about SIT-1 and 2 is you can short the output all day long if you like.
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Old 16th October 2017, 05:05 PM   #6
rickmcinnis is offline rickmcinnis  United States
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After more consideration I can see that trying to use the FOSTEX t500 and the 2441 in parallel unless I attenuate the drivers with more series resistance than I want to use I will never get much above 2 ohms from around 6 kHz on up. Below that it rises steadily to well over 100 ohms below cutoff.

I guess my question is - on an ideal basis - I figure it is better to have the impedance higher than this for sonic reasons?

Tonight I am going to try the drivers in series which brings the impedance up to a bit over 4 ohms in the same region.

The t500 measures a bit over 6 ohms for almost all of its range, not the 8 ohms their graph shows. There is a little blip where it rises slightly.

Would the amplifier sound better with less of a load?

Starting to think my idea of using an ACA for the Fostex might be more worthwhile than I originally thought! Need to put it together and find out but I cannot think that the ACA would sound better than the SIT even in this situation.

Any chance of a preview of the next SIT iteration?
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Old 16th October 2017, 05:45 PM   #7
Nelson Pass is offline Nelson Pass  United States
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SIT 1 and crossovers
Quote:
Originally Posted by rickmcinnis View Post
Would the amplifier sound better with less of a load?
It will sound a little different. You are the one to decide if it is better.
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Old 16th October 2017, 05:47 PM   #8
Zen Mod is offline Zen Mod  Serbia
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SIT 1 and crossovers
well , loading SIT with 2R is certainly waste ........ of everything
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Old 16th October 2017, 06:15 PM   #9
rickmcinnis is offline rickmcinnis  United States
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ZM: I worry that might be the case though I would have no idea why!

I guess I am trying to find out what the amplifier would prefer or it the amplifier couldn't care less!

I assume the amp produces little power with that kind of a load and I do not need much power way up there! So that could be a wash.

Yes, it sounds just fine as it is but then I now see that I have presented this load to the amplifier pretty much since I got it so I have no basis of comparison.

So is there a "rule of thumb" (don't we wish DIY audio was chock full of these?) for an optimum load for the SIT amplifiers?

THANKS for all counsel,
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Old 16th October 2017, 07:31 PM   #10
Zen Mod is offline Zen Mod  Serbia
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SIT 1 and crossovers
you know , if you connect them (parallel) in a way that you're satisfied with sound , I'm pretty sure SIT-1 will endure and deliver

are they (fostex and jbl) really working in same range,and why ?
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