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Robbie010

Type A Crossover - Capacitors in series?

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4 hours ago, JohnA said:

Type A Circuit2.jpg

 

The resistor is wrong at the autoformer.  Rather than in series, it should be across some winding (don't know which at the moment).

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Not really.  The resistor and inductor are simulations for the primary winding's wire resistance and inductance.  I suppose it could also be simulated by a dozen small resistors and inductors in series.  I look at that as an unnecessary complication to the demonstration that the autoformer presents a node between the 2 caps. 

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Unless I'm doing something wrong, this comes very close to what I'm seeing if you assume the K-55 is a 16 ohm load.

 

13uf cap impedance + ((double the K-55 impedance // (2uf cap impedance + K-77 impedance))

 

 

 

 

Type A.jpg

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On 1/24/2020 at 12:12 AM, MechEngVic said:

Ha! I didn't notice it until you drew it out. The autoformer, while not offering attenuation in series with the two caps, acts like an inductive roll-off, and a third component in the tweeter circuit. That makes it a third order, 18db slope. 

 

I think you may be correct, but it is operating so far below the 9k f3 of the tweeter cap its hard to say how much effect it has.  Three pole, yes, 18 dB, hmmm, douno.  Some of the later networks label the autoformer as 2.7 mH. 

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Okay, but pertinent to what I've been saying, it would be interesting to see comparison between that black trace, a version of same without T2A connected between the caps, and a version of same with the supply side of C2 connected instead to the other end of C1.

 

The difference should not be great in any event (2uF vs 1.7uF), but whichever new (dis-/alt.) connection more nearly follows what you have already shown would indicate whether or not the two caps act (more) as "series caps" or not.

 

Thanks.

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4 hours ago, John Warren said:

Here's a sim that breaks the autoformer into it's parts per JohnA approach:

 

http://www.northreadingeng.com/Audio/group/viewtopic.php?f=12&t=10&p=38#p38

Looks like that autoformer simulation creates an electrical band pass across the 13 ohm resistor.  Was that your intent?

 

An accurate simulation would result in the voltage across the K-55 rising to 0db along with the K-77.

 

I did find this, but I'm not smart enough to determine if it works.

 

https://www.quora.com/What-is-the-equivalent-circuit-for-auto-transformers-impedances

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You all make me think!  It's been quite fun! 

 

There has always been a question in my mind whether the T2A is a tapped autotransformer (like a tube amp output) or tapped transformer (like a substation transformer).  That would affect the circuit configuration in a model.  Being from the power industry, I drew it as a transformer, but looking at John's model, that may not be correct. 

 

Good to see you pop in, John! 

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2 hours ago, mboxler said:

Looks like that autoformer simulation creates an electrical band pass across the 13 ohm resistor.  Was that your intent?

 

An accurate simulation would result in the voltage across the K-55 rising to 0db along with the K-77.

 

I did find this, but I'm not smart enough to determine if it works.

 

https://www.quora.com/What-is-the-equivalent-circuit-for-auto-transformers-impedances

I think that's right. The 13ohm resistor is the midrange driver so it would have both a high pass and low pass. 

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On 1/21/2020 at 10:55 AM, JohnA said:

The schematic is correct.  It is an older design technique, but it works.  My understanding is that it increases the crossover attenuation slope at a point below the tweeter's crossover point. 

This type of crossover is called "improved three-way" by Don Davis. In addition to increasing the order of the tweeter network it is "a true constant resistance network".  Also, "The summed frequency response is ideal in that the sum of the transfer functions is identically equal to one. The summed power response is also identically equal to one". 

 

Quotes are from Sound System Engineering by Davis and Petronis, third edition, page 299.

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31 minutes ago, JohnA said:

You all make me think!  It's been quite fun! 

 

There has always been a question in my mind whether the T2A is a tapped autotransformer (like a tube amp output) or tapped transformer (like a substation transformer).  That would affect the circuit configuration in a model.  Being from the power industry, I drew it as a transformer, but looking at John's model, that may not be correct. 

 

Good to see you pop in, John! 

Can you explain the differences between those two transformers to us M.E.'s?

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2 hours ago, mboxler said:

An accurate simulation would result in the voltage across the K-55 rising to 0db along with the K-77.

 

FFT at the same location as Tap_4_-3dB.

 

Photo of test setup:   http://www.northreadingeng.com/Audio/group/viewtopic.php?f=12&t=10

 

If you want to  continue the discussion please log into my site otherwise cheers!  

T2A_-3dB.jpg

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On 1/23/2020 at 2:46 PM, Deang said:
On 1/23/2020 at 12:37 AM, JohnA said:

 

😀 Not with a node between the 2 caps.  The autoformer makes a node that separates the 2 caps into 2 *filters* in series. 

 

 

That's what I think. Didn't Al test this out here years ago? 

 

It seems that about every year or so someone comes up with this question. The circuitry is 2 filters in series,  not a simple series capacitive circuit.

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I’m with Don. Well, not like that, if you know what I mean. 

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2 hours ago, MechEngVic said:

Can you explain the differences between those two transformers to us M.E.'s?

 

I don't get it either.  I made a fair amount of money working on/with, and testing, many kinds of transformers from feeder substations to plant distribution.  There are many winding configurations, but an autoformer is still just a transformer.

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Yup. Just a long piece of wire around a core with taps. I have no idea what the excitement is all about. Well, the newer ones we use are bifilar wound, which is nice I suppose. 

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Autoformer.  Here auto means single or alone, from Greek.

 

600px-Tapped_autotransformer.svg.png

 

Transformer

880px-Ideal_transformer.svg.png

 

 

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From Wikipedia: 

 

In an autotransformer, portions of the same winding act as both the primary winding and secondary winding sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which are not connected to each other.

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An autoformer is a voltage divider for ac with very little dcr.

 

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6 hours ago, JohnA said:

Autoformer.  Here auto means single or alone, from Greek.

 

600px-Tapped_autotransformer.svg.png

 

Transformer

880px-Ideal_transformer.svg.png

 

 

 

Absolutely.  In basic terms the only difference between "auto" (single, or "alone") winding transformers and two-winding transformers is the latter provides electrical isolation between the primary and secondary circuits.  You can divide or multiply voltage with either.

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