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    Magneplanar MMG's on the 8-ohm tap?

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    StevieRay

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    Magneplanar MMG's on the 8-ohm tap?

    Post by StevieRay on Fri Dec 21, 2018 11:54 am

    I recently purchased and built a VTA ST-120.  'Still only have about 50 hours on the tubes (Kt-120).

    The amp is wonderful, especially so on my Quad ESL-57's -- you can basically 'tune' the sound by trying the different output taps.

    I'll wax poetic about the sound after the tubes have at least 100 hours on them....

    But for today, just a question; maybe not a 'simple' one:
    When using the MMG's, of course they're rated for 4 ohm's, would there be any harm in trying the 8-ohm taps, which should have a higher output voltage?

    Because of the flat impedance curve of the MMG's, I don't expect there to be any frequency response changes (unlike the Quads), but by using a 4 ohm speaker on the 8-ohm tap, is there any chance of damage to either the output transformers or output tubes with too much current?

    60W on the 8-ohm tap is ~22v at 2.75A.  Running that 22V with 4-ohms is now 5.4A and 120W.

    Will the voltage sag (in other words can't get there from here) or is that increased current gonna smoke something if driven hard?  Probably not gonna drive it that hard, but looking for a little more 'punch', or to just experiment for s**ts and giggles.  'But don't wanna let the smoke out....

    Using SS rectifier (Weber WS-1 w/thermistor); line voltage is 115vac.  All AC and DC voltages in the middle of the ranges per the manual.

    Thanks for your thoughts!
    Peter W.
    Peter W.

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    Re: Magneplanar MMG's on the 8-ohm tap?

    Post by Peter W. on Fri Dec 21, 2018 12:07 pm

    OK - the Magnepan MMG speakers, both versions, are nominal 4-ohm speakers. So, technically, either tap will work. As you point out, there is no perceptible swing from nominal to actual with Maggies.

    With that in mind:
    a) You will not damage the speakers. They really don't care which tap they see.
    b) By connecting to the 8-ohm tap, you will actually be very slightly reducing the load on the OPT.
    c) And, at the same time, you are also reducing (in watts) the power to the speaker.

    Cutting to the chase, there is no inherent danger with either choice. BUT:

    Maggies are power-pigs. I have the MG-III(probably a) versions, that were dull as the proverbial dish water even on a 75-watt (per channel) tube amp, but came alive when driven by a 225 watt brute-force solid-state amp. Depending on the size of your room and your preferred listening levels, this may be an issue. I have a very large room, so the power is both necessary and convenient.

    Enjoy!
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    wildiowa

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    Re: Magneplanar MMG's on the 8-ohm tap?

    Post by wildiowa on Fri Dec 21, 2018 12:21 pm

    Never worried much about fine tuning impedance on the tube stuff -- old school, very forgiving unless you had no load at all on them. The new Class D solid state stuff is weird. I have Class D amps rated at 800 watts a side at 2 ohms..yet at 4 ohms they are 400 watts, and at 8 ohms they are 200 watts. Don't think there is a danger of meltdown with a mismatch, but man the power rating is sure affected. Not sure whats going on in there...mystery chips and circuits/theories far beyond my understanding. But to maximize your power with this stuff matching impedance seems to be important.
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    StevieRay

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    Re: Magneplanar MMG's on the 8-ohm tap?

    Post by StevieRay on Fri Dec 21, 2018 1:06 pm

    Peter, in regards to item "c" in your list, I don't know if I agree -- IF (and that's a BIG IF) we assume that the amplifier is a voltage source, the 60W at 4 ohms is ~15.4v.

    But to get 60W from the 8 ohm tap we now need ~22v (assuming the speaker was 8 ohms).  The different OP transformer winding gets us that higher voltage (but at a lower current for 8-ohm load at 60w).

    But if you put a 4-ohm speaker on the 8-ohm tap, AND IF the amplifier output voltage doesn't sag, you'll now be generating 120W (22v, 4 ohms, 5.4A) instead of 60W.  So will the OPT 'limit' the curent, or will the voltage sag, or will the OPT windings burn out (or the tubes)?

    So at double the current, something's got to give.  Either the amp will sag (because of power supply AND/OR the feedback, which will be different for different taps) or current will double and be exceeded.

    This is a complex subject, nonetheless because I'm used to solid state amps fully rated to 4 ohms (or less) and won't give up at low impedance.  Just looking for answers -- no harm in trying but I just don't want to blow the amp up.

    And the power problem -- I do have an old hafler DH-200 that can whip out ~200w into 4 ohms, and a c-j MF-2250 (lovely sound, but will probably sell now) that cranks the Maggies up.  Both of those however are too much power and too bright with the Quads.  The c-j is very tolerable with the Quads but NOT the hafler without a tone control......

    The Maggies are the most power hungry things I have ever dealt with -- but worth it I agree, and the MMG's are a bargain.

    So i do have an amp suited to the MMG's, but just trying to get some more 'magic' out of them with the ST-120......
    WLT
    WLT

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    Re: Magneplanar MMG's on the 8-ohm tap?

    Post by WLT on Fri Dec 21, 2018 1:08 pm

    It should work out fine and it mayl change the sound some. Look at what is happening from the output tube perspective.

    The speaker load now appears as half the restive load than if the amp was set at 4 ohms. This reflects back thru the output transformer so the output tubes have a lower resistance load line on their volt/amp curves. There are many places to look up the effects of this from the RCA Receiving tube handbook to the Radiotron Designers handbook.

    The bottom line is you will not hurt anything but it will change the total power out capability some and the distortion level slightly.
    Peter W.
    Peter W.

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    Re: Magneplanar MMG's on the 8-ohm tap?

    Post by Peter W. on Fri Dec 21, 2018 1:28 pm

    OK - Let's look at the "Nature of the beast".

    Tube amps are generally constant-current devices. This because of the output transformer.
    Solid-state amps are (with some remarkable exceptions c.f. McIntosh) are generally constant-voltage devices.

    Once you get past this, it makes sense. Your ST120 is putting out a (nominal) 60 watts whether at 4, 8 or 16 ohms. Think it through and look at how an OPT works. Were it any different, where does all that heat go, otherwise? The input current determines the output current.

    On the other hand, it makes equally good sense that a solid-state device unimpeded (pun intended) by a transformer should have a direct, usually linear, correlation of output power (in watts) to speaker impedance.
    If you have a sensitive, accurate VOM, put it on the outputs at the two taps (WITH speakers connected, of course) and measure AC volts.
    Peter W.
    Peter W.

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    Re: Magneplanar MMG's on the 8-ohm tap?

    Post by Peter W. on Fri Dec 21, 2018 1:37 pm

    A few points to add:

    I keep three (3) Tube-based (vintage) power-amps. A Dynaco ST35 in daily use in my office, a ST70 in occasional use in my radio room - it completes with three other options, and a Scott LK150 that gets used least of all, but enough.

    Speakers driven are a mixture of AR and Dynaco low-efficiency speakers running anywhere from 83 dB @ 1 watt to 87 dB @ 1 watt, and in generally reasonably sized rooms. The AR3a is rated at 4 ohms and dips below 1 ohm in actual impedance. So amplifier stability is critical. I run all speakers from the 16-ohm taps on the 70 and the 150, and my AR Athena system from the 8-ohm tap on the 35. Things tend to run a bit (just a bit) cooler, and stability is very good.
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    StevieRay

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    Re: Magneplanar MMG's on the 8-ohm tap?

    Post by StevieRay on Fri Dec 21, 2018 2:54 pm

    I think I'm getting some of this now -- but a LOT of it depends on output topology (single-ended, ultralinear, triode, and lots others including OTL), winding load for the OPT on the output tubes, and so forth.  I have curves for the KT-120 tubes but I don't know winding ratios for the ST-120 OPT taps.

    For comparisons, see the excerpt from a Stereophile review of the Rogue Stereo 100 amplifier (rated 100WPC Ultralinear, with triode switches, etc.) for MAYBE a possible comparison of what the VTA ST-120 is doing:

    "Rogue specifies the Stereo 100 as offering 100Wpc (20dBW into 8 ohms) in Ultralinear mode. Defining clipping as when the THD+noise in the output reaches 1%, my measurements of the Rogue's clipping power were lower than this. Into 8 ohms from the 4 ohm tap in Ultralinear mode (fig.7), the Stereo 100 clipped at 56Wpc (17.5dBW) and 83Wpc into 4 ohms (16.2dBW, fig. 8 ), both with both channels driven. The clipping power from the 8 ohm tap was 80Wpc into 8 ohms (19dBW) and 85Wpc into 4 ohms (16.3dBW). As anticipated, less power was available in Triode mode. The Rogue clipped at 37Wpc with the 4 ohm tap driving 8 ohms (15.7dBW, fig.9), and at 47Wpc into 4 ohms (13.7dBW, fig.10). The maximum output from the 8 ohm tap into 8 ohms in Triode mode was 40Wpc (16dBW). Figs.7–10, which were taken with the left channel, reveal that the Stereo 100 has respectably low distortion at low powers."

    Read more at https://www.stereophile.com/content/rogue-audio-stereo-100-power-amplifier-measurements#Oe6RLjeAw6GGi3yb.99

    From what you see there, a 4-ohm load on the 4 ohm tap got you to 83Wpc, and on the 8 ohm tap a 4 ohm load got to 85Wpc -- not really any difference.  But I can't find any spec for the feedback in the Rogue (spec'd as 13 or 14db for the ST-120?).

    FYI -- for fans of the VTA ST-120 (of one I am VERY MUCH SO) you get about the same amplifier for less than half the price.  The casework on the Rogue is pretty, and a nice built-in bias meter, but if you're crafty enough to make your own chassis, you can do anything.

    The ONLY thing I see missing is a complete set of measurements on the VTA ST-120.  Having data like that above would probably sell more amps in the long run.  The 'Nerds' like me want to know.....especially the measurements on a 'typical' KIT amp.  You always have that fear in the back of your head with kits, even though if given the choice I'll always take the kit -- it's just fun and at the end of the day I KNOW the quality of the build!
    corndog71
    corndog71

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    Re: Magneplanar MMG's on the 8-ohm tap?

    Post by corndog71 on Sat Dec 22, 2018 10:41 pm

    Another way to get more “magic” out of your MMG’s is to install better crossovers.

    I happen to know a guy who can provide them for you.

    Call Danny Richie

    http://gr-research.com

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    Re: Magneplanar MMG's on the 8-ohm tap?

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