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Old 10th May 2010, 03:16 PM   #1
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Default making a zobel output RL, options

Dear,

One quick question.

To make a RL in series with an amplifier output. Is there any disadvantage to make 14 turns around a 10ohm 5 watt resistor? Or is it better to make a air inductor separate from the resistor?

If the last option, If I make a inductor by 14 turns of magnet-wire, does it matter how thick the cylinder is I turn the manget-wire around to form an inductor? (after the turns are done I remove the cylinder of of course)

With kind regards,
Bas
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Old 10th May 2010, 03:36 PM   #2
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Quote:
Is there any disadvantage to make 14 turns around a 10ohm 5 watt resistor?
The coil, as small as it may be, will induce some current on the conductors in that resistor. That current will be out of phase with the current through the resistance, meaning you're inducing distortion and noise in there.

Why would you wind the coil around the resistor?
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Old 10th May 2010, 05:02 PM   #3
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Originally Posted by jmaf View Post
The coil, as small as it may be, will induce some current on the conductors in that resistor. That current will be out of phase with the current through the resistance, meaning you're inducing distortion and noise in there.

Why would you wind the coil around the resistor?
Dear jmaf,

First because I saw it at the Leach site. Second it will save space on the PCB.

With kind regards,
Bas
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Old 10th May 2010, 05:35 PM   #4
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If you change the length,diameter,or number of turns the inductance changes.
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Old 10th May 2010, 05:55 PM   #5
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Originally Posted by woody View Post
If you change the length,diameter,or number of turns the inductance changes.
Thank you. I will seek for an online diameter/length ratio calculator online, since I forgot most of my math (Admitted being spoiled by software who calculates everything for you)

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Bas
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Old 11th May 2010, 02:46 AM   #6
jaycee is offline jaycee  United Kingdom
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I would say it is negligible. I have done both and not seen any difference in performance. The amount of inductance is not really that critical, it is just enough to isolate the capacitive effect of the cable, and the resistor acts as a stopper against RF ingress.
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Old 11th May 2010, 03:18 AM   #7
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Originally Posted by jaycee View Post
I would say it is negligible. I have done both and not seen any difference in performance. The amount of inductance is not really that critical, it is just enough to isolate the capacitive effect of the cable, and the resistor acts as a stopper against RF ingress.
Pretty interesting. I've seen similar tricks in guitar amps, like taking hookup wire soldered to +B on one of the power tubes and winding the other end of it around the input cable(no connection), to stabilize the amp, but never around a resistor.
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Old 11th May 2010, 04:21 AM   #8
jaycee is offline jaycee  United Kingdom
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well, winding it around the resistor isnt important, its just a good way to save space. The inductor is what usually passes the signal, but at higher frequency the resistor comes into play. It forms a parallel RL circuit.
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Old 11th May 2010, 09:53 AM   #9
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by jaycee View Post
I would say it is negligible. I have done both and not seen any difference in performance. The amount of inductance is not really that critical, it is just enough to isolate the capacitive effect of the cable, and the resistor acts as a stopper against RF ingress.
The resistor is there to reduce the Q of the coil - i.e. it acts as a damper and ensures that the total coil+resistor impedance can never be higher than the resistor value - it really does not help with RF ingress. If you are worried about RF getting into the feedback summing node via the feedback resistor, you really have to try some other tricks . . . but most people here would frown at that. How about a Murata RF ferrite for example?
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Old 11th May 2010, 10:39 AM   #10
AndrewT is offline AndrewT  Scotland
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air core is theoretically better.
Larger diameter core gives more inductance and more copper length.

I like the full Thiele Network of R+C & R//L
But I have combined that with the other version of R//L & R+C across the speaker terminals.
This results in a Pi filter of sorts.
47nF to 100nF + 4r to 10r fitted very close to the output devices to minimise inductance.
Then in the cable route from amp to speaker terminals air core L//R. 1uH to 2uH and 4r to 10r.
Finally across the speaker terminals 68nF to 150nF + 4r to 10r.

It seems to sound alright and seems to perform OK.
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