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In Reply to: RE: A few quick(ie) questions..... posted by Dr. Toobz on September 13, 2009 at 20:34:25
a) Can the acrylic base be painted as-is
Sand it and you should be fine. Don't trust the Krylon paint that is made for plastics, it sucks.
b) Does one need to use a coupling cap to keep DC off the transformers if using the same Specos from the SEX amp?
Yes, we use the stock coupling caps and wire the Speco as an autoformer.
c) I would want to go from 8kohms to 250 ohms to power my Beyerdynamic DT880's.
No prob, go 8K:500, the reflected load will work out. Do wire the secondary in series with the primary to use the entire winding.
d) Would series resistors have to be used on the headphone jack if the output impedance ended up being lower than 250ohms?
Try it and see, but I would say no. Low impedance/high sensitivity cans seem to work OK, but that is somewhat subjective.
e) any ideas on what size Hammond box would cover the entire battery arrangement?
Why? It would make them pretty tough to get to and change. If you bought a Seduction base, you could build the kit with the batteries on the bottom...
Follow Ups:
Thanks for all of the great answers! I'm feeling pretty good about this turning into a decent headphone amp.
As for the Hammond box, my idea was to flip one upside down over the batteries, but not attach it - similar in fashion to a transformer bin (though unsecured). It would be for looks as opposed to utility - all I would have to do to change the batteries is lift the box/bin. If I wanted to get fancy, I could put tabs on the box and make a few holes for it to "snap" in place, but I don't see much benefit. I just think the amp would look cooler with a hammertone plate and black matte battery box as opposed to bare cells - I'm more for a refined, pseudo "high end" look myself.
I may just mount the batteries underneath as some have suggested, since I never secure the chassis plate(s) on anything I build (I prefer them to "float" so I can get underneath easily).
One more thing: at only 3VDC for the filaments, do these tubes even glow? (Half of the fun is seeing the tubes light up in a dark room while listening to music - which is why I wish I had something with big 300B, 6L6 or 6AS7 tubes to show off. The 0D3 rectifier tube used in the Foreplay would look awesome as well....)
The filament voltage is 1.5v, and with the size of the tubes and the top getter, you don't see much filament glow. Heck, they don't even really get hot!
If you really want a hammertone plate, PM me and I can discuss some options with you since I do the plate machining.
-Paul
You might want to experiment a little with the loading as Paul suggested. I have a pair of DT880's (250 ohm) and for some music I like to use an adapter plug which has a 120 ohm resistor built into it. I used it with an OTL amp with a 120 ohm output. With the adapter the sound of the highs become a little softened and the bass becomes a little more prominent. I liked it that way with digital sources. The 880's seem to lend themselves to "tuning" like this more than any other phones I tried this with.
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