Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Hammond 125CSE, Type 42 Amp, DIY Fostex 126En Speakers Come Together As One!

When you buy Hammond transformers (or most any transformer for that matter) you get a little schematic sheet that shows how to hook it up.

Whatcha do is find the impedance of the speaker you're using and the primary load.  I'm going to run an 8 ohm speaker - and I need a load of 7,000 ohms.  There isn't an exact match, which isn't too much of a concern for our amp.  I'm going to go with a 10K load - this will give a bit less power and (theoretically) a better bandwidth.  So I'm going to just connect the GRN (aka green) and black wires to my speaker terminals.

There, there now.  This won't hurt a bit.

I'm soldering the wiring for the new output transfomer.  I got this way cool Opti-Visor with LED lights on it.

Now we have the new transformer running on the amp and hooked back up into my "lab" bedroom system.

With the stuffing in the speaker, and the bigger output transformer on the amp, I actually have bass extension!  Not a ton, but a nice amount for such a small driver.

The speaker is really detailed - string quartets sound pretty amazing.  You can hear little changes in bowing! 

The sound is a bit more congested playing full symphonies, but that's to be expected.  Still, not bad at all.  I find myself really listening to the music - I think one would say it's "more involving."

I believe I mentioned the baffle step correction circuit.  I read a couple of articles such as the one here.  Then I cooked up this one

This goes in the line between the preamp and amp.  The pot lets you adjust a cut of up to 6 db at my "center" frequency, which I calculated to be 624 Hz (!).  Basically you calculate for the value of the capacitor - in my case it was .017uF.  I went through some .02 caps and measured them and found one that was about .018uF.

Martin King has done a lot of research on diffraction loss as well.  Most of the BSC designs I've seen use a circuit in the speaker line, which compensates for the rolloff, but also cut the speaker's efficiency.  I wanted to avoid that and so used the Elliot Sound Products circuit. 

It really works in practice - I find about half rotation (maybe 3 db?) of the pot better balances the highs and mids.

I've been running my FM-3 (on the top left in the picture of the whole 'system') with the cover off.  This is what the EMM-801 tuning eye tube looks like from the top - pretty cool.


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