What is on your workbench right now?

[restorer-john]

If they're anything like the Chinese cheapies I bought from Amazon for my dummy load, quite low.

Same with Chinese ones I bought a while back. I was really surprised. But I do have some otherwise very nice 4R ceramic ones which aren't as good in that regard.

 

[jasonhanjk]

Just finished DIY yesterday.
JRC2068 and JRC4556.

 

[mansr]

I'm pretty sure I've identified the problem with this Proel PRL1400 amp.
View attachment 90346

In case anyone is wondering, it was indeed those large filter capacitors. I replaced them and, just in case, the smaller electrolytics in the background. The amp has been running fine for a few days now.

 

[Rick Sykora]

Same with Chinese ones I bought a while back. I was really surprised. But I do have some otherwise very nice 4R ceramic ones which aren't as good in that regard.

These are from Parts Express and would not call them inexpensive (relatively speaking the Arcol ones seem a much better deal in hindsight). They are claimed to be low inductance...

Using my impedance rig for REW (not sure how accurate it is at low inductance) , I measure 2-15 uH up to about 10kHz and then rises to about 90 uH at 20kHz. See later post as cannot repeat these measurements...

 

[LTig]

These are from Parts Express and would not call them inexpensive (relatively speaking the Arcon ones seem a much better deal in hindsight). They are claimed to be low inductance...

Using my impedance rig for REW (not sure how accurate it is at low inductance) , I measure 2-15 uH up to about 10kHz and then rises to about 90 uH at 20kHz.

Strange. Does inductivity depend on frequency?

 

[Rick Sykora]

Strange. Does inductivity depend on frequency?

The short answer for real world inductors is yes it depends (on application).

At what frequency it happens is a function of the Q of the inductor and its resonant frequency. Btw, am about 35 years past my AC circuit theory class, so had to get a online refresher.

 

[Doodski]

Strange. Does inductivity depend on frequency?

XL = 2piFL

So... the inductive reactance is directly proportional to frequency and inductance.

 

[LTig]

XL = 2pieFL

So... the inductive reactance is directly proportional to frequency and inductance.

This I know, I wondered about the dependency of the inductance L on frequency F.

 

[Blumlein 88]

This I know, I wondered about the dependency of the inductance L on frequency F.

As frequency goes up so does inductive impedance. Or are you asking if the inductance itself changes with frequency? The answer to that should be a no.

 

[LTig]

The short answer for real world inductors is yes.

At what frequency it happens is a function of the Q of the inductor and its resonant frequency. Btw, am about 35 years past my AC circuit theory class, so had to get a online refresher.

The inductance of a real world resistor is constant, but there is also a stray capacitance parallel to the serial connected resistor and inductor (see Horowitz/Hill Art of Electronics - The X chapters page 20). This circuit has a resonance frequency, and knowing it helps to calculate the capacitance. I think you miscalculated the inductance when you assumed that the complex impedance of this circuit results from the inductance alone.

However, looking at the graphs on page 22 in said book the resonance frequency of all shown wirewound restores is far above the audible frequency range.

 

[LTig]

As frequency goes up so does inductive impedance. Or are you asking if the inductance itself changes with frequency? The answer to that should be a no.

Inductance itself.

 

[Rick Sykora]

The inductance of a real world resistor is constant, but there is also a stray capacitance parallel to the serial connected resistor and inductor (see Horowitz/Hill Art of Electronics - The X chapters page 20). This circuit has a resonance frequency, and knowing it helps to calculate the capacitance. I think you miscalculated the inductance when you assumed that the complex impedance of this circuit results from the inductance alone.

However, looking at the graphs on page 22 in said book the resonance frequency of all shown wirewound restores is far above the audible frequency range.

Agree, and I went back and remeasured the power resistor. For most of the audio range the inductance is 15 uH for my 4 ohm power resistor. Not sure what happened with the earlier measurement, but it was early and I must have messed up as I cannot reproduce the same readings.

 

[Rick Sykora]

Now dual resistor dummy load and twice the fun!

 

[Rick Sykora]

As frequency goes up so does inductive impedance. Or are you asking if the inductance itself changes with frequency? The answer to that should be a no.

Amending my earlier statement, apparently inductance can rise with frequency but is application dependent. It does not appear to be an issue at audio frequencies. Here is the reference...

Kenneth Lundgren
, M.S. Electrical and Electronics Engineering, Northwestern University
Answered September 8, 2017 · Author has 6.5K answers and 2.5M answer views

All the answers here except Roy MaCammon’s ignore what the OP has asked. All of the math about the reactance increasing with frequency is true. However, every inductor has a self-resonant frequency which is caused by the stray capacitance within the inductor. At the self-resonant frequency, the reactance will be much higher than predicted by X=jωL. So as the frequency approaches the resonant frequency, the equivalent value of L will actually increase. In a high Q inductor, L may appear infinite.
At frequencies above the resonant frequency, the inductance will actually appear negative, i.e. capacitive.
INDUCTOR IMPEDANCE vs FREQUENCY:

APPARENT INDUCTANCE vs FREQUENCY:

If the inductor is being used as a choke - to block high frequency AC, you can use this to advantage, by selecting an inductor with a resonant frequency close to the frequency that you want to block.
There are special inductor designs to maximize the self-resonant frequency by minimizing the self-capacitance.

There may be several self-resonant frequencies.

 

[LTig]

Agree, and I went back and remeasured the power resistor. For most of the audio range the inductance is 15 uH for my 4 ohm power resistor. Not sure what happened with the earlier measurement, but it was early and I must have messed up as I cannot reproduce the same readings.

Don't worry too much, you're neither the first nor the last to fall into a measuring trap.

 

[ajawamnet]

Same with Chinese ones I bought a while back. I was really surprised. But I do have some otherwise very nice 4R ceramic ones which aren't as good in that regard.

Back in the day at Opus One, we had banks of Arcol or Dale non-inductive. Here's a portable one I designed. It has a transformer coupler that I can use to feed test gear or bench amps to hear what's going on.

Has a carrying case for taking to jobsites... I've run 180 watts (DC in fact) for hours as part of a test of some non-audio gear.

 

[pma]

Designing a new preamp ....

 

[dfuller]

Got two guitar amps in right now.

1. A Fender Twin Reverb reissue that was no-loaded by a faulty speaker cable and nuked some socket mounted resistors bad enough that I'm just replacing the sockets rather than trying to clean the carbon off.

2. A Bogner Uberschall Twin Jet with what I'm almost entirely certain is a bad plate resistor on the input stage.

 

[sloth_kwj]

iPad : distortion of -0.5dBFS 1kHz sine, 16bit, TPD dithered, 3.5mm jack analog output. Volume slider about 80%.

View attachment 78884

What model of ipad?

 

[jasonhanjk]

Well, not right now but it's yesterday picture. TPA6120 on proto board, 1 channel complete.
Output resistor 0 ohm with non-inverting gain of X1. No oscillation detected with input shorted.