Add balanced input to M-125?

Hi all,

Apologies up front if there is already a thread on this, but I did not find the answer after a cursory search of the board.

Has anyone tried adding a balanced XLR input (in place of or in addition to the unbalanced RCA ins) on the M-125 monoblocks?  

If space allows, I would be interested in adding some XLR jacks and a Jensen transformer, but would like to know if others have attempted this.  What are some potential problems one should consider before attempting?

Thanks for any ideas.

Best,
Paul

ptstucky wrote:Hi all,

Apologies up front if there is already a thread on this, but I did not find the answer after a cursory search of the board.

Has anyone tried adding a balanced XLR input (in place of or in addition to the unbalanced RCA ins) on the M-125 monoblocks?  

If space allows, I would be interested in adding some XLR jacks and a Jensen transformer, but would be interested to know if others have attempted this.  What are some potential problems should one consider before attempting?

Thanks for any ideas.

Best,
Paul

Hi, Paul, space would be a consideration, for one thing. I believe Bob L. did address this matter at some point. Might PM him.

Anyone do the XLR’s on a M125?

B&WTube wrote:Anyone do the XLR’s on a M125?

That does seem like a reasonable thing to do. It galvanically isolates the input and eliminates ground loop issues. They may cause a slight sonic change.

It's probably easier than lifting the audio gnd off the chassis.

two ways to do balanced inputs, one is simply use an XLR connector, and tie the GND and - legs together.
The better way is to use an isolation transformer, a good one costs around $80 each (Lundahl or CineMag)

Funny I missed this thread before as it's something I'm interested in doing.  I mounted XLR's next to the RCA's before building as there is plenty of room.  I also considered just using the RCA hole as a lead hole for the XLR punch, but there is not enough room to do that.

Since I have a long run from preamp to amps I was expecting at least a little hum, but in fact the amps are dead quiet with my ear on the speaker (very power hungry Martin Logans), so I have put that project on the back burner for now.  But I have a pair of Jensen JT-11's with wire leads coming out of the case (they come in various styles) and it looks like they would mount using the included brackets on one corner of the driver board.    If I try it I'll let you know how it works out.  I've also considered just connecting pins 1 and 3 in parallel with the RCA,  not a true balanced connection.

Another option would be for Bob, Roy, or someone else to redesign the driver board for true differential input.  Since one half of one dual triode is doing nothing, that would seem to make sense.  I thought about ways the present design could be modified, but the biggest challenge to me is how the feedback feeds into the cathode of the voltage stage.  That would have to be rethought since feeding a differential stage like that would result in the same phase coming out of each half, assuming the common cathode resistor required for a differential pair.  But if they came out with a new driver board kit with differential input I'd consider buying it.

Looking at a schematic for the Dynaco Mark VI I found somewhere on the Tubes4HiFi site, it did have a differential input and XLR connector, as well as a rack mountable panel, suitable for pro installations.  It used just one stage for both voltage amplification and phase splitting, very different from the M-125 design.

XLRs on an amp or other audio equipment add a sense of heavy duty design and imply a pro studio pedigree. You use balanced low impedance mics when you have a long cable run but a typical mic level signal would only be a few thousandths of a volt. A preamp would put out at least one volt I would imagine, probably more, so the issue of losing signal due to long cable runs between the preamp and picking up hum is not a big deal. Feeding high impedance line levels from the preamp to the amp through two conductor RCAs would seem to be fine. Adding balanced XLRs would look impressive but not sure it would have any practical improvement to the system.

Bob Latino wrote:XLR inputs for the M-125 monoblocks ? The general rule with all the VTA amps is that if what you want to add in does not impact the sound quality of the amp, you won't get this item with the standard VTA M-125 kits. No XLR inputs, no VU meters, no bias meters, no autobias boards and no tube cage. None of these 5 items will make any real impact on or "upgrade" the sound of the amps. To my knowledge, there are no other USA made 125 watt tube monoblock amp KITS.

Read the link below about a comparison of the VTA M-125 with the original Dynaco Mark VI.

VTA M-125 vs. Dynaco Mark VI

Bob

I get the philosophy of not adding unnecessary complications, but it's not really fair to lump balanced inputs into the same category as VU meters.  In the first place an XLR is inherently just a better, more reliable connector.  In the second, common mode rejection can make a difference in hum or noise pickup or ground loops even at line level, though it doesn't appear to in my particular case.  I have definitely had situations with buzz using unbalanced runs but not with balanced.  Some people feel balanced connections sound better, possibly from low level noise not specifically audible by itself, but it does necessitate either an extra transformer or more complicated circuitry.

You're right about tube cages not improving sound.  They can also interfere with ventilation and cause heat build-up, depending on the design, as well as interfere with the visual appeal of the glowing tubes.  However, they can make equipment more child safe or pet safe.  I once had a cat land directly onto a hot griddle.  He appeared to be unscathed, and never repeated that mistake.  My current cats haven't learned any hard lessons so far from the amps that I know of.  If they did, I expect the worst consequence would be a nose burn that would heal.  Still if there were coarse cages available, such as used by PrimaLuna, I'd consider them, but I'm not worried enough to consider fabricating them on my own.

I have a clear memory of about age 3, touching a tube of my Dad's Pilot integrated amp (never saw that brand in my vintage equipment browsing but I grew up with it).  Since I clearly wasn't supposed to do that, I was able to conceal the 2nd degree burn blister I got from it.  I survived, and never repeated that mistake.  The amp lived in a cabinet next to records, which occasionally would lean on the amp and get melted.

From your link, it looks like my memory of the MK VI was incorrect.  It used a pentode voltage amp and split load phase splitter.  Pins 1 and 2 are just tied together (though it's hard to see from the text printed over it).  You still got to use a better connector though.