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In Reply to: RE: Follow up on my previous post... posted by pix on February 09, 2016 at 00:08:13
I think you are right pix, surely all those things apply. I think Bob represents them as "Better than what Klipsch supplied", but I don't know the specs of the Klipsch units either. Here is a doc showing how to wire the 3636 for different levels of attenuation: http://www.critesspeakers.com/3636atz.pdf
I have a pair on order too.
G
Follow Ups:
The stock T2A I measured was about 35mH from top to bottom (spec is 40mH), the universal replacement is similar.
(I own a pair of the Crites units)
I assume the autoformer shall be connected after the x-over, hense it could be optimized depening on the x-over frequency.
I need to lower my mid/hi horn from 112dB to 101dB = 11dB which calls for a winding-ratio of 3,5 : 1 (step down)
My x-over point is 350Hz where an 4mH chocke would load the amplifier with 8,8 Ohms (driver load not included yet)
The Windingratio of 3,5 : 1 call for a impedansratio of 12,25 : 1, which would mirror the 8 Ohm driver as a 98 Ohm load
But since the 8 Ohm load is 1/3,5 down the total load seen from the amplifier is 6,6 + 2,2 (parallell to) 98 = 8,56 Ohm
Result:
A 4mH winding with an 1 : 3,5 tap
Right or wrong?
Hey,
The short answer is Yes... However there is a 600 pound gorilla in the corner is begging for some attention. Here is a basic simulation showing that the behavior with an autoformer can be made to behave the same as an L-pad.
While being rather small the thing that everyone (except fostex) overlooks is the 8.5 ohm R2 in the autoformer example. This resistor does two important things and gives some unexpected results if left out. First it sets the crossover impedance and second it damps the CL resonance that all crossovers have. Here is the same siumulation run with a value of 8.5 ohms and 1 meg for that shunt resistor. (1 meg essentially takes it out of circuit)
Well the first thought is YIKES... but then you realize that the autoformer is reflecting back a higher impedance than the 8 ohms the driver presents so of course the crossover needs to change. So Lets change the crossover values for the 100ish ohm reflected impedance and see what happens.
OK now we are back in business except for one small problem and that is the non-linear impedance curve of a driver. Lets say that out nominal 8 ohm drive starts to increase in impedance as frequency goes down and is actually 12 ohms in the crossover region. The L-pad handles this nicely but the autoformer "magnifies" the impedance change and that nasty LC resonance starts to show up.
This was just a simple "first cut" at some of the things autoformers do that typically get ignored and the moral of this short story is to remember to use a damping resistor across the primary to avoid the gremlinsh that are sure to pop up.
(The people at Fostex knew all of this and if you look inside the T100 you will se a couple of 8 ohm resistors across the primary)
dave
Thank you, Dave!
Wow!
I never expected such great responses to my posting.
Your expertise far exceeds my own here, Dave.
Can you simplify things a little bit?
Do I need to add R2 = 8 ohms, and then change my crossover values for the high pass section going through the autotransformer?
I am crossed over at 800Hz 3rd Order into 8ohms.
Could you suggest some values?
If I am reading your posting right, C1 changes from 40uf when using the autotransformer? Plus, I add R2?
Thank you.
"Ad Astra Per Alia Porci", - To the stars on the wings of a pig: John Steinbeck
Do I need to add R2 = 8 ohms, and then change my crossover values for the high pass section going through the autotransformer?
Some value of R2 is needed and the crossover values will remain the same. Typically the value for R2 will be about 10% above the nominal crossover value and all of this should be verified by measurement.
I am crossed over at 800Hz 3rd Order into 8ohms.
how much attenuation and what is your speaker impedance. LTspice is free and easy to learn and a good starting point.
dave
Current HP filter values:
C1 = 15uf
C2 = 56uf
L1 = 1.2mH
4db attenutation w 3636 autotransformer
"Ad Astra Per Alia Porci", - To the stars on the wings of a pig: John Steinbeck
This is a perfect example of what I was saying. Here is the simulation of your crossover by simply replacing a 4dB pad with a 4dB autoformer.
The underdamped LC resonance gives a 4 dB bump at the crossover frequency which will definitely change the sound. Simply changing the value of the shunt resistor from 1E99 to 13 brings the response back to match the L-pad situation. (1E99 is a simple way to take the shunt resistor out of circuit) In the sim below you can see the autoformer response wiht and wihtout the shunt resistor compared to the L-pad.
The LT spice file is at the link below. Simply download LTspice and run it to get playing.
dave
I went back through these graphs.
It is very likely the mid voice coil was already going bad.
"Ad Astra Per Alia Porci", - To the stars on the wings of a pig: John Steinbeck
Thank you again for all of the great help, Dave.
I can't thank you enough!
Here is another possibly very important thing. Could you answer this?
One of my EV DH1/D1012A mid diaphragms went bad and I haven't played the system very loud. Luckily, I still had a really old replacement to use.
Ugh.
I have had trouble from this one rubbing before so maybe it was ready to go bad.
BTW I am using the autotransformers in a DIY high efficiency system, not as mods to Klipsch or others.
When I hooked up the autotransformer without changing my high pass crossover values or using a shunt resistor, was the crossover point shifted down due to the resulting impedance change?
Maybe to a point unsafe for the voice coil?
Thanks again.
Tom Purdon
"Ad Astra Per Alia Porci", - To the stars on the wings of a pig: John Steinbeck
There is a noticeable difference after adding R2 = 13 ohms.
If this is normalizing the impedance seen by the filter, then isn't the overall system impedance seen by the amp improved?
Adding the appropriate valued shunt resistor for the amount of cut in db seems like a valid idea.
Doing so seems to have smoothed out a hump just above the 800Hz crossover point that I have to admit not noticing very much until it was gone.
"Ad Astra Per Alia Porci", - To the stars on the wings of a pig: John Steinbeck
The load presented to the amp in both situations is the same however the source impedance driving the speaker is different and lower in the case of the autoformer. It always bothered me how people rant about needing a low source impedance to drive a speaker and then insert an L-pad to match level. In your case it isn't too bad since you only needed 4dB of attenuation and the results are below.
However if we go to the case where the pad is taking a driver down 12dB it becomes a bit more clear what the autoformer is doing.
Note that the source impedance of the amp in this case is 1/10th of an ohm yet the speaker is being driven by 2-3 ohms in the L-pad case and a fraction of that in the autoformer case. I'm not suggesting that it is always a good thing to drive a speaker with a low source Z but it is something to be aware of. Just for fun lets assume we are driving this crossover from one of those antiquated tube amps with a 2 ohm output Z and see what happens.
not much change... lets look at both values on the same plot and it becomes clear that the output impedance of an amp has little effect on the damping factor of a driver.
I find it kind of comforting that even with a high output Z of a tube amp that an autoformer can still feed the driver from a fraction of an ohm.
dave
My mind is totally blown.
That is amazing, Dave!
Thank you, again for all the help.
"Ad Astra Per Alia Porci", - To the stars on the wings of a pig: John Steinbeck
Thank you, Dave!!!!!
I am heating up my soldering iron and getting out my bin of resistors.
13 ohm pad. Is that right?
Thanks for the link too. I will experiment with it and see if I can figure it out.
Thanks a million, Dave!
All the best!
"Ad Astra Per Alia Porci", - To the stars on the wings of a pig: John Steinbeck
There are numerous how to videos on youtube on how to use ltspice I
know because I needed help myself.
Cal
Thank you, Dave.
4db of attenuation.
Does adding R2 negate the need to change high pass crossover values?
"Ad Astra Per Alia Porci", - To the stars on the wings of a pig: John Steinbeck
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