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Multi-Way Conventional loudspeakers with crossovers

Placing autotransformer in crossover
Placing autotransformer in crossover
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Old 5th December 2018, 06:54 PM   #1
dave123 is offline dave123
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Default Placing autotransformer in crossover

Hey guys,
actually I'm building a two way speaker and I wanted to attenuate the tweeter using an old school autotransformer. first of all I located it after tweeter HPF but when I measured all taps frequency response I felt something is wrong. Autotransformer should not change the frequency response like this so I removed it and connected it before the HPF and the frequency response seems correct.
I want someone tip me off where is the right place to connect autotransformer in a XO network and if someone can explain the reason of this frequency response it would be great. (I suppose autotransformer after a LR2 filter would we a second inductor in parallel with the first inductor)

Placing autotransformer in crossover-01-png
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Placing autotransformer in crossover-02-png
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File Type: png 02.png (97.5 KB, 314 views)

Last edited by dave123; 5th December 2018 at 07:09 PM.
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Old 5th December 2018, 07:06 PM   #2
GM is offline GM  United States
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FWIW here's an ancient Altec two way autoformer schematic: http://digilander.libero.it/stevevis...N800schema.jpg

GM
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Old 5th December 2018, 07:18 PM   #3
olsond3 is offline olsond3  United States
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I am no expert on transformers, but the crossover network (cap and coil) is designed around the assumption you have a very low input impedance and a fixed known output impedance. So if different setting on your autotransformer change the impedance seen by your crossover, it's response will change. That is why you see L-Pads used in crossovers, as an L-Pad is designed to provide a fixed impedance regardless of attenuation setting.
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Old 5th December 2018, 09:08 PM   #4
Galu is offline Galu  Scotland
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This may be of help:

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Old 6th December 2018, 03:26 AM   #5
abraxalito is online now abraxalito  United Kingdom
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What @olsond3 said. With the autotrafo after the 2nd order filter you'll have varying damping as you change the taps. The resistive component of the tweeter's impedance is providing the damping and this is changing (as presented to the filter) with differing attenuations. Reducing the tweeter output is going to result in too little damping.

The best place to have your autotrafo is before the filter - its being then driven by a low impedance (your amp) and trafos give less distortion in such a circumstance. Its output impedance is going to be lower than the input (ignoring losses) hence more than good enough to drive the LC filter.
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Old 6th December 2018, 04:24 AM   #6
JMFahey is offline JMFahey  Argentina
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Might write pages on it but the short answer is: in principle avoid autotransformers in crossover networks, you are opening a huge can of worms if you try to.

Since you want to *attenuate* , just go the tried and true resistive way.
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Old 6th December 2018, 09:44 AM   #7
marco_gea is offline marco_gea  Italy
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Default Oh, dear!

So little really useful information in this thread so far!

Contrary to what has been said:

- autoformers are possibly the best way to attenuate a tweeter;

- they should be positioned AFTER the high-pass section, to protect them from high-energy low frequencies, which can saturate their core and lead to distortion.

But, also:

- attenuating a tweeter using an autoformer results in a much higher load impedance to the high-pass filter, and one that changes for each different attenuation step;

- THEREFORE, autoformers must be used with a shunt resistor in parallel to their input, and in principle the resistor value must be different for each attenuation step, so that the resulting total load impedance remains the same as the one assumed for the calculation of the high-pass filter.

Having said this, for large attenuation values (i.e., when the tweeter is attenuated more than -10dB, such as is the case when compression drivers are used with direct-radiating woofers), the (autoformer+tweeter) impedance is so high, that in practice a fixed shunt resistor of 8.2 Ohms results in an overall ~constant 8 Ohm load all the way down from -10dB to -20dB or more.

That's all there is to it, really - I hope this clears things up for the OP :-)

Marco

[EDIT] for the OP: I have attached a revised schematic highlighting the resistor that you were missing!
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File Type: png 01.png (30.9 KB, 213 views)

Last edited by marco_gea; 6th December 2018 at 09:48 AM.
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Old 6th December 2018, 01:16 PM   #8
Pano is offline Pano  United States
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Placing autotransformer in crossover
I've seen and heard the above scheme used to good effect. But I have to wonder - why not just use two resistors?
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Old 6th December 2018, 04:00 PM   #9
marco_gea is offline marco_gea  Italy
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Quote:
Originally Posted by Pano View Post
I've seen and heard the above scheme used to good effect. But I have to wonder - why not just use two resistors?
Sure, two resistors in an L-pad will do the trick quite nicely, and with minimal fuss.

However, when a lot of attenuation is required (e.g., -22.5dB in my own system), an autoformer-based solution just sounds a little bit better, IMHO: more dynamic, essentially, and possibly subjectively slightly more "refined", for whatever that may be worth.

In any case, I wasn't suggesting that one must use an autoformer, but only pointing out how to use it properly, and explaining why the OP was getting weird results.
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Old 6th December 2018, 04:51 PM   #10
olsond3 is offline olsond3  United States
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Default Why?

Quote:
Originally Posted by marco_gea View Post
Sure, two resistors in an L-pad will do the trick quite nicely, and with minimal fuss.

However, when a lot of attenuation is required (e.g., -22.5dB in my own system), an autoformer-based solution just sounds a little bit better, IMHO: more dynamic, essentially, and possibly subjectively slightly more "refined", for whatever that may be worth.

In any case, I wasn't suggesting that one must use an autoformer, but only pointing out how to use it properly, and explaining why the OP was getting weird results.
Help me understand this. What known electrical properties of the autoformer make it sound different and what would make those differences better? I assume it can only have more distortion than a resistor divider if undersized (saturation, hysteresis) and some frequency response variations. Resistors could have a tiny amount of inductance, some miniscule variation with heat if undersized, self noise below 100 ohms isn't going to matter.
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