M-125 Output Impedance Tricks?

gktamps wrote:Yes, indeed there is. Although it doesn't lower the output impedance of the amps, it does essentially raise the impedance of the speakers.

I used these with my custom built speaker cabs with Seas Prestige drivers, because nominal impedance was 4 Ohms, dipping down to almost 3 at times. There was a noticeable improvement in sound quality. My current speakers are 8 Ohm, so these are not being used at this time.

https://anticables.com/store/%E2%80%9CNaked%E2%80%9D-ZERO-Autoformers-p14644819

sayntjack wrote:It sounds like your M-125 is not quite kicking butt to the level you want.  I'm wondering which VTA driver board you're using as I have a thought on why.  This year I completed a rebuild of a pair of Mk IIIs for a friend using the M125 V19 driver board.  The amps sounded nice but simply did not have the energy / dynamics of other MkIII rebuilds I've done in the past using other driver boards or my own design.  I've been scratching my head over this for quite a while and am coming to the conclusion that the driver board simply does not have enough gain to allow adequate global negative feedback for the KT88s to drive the loads with optimum dynamics.  I'm following this up with Roy but as yet haven't got into the gritty details of same.

corndog71 wrote:When was the last time you had your ears checked?  I ask this seriously because earlier this year I saw my doctor for a check up and he cleaned a bunch of ear wax out of my ears and that improved my perception of higher frequencies not to mentioned an imbalance.

Another option is to try different coupling caps on your amp.  My current favorite is Miflex copper caps.

You might also consider room treatments.  Diffusers can help open up the sound of your speakers.

sayntjack wrote:Hi ttocs.  Driver tubes:  I have not found that changing the driver tubes effects high frequency, only soundstage size and depth, bass and dynamics.  (I have the VTA M125 V19 driver board which uses two 12AU7s.)  In the phase splitter position I've found that a 12BH7 is superior in delivering a more dynamic and involving sound.  It doesn't seem to matter what brand.  Swapping the amplifier position 12AU7 with different brands and vintages I have found affects the bass and overall dynamics.
One component that can have a significant effect on sound are the coupling capacitors.  These range in price from a couple of dollars to 100+.  For clean extended highs and good dynamics I have found the MIT Multicap PPFXS to be very good.  Like tubes, capacitors need settling in time to reach their characteristic sound.  This can range from 10 or so hours to 200+ for some Teflon capacitors.
Given that you have ample punch and I don't I wonder if you could post the version of your driver board and any mods that you've made to it.  It should be printed right on the top of the board, eg. M-125 v19.

sayntjack wrote:Hi ttocs.  You say that the lack of high frequency occurs on your Martin Logan Expressions.  I looked into the ML specs - they have a built-in high power class D amp so you would only be driving the high frequency section.  According to the specs, this load is 4 ohms at 91 dB efficiency and needs only a 20 Watt amp.  So, there should be no problem with your M-125s.  When you say that the high frequency is rolled off is this from listening,  measurement with a mic in front of the speakers or measurement at the input terminals of the speaker?  Does it sound like the highs are rolled off?  There are complications to making electrical measurements on the amp output due to the complexities of the impedance characteristics of the amps and speakers and how they interact.  

ttocs wrote:

This "quoted" impedance is optimistic. The stat panels go as low as 0.7Ω.

The highs don't really sound rolled off, but they aren't as sparkling as when the Krell is used, but the Krell cannot be described as "Real" like the M-125's provide. My friends say "it's like I can reach out and touch the musicians".

Bob Latino wrote:

2. Re > "the Krell cannot be described as "Real" like the M-125's provide. My friends say "it's like I can reach out and touch the musicians".

Is this not the more important aspect of the music ?

Bob

AmpedUp wrote:Hi ttocs,
I have to respond here as we have almost the exact same audio system.  I have the M125 pair with a Don Sachs preamp.  Because my listening room is only 12x22" I went with the Impressions instead of the Expressions and I only need a single sub, Dynamo 1500 to fill my room up with sound.  That feeling that you can touch the musician is where the electrostats shine.  These and Planers are known for their superb clarity.  They do lack dynamics of some other speakers, but the ML hybrids like ours are plenty good for me.  Every instrument that I listen too sounds "in the room" with one exception - the snare drum.  I do not get the high frequency snap that comes from a live drum set.  I am not sure what you are trying to improve what you hear from your system, but there are amps that can deliver these higher frequencies but they cost way more than our VTA amps.  And those amps require speakers much more expensive our our Martin Logans to deliver that exceptional of sound quality.  I am proud of and love the sound and value of my VTA amps with the ML speakers.  Yes they could always be better but the cost of just a little improvement would be in the hundreds of dollars and many hours if I start swapping internal components and in the tens of thousands if I start swapping amps and speakers.

I love your question.  I appreciate anyone that looks to improve their VTA equipment for just a little better sound quality and shares that knowledge.  I see that you got different advice.  All will make subtle differences to what you hear.  12BH7 vs 12au7 driver tubes, Weber SS vs GZ33 vs 5AR4 rectifiers, I also run K-120 power tubes.  I could have mentioned the difference in sound with cables but that would have opened up a hornets nest of opinions.  IMHO Corndog might have given you your best options here in another article with his opinion with different coupling caps.  If you are willing to spend the $ for just a little extra quality in sound measurements, the Juniper and Mundorf caps will cost you significantly more than the K42Y caps that Bob supplied but are supposed to be of better quality.  I followed Bob's advice and I'm 100% satisfied with mine.  I do not know if these will improve your measurement tests, but just like tube rolling it can be fun experimenting.

I mentioned my experience listening to a snare drum to point out that what you hear is more important than any measurement.  In my case I do not know the measurements of my system, but I used room treatments and the PBK kit to balance my system and the music in my life is very good.  I hope that yours is also...  The $100 for the ML PBK kit is worth the time to setup as if flattened my bass curve to really dial in the best sound.  I have 2 kits and will give you the second if you would like it.

Mike...

audiobill wrote:ttocs -

For most realistic listening experience, you actually want your measured in-room response to gently decline about 1db/octave from 1K out to 20k.  Please research "Target curves".

Best,

Bill

tubes4hifi wrote:agree with amped up and corndog in the other post.
Change to a 12BH7 phase splitter and replace the output caps with either Mundorf silver-oil . . . .
Also, change R28 on the board from 10 ohms to 8.2 ohms to increase the current thru the driver.

ttocs wrote:

https://anticables.com/store/%E2%80%9CNaked%E2%80%9D-ZERO-Autoformers-p14644819

Thanks.
I looked at these a couple years ago, but am still unsure how these interact with electrostatic speakers, if at all. Still considering them, however, as a speaker-side helper.

gktamps wrote:

ttocs wrote:

https://anticables.com/store/%E2%80%9CNaked%E2%80%9D-ZERO-Autoformers-p14644819

Thanks.
I looked at these a couple years ago, but am still unsure how these interact with electrostatic speakers, if at all. Still considering them, however, as a speaker-side helper.

You could always get in touch with the company and ask; I did the same with my own specific questions. My own understanding is that they would interact properly with your electrostatic speakers, since these affect the ac domain of the interactive impedance your amps see with the speakers; IOW, this is not dependent on the differences between cone or stat impedance curves. In fact, I used these with my cone speakers because the impedance dipped low, but have never needed them with my Eminent Technology LFT-8b panel speakers, which have a nominal 8 Ohm impedance.

You seem to have identified the high frequency rolloff to be the result of using your ML speakers. My approach would not be to modify the amplifiers, but resolve, modify, or replace the speakers causing the effect, if other speakers do not exhibit this undesired trait.

However, if you want to see if minimal changes to the amplifiers will make them play better through your MLs, I agree with the driver tube change recommendations - those tubes do affect the sound in a noticeable way. I would not start changing capacitors, given the information you've provided.

I use my M125s with KT120s with JAN5963 and EH gold pin 12BH7A tubes with my LFT-8bs. Efficiency is lower than your MLs, but impedance does not dip nearly as low as your MLs, and there is no high frequency rolloff compared to my cone speaker cabinets. I avoided MLs out of this concern, even though I also use a bunch of different solid state amps (I rotate through systems), all of which can handle the impedance challenges of the MLs.

Yeah. I'm with Bob. Issue is the speaker impedance/frequency response. The autotransformers may be just what you need.

ttocs wrote:

I'll try some tubes first since this is the least invasive approach.

Next, I'd have to try the Zeros. Or maybe do this first?