How to HF boost an compression driver behind an autoformer?

discussion of magnetic volume control design for both line and speaker levels.

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docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

How to HF boost an compression driver behind an autoformer?

Post by docali »

Hello Dave!

Maybe you have an idea to this issue. I have a compression driver that slighty falls off after 10 kHz in the used CD-horn. The current solution to overcome this in the cross-over is to use an series resistor which levels down the efficiency to adapt the compression driver to the woofers and then to bypass the series resistor with a small capacitor.

I have played around with some autoformers but did not found a comparable solution. Bypassing the autoformer with a capacitor does not work well. Do you have any ideas?

BR!
Bernd
ultramarin
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Joined: Fri May 15, 2020 1:57 pm

boosting HF

Post by ultramarin »

is it a possibility to use the shunt restistor in series with a small choke ?

Grüße
Bernd
ultramarin
Posts: 5
Joined: Fri May 15, 2020 1:57 pm

Other use of shunt resistor

Post by ultramarin »

moin, moin aus Hagenow,

in my speaker with fielcoil Altec 288 I use a TVC for ~12dB down, and no shunt resistor. For a 6dB filter 500Hz I use 3uF cap insted of a much bigger cap when TVC is shunted by correct resistor for 16Ohms. But I still want a constant Impedance over frequenz. Do you have a formula to calculate a RC comination that would give me a constant impedance over frequency ?
I like to use copper-foil caps, and 3 uF a much cheaper than ~30uF.

grüße
Bernd :D
dave slagle
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Post by dave slagle »

Hey Bernd (etf Bernd??)

I have a calculator at the link below.

http://www.intactaudio.com/Speakerformer.html

just set the "crossover impedance" to the desired crossover impedance and enjoy the benefits of a smaller cap. I do find that some shunt R is needed to "tame" the impedance curve that can get out of hand if the step down ratio gets to be to large.

If you play with the calculator you quickly see the tradeoffs at hand and the really nice thing is you can actually use the shunt resistor to tweak the crossover value.

dave
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ultramarin
Posts: 5
Joined: Fri May 15, 2020 1:57 pm

shunt R

Post by ultramarin »

Hallo Dave,
yes ETF Bernd. actually I´m using 15Ohm secondary with my 16Ohm driver, helps to get driver impedance a bit more linear. Do you think this is enough damping for the autoformer ?

Grüße
Bernd :wink:
ultramarin
Posts: 5
Joined: Fri May 15, 2020 1:57 pm

crossover

Post by ultramarin »

Moin,

to have it complete :
it s a 500 Hz 6dB crossover,
Altec 416a TT
WvL 284 with Ejmlc 300 Hz Horn
for TT 12mH and 15Ohms and 15uF RC
for HT 3uF Autoformer for -12dB and 15 Ohms parallelto 16Ohm driver.
Iwant to ad a correct RC in front of the 3uF.
when I calculate with basics I add 15 Ohms ~48 uf infront of the 3uF.
Do you agree ?

Grüße
Bernd :?:
dave slagle
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Post by dave slagle »

Hey Bernd,

When you say 15Ω TVC it is unclear how that number is determined.

the 12dB cut will make the 16Ω driver appear to the crossover as 256Ω. If you then have 15Ω in parallel with the autoformer (or transformer primary) then the crossover should be treated as if it were 256||16 or 14.2Ω

dave
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ultramarin
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Joined: Fri May 15, 2020 1:57 pm

resistor

Post by ultramarin »

The 15 Ohm resistor is parallel to the drivers voice-coil.
So the autoformer sees ~ 8 Ohms and the impedance peak of the driver are a bit linearer this way. so the amp sees 16*8 Ohm = 128 Ohms with crossover ~500Hz. I want to add a RC parallel to th crossover to bring the 128 Ohms down to 16 over 500 Hz. there for 15 Ohms and 48 uF ?
my brain is blocked in the moment, so sorry for beeing stupid.

Grüße
Bernd
dave slagle
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Post by dave slagle »

Hello Bernd.

A 12dB reduction of voltage is a 4:1 stepdown. the impedance ratio is the turns ratio squared or 16:1. so your 16Ω load will appear as 256Ω at the primary of the autoformer/transformer.

the "shunt" resistor must be placed across the primary and not on the driver directly.
Image

Placing a 15Ω resistor in parallel with the 16Ω driver will give you a 7.74Ω load to reflect back across the 16:1 and give you 123Ω. The high value isn't the problem, it is how much that value changes when you switch taps. Assuming the 7.74Ω load your crossover will drive the following impedance for the following dB cut.

10dB 78Ω
11dB 98Ω
12dB 123Ω
13dB 155Ω
14dB 195Ω

It is this variation that causes the crossover frequency to shift as you attenuate and the difference in crossover frequency from a 3dB change is attenuation is an octave.

Placing a 17.1Ω resistor across the primary to set the crossover impedance to 16Ω nets the following results.

10dB 15.4Ω
11dB 15.7Ω
12dB 15.9Ω
13dB 16.1Ω
14dB 16.4Ω

The variation of 1Ω over the 5dB range isn't going to have a noticeable effect on the crossover.

There is an easy to use calculator towards the bottom of this page.
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