Bob Fitzgerald MPX shootout

[drknstrmyknight]

I finally got around to wrapping one of Bob's MPX adapters and am
testing it. First up was to compare it to a Foster Blair modded Eico
MX99 on an Eico HFT-92 that Foster also updated.

Initial impression is it is about the same sonics. I;m going to run
this against an unmodified Sherwood 3000 and Stromberg Carlson that Craig fixed up and let you know.


http://www.acousticpsychos.com/coppermine/thumbnails.php?album=9

[Bob@FM]

Nice Work on getting it in the box. Where did you buy that box?
Did you have any problems building it, or making the adjustments?

Bob

[mhardy6647]

Is that "BFitz"?! Didn't know you were here! Welcome...

There was a nice thread on this MPX at AA a week or so ago.
If all the bits are readily available, I need to work up the gumption to build one and try it on one of my much-beloved Sherwoods.


http://img.photobucket.com/albums/v223/mhardy6647/P1020751.jpg

[drknstrmyknight]

Partsexpress:
262-070 Penn-Elcom R1196/100 Heat Sink Box 3.54" x 2.0" x 4" L $8.52
2 262-078 Penn-Elcom R1198 End Plates For R1196 Heat Sink Box @$1.92 ea

I had no problems at all, just wired her up and she lit up like a chrismas tree on an fm stereo signal. This board came to me stuffed and tested. The only challenge was finding a 12v wall wart that didn't put out 20 volts as I was warned 16+ volts would fry it. After buying 2 I found one in my junk pile.
As I got no docs with the board, I'm not sure what adjustments there are.

[Bob@FM]

Hi Mark;

Yes, BFitz here. I've tested it with the S3000 and it works great.

My tests with the S3000II showed the MPX out level on that tuner was a bit too high, but I did not investigate a simple divider yet. The Scott tuners may need a boost or cap change, the bass on my 310D was lacking and the level low. Scott experts predicted this, apparently the wideband detector has a low output level compared to other tube tuners. The Fishers tube tuners I tested also work fine.
Bob

[Bob@FM]

Hi Frank,
Thanks for the details on the box. That seems like decent deal . I now recall you have one of the early handmade prototypes without onboard regulation . If you want to swap it even for a current unit based on the current production board (N/C) send me an email. It should fit right into your existing box.
Bob

[mhardy6647]

I have an S-2000 (shown above) and an S-3000 (both unsuffixed).
The only mono Scott I have is an LT-10.
Did you try really "old school" Fishers? I have an 80R (AM/FM mono).

[Bob@FM]

Mark -
I tried a FM100R. The 80R should work, if not the workaround is not that hard - a little front end high impedance buffer or amp. I also have an FM90X here, but did not try it yet. The LM4500 input impedance is 50K ohm. The universal solution is to use a 500k or 1M input divider, followed by an amp or buffer. In many cases the coupling cap in the tuner needs to be bigger if you try and drive the 50k input on the LM4500 directly, or you will lose some bass. I have not done extensive trials on this, so would welcome eXperimentation results from people.
Another solution to a lower impedance MPX output would be to rewire the tuners existing (usually) cathode follower mono audio output. You would simply need remove the 75uS de-emphasis cap, or even switch it in and out, so you would have a flat output response to 53kHz.
Bob

[drknstrmyknight]

Hi Frank,
Thanks for the details on the box. That seems like decent deal . I now recall you have one of the early handmade prototypes without onboard regulation . If you want to swap it even for a current unit based on the current production board (N/C) send me an email. It should fit right into your existing box.
Bob

Hey - I didn't pick up on your handle, welcome!

That's nice of you, but this works and I'm lazy :thmbsp:

I hauled out a Stromberg Carlson SR-445 to try it with - what's the
DETECTOR output? I think I recall one on my Accuphase T-101 too, but never got around to investigating what it is.

[mhardy6647]

sorry I hijacked your thread, too!

[drknstrmyknight]

Not at all. Tube MPX unit prices are nuts and apparently the 19kc transformers are unobtainium, and I seem to be plagued with them failing, so a working MPX adaptor is a good thing, and knowing what they work with is valuable.
Here's the info on Bob's design
http://fmmpx.com/index.html

[Bob@FM]

I hauled out a Stromberg Carlson SR-445 to try it with - what's the
DETECTOR output? I think I recall one on my Accuphase T-101 too, but never got around to investigating what it is.

In an FM tuner, the detector circuit is what turns the 10.7MHz FM signal into the composite audio signal. Composite audio has the L+R information between 0-15kHz, and the L-R between 23-53kHz. So the detector output is exactly the same as the MPX output. It gives you the wideband composite signal with no filtering that the MPX circuit needs to create stereo. On later solid state tuners, it's also the H or Horizontal scope output.
An outboard MPX adapter can also help the sonics of solid state tuners quite significantly, for a number of reasons. It may be a more modern MPX chip with lower distortion, higher separation, and lower noise. But also it can bypass a lot of the internal wiring, switching, muting, filtering, and old electrolytic caps in the audio path. The downside is you lose muting, but it's worth it for the sound quality. It's quite easy to compare by running both outputs (tuner and outboard MPX adapter) into the same preamp and switching back and forth.
Bob

[KentTeffeteller]

Hi,

Wonder if this would work with my old REL Precedent mono tuner at work? It does have a multiplex out. Nice to know there is an alternative MPX unit for mono era tunas.

[drknstrmyknight]

It works well with my Stromberg Carlson SR-445 tuner, and the Eico HFT-92 and the Sherwood 3000. You need a strong signal, but that's true of the Eico MX99 as well. The seperation is better on Bob's box, though it's a bit brighter IMO - not unpleasantly so.

[Paul C]

You can easily regulate those wall warts (or old laptop ac adapters) to 12v using a 7812 regulator chip... or more if you need more than one amp... and a few capacitors.

This chip is in a TO-220 package, with three leads. It looks like a transistor.

One lead is in, middle is ground, and the other lead is out. A cap from in to ground, and another from the 12v output to ground to take care of any residual ripple, and you are good to go. If you need more than just 1 amp you can wire any number of them in parallel.

[Bob@FM]

Hi,

Wonder if this would work with my old REL Precedent mono tuner at work? It does have a multiplex out. Nice to know there is an alternative MPX unit for mono era tunas.

Yes, it will work fine. Just be sure there really is a signal output from that jack. The REL I had here had the jack, but it was not connected to anything. There was a service manual update from REL that had the recommended MPX output circuit. Make sure that circuit is installed.
Bob

[Bob@FM]

You can easily regulate those wall warts (or old laptop ac adapters) to 12v using a 7812 regulator chip... or more if you need more than one amp... and a few capacitors.

This chip is in a TO-220 package, with three leads. It looks like a transistor.

The latest LM4500 board uses an onboard LM317 type adjustable regulator. When the LM317 circuit uses a cap from adjustment pin to gnd, it lowers the noise and increases the stability. This circuit enhancement is also implemented on the LM4500 board, and is recommended.
Bob

[mhardy6647]

I got the mission critical bits to build one from Bob about a week ago... thanks Bob!

There are a couple of projects in the queue ahead of it, though :-(

[KentTeffeteller]

Hi Bob,

Thanks! Our Precedent had that update circuit installed. Tried a Fisher MPX unit with it. Works in Stereo. Now I can order one. :tresbon:

[DaWoofer]

Bob- if you are still monitoring this thread, would you know if your circuit works with the McIntosh MR55A? It has the Multiplex jack and it is wired. Thanks. I will also check the site listed above.