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    M125 Resistor Rolling

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    Denizen

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    Join date : 2017-08-15
    Location : Berlin, Germany

    M125 Resistor Rolling

    Post by Denizen on Wed Feb 14, 2018 4:09 pm

    While capacitor rolling is always a hot topic, other than type I hadn't heard any talk about resistors (outside of use in speaker crossovers). Recently though I've heard mention of it and it's piqued my interest. I'm lucky enough to have a fellow diy / hi-fi friend here in town and he's got quite a selection I can choose from. Question is, which resistors will give the most audible change? R33 catches my eye as it's last in the chain of the driver tube but truth is that that's a wag on my part since I'm still quite new to diy having bought my first soldering iron only a little over a year ago.

    Anyone care to enlighten me as to which and why?

    Huge thanks in advance,
    D.
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    tubes4hifi
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    Re: M125 Resistor Rolling

    Post by tubes4hifi on Wed Feb 14, 2018 7:46 pm

    R33 & R35 could be Takman carbon film. R2 is actually the most important resistor in the whole chain, before any amplification takes place, and R8 in the NFB loop,
    those should be top quality metal film, like PRP or better. Shinkoh or AudioNote tantalum if you want to spend big bucks, and who know if you can hear any difference.
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    Denizen

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    Re: M125 Resistor Rolling

    Post by Denizen on Thu Feb 15, 2018 2:54 am

    That makes much more sense than what I wrote. After all, the resistors I will experiment with first will be the loading resistors for my mc cart. Also, where the signal is weakest is where I already concentrate the most on keeping wires short and of high quality. Makes sense that that thinking carries all the way through the chain.

    Thanks.
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    Denizen

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    Re: M125 Resistor Rolling

    Post by Denizen on Fri Feb 16, 2018 2:40 pm

    Digging around, I found that I have some 7k5 Shinkoh so I went ahead and popped 2 of those into r8. Not enough difference to say if it's the resistors or my imagination but there does seem to be a bit more depth and a slight bit more richness. I hope to get some z foil "nudes" this weekend that I'll try in r2 and hope for a more dramatic change.
    To be clear though, the m125 are already everything I could have hoped for, it's just that it's addictive trying to wring a few more drops of HiFi goodness out of them. Nice to find an avenue of tweaks that are cheaper than tubes. Especially now that I found Heerlen Holland ecc82's to be the perfect front tube for me, making tube rolling moot.

    Speaking of... As I am running Magnepans (inefficient so the change is more noticeable), a few weeks ago I went ahead and swapped out the Weber solid state rectifiers for a pair of TAD gz34 and did enjoy a further opening up of the sound. Not a night and day difference but definitely noticeable. So for any of you with inefficient speakers (especially if you've done the diode mod), it might be worth trying at some point.
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    pedrocols

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    Re: M125 Resistor Rolling

    Post by pedrocols on Sun Feb 18, 2018 8:04 pm

    I did changed the R2 to the Vishay and to be honest I could hear more difference rolling tubes. I put them in because I had them laying around from another project and I was not really going to use them for anything else.
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    Denizen

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    Re: M125 Resistor Rolling

    Post by Denizen on Mon Feb 19, 2018 3:10 am

    Good to know. I'll focus on my phono amps more anyway. Just such a quick and easy swap I thought it'd be worth a shot.
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    pedrocols

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    Re: M125 Resistor Rolling

    Post by pedrocols on Mon Feb 19, 2018 4:37 am

    Yes quick and easy swap! Just try. It is just one resistor so why not.
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    Peter W.

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    Re: M125 Resistor Rolling

    Post by Peter W. on Mon Feb 19, 2018 11:59 am

    Coming late to this, I would add this discounted $0.01 worth of observation:

    Resistors want to be:

    a) Thermally stable. Most conductors increase in resistance with temperature. A resistor used in sensitive electronics wants to be thermally stable within the entire expected operating temperature range.
    b) Free of additional effects. Wire-wound resistors add minute amounts of inductance, and can pick up stray inductance from nearby sources. If this is an issue - they should be avoided.
    c) Long-lived. They should not drift with age.
    d) They should have a predictable failure mode. Some resistors in some circuits are as much fuses as they are resistors. And, if that is a desired side-effect, they should behave in the most desirable way when forced to failure.

    After which - a resistor is, by its nature, a predictable clog in the pipe. No more, no less. Whether they are rolled on the thighs of virgins on Walpurgisnacht, or cut from a piece of HB drafting graphite, if they meet a-d, they will be fine.
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    sKiZo

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    Re: M125 Resistor Rolling

    Post by sKiZo on Mon Feb 19, 2018 4:41 pm

    Any thoughts on using VRs instead of fixed value resistors? Add a test point similar to that used for bias adjustment at each position for dialing it in, then tweak the value by ear. Seems to me the design allows quite a wide range befire you end up with any potential of damage.

    And remind me - what is it the magical R39 does? Seems to me that would be a good location for the mod ...
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    Bob Latino
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    Re: M125 Resistor Rolling

    Post by Bob Latino on Mon Feb 19, 2018 6:48 pm

    sKiZo wrote:

    And remind me - what is it the magical R39 does?  Seems to me that would be a good location for the mod ...

    R39 is the main bias resistor on the board. What it does is set the "range" for bias adjustments. On the M-125's this is a 1K resistor. Having a 1K resistor there, allows the amp to crank in a lot of negative DC bias voltage to better control the output tubes. This resistor is not in the signal path and changing it out for a "better" resistor will have no effect on the sound of the amp. I also do not recommend altering the value of this resistor.

    Bob
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    Peter W.

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    Re: M125 Resistor Rolling

    Post by Peter W. on Mon Feb 19, 2018 6:53 pm

    [quote="sKiZo"]Any thoughts on using VRs instead of fixed value resistors? Add a test point similar to that used for bias adjustment at each position for dialing it in, then tweak the value by ear. Seems to me the design allows quite a wide range befire you end up with any potential of damage.

    a) Variable resistors will be considerably larger than a fixed resistor of the same size and capacity.
    b) Variable resistors add an additional failure point.
    c) Variable resistors can drift over time, or become contaminated, or become intermittent.

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