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Author Topic: SMF-1: analog dry for digital boxes.  (Read 3360 times)

Kaz Kylheku

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SMF-1: analog dry for digital boxes.
« on: December 03, 2012, 06:26:50 PM »
Here is a little project of mine to build a "last piece of the puzzle" piece of rack gear that I badly needed.

It went very well.

http://www.kylheku.com/~kaz/smf-1/
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joecool85

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Re: SMF-1: analog dry for digital boxes.
« Reply #1 on: December 04, 2012, 04:18:32 PM »
Here is a little project of mine to build a "last piece of the puzzle" piece of rack gear that I badly needed.

It went very well.

http://www.kylheku.com/~kaz/smf-1/

Killer project, came out great.  Only thing missing is a new faceplate for that DOD box.

Also, I moved this into the Preamps and Effects section as it fits better here.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Kaz Kylheku

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Re: SMF-1: analog dry for digital boxes.
« Reply #2 on: December 19, 2012, 12:27:14 AM »
A few days ago I had a "bad tone day", which turned into two days, then three ...

Man, I was going crazy. Why is the sound so harsh?

I adjusted things. Unplugged jacks, plugged them back in.

Then I traced it to the SMF-1.  I noticed the frequency knobs seemed to have become less effective. Turning down the presence somehow wouldn't filter out all the harshness.

Pop it open. Oops, voltage regulators are hot to the touch. Putting a finger on them soon brings it near the threshold of pain.  Voltages seem okay though.

So, long story short, put heatsinks on them, and back to awesome tone. Creamy raging sustain; three dimensional cleans, etc.

The regs must have been near their limit, and so probably kicking some current regulation in and out, causing distortion.

Absolute night and day difference, the imaginary equivalent of which would require the combined confirmation bias of 1000 audiophiles.   :lmao:

On a different topic: making progress on the faceplate. It is designed, and I have procured the 19" by 1 3/4" aluminum. I am going to try printing the artwork on a color laser printer, and transferring it via heat, then clear-coating some acrylic over that.
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phatt

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Re: SMF-1: analog dry for digital boxes.
« Reply #3 on: December 19, 2012, 06:50:36 AM »
The reg will run very hot if it has to drop a lot of voltage.
You may need some series resistance before the reg.
Phil.

Kaz Kylheku

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Re: SMF-1: analog dry for digital boxes.
« Reply #4 on: December 19, 2012, 10:02:53 PM »
Yes, the regulators do drop a lot of voltage. This is inherited from the device.  The original circuit board used the same type of voltage regulators (without any series resistor).   The transformer is such that the filter caps charge to 30 volts, for a R-to-R of 60!  Totally crazy drop-out down to the desired +/- 15V.

I played it loose by not stealing the heat sinks from the very beginning.

The original board has 12 IC's, most of which are RC4559's. But my four NE5532's can easily have about the same total current draw as all the IC's on that board.

Anyway, the wrap up of this story is that although the heatsinks helped, there was still a problem. I noticed a degradation in sound quality that happened after warmup. I could easily reproduce it by turning the unit off for a few minutes and then on again. Within a minute or two something would change in the tone (though not as drastic as before I put on the sinks). So I concluded that perhaps one or both of the regulators are shot.

I ended up transplanting the regulators from the original board.  They are Motorola-branded units! Vintage.  8| (This was built about six years before Motorola stopped making semiconductors and spun off On Semi.) With these regulators, it sounds fine.

The heatsinks I took from the original unit had legs and were soldered to pads, but I broke those off. The sinks are light and the regs can just hold them up.  On the other hand, I did something better: I screwed the sinks tightly to the regulators with small nuts and bolts. In the original unit, they were just slipped on and held by a very light leaf-spring tension.  The heat transfer is better with the tight bolting. The matching hole for a bolt was already manufactured in the heat sinks, just not used.

If those particular voltage regulators survived 19 years with those heat sinks and that current draw, I figure they are proven in those conditions.

But if I build another one of these units, for sure I will for sure avoid a transformer that puts anything over 20 volts on the filter caps.
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sim0n

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Re: SMF-1: analog dry for digital boxes.
« Reply #5 on: December 20, 2012, 10:29:01 AM »
The reg will run very hot if it has to drop a lot of voltage.
You may need some series resistance before the reg.
Phil.

zener diodes also work well

Kaz Kylheku

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Re: SMF-1: analog dry for digital boxes.
« Reply #6 on: December 24, 2012, 02:28:42 PM »
Faceplate progress.

The aluminum panel is complete, but I'm having difficulties transferring the artwork via toner transfer.

So far, the best results (though not nearly good enough) were achieved by heating the whole thing in an oven after ironing over the transfer paper, and then ironing some more. One problem is that the 1/8" slab of aluminum acts like a big heat sink with regard to the clothing iron.

The surface is wet-sanded up to 400 grit paper.

I installed the power switch the right way: I turned it 180 degrees so that you push the rocker upward to turn it on. Also, I discovered that the original hex bolts that attach the panel to the body were poorly threaded: very difficult to turn.  I replaced them with attractive stainless steel machine screws with a 6x32 diameter/pitch which screw in perfectly easily.

Turns out there is a reasonably priced water jet cutting shop near our work; next time I will have this kind of thing neatly cut by machine rather than huffing and puffing.
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