MPX3 Preamp Use

[n_maher]

I couldn't decide if this was a DIY thing or not, but I'm going to post this here since it also involves tube-configuration questions and most folks might not read a DIY thread.

So my lawyer has purchased an MPX3 that includes the 5687 adapters. As a part of my yearly retainer I will be attempting to add preamp outputs to this amp. The question came up during negotiations as to what the best tube combo would be given that this device will be much more preamp than headphone amp. So what say you, masters of the SP domain?

Also, can anyone come up with a plausible reason that I can't just tap the headphone output for the preamp output? I've read other SP modding threads saying that this is all Mikhail does and that certainly sounds easy enough.

Thanks in advance.

Nate

[tkam]

You should be able to do that no problem. That said I know myself and many others have run into the problem of the SP amps just having way too much gain to have a reasonably useful range on the volume control. Of course a lot with depend on the power amp in question and how loud the person listens.

[PaulB]

Yes, too much gain can be a small problem when using MPX3 as preamp, especially with small room, high voltage output source, relatively higher power amp, and more sensitive speakers. A step attenuator would be a good idea to make sure it sounds good even at lower marks.

With advice from SP masters I enjoy the preamp sound with 396A (on 2c51 adaptor) as driver, either 6BL7's or 5687's (with adaptors) as output tubes. 6BX7's are coming soon.

Beside and despite the gain small issue, the MPX3 sounds damn good as preamp IMHO.

[luvdunhill]

Too much gain you say? Perfect for a pair of these:

http://www.firstwatt.com/products/f4.htm

and a 0.24mV output moving coil cartridge....

Nate: Please keep me posted on this, as I'm going to do the same thing. First things first, how do you get those damn screws off? Honestly, this is what's been holding me back from doing this.

[philodox]

Cool, those things would be perfect with my headamp as a pre, since it puts out a crazy amount of voltage swing already.

[tkam]

First things first, how do you get those damn screws off? Honestly, this is what's been holding me back from doing this.

If Mikhail is using the same screws on everything these days you'll need either a 1/16 or 5/64 hex bit.

[luvdunhill]

Cool, those things would be perfect with my headamp as a pre, since it puts out a crazy amount of voltage swing already.

my plans exactly... I have about 50 lbs of Plitron toroids and four sets of PCBs ready for the dual dual mono version (can I coin a term? basically, bi-amping each speaker)

[philodox]

Is the design available? I did a quick look and couldn't find it.

[luvdunhill]

If Mikhail is using the same screws on everything these days you'll need either a 1/16 or 5/64 hex bit.

thanks! I looked at some point at Lowe's / Home Despot and they didn't have anything. Perhaps I need something like this:

[luvdunhill]

Is the design available? I did a quick look and couldn't find it.

http://www.firstwatt.com/downloads/f4_om.pdf

[philodox]

Sweet, thanks.

[luvdunhill]

Sweet, thanks.

two other helpful links for obtaining parts, etc...

http://www.diyaudio.com/forums/showthread.php?s=&postid=1374762#post1374762

http://www.diyaudio.com/forums/showthread.php?s=&threadid=110654

[n_maher]

Could you snap a quick picture of one of the fasteners?

thanks! I looked at some point at Lowe's / Home Despot and they didn't have anything. Perhaps I need something like this:

[luvdunhill]

Could you snap a quick picture of one of the fasteners?

They are exactly has tkam described, they require a very small (I had no idea how small, or even torx, fractional hex, metric hex, etc. because it's so small and hard to ascertain the shape) hex key. Something like this:

Honestly, I don't think a camera will get an accurate picture.

[n_maher]

If your camera has a macro mode it might. It is often represented by the graphical symbol of a flower, like this:

[luvdunhill]

nate:

perhaps, but remember tkam is suggesting a 1/16" bit, so it will be quite small, and the screws are black and the chassis is glossy black.

I'm going to pick up one of those 100pc security bit thingys at harbor freight (it's on sale!) this weekend and I'll report back

[n_maher]

Cool, any reported results would be great. And the only thing I was hoping to find out via the picture is whether or not a security bit is needed. I have the necessary Torx bits already, but they're standard types and don't have the center hole usually associated with a security style fastener.

[grawk]

They're hex and not torx tho. Think REALLY small allen wrench.

[luvdunhill]

Cool, any reported results would be great. And the only thing I was hoping to find out via the picture is whether or not a security bit is needed. I have the necessary Torx bits already, but they're standard types and don't have the center hole usually associated with a security style fastener.

definitely not a security bit.. don't forget there are SAE hex and fractional hex as well.

[Voltron]

I just noticed this thread after PM'ing you and your lawyer about the topic of the best tubes for preamp usage. I have found that the 2c51 driver and 6BL7GT output is really good and still allows for a reasonable volume range. I previously found that all 6SN7s with my old MPX3 was a bit soft and unfocused, but with 6414 driver and 6BL7GT outputs that one sounded good but the gain was high. If he runs into trouble with the 2c51, he can always try a K-R black glass or Tung Sol round plate 6SN7 as the driver and see if that calms it down for quieter listening.

As I said elsewhere, he was genius to ask for your services rather than sending the amp to SP for the pre-outs. I am not a DIY guy, but I really don't see why you couldn't wire them just as you describe.

[n_maher]

They're hex and not torx tho. Think REALLY small allen wrench.

Cool, I've got a fine selection of metric and standard hex wrenches.

[luvdunhill]

Cool, I've got a fine selection of metric and standard hex wrenches.

so, they might work. I'll share my experience... There was no way in hell those screws would move with a normal Allen key type wrench. In fact, I'm pretty sure I stripped one trying, so this could be interesting...

Also, a few more concerns. Depending on if it's a P2P wired or PCB MPX3, drilling holes for new jacks may be a PITA, as everything is top mounted and the whole enchilada is quite heavy at that. It may be easier to convert one of the jacks, assuming he has two with source selector knob on the front. This is what I'm planning on doing.

[luvdunhill]

I have found that the 2c51 driver and 6BL7GT output is really good and still allows for a reasonable volume range.

seems reasonable. I was thinking about something like a 12AY7 driver tube into a pair of 6BX7s. Just curious, do you actually mean the 6BL7GT or perhaps the GTA? If the former, then you're probably the only person I've met that has recommended these tubes. I love 'em, no one else will try them

[Pars]

You need something like these. If you can find a brand called Bondhaus they are nice, but I also have some similar to these and they work fine.

Regarding the key sets at Harbor Freight... some of these sets have really soft metal and don't take very well to much force.

[luvdunhill]

You need something like these. If you can find a brand called Bondhaus they are nice, but I also have some similar to these and they work fine.

Regarding the key sets at Harbor Freight... some of these sets have really soft metal and don't take very well to much force.

I just picked up the driver set from HF. It is definitely the 1/16 size. I'm going to try it in a hand held screwdriver first (depending on my fustration level, then I'll whip out the DeWalt) and will report back.