The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Impedance mismatch question

    Share

    1bigpig

    Posts : 11
    Join date : 2015-03-12

    Impedance mismatch question

    Post by 1bigpig on Mon Apr 06, 2015 11:57 pm

    I have a question that I cannot seem to find anywhere.  I know that if you hook up an 8 Ohm speaker to the 4 Ohm tap, you are suppose to get a little more power out of your amp and a little tighter bass at the expense of some top end roll-off.

    So what are the sonic ramifications of hooking up a 4 Ohm speaker to the 8 Ohm tap of the amplifier?  As for "Just try it out and see...", well I have to take my amplifier semi apart to change the speaker taps (Bob's ST-120 amp) so it is easier for me to ask then spend 30+ minuted mucking with the amp.

    Thanks,

    Bruce

    PS Is there a difference between the VTA driver board for the ST-70 and the ST-120?

    Kentley

    Posts : 335
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: Impedance mismatch question

    Post by Kentley on Tue Apr 07, 2015 1:40 am

    Wonderful question. And the answer is:
    Contrary to logic, hooking up a so-called 8 ohm speaker to 4 ohm taps and getting well-defined results depends on so many factors that a definitive answer is impossible. But it's a better bet than the reverse.
    In general, from experience, one gets better control over sound quality when you work "upward" because most speakers RATED at 8 ohms actually dip in impedance significantly lower at certain frequencies, often the upper lows or lower mids. Which causes stress on the output load of your poor ST-120. It's like asking a 90 lb. mule to carry a 120 lb. bag of cement.
    A practical limit is reached when the mule is carrying a 20 lb. load, and is used to 100 lbs. What happens is that he grows complacent, and delivers ho-hum sound. The upper and lower edges of his potential response suffer, to stretch the metaphor.
    I recently got an ST-120. I have B&W speakers rated at 8 ohms. They sounded bloated on the 8 ohm taps. On the 4s everthing is tighter and I can sense that the amp is happier.
    But every speaker is different. That's why it is said "try it". In general: your ears, and the amp, will be happier when the impedence of the speakers is somewhat higher than the amp.
    This may or may not make any sense. Your move.

    bluemeanies

    Posts : 129
    Join date : 2015-02-09
    Age : 66
    Location : Folsom Pa.

    Re: Impedance mismatch question

    Post by bluemeanies on Tue Apr 07, 2015 1:14 pm

    Very interesting conversation.  
    I am new to all of this but have been doing some research on line and via phone with B&W so I thought I would share.
    I have the B&W804S speakers and contacted B&W about running the 804s on a 4ohm tap.  B&W recommended that the best performance for the 804's will be better served by connecting to a 8ohm tap.  While B&W leaves the choice open to the individual they stand by the nominal impedance of the 804s at 8ohms and while they did not tell me by how much the they did say that in motion the 804's impedence does DROP.  Another factor depends on how well built the amplifier is handling the 4ohm mode.  Of course this all revolves around the B&W804S speakers and not the overall spectrum of a particular speaker company.
    Therefore as you have staed Kent nothing here is definitive.  What one set of ears may hear, might be a sour note in crictical listening to another listener.  
    The 804S speakers impedance from what I have read can drop to 6.5 ohms depending on frequency and speakers in general can drop as low as 4ohms for the same reason.

    Kentley

    Posts : 335
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: Impedance mismatch question

    Post by Kentley on Tue Apr 07, 2015 1:33 pm

    I'm glad you are taking it logically, Mr. Meanies. B&W is still a wonderful company, even tho they have been pandering, out of necessity, no doubt, to the HT crowd.
    Perhaps it would be proper to note that my listening habits (largely "serious music i.e. "classical") give me a different set of criteria for my ideal sound. For me, less is more, and control is more vital than volume.
    And with the VTA ST-120 or the M-125s, I am confident that the performance will be solid using either set of taps.
    There seem to be conflicting opinions on whether impedance mismatch can cause actual harm to the circuits. I still maintain that one is safer when the amp runs into a slightly higher impedance load than the other way around. Perhaps our esteemed Team Leaders would care to weigh in on this {hint hint}. Also be aware that with SS amps, the problem is practically nonexistent. They sound equally "good" into most any load.
    I imagine that many of us here came into the tube realm via Fenders and Marshalls. There the whole game is different. Deliberate mismatches and overloads are part of the magic of the sound. Some of that ideal may be held over in our listening habits - our ears are accustomed to the "ballsy" sound of our Fender Twin blowing the gaskets off our eardrums. To each his own. Just beware the Tinnitus Monster. affraid

    Kentley

    Posts : 335
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: Impedance mismatch question

    Post by Kentley on Tue Apr 07, 2015 2:19 pm

    It just occurred to me that my present experiences with impedance matching the B&Ws with the VTA might be related to the possibility that my crossover caps have drifted from original values. {see my thread re:Xovers) It may be that my situation is unique, and will change to a more conventional one when I recap the speaks. I'm glad I ordered the optional two-tap choice now so that it will be simple to compare and contrast.

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Impedance mismatch question

    Post by sKiZo on Tue Apr 07, 2015 2:28 pm

    bbbbbbbrrrrrrrIIIINNNNGGG!
    WOULD SOMEBODY PLEASE ANSWER THAT PHONE???

    As mentioned, lower impedance should usually produce stronger bass, but that's dependent on a number of factors. In my case, I wouldn't be surprised if my system tested at 2 ohms to the big drivers under heavy load using the 8 ohm taps ... synthed bass at 11 kinda thang. So, as with everything, we compromise and aim for the best overall results across the audio spectrum, and let the crossovers sort it all out. Their sole purpose is to keep all the drivers happy with a proper load. Worst possible load will usually occur at the crossover points where their sonic impact is minimized.

    (So yes ... drifting cap values could have a significant impact on what you're hearing.)

    So, no ... I personally wouldn't use 4 ohm taps with an 8 ohm load, but whatever works for you. If anything, a finicky speaker might be best served with bi-amping.

    And ya ... best way to experiment would be to make all the taps available for quick A/B comparisons.

    Speaking of crossovers ... another project on the hit list as my XR16's are pushing 35 years old now.



    Those bad boys will require a daughter card and additional bracing for support as they're a tad bit bigger than the OEM caps ...  clown

    Drivers have all been refurbed, so that should make the speakers like new. May also reseal the boxes as long as I'm in there ... not uncommon for them to leak some at the terminal plates.

    PS ... speaking of drivers ... a LOT of the benefit you seem to be getting from the 4 ohm taps could be due to stiff action on the cones. My surrounds didn't look all that bad, but I had one of the big woofs go open, and figured as long as I was at it ... results were pretty spectacular when it came to defined bottom. Amazing what we can get used to over time.

    Cubdriver

    Posts : 68
    Join date : 2014-01-21
    Age : 51
    Location : Southeastern Litchfield Co, CT

    Re: Impedance mismatch question

    Post by Cubdriver on Tue Apr 07, 2015 4:52 pm

    sKiZ, is the battery for testing the driver phasing? Wink

    -pat

    Kentley

    Posts : 335
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: Impedance mismatch question

    Post by Kentley on Tue Apr 07, 2015 4:57 pm

    Sharpen up the needlenose, Jed. sKiZo's a-comin'....

    1bigpig

    Posts : 11
    Join date : 2015-03-12

    Re: Impedance mismatch question

    Post by 1bigpig on Tue Apr 07, 2015 5:01 pm

    Ok, here is what I got and here is the problem...

    I have some old (early 90's) KEF Q95 speakers.  I am 99.999% sure that the ferro fluid in the tweeters is all dry and they have been in storage for the last 10 years or so.  They are a 4 Ohm resistive load (RCRC crossover network).  They already sound dark flat with an old Carver 1.0t amp.  When I hook them up to the ST-120, they don't sound any better and I am trying to see if these speakers are just at the ends of their life or if the impedance mismatch is causing them to still sound dark with the tube amp.  On my other speakers, the tube amp really brings the stage forward and a nice warm tone but not with these KEFs.  They still sound cold.  

    I know the both amps are fine because my main speakers, a pair of Paradigm Studio 20's, sounds good with the Carver and awesome with the ST-120.  I also realize that there is about 25 years of technology difference between the the sets of speakers and that KEF, being English, was voiced to be more flat in response, but these just sound cold...  I have verified that all the drivers are "good" (impedance is correct for each driver) but I have not messed with the crossover.  So that is another potential area of investigation...

    These are not valuable, rare, or overly special, just trying to decide if it is even worth spending any money on, Craigslisting them or having a nice bonfire.

    And, no, I did not get into tubes from a Fender or Marshall.  I got into tubes after I purchased a Schiit Vali headphone amp last year.  I have never heard something sound so good as that headphone amp.  It does help that I have a pretty good set of headphones but that thing just rocks...  Figured if tubes can do that to headphones, imagine what it could do to a set of real speakers!

    Bruce

    Kentley

    Posts : 335
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: Impedance mismatch question

    Post by Kentley on Tue Apr 07, 2015 5:11 pm

    Bruce, I would definitely investigate your tweeters. When I replaced my 25 year-old diaphragm/coils, the ferrofluid resembled nothing so much as pine-sap. Really. And the job is a lot easier with most speaks than mine - and I managed to do two without incident. And I'm a klutz with tiny things.
    There is a small dealer who sell pre-measured doses of fluid for various sized tweets. Takes all of the guesswork out of the job. If you'd like, I'll look him up fer ya.
    What cans do you have?

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Impedance mismatch question

    Post by sKiZo on Tue Apr 07, 2015 5:21 pm

    Cubdriver wrote:sKiZ, is the battery for testing the driver phasing?  Wink

    -pat

    Sure ... why not! It's even audiopile grade! clown

    More for a sense of scale ... them caps is HUGE compared to OEM ...


    Cubdriver

    Posts : 68
    Join date : 2014-01-21
    Age : 51
    Location : Southeastern Litchfield Co, CT

    Re: Impedance mismatch question

    Post by Cubdriver on Tue Apr 07, 2015 5:30 pm

    sKiZo wrote:
    Cubdriver wrote:sKiZ, is the battery for testing the driver phasing?  Wink

    -pat

    Sure ... why not! It's even audiopile grade!  clown

    More for a sense of scale ... them caps is HUGE compared to OEM ...


    That's what I figured, but had to be a smartarse. Smile

    -Pat

    1bigpig

    Posts : 11
    Join date : 2015-03-12

    Re: Impedance mismatch question

    Post by 1bigpig on Tue Apr 07, 2015 8:59 pm

    Kently,

    I agree that they should be refilled but...(there is always a but)...these are the first of KEF's Uni-Q speakers and you have to desolder and disassemble the midrange to get to the tweer.  Second, I think the ferrofluid mainly effects the power handling, not the sound.  But I could be wrong.

    I have some Hifiman he-560s.  I later purchased a Schiit Lyr 2 and some NOS Tesla tubes and the Vali still sounds better to me!  I listen to them both after the wife goes to bed.  The Lyr 2 is definitely louder --probably cleaner too-- but damn if that Vali doesn't put a smile on my face... I mean the Lyr 2 has that audiophile "sound" but the Vali has that tube warmth that just makes everything sound that much sweeter.  Oh they are both driven by a Schiit Bifrost Uber DAC.  A REALLY nice DAC for the money.  It is also the DAC for my ST-120.  I have gone all digital.

    Bruce

    Kentley

    Posts : 335
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: Impedance mismatch question

    Post by Kentley on Tue Apr 07, 2015 9:24 pm

    Bruce -
    I'm a Sennheiser geek myself. 650s now, 800 someday. Nothing I've ever heard seems as natural AND comfortable. The "fatigue factor" is always a problem with cans. And the ears are punished by the near-field pressure levels. Pete Townsend swears that his tinnitus was caused not by the bone-crushing stage audio, but by 'phones in the studio. I use a cheap Chinese 12AU7amp, but I run it off my tubular pre first, so it is effectively a simple impedance-matching device.
    My tweets in the B&Ws were 25 years old, and benefitted dramatically from the fluid change. The stuff is like motor oil. Yes, its primary function is to dissipate heat, but... I don't think old gluey oil helps the tweeters' mobility. It was like removing several layers of terrycloth from the front domes.
    I do sympathize with the potential difficulty. I was lucky in that my tweets are mounted on top of the cabs, and are accessed easily. TOO easily...
    It may be time to drop your KEFs at Goodwill - or sell 'em as-is on EEEWW-Bay.

    1bigpig

    Posts : 11
    Join date : 2015-03-12

    Re: Impedance mismatch question

    Post by 1bigpig on Wed Apr 08, 2015 12:11 am

    I agree about headphone "fatigue factor" as my ears get very hot and sweaty after an hour.  Also, while I "turn it up" as far a volume goes, it is still not very loud.  Since they are open headphones, she can hear it if it is too loud.

    Yea, I think the KEFs may find a new home...

    Bruce

    Sponsored content

    Re: Impedance mismatch question

    Post by Sponsored content Today at 4:03 pm


      Current date/time is Thu Dec 08, 2016 4:03 pm