MPX3 Preamp Use

[Voltron]

So I moved the MPX3 back into my main rig tonight and did some shuffling of components so that I can use it to do a little relative comparing of my two primary sources. Made a somewhat troubling discovery, with the MPX3 being fed by my DAC I'm on the first click of the stepper and it's plenty loud with my HD650s. Two steps was too much. Granted, I listen softer than the average person but having no adjustability is a little scary. Any alternate tube suggestions that Mike might try? I'm guessing he might want to investigate them.

What combo were you using Nate? If it is the 5687s as output with a 6SN7 input, then I would switch to a good pair of 6SN7s as output. The traditional MPX3 setup is 3 x 6SN7 and a favorite combo is a Ken-Rad VT-231 or Tung Sol roundplate driving Sylvanias. Not sure what combos the seller included though. The EH tubes are not in the same league at all.

[tom_hankins]

What combo were you using Nate? If it is the 5687s as output with a 6SN7 input, then I would switch to a good pair of 6SN7s as output. The traditional MPX3 setup is 3 x 6SN7 and a favorite combo is a Ken-Rad VT-231 or Tung Sol roundplate driving Sylvanias. Not sure what combos the seller included though. The EH tubes are not in the same league at all.

I agree on the 6sn7s. With the Singlepowers I have had, I have always come back to a KenRad VT-231 or TungSol roundplate in front of some kind of Sylvanias ( VT-231s, badboys, 6sn7w)

When I had the SDS-XLR here to try as preamp I liked the 2c51 in front of 6bl7 gta.

[n_maher]

Thanks guys, I'll see what the seller included as well as what 6SN7s I have kicking around. The EH combo was definitely not worth trying again. And it's also been clarified that I'm a church mouse when it comes to my listening levels. Last night I spot checked levels and my average was around 65db on the first click of the stepper, the next two steps each raised the level 5 to 6db with each step, which made things a bit too loud for my ears. What can I say, I live in the country with almost no ambient noise and I like my hearing.

What combo were you using Nate? If it is the 5687s as output with a 6SN7 input, then I would switch to a good pair of 6SN7s as output. The traditional MPX3 setup is 3 x 6SN7 and a favorite combo is a Ken-Rad VT-231 or Tung Sol roundplate driving Sylvanias. Not sure what combos the seller included though. The EH tubes are not in the same league at all.

I agree on the 6sn7s. With the Singlepowers I have had, I have always come back to a KenRad VT-231 or TungSol roundplate in front of some kind of Sylvanias ( VT-231s, badboys, 6sn7w)

When I had the SDS-XLR here to try as preamp I liked the 2c51 in front of 6bl7 gta.

[guzziguy]

What can I say, I live in the country with almost no ambient noise and I like my hearing.

WHAT?

[Dusty Chalk]

whimp. it's a scientific fact that you can't hear all the detail in music until you hit 115 dB.

No, it's a scientific fact that you can't hear all the detail in music until you hit your mama.

[n_maher]

whimp. it's a scientific fact that you can't hear all the detail in music until you hit 115 dB.

Just for you Reks, I'll try the second step of the stepper this afternoon.

[luvdunhill]

so, I'm looking inside yet another MPX3, this one has a "XLR" jack wired up in the back. The pin 2 (left when looking at the back of the jack) is wired to the input signal and the ground is taken, not from the remain two pins, but the brass tab that is tied to the jack itself. Symptom is that when using this jack or the other RCA jacks, a bit of the other signals leak into the selected input if multiple inputs are turned on.

So, question is, is this wiring scheme for a XLR->RCA converter valid? Is this the source of this problem, or is it related to the switch itself?

[luvdunhill]

so... I finished adding all Black Gates to my MPX3 last night... all the low voltage caps are upgraded now... good thing is, I just have to keep it on without signal to burn 'em in, as the CRC heater supply was effected mostly. I'm not sure if this will make much a difference, but what the heck.

[tyrion]

My MPX3 arrived after making the trip from Alaska to New Hampshire to Florida. I immediately through it into my system as a preamp for the Berendsen STA-150. I am picking up a slight hum through the speakers that Nate had described to me. It certainly isn't noticeable when the music is playing and non existent through headphones. I'm using a couple of Sylvania 5687s and a Tung Sul 6SN7. I just grabbed the first pair of 5687 and the first 6SN7 I came across and threw them in. The gain is a little high but workable. It would be better if I didn't have the stepped attenuator but still not a big problem. I've only just begun to listen to the combination and so far I like what I hear. The one thing I noticed is that the imaging is really amazing. I'm listening to Patricia Barber Constntinople and the I am really getting a sense for the placement of the instruments. Also, the nice deep controlled bass also stands out at the moment. There is a nice sparkle on top with a real nice sense of decay. Just some preliminary impressions. I can definitely live with this sound, despite the ever so slight hum when the music is paused. It will take more getting used to not having a remote. I sit so close and my chair rolls that it's not an issue. I also use my phone as a remote for itunes and can control itunes volume if I need to get it quiet fast.

Nate, another great job! Thank you.

Here are a couple of pics:

[n_maher]

Sweet, Mike, glad to hear that you got it all up and running and that it survived the journey ok.

If the hum becomes bothersome I'll talk with Mikhail and see what he recommends. I routed the wire along with the signal wire but I'm wondering if in a chassis that tight that it really requires shielding of some sort.

[tyrion]

Sweet, Mike, glad to hear that you got it all up and running and that it survived the journey ok.

If the hum becomes bothersome I'll talk with Mikhail and see what he recommends. I routed the wire along with the signal wire but I'm wondering if in a chassis that tight that it really requires shielding of some sort.

It's not bothersome at all. I don't think it has any effect on the sq. When you are 6 or 7' from the speakes, you really have to concentrate to hear it.

[n_maher]

It's not bothersome at all. I don't think it has any effect on the sq. When you are 6 or 7' from the speakes, you really have to concentrate to hear it.

Hey, as long as it's not annoying you I'm ok. I wish I could have found its root cause but with limited testing scenarios (not to mention that the one speaker amp i do have has a slight hum) meant not knowing if there was even an issue. That's what you get from the back alley audio group.

[tyrion]

Hey, as long as it's not annoying you I'm ok. I wish I could have found its root cause but with limited testing scenarios (not to mention that the one speaker amp i do have has a slight hum) meant not knowing if there was even an issue. That's what you get from the back alley audio group.

I think I made up my mind, I will go with the MPX3 as a preamp, keep the power amp and return the integrated. You and Mikhail can brainstorm it at CanJam.

[tkam]

Hey Mike, it's cool that the MPX3's usable range is working out for you. The SP amps do make wonderful pre-amps if the situation makes them feasible.

[tyrion]

Hey Mike, it's cool that the MPX3's usable range is working out for you. The SP amps do make wonderful pre-amps if the situation makes them feasible.

I would like to have more range but it's not a big deal. The real test will be if the lower volume is soft enough for a Sunday morning when it's real quiet and the kids are sleeping. I don't listen to speakers too often under those circumstances but there are occasions when I do.

[tyrion]

if the range is too limited, you could always get some attenuators...

That's worth a try.

[Dusty Chalk]

Or you could just pretend there's someone in the room who doesn't know the words...and no sense of melody...

...and a really low voice...

[tyrion]

Or you could just pretend there's someone in the room who doesn't know the words...and no sense of melody...

...and a really low voice...

I don't have to pretend, you just described me.

[luvdunhill]

If it were me using the MPX3 as a preamp, I'd try a few 3 6SN7 combos and think about ditching the 5687. They are great sounding tubes, but in a preamp context I think most "standard" recommendations might go out the door. I'm going down this route as well, and can comment later on as well.

[tyrion]

If it were me using the MPX3 as a preamp, I'd try a few 3 6SN7 combos and think about ditching the 5687. They are great sounding tubes, but in a preamp context I think most "standard" recommendations might go out the door. I'm going down this route as well, and can comment later on as well.

I plan on doing that. Right now all the 6SN7 tubes I have are different so I don't have a matched pair to through in at the moment but do have matched pairs of 5687s. Time to search for another pair of Ken Rads.

[hirsch]

A few quick thoughts...

First thing to do when there's hum in almost any audio product is to check for a ground loop. Try using a cheater plug from Rat Shack so that the ground pin isn't connected (the MPX-3 will still be grounded through the ground of the interconnects from your source). You can also plug the MPX-3 into a circuit (not just a socket, but the whole circuit) with nothing else plugged into it, and see if there's still hum. If either of these gets rid of the hum, then it's simply a matter of tracking down which component is interacting with the MPX-3 to create the ground loop. All components of your audio rig should be hooked into the same wall socket/power strip/conditioner/whatever you like so that they are all grounded at the same place to minimize the chance of a ground loop.

Another common source of hum is EMI/RFI through unshielded interconnects. If your interconnects are not shielded, try one that is. If that eliminates the hum, then you've identified the problem, and can deal with it.

If neither of the above works, I'd try different tubes to insure that a noisy tube wasn't affecting things.

[n_maher]

with my fist PPX3 Slam i did all of those things and still had a hum (this was when i was using CD3000s). i ended up slapping the ety 75 ohm adapter on the end, and it killed it, as the hum was essentially a low level signal, and the added removed the phone's ability to pick it up, so the attenuators would probably be worthwhile if none of hirsch's very good pieces of advice don't work out.

The weird thing is that the headphone is pretty damn quiet, certainly quieter than the preamp outputs that I added even though they're essentially the same thing. So I'm pretty sure that it's not a ground loop although now that I think about it the amp does have a slight issue with the grounding of the volume pot. Not sure why this would effect the preamp outputs more...

[tyrion]

I have to pick up a second cheater plug tomorrow and I will try out Hirsch's suggestions. If that doesn't work then I will get the attenuator and see if that does the trick.

[grawk]

I have to pick up a second cheater plug tomorrow and I will try out Hirsch's suggestions. If that doesn't work then I will get the attenuator and see if that does the trick.

Cheater plug won't fix the too much gain problem

[tyrion]

Cheater plug won't fix the too much gain problem

No it won't. Now that you mention that, I just set up a tt. With the gain on the phono set at low, the gain for the mpx3/power amp is high, but not too bad. The problem is when I use either the mpx3 headphone out or he 007t/O2, the gain is way too low, especially with the 007t/O2. If I move the phono to med gain, the gain will be too high with my speaker amp and perfect with the headphone amp. This might be a problem.

Gratuitous tt pic: