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RC filter component selection
RC filter component selection
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Old 27th October 2017, 08:40 PM   #21
rayma is offline rayma  United States
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Quote:
Originally Posted by robinlawrie View Post
i just need to specify an rc filter that works with the minidsp and the amp with the lowest attenuation possible.
The output impedance is 560R, so 2.5k for the series R
gives 1.53Vrms (-2.3dB) output with a load of 9.9k.

If we depend on the 560R output impedance only, with no extra series resistor,
then 2Vrms input gives 1.89Vrms output (-0.49dB).
The low pass capacitor would then go in parallel with the amplifier input.

Last edited by rayma; 27th October 2017 at 08:47 PM.
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Old 28th October 2017, 07:47 AM   #22
robinlawrie is offline robinlawrie  United Kingdom
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I like the sound of using the output impedance, less attenuation to deal with. Are there drawbacks to this approach?

One other question: surely the output impedance is already considered? I. E. The output will be 2v *after* the 560R output impedance, so attenuation would be zero?

Thanks for the great help.
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Old 28th October 2017, 09:01 AM   #23
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by robinlawrie View Post
.................... surely the output impedance is already considered? I. E. The output will be 2v *after* the 560R output impedance, so attenuation would be zero?

Thanks for the great help.
the output voltage or emf is 2V, the source impedance is 560ohms.

If you feed that output to an infinitely high load impedance the load sees the full 2V of output.
If you feed the output to a 560ohms load, then the emf gets split exactly into two equal parts.
1V is dropped through the internal source impedance and the other 1V is dropped across the load.
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Old 28th October 2017, 09:37 AM   #24
robinlawrie is offline robinlawrie  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
the output voltage or emf is 2V, the source impedance is 560ohms.

If you feed that output to an infinitely high load impedance the load sees the full 2V of output.
If you feed the output to a 560ohms load, then the emf gets split exactly into two equal parts.
1V is dropped through the internal source impedance and the other 1V is dropped across the load.
right. I think i actually understand! Thanks for the basic lesson

So this means, given the 9.9k input impedance of the amp and the 560R output impedance of the dsp, and the 2v rms output, the voltage i will see across the amp input will only ever be 1.89v. Rms.i think I can live with that.
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Old 28th October 2017, 11:39 AM   #25
robinlawrie is offline robinlawrie  United Kingdom
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So. Am i right that a 90 nf capacitor across the amp input will provide a 3.16khz lowpass? Based on the 560 ohm output impedance of the minidsp it seems right, assuming its that simple.
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Old 28th October 2017, 12:16 PM   #26
AndrewT is offline AndrewT  Scotland
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Yes, the 1.89Vac from a 2Vac source. That's a loss due to the 560r of 0.48dB

Yes, 3.16kHz for 560r & 90nF
Instead use 100nF with 510r (3.12kHz) or 470r (3.39kHz) or paralleled 1000r (3.18kHz)
or 510r||39k (3.16kHz)

or keep the 560r with 100nF and add an adjusting resistor to give the F-3dB frequency you require.
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Old 28th October 2017, 02:06 PM   #27
rayma is offline rayma  United States
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Quote:
Originally Posted by robinlawrie View Post
I like the sound of using the output impedance, less attenuation to deal with.
The output will be 2v *after* the 560R output impedance, so attenuation would be zero?
The output impedance is probably just a series 560R resistor at the output.
Using it only will probably work.

The rated 2V output is usually the either open circuit voltage, or the voltage
into a 10k load, so it should be ok.
.

Last edited by rayma; 28th October 2017 at 02:10 PM.
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Old 28th October 2017, 02:17 PM   #28
robinlawrie is offline robinlawrie  United Kingdom
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Quote:
Originally Posted by rayma View Post
The output impedance is probably just a series 560R resistor at the output.
Using it only will probably work.

The rated 2V output is usually the either open circuit voltage, or the voltage
into a 10k load, so it should be ok.
.
Well the components around the output of the minidsp are all tiny surface mount. Black is usually resistor right?
Seems to be one in series directly before output.


No actual markings or identification to get a brand i can see. I guess i will finish my build, add a 90 nf or thereabouts capacitor across amp input, and measure response. Ive got a minidsp and measurement microphone after all
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Old 28th October 2017, 03:01 PM   #29
robinlawrie is offline robinlawrie  United Kingdom
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Here is a closeup of the output section of the minidsp.
Attached Images
File Type: jpg 2017-10-28 16.59.18.jpg (385.2 KB, 56 views)
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Old 28th October 2017, 03:25 PM   #30
00940 is offline 00940  Belgium
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The resistors are the ones below, with white markings. Looks like there is a LCLC filter before the output. Plain black is usually inductor or ferrite bead.

But before that, you have two 270R resistors in serie with the output, with a muting transistor in between. That's your 540R output impedance, plus some resistance in the inductors or ferrite afterwards.
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