I am new to amplifier circuit design so please excuse my ignorance.
I noticed that some amplifier circuit have an output inductor parallel with a 8 to 10 ohm resistor. Could someone educate me as to the function of the inductor? What is the benefit of this part? For a high current amplifier driving a low impedence load, this part would seem to contribute considerable amout of DC resistance or cause the inductor to go into saturation. I may be mistaken about this saturation stuff since it is an air core inductor.
I noticed that some amplifier circuit have an output inductor parallel with a 8 to 10 ohm resistor. Could someone educate me as to the function of the inductor? What is the benefit of this part? For a high current amplifier driving a low impedence load, this part would seem to contribute considerable amout of DC resistance or cause the inductor to go into saturation. I may be mistaken about this saturation stuff since it is an air core inductor.
The inductor is there to isolate the internal feedback loop of the amplifier from external RF signals picked up by speaker wiring, crossover coils, driver voice coils, etc.. and from the own RF resonances of these elements.
Above audio frequencies, the internal RC dummy load should always dominate against external loads.
Above audio frequencies, the internal RC dummy load should always dominate against external loads.
Thanks to all who replied. Okay, I am beginning to understand better now. The inductor is to prevent amplifier oscillation when driving "difficult" loads that have high capacitance.
I would conclude that amplifiers with small or no feedback would be less susceptable to such speaker load and could do away with it.
Anyone know what a recommended value of inductance it should be?
I would conclude that amplifiers with small or no feedback would be less susceptable to such speaker load and could do away with it.
Anyone know what a recommended value of inductance it should be?
caesar148 said:
In the quoted graph bellow, appears the ringing caused from a resistive 8Ù dummy load paralleled with a 2ìF capacitor connected and damped via a 8ìH inductor in parallel with a 4,7Ù resistor in the output of amplifier. The test signal is a square wave.
An externally hosted image should be here but it was not working when we last tested it.
Eva said:
RF picked up by the speaker wiring ?
How on earth is a 4 ohm impedance going to pick up RF ?
Yes have an RF filter on the high impedance input but never on the output.
You can get oscilation on the output due to capacitance and inductors ringing hence adding an inductor to decouple it from the feedback loop.
Actually RF very frequently couples effectively into the low impedance speaker wiring, your amplifier output may not look like a low impedance (and your speakers don't either) at a couple of MHz and above where even a few inches of wire may have an effect. Often rf picked up on the output wiring will couple through the feedback network particularly if there is a cap across the feedback resistor right back into the input stage which in most cases will rectify it, resulting in unwanted interference with the audio signal at the output.
Jfet based inputs generally don't exhibit this problem to the same degree as bipolar inputs. (no intrinsic diode junctions and higher slew rates, etc.)
This was an all too common problem at the large mid-fi manufacturer I spent most of the last decade at. Extensive testing was done in RF shielded chambers to mitigate this problem.
The inductor is there primarily to isolate capacitance from the output and feedback loop, but it may stop some rf from getting back to the input stage of the amplifier.
Jfet based inputs generally don't exhibit this problem to the same degree as bipolar inputs. (no intrinsic diode junctions and higher slew rates, etc.)
This was an all too common problem at the large mid-fi manufacturer I spent most of the last decade at. Extensive testing was done in RF shielded chambers to mitigate this problem.
The inductor is there primarily to isolate capacitance from the output and feedback loop, but it may stop some rf from getting back to the input stage of the amplifier.
I agree.
For a typical amp, 100Wrms, I just wrap some 22WGA magnet wire around a 10 Ohm 1/2W resistor and solder it at the ends. Use a 1/2W metal film resistor in series with the 100nF cap for the Zobel before the inductor. Don't think AC current can't flow through the 100nF cap because its value is small. I used a cheap 1/4W 11 Ohm carbon resistor in series with 100nF at the output of a test amp I built. I was fiddling with it once while it was on and it broke out into full RF oscillation and the 1/4W resistor suddenly burst into flames and burned up.
. Once the funky smoke cleared, I replaced it with a more suitable component.
For a typical amp, 100Wrms, I just wrap some 22WGA magnet wire around a 10 Ohm 1/2W resistor and solder it at the ends. Use a 1/2W metal film resistor in series with the 100nF cap for the Zobel before the inductor. Don't think AC current can't flow through the 100nF cap because its value is small. I used a cheap 1/4W 11 Ohm carbon resistor in series with 100nF at the output of a test amp I built. I was fiddling with it once while it was on and it broke out into full RF oscillation and the 1/4W resistor suddenly burst into flames and burned up.
. Once the funky smoke cleared, I replaced it with a more suitable component.
CBS240 said:I agree.
<snip>
Don't think AC current can't flow through the 100nF cap because its value is small. I used a cheap 1/4W 11 Ohm carbon resistor in series with 100nF at the output of a test amp I built. I was fiddling with it once while it was on and it broke out into full RF oscillation and the 1/4W resistor suddenly burst into flames and burned up.
Me too! (and more than once) Quite exciting...
Don't use fusible metal oxide resistors either... You could loose your zobel network at an inopportune time..
nigelwright7557 said:
Load impedance is no longer 4 ohms at RF, and it's not purely inductive or capacitive either at these frequencies.
Parasitic capacitances and inductances due to wirings and crossover coils, capacitors and resistors produce a complex load impedance that may exhibit a few high-impedance and low-impedance resonant modes at various RF frequencies. This may easily force an amplifier to oscillate at these modes if some load isolation at RF is not provided.
RF behaviour of speaker wires, crossover coils and capacitors and speaker voice coils is obviously behind the understanding of the average forum member, but it can't be just simplified to "4 ohm" or to a plain "load capacitance".
Have you ever tried to measure amplifier output impedance at, say, 4Mhz? The R component from the RC network will dominate, but if you remove it, the output stage itself will behave much like an inductor (>50 ohms) with output transistor parasitic capacitances in parallel (thus becoming a resonator/oscillator with a high impedance peak at some frequency!). Fortunately the RC network masks this behaviour.
BTW: Note that parasitic capacitances increase when the output gets closer to the rails, where oscillation is more likely to happen.
BTW: Note that parasitic capacitances increase when the output gets closer to the rails, where oscillation is more likely to happen.
without changing the subject much.
could the output inductor be placed in series with the 8ohms or 10ohms resistor that is usually placed in parallel with that coil, an still keep its functionality as an RF filter?.
reason is, im looking for a cheap way to protect the output from speaker shorting out or simply human error of briefly shorting out the output.
that way that 8ohm or 10ohm resistor would serve as a load to the amp shorted output, and keep the output transistors or fets/mosfets from burning up, at least till the amp is turned off, and fixed. its to be used on an amp that doesn't have any form of output protection. sure, the resistor its going to have to be rated higher then 1/2w or what ever is usually rated at, to keep it from going up in smoke.
just not sure if it would change the functionality of the coil, if its set in series rather then the normal parallel to the resistor.
any other advise would be greatly appreciated.
thanks in advance.
laters
could the output inductor be placed in series with the 8ohms or 10ohms resistor that is usually placed in parallel with that coil, an still keep its functionality as an RF filter?.
reason is, im looking for a cheap way to protect the output from speaker shorting out or simply human error of briefly shorting out the output.
that way that 8ohm or 10ohm resistor would serve as a load to the amp shorted output, and keep the output transistors or fets/mosfets from burning up, at least till the amp is turned off, and fixed. its to be used on an amp that doesn't have any form of output protection. sure, the resistor its going to have to be rated higher then 1/2w or what ever is usually rated at, to keep it from going up in smoke.
just not sure if it would change the functionality of the coil, if its set in series rather then the normal parallel to the resistor.
any other advise would be greatly appreciated.
thanks in advance.
laters
How much effect does the output inductor have on the sound quality? Has anyone noticed a difference in sonic quality when the inductor is present or absent?
I would prefer not to have the output inductor and resistor if possible. Adding a reactive load to the output will cause a non-linear phase shift to the sound.
To do this, I believe the following needs to be applied:
1. Bandwidth limit the amplifier to within the audio frequency. High bandwidth amplifier that go to 100KHz and more serves no purpose except to become an RF amplifier.
2. Limit the amount of feedback.
I would prefer not to have the output inductor and resistor if possible. Adding a reactive load to the output will cause a non-linear phase shift to the sound.
To do this, I believe the following needs to be applied:
1. Bandwidth limit the amplifier to within the audio frequency. High bandwidth amplifier that go to 100KHz and more serves no purpose except to become an RF amplifier.
2. Limit the amount of feedback.
Re: without changing the subject much.
It is a very bad idea for other reasons. At audio frequencies you can ignore the inductor, so in this case you get an 8 Ohm resistor in series with an 8 Ohm speaker. This means two things.
1) The amp output stage suddenly sees a 16 Ohm load instead of an 8 Ohm load, so you get only half the maximum power out compared to the normal approach.
2) The output voltage is split between the resistor and the speaker, so they each get half of the output power.
Say your amp outputs 100 W in 8 Ohms. If you you now put the resistor in series with the coil instead of parallel, you only get half och half the power, that is 25 W, in the speaker. Furthermore, the resistor must now be rated for 25 W.
boricuaso said:
It is a very bad idea for other reasons. At audio frequencies you can ignore the inductor, so in this case you get an 8 Ohm resistor in series with an 8 Ohm speaker. This means two things.
1) The amp output stage suddenly sees a 16 Ohm load instead of an 8 Ohm load, so you get only half the maximum power out compared to the normal approach.
2) The output voltage is split between the resistor and the speaker, so they each get half of the output power.
Say your amp outputs 100 W in 8 Ohms. If you you now put the resistor in series with the coil instead of parallel, you only get half och half the power, that is 25 W, in the speaker. Furthermore, the resistor must now be rated for 25 W.
caesar148 said:
nigelwright7557 said:
The resistances and inductances of speaker voice coils and crossover components are always in the signal path.
Even the voice coil of a tweeter is 10 to 100 times more inductive than our little inductor, and a woofer may be 200 to 2000 times more inductive (ignoring RF resonances). Furthermore, voice coil DC resistance is huge in comparison with the output impedance of the usual solid state amplifiers, and even in comparison with the resistance of any other element in the circuit (including the RF coil). This almost always limits effective damping factor to 2.
To make it more ironic, the resistances of speaker voice coils are modulated by signal level and they may easily increase by 15% during loud passages due to self heating.
Now you can continue blaming the little RF-filter coil for nothing. I wish everything was as harmless as this coil.
EDIT: Oh, I forgot to mention that voice coil inductance may easily change by another +/-15% with cone or dome excursion. Nowadays sound quality is something about speakers most of the time...
caesar148 said:
I suggest you to include the output inductor. In this link you can find ready calculators for any type of inductor and much more:
http://www.pronine.ca/links.htm
Output coil can effect the sound output WITH a loudspeaker. I have had it demonstrated to me in my own lab, by others. IF the inductance of the output coil is MORE than 2uH, you are in trouble. IF 1uH, maybe OK, and .5uH is probably essentially inaudible. This has been shown by experience of a number of audio designers. Arguing from mere measurement or first principles avails you little in this case.