Welcome! Need support, you got it. Or share you ideas and experiences.
173.28.169.173
The "building bug" is hitting me again, and my new alumimum iMac is getting lonely and needs a tubed companion to soften the harsh, solid-state sounds from its headphone jack. So, I'm thinking of slapping together a Quickie for headphone use, something that would look classy next to my iMac on a dark, tobacco-red colored wood bankers desk. I'm thinking of putting one of these into a Seduction base, staining the base a very dark red or mahogany, covering the batteries with a black Hammond box, and using my spare S.E.X. non-shorting phone jack left over from when I put a shorting jack in. Here's a few questions, though:a) Can the acrylic base be painted as-is with hammer-tone spray paint, or will it need to be primed (or sanded) first? I'm assuming it is pretty slippery....
b) Does one need to use a coupling cap to keep DC off the transformers if using the same Specos from the SEX amp? Or, considering that we're looking at 2 or 3mA here, would simply going into the primaries as-is work ok? If the former, what value of capacitors would I need? (I'm assuming in the neighborhood of 1uF?)
c) I would want to go from 8kohms to 250 ohms to power my Beyerdynamic DT880's. (I also have Grados, but I don't think this pre-amp would like the 32ohm impedance.) What taps on the Specos would I use to go from 8k to 250k instead of 8k to 8ohms like on the S.E.X. amp?
d) Would series resistors have to be used on the headphone jack if the output impedance ended up being lower than 250ohms? Conversely, will loading a 500ohm tap with 250ohms drag down the load that the tubes "see" on the primary side enough to cause problems? (I'm assuming not - 8k/2=4k, which doesn't sound that stressful).
e) any ideas on what size Hammond box would cover the entire battery arrangement? I've not worked with their enclosures before, and have no idea prior to seeing the amp how much area the battery boxes occupy.
I apologize if some of these questions have been asked elsewhere - hopefully, this thread might summarize some of the common ones so that others building their Quickies in a similar way might be able to have a reference.
Edits: 09/13/09 09/13/09Follow Ups:
Can somebody tell me what the overall dimensions of the plate are, and how deep below the plate do the components protrude (assuming that a battery pack would be elswhere)? I've got a nice little wood box and would like to build a small headphone amp out of this kit.
-- Jim
The plate is a 10"x6" and 1/8" thick (give or take). The components on the bottom need around 1.5" to be really happy.
If you are going for headphone use, you will want 2" of depth.
Thanks Paul. Looks like the box I have isn't big enough, but as soon as I can scare up $100, I'll probably get one anyway. Who couldn't use another headphone amp? :-)
-- Jim
b) Does one need to use a coupling cap to keep DC off the transformers if using the same Specos from the SEX amp?
You could in theory replace the 4K plate load resistor with the Speco if you want. Use the 8K tap (brown = "0.625 watt"). But it will put the full plate voltage (36 volts) on the headphone wires, since you must use the primary 10W tap as an autoformer. And it rasies the plate load - see below.
d) Would series resistors have to be used on the headphone jack if the output impedance ended up being lower than 250ohms?
If you are using 8k:500, that's effectively 4K:250, which matches the tube's desired 4K load. The tube's 4K plate resistance combined with the 4K plate resistor makes 2K, so the output impedance on the 500 ohm tap (which is now the 2540 ohm tap) is half the load, or 125 ohms. Close enough to 120 ohms!
You might have noticed the tube now sees 2K, which is on the low side. Obviously not a problem, since that has worked pretty well. Fortunately, triodes are pretty tolerant of load mismatches! You could replace the 4K resistor with the old SEX amp plate choke; that raises the output impedance to 250 ohms but increases the power and loads the tube better.
e) any ideas on what size Hammond box would cover the entire battery arrangement?
PB's suggestion of putting the batteries below the chassis is OK - that's how I did the prototype. Just don't glue the chassis to the base! Personally, I like the batteries on top, I think it looks cool, but gustibus and disputantum after all...
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...
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.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: