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Why shouldn't I use inductors to build passive tone control circuit?

I designed an RLC circuit to boost about 6 dB at around 80 Hz at audio line level. I checked it with SPICE and it did exactly what I wanted. Ordered the parts, built it and it seemed not to do much of anything. Then I started reading that RLC circuits are not commonly used for this kind of application because of the size and cost of the inductors that one would need. This set me thinking that I must be completely misunderstanding something very fundamental about inductors because the ones I used are about the size of M+Ms and cost about 75 cents each, so I guess the type I have are completely wrong. They are 40 milli-Henry, capable of handling several volts.

Can someone enlighten me please?

Francis Elers

snipped-for-privacy@earthlink.net

Reply to
Francis Elers
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Can you post a schematic of your circuit? What source and load impedances did you design for / test with?

Reply to
Andrew Holme

Part numbers?

Are you sure they aren't 40 micro Henry? I have trouble believing that you're getting that much inductance in that small of a package, and if you are I have trouble believing that your DC resistance isn't through the roof.

At 80Hz a 40 mH inductor would have an inductive reactance of 20 ohms, which means that you would have to have a DC resistance significantly below 20 ohms for the thing to be at all useful. Such an inductor would be pretty darn big -- I'd be surprised at anything less than a few cubic inches, and if you showed me something that weighed 20 pounds I wouldn't be surprised.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

Thanks for your help. I admit I didn't really concern myself with that aspect at this stage, When I tested it with LTSpice it seemed to do exactly what I wanted so I just went ahead, intending to use a solve impedance problems later. I don't know what load LTSpice assumes for it when I run the simulation. In the event it was placed in the pre-amp out/power-amp in path of a standard Hi-Fi amplifier.

As you will have realised by now I am a complete novice. Please accept my apologies if this stuff is annoyingly basic. Thanks again

Francis Elers.

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Reply to
Francis Elers

I actually had to use a 22 and an 18 in series. I just checked the Futurelec invoice and it says 18000 uH and 22000 uH (but the Greek Mu symbol) so I think they believe they sent me the right ones since 18000 micro is 18 milli, isn't it? But as I say they are suspiciously small - no more than 1/2 inch largest dimension.

Many thanks for your help,

Francis Elers

Reply to
Francis Elers

What's the DC resistance?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

Hello Sir,

105 Ohms. Hopelessly high, I guess?

Francis.

Reply to
Francis Elers

You might add that bit of info to your simulation and see if it then agrees better with reality.

--
Regards,

John Popelish
Reply to
John Popelish

"Francis Elers"

** Yep.

For that job you really must use a " gyrator " - can be just one op-amp, two Rs and one C.

For 80 Hz, I suggest you go for a lot more than 40mH too, more like about

10H to give a reasonable Q factor of say 5 to 10.

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..... Phil

Reply to
Phil Allison

Well, inductors are prone to picking up hum etc.

Also they have poor tolerance and may be quite non-linear. Use active filters.

Graham

Reply to
Eeyore

Thank you all very much for your help. I investigated gyrators and SPICE modelled a circuit with one and it seems perfect so I'm going to read up on that solution and forget about inductors for this project.

Much appreciated,

Francis Elers.

Reply to
Francis Elers

I saw the (binary) schematic you posted. Note that most servers don't allow binary attachments in s.e.d. Use

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You had 30 ohms + 40 ohms in series across the source. Few audio line outputs will drive such a low impedance.

Reply to
Andrew Holme

Thank you. I am very unclear generally about source/load impedance and how to modify (buffer?) it if it is not naturally what you need it to be. I will read up on this as I am beginning to realise that it is essential. Thanks also for the tip about posting binaries

Francis Elers.

Reply to
Francis Elers

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