Newbie question about through hole inductors in audio path - what parameters to focus on?

[MichaelNuzum]

Hey all- I am sort of embarrassed to ask because I feel this is basic stuff I should either know or be able to figure out. But I am having real trouble finding the information on my own. I have done some web searching, and I can find a good bit of info about iron core/ air core type inductors, but next to nothing about the small through hole type. The type that are like a medium sized resistor.

I need to find a replacement inductor for an Aphex 124a. (I can't find a schematic, sorry) I believe I am looking for a mil spec 2KuH(?), the color code on the failed component is silver-red-black-red-silver. It is component L206, but again sorry, can't find a schematic for that designation to mean anything.

So, my question- looking at data sheets, what are the specs I want to focus on for better audio performance? The inductor is in the signal path, so I need to find one best suited for audio application. What parameters should I be looking at and comparing to find the most suitable replacement for this job? Or is that not a thing, will any work as well as the other?

Again, sorry for what feels like such a basic question, I tried to self educate and have run into a brick wall, so was hoping I could find some illumination here. Thanks in advance for any insight anyone can lend.

[Enzo]

Frankly, I'd focus on finding a part with the right inductance, and of physical dimensions that fit your board. Just my opinion, but the core chemistry and other details will make very little difference.

[Helmholtz]

It all depends on the purpose/function of the part in the circuit.
Main parameters are inductance and max. current, but sometimes also HF damping/attenuation and DCR matter
2kµH would be 2mH.

[MichaelNuzum]

Thanks Enzo, this is the type of stuff I need to know to help fill in the gaps in my knowledge. When researching inductors in audio path circuits, a great majority of the info you can find pertains to audio crossovers and the large format iron and air core type inductors. Not a lot of info about the effects of small format inductor type in other positions.

I was concerned because I know lots of folks have pretty strong opinions about which types of capacitors to use in certain audio applications. I was wondering if similar considerations would be relevant here. From your comment I see the answer is not really.

[MichaelNuzum]

Thanks Helmholtz. I admit, I can't describe the function of the component in the circuit. That's definitely way above my pay grade. All I know I know is the audio signal passes through it. Why, I couldn't say.

I have the inductance value (thank you for that conversion, also). I would imagine max current is pretty low, as it is inline from the rca jacks before any buffering or amplification. So it will only ever see program signal from a -10dBV audio output.

[Helmholtz]

. All I know I know is the audio signal passes through it.

. I would imagine max current is pretty low, as it is inline from the rca jacks before any buffering or amplification. So it will only ever see program signal from a -10dBV audio output.

Agree. If it's in series with the signal, it would be part of a low pass filter.

[g1]

A little surprised that no one like mouser or digikey stocks that value in a resistor style package like that. Make sure you are 100% on the value being correct.

[Enzo]

Signal path describes a large part of a circuit. If it is in some sort of tonal circuit, then the value matters. But it could simply be an RF arrester in the input line. In that case, it is less critical. That is what yours appears to be. In many amps it is a ferrite bead rather than an actual inductor.

[MichaelNuzum]

G1- I am 0% sure to be honest. The component I pulled sure looks like silver-red-black-red-silver. But I can totally see one of these bands being a very red shade of brown. That would be 1000uH/1mH, a much more common value, yes?

[MichaelNuzum]

Enzo- I get what you're saying. The component is one of the first four or five components the signal sees after the input.

This unit is just a line matcher for old -10dBV gear to properly interface with modern +4dBU gear. No tone controls, just a pretty simple back and forth level and impedance matching circuit.

Again, sorry I don't have a schematic to share for reference. I'd love to learn the actual "why' as to the use of this part. But you and Helmholtz have both put forth an explanation of what it probably is for. I'll accept that.

[Enzo]

Is the part broken? Or are we "upgrading"? You say it is in series with an input from an RCA jack? Try a plain old 100 ohm resistor, just to see if the rest of it all works.

The thing about "common values" is that all the other values are used all the time. Designers use "standard values" which is not the same thing. Resistors for example have standard values like 1k, 1.5k, 2.2k. 3.3k,4.7k, 5.6k, and so on. There are some circuits where value is critical, so it might need a 2.45k, in which case you special order that value. But 99% of the time 2.2k would be close enough. On the other hand, if we wanted a 2.45k resistor, we wouldn't use a 1k instead just because it is more common.

[MichaelNuzum]

Part is broken. Knowing anything about upgrading parts like this is way beyond my skillset. It's just a type of interface I use in my hybrid vintage/analog recording studio. I bought this one knowing one channel was out, but not how or why it died.

But, that's the cool thing about this unit, it is two channel, and channel one is working. So I was able to trace the signal up to and through the first channel inductor to the following op amp. In channel two, I can trace signal up to the inductor, no signal at the other side.

I don't have an LCR meter on hand to verify the status (or value) of the components. Just purchased one because of this job, but it hasn't arrived. Then I can pull channel one's part and measure it's value to confirm or deny.

[Enzo]

Do you have a plain old meter? A DMM? your inductor is either open or it is not. SInce it is broken, I will assume open. Your ohm meter will tell you that. The inductance value won't be "wrong".

No resistor? Even a piece of wire will work enough to test the rest of the unit's function. At the very worst, you might hear some local radio station leaking into the sound. That is the sort of thing the inductors prevent. Not only that, any digital stuff inside the unit has potential to make digital noise, and such an inductor helps prevent the internal noise from being radiated out through the input jacks. But if we get some noise during testing, it won't matter once everything is back to 100%

[MichaelNuzum]

Enzo- yes, sorry, I failed to mention that the inductor tested open on my DMM. I just followed your suggestion and tacked in a solid wire jumper, and with it installed the signal flows from end to end as it should. So once I get the LCR meter, I will measure the corresponding inductor from channel one to determine the value I need to purchase for channel two.

If I might pick all of your brains one step further: knowing you don't have a crystal ball answer for this specific unit, what sort of situations would cause an inductor to fail open? I realize that is a crazy general and broad concept, and maybe not answerable, but I am always curious as to what the "why" is whenever a component fails.

[Enzo]

Just speaking for me, in the shop, when I say something is broken, it means physically broken, as in cracked or broken in half. If the part is intact, I would just call it open. Others may not try to adhere t that rule.

On a dry winter day, ever walk across the carpet, and as you reach for the door knob, a spark jumps from your hand? That is actually a static charge on your body of several thousand volts. WHen we are flopping around plugging things in, if something has a different static charge from your unit, then when the plugs touch, several thousand volts can hit that part. Or whatever connects to this unit could have a charged cap on its output, and discharges through that part.

But mainly we face...tada...RANDOM COMPONENT FAILURE.

SO with a wire jumper, signal flows end to end? I hope you mean end t end of the whole unit, no just end to end of the piece of wire