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Wtd.. 2.8R 10w or 15w resisitor?

Discussion in 'd.i.y.' started by The Captain, Jan 2, 2008.

  1. The Captain

    The Captain ~~~~~~~~~~

    Hi I've some puny 1/2w resistor in heybrook speakers- Im after higher wattage replacements, but n/a it seems..

    would anyone have such a resisitor lurking?

    thanks, Capt
     
  2. trancera

    trancera pfm Member

  3. PigletsDad

    PigletsDad pfm Member

    The difference between 2.7 and 2.8 won't matter.
     
  4. The Captain

    The Captain ~~~~~~~~~~

    Thanks I did the RS/ maplin/ farnell search but forgot about Wilmslow!

    Would anyone remind me how changing the 2.7r 1/2w tweeter resisitor (its in series on x-over with a 4u7) might affect the general sound..

    for a 10w or even a 20w?

    thanks
     
  5. martin clark

    martin clark pinko bodger

    If you aren't already caning the speakers (so the 1/2w resistor isn't running silly-hot and drifting up markedly in value) - no detectable change will result.
     
  6. Robert

    Robert Tapehead

    Hi Rory,

    It won't effect the sound if all you are doing is upping the wattage.
    1/2w sounds horribly low but given the sort of power requirement up above 2-3khz in music it wont be an issue for normal use.
    If you drive the speakers hard for long periods with lots of HF energy you might heat the resistor and that could change the sound as the resistance will change.
    The power handing of many soft domes (the tweeter in the HB1 is quite elderly) is only around 8w RMS.

    You should be able to pick up 3w resistors from Maplin and RS - that's ample.
     
  7. markt

    markt hello

    Rory,

    From a subjective point of view a larger resistor has always sounded less grainy IME, it might not be measurable, it might be a shocking thing to think about, but it's true.

    Just don't go overboard so much that its self inductance changes frequency response, I have had improvements from impractically large resistors, if a 20w is cheap then use it.

    The white ceramic ones from Wilmslow are much better than the cement coloured ones I have found, metal oxides much better again.

    Don't believe that it makes no difference, it does!

    Not just a difference but probably the largest difference of any cheap component in the speaker.
     
  8. The Captain

    The Captain ~~~~~~~~~~

    Interesting views here.. I'll have to go and try it, especially as a 2.7r 20w std ceramic wirewound Ive just seen on ebay for £1.

    Thanks folks- they're for heybrook Heylo diddy floorstanders if of any interest. Little crackers!
     
  9. James

    James Lord of the Erg\o/s

    Has anyone considered why Heybrook put 1/2w resistors on the tweeter circuit? It could be a protection measure to prevent the tweeter from getting fried by too much power. Resistors are cheaper than tweeters.

    In my experience, I have never run anything resistor rated more than 10W on a tweeter circuit, and even when running flat out, mine never get warm to the touch. I honestly think anything more than a 5W would be overkill.

    James
     
  10. The Captain

    The Captain ~~~~~~~~~~

    Oops just bought them.. do you think the 1/2w may have something to do with these Heylos thin horrible solidcore internal cabling? (which Ive just ripped out for some half decent stuff)

    I use the same tweeter in the HB1 s3- a vifa D26- which has a 18 ohm 3w in series with a 4u7: the tweeter being an 8 ohm version in the hb1, as opposed to these Heylos which are 6 ohm.
     
  11. markt

    markt hello


    James,

    I think the opposite, a resistor as a fuse is wrong, if you need a fuse, use a fuse, small resistors sound awful in crossovers, tweeter or woofer.


    btw, happy new year.
     
  12. The Captain

    The Captain ~~~~~~~~~~

    Mark Im all concerned now! its a good point though, ie why on earth did they choose a 1/2 watt..
     
  13. markt

    markt hello

    Rory,

    if the design is relying on resistor heating for some weird reason, or a resistor is being used as a fuse, the first thing I'd do is throw the speakers in the bin!
     
  14. markt

    markt hello

    I apologise, that was unhelpful, it's just that it's an area that makes all the difference IMO, it should be taken advantage of and I just can't understand the need to use a small resistor, seems wrong.
     
  15. LesW

    LesW Making electronics make music

    I've probably got some thick film 25W jobbies somewhere in the store so I'll look for them when I open up tomorrow......
     
  16. The Captain

    The Captain ~~~~~~~~~~

    Thanks Les- btw what's your opinion on 10w or 20w replacements for the 1/2w here?

    all helpful Mark (Ive yet to launch them in the bin)- obviously more than one school of thought, so its worth taking care with this.
     
  17. James

    James Lord of the Erg\o/s

    Hmmm ... what's the fundamental difference between a fuse and a low-wattage resistor? Sure, I know that the resistor adds resistance but whether you put a 100W 2R7 or a 0.5W 2R7, both will result in the same voltage drop. The only difference is that the 100W one will pass a lot more current before it fails. So, what's the advantage of putting in a separate fuse over a lower-wattage resistor?

    Happy new year to you too.

    James
     
  18. markt

    markt hello


    James

    Putting aside the question of why a fuse is needed in the first place, the answer must be noise, resistors are noisy, small resistors are worse, I'm not going to convince you of this because you are unlikely to experiment.

    I wouldn't put a fuse in, full stop. It's unlikely to have a positive effect and preventing a negative effect would mean soldering it in place, I simply wouldn't consider either but a resistor would sound the worse of the two as a guess, a ridiculous situation involving a soldered in fuse is unlikely to be harmful to the sound (no more so than capacitance across a PCB or inductors held down with tie wraps) but why it should be needed is what bugs me!

    I have wondered why big resistors sound subjectively better, in some circumstances its possible that they might have a mild filtering effect outside the audioband via inductance, possibly keeping the amplifier more stable.

    Or maybe the thermal noise of a signal current has a greater effect at low level currents than is thought.

    Might be vibration, a large resistor has some advantage here, a bit of mass added to parts gives interesting results.

    It is embarrassing to sit on the subjective side of things, I really don't like it at all, but without hard proof there is no real way of putting the big resistor case forward objectively so I won't attempt it.

    Fuses suck, small resistors suck (relative to what they have passing through them) and being on the subjective side of things also sucks.

    An even happier new year to you!
     
  19. Albertb

    Albertb pfm Member

    I am sorry but I have to politely but fundamentally disagree with some things being said here.

    A fuse and a resistor are totally different beasts in every way and no decent designer will use a resistor in place of a fuse. And without a doubt, though design may have moved on, Heybrook were and are a more than decent manufacturer. Just because the resistor will burn out and cut the circuit with the same ultimate effect as the fuse does does not make them behave the same in doing so.

    Bear another thing in mind here, re-read Piglet's Dad's response earlier about 2.7 or 2.8R. He is known for speaking sound technical sense, (usually ;) ), so take his opinion as having merit. Now let's spare a thought to that resistor which you are discussing in terms of its change of resistance on heating.

    Not having such things at my fingertips I did a very quick look at some specs just to get a feel, (Farnell catalogue!). I looked for the worst cases and considered a humble carbon resistor, 2.7R, 0.25W. These generally had a temperature coefficient of around 350ppm/deg C. (The poorest temperature coefficient I could find was 1500ppm for a very different beast). Let's say the temperature rise will be 100deg. That means that the resistance will change by 2.7 X 350 x 10^-6 x 100. This gives less than 0.1R, or less than from 2.7R to 2.8R. Now what was it PD said?

    And please let's not spread smoke about dynamic changes versus static changes, we are talking about components with considerable thermal inertia here being swamped by the same type of effect in the voice coils and other circuitry.

    The percentage power delivered to a tweeter through a half decent crossover is somewhere in the region of 2%-15% depending on source material and crossover frequency. Let's say we have a 100W system driving it. Thinking average power here and not 100W continuous, with a 20dB or 10X difference between peak and average voltage levels, (normally considered sensible), that means a 100:1 ratio of power, (V^2), or about 1W average for 100W peak. Of this let's say 10%, (highish!), is taken by the tweeter branch and about 1/3 is taken by the resistor, (2.7R:6R). That means the resistor is experiencing an average power just below 35mW.

    Or I could easily be wrong somewhere, very genuinely, if so please point it out to me as this sort of topic intrigues me. 20W wirewound resistors in place of 0.5W to dissipate 0.035W seems a little - er - "skewed thinking" to me.

    Anyone care to comment on the audibility of running your speakers without a lagging overcoat and scarf in winter? ;) :D
     
  20. James

    James Lord of the Erg\o/s

    I fully agree that fuses are redundant in a competent design. I'm interested to hear that resistors are inherently noisy and that the smaller they are, the noisier they are. I thought the noise of resistors had more to do with how they are made, where metal-film types are superior to metal oxide, which in turn are better than wire-wound types. My minimum standard in passive XOs are Eagle Metal Oxide resistors, which are rated 10W.

    James
     

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