wirewounds for audio ?

[All Ears]

Apologies if this has been asked before many a time...

Is there any inherent problem in using wirewounds in audio signal parts of circuits ?

Can the inductance interact with other local components and result in a tuned oscillation or somesuch ?

Also, how much real difference in inductance is there between a wirewound and a metal film resistor, given that when i have scratched off the outer coatings of some metal films I have found a spiral film layer, which presumably also has inductance.

I just ask since I have a large pile of old style 5W green enamel coated wirewounds (various values less than 10K) and I was wondering if I could find some use for them in a tube build I am working on at the moment (namely the Redd 47 pre). I had already figured I will need a 5W-10W wirewound for the plate dropper resistor, but I was wondering what the effect would be of using wirewounds in other parts of this circuit.

Also, I wonder are wirewounds more or less reliable over time than metal films of the same wattage rating ? I would have thought wirewounds would be more beefy, but think I can recall some people saying wirewounds can vibrate and perhaps that would lead to internal breakage. (?)

Noisewise, I would have thought WW's would win the day in audio circuits, but of course there are many more other considerations than just noise to take into account. I rarely see WW's used in audio signal parts of circuits so there must be some reason not to use them.
Curious to know what those reasons might be.

 

[emrr]

Use them if you have them. 

All of the RCA and Western Electric pre-war broadcast gear use wire wounds in every position.  They measure spot-on today, in every case I've measured.  They beat the crap out of carbon for value stability and noise.  They also had to cost a small fortune in comparison.  Today they still cost a small fortune in comparison to metal film.  I don't know how they stack up today in noise and stability, surely much less distance between.  If you look up modern low wattage WW, they appear to go into military and industrial apps, and you'd never spend the money for them. 

 

[JohnRoberts]

The inductance can be managed by counter winding the resistive element so there is a first order cancellation of any magnetic field, while you might try measuring the resistor's inductance for sensitive applications.

A little extra inductance in a series build out application could be a good thing, unwanted inductance in a filter or EQ seems undesirable.

JR

PS If you have a pile of these you might be able to convert them to a pile of money instead selling them to audio-phools who think they are better.

 

[DaveP]

I would guess any inductance would be in the uH range if that?  You could check out that reactance at various frequencies to see if that affects the circuit, radio work might have a problem but I can't imagine audio being affected.

In a non-feedback circuit the inductance might cause a peak somewhere in the RF region where your amp has no response anyway.

If there is a capacitance in parallel or series you will get a tuned circuit, but again it may be in the RF region.  A screened wire may be 150pF per metre so that's the worse case, check out the formula.

The spirals on a metal foil are very few compared to a wire wound and can be ignored for audio work.

Inductance will only cause oscillation if it contributes to a 180 degree phase shift in a feedback circuit somewhere in the amp.  As Doug says, if WW was good enough for RCA and WE, then feel free to go ahead.

best
DaveP

 

[All Ears]

Thank you all - this is why I love groupdiy - so much knowledge and experience accumulated here amongst generous ppl willing to share it.

I will try out the WW's in as many positions as possible in the tube pre - if there is anything unusual I will post it back here.

Perhaps I should add "If it's good enough for WE and RCA, then it's good enough for me" as my byline.

 

[ruffrecords]

Altec used to specify wirewounds for cathode resistor for their stability and the ability to handle overload when tubes went bad.

Cheers

Ian

 

[PRR]

Resistors are the CHEAPEST part of your build. Don't ask us to approve your parts-pinching; go with the flock and use the "right" parts. (Yes, this changes every decade but we gotta keep up with the fashion trends...)

Above 1K there is probably "no inductance" in a wire-wound for audio purposes.

A 10K plate-load might get a little "better" (above 100KHz?) with a little inductance.

8 Ohm Wire-Wound is somewhat notorious for causing wrong results in power-amp testing. Next to 8 Ohms it does not take much inductance to slide the actual impedance higher than it appears. 100 micro-Henries is over 8 Ohms at the top of the audio band.

I have had burn-up trouble with 10 Ohm WW cathode resistors and a plate-lead run too close. This became a in-phase transformer, and the thing went bonkers at some supersonic frequency. Better hygiene (keep plate leads away from all else) was the answer in this strange case.

> metal films I have found a spiral

10 or 20 turns?

A high-value wire-wound has hundreds of turns. Inductance goes as square of turns. So hundred of times more inductance in a WW than a MF.

> counter winding the resistive element

Yes, but you never find that in the Big Brown Lump WW power resistors.

I do think Shalco (and others) did do a reverse-wind on high-value precision fader resistors.

 

[emrr]

Sometimes the really nice old ones are multi-section to manage inductance.  I'd imagine it's true of the new military stuff for high impedance work. 

Here's the inside of a Daven attenuator from the late 1930's, one place that stable precision results were desired.