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

F

Francis Elers

Guest
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

fecommerce@earthlink.net
 
"Francis Elers" <franciselers@earthlink.net> wrote in message
news:iISdncHzGLAuYMrVnZ2dnUVZ_qvinZ2d@earthlink.com...
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?
Click to expand...
Can you post a schematic of your circuit? What source and load impedances
did you design for / test with?
 
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
 
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
 
Hello Sir,

105 Ohms. Hopelessly high, I guess?

Francis.

> What's the DC resistance?
 
"Francis Elers"
105 Ohms. Hopelessly high, I guess?
Click to expand...
** 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.

http://en.wikipedia.org/wiki/Gyrator

http://sound.westhost.com/dwopa.htm#inductor


...... Phil
 
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.
 
"Francis Elers" <franciselers@earthlink.net> wrote in message
news:RcWdnZ9UfbTCisTVnZ2dnUVZ_ozinZ2d@earthlink.com...
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,
Click to expand...
I saw the (binary) schematic you posted. Note that most servers don't allow
binary attachments in s.e.d. Use http://www.tech-chat.de/download.html

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

fecommerce@earthlink.net


Part numbers?
Click to expand...

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
 
Francis Elers wrote:
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


What's the DC resistance?
Click to expand...

--

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
 
Francis Elers wrote:
What's the DC resistance?

105 Ohms. Hopelessly high, I guess?
Click to expand...
You might add that bit of info to your simulation and see if
it then agrees better with reality.

--
Regards,

John Popelish
 
Francis Elers wrote:

Hello Sir,

105 Ohms. Hopelessly high, I guess?
Click to expand...
Well, inductors are prone to picking up hum etc.

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

Graham
 
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