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Crack;Bottlehead OTL - Page 107

post #1591 of 6029

I tried polishing the top plate once... gave up shortly thereafter and went with paint instead rolleyes.gif

 

Although now that I think of it, a mirror finish paint might work... 

post #1592 of 6029

Has anyone ever tried putting an adapter on the headphone out to make this work better with a lower impedance headphone (like the D7000)?  Something like a 70Ohm resistor ?

post #1593 of 6029

I've tried it with both ~70 ohm and ~250 ohm resistors. Both seemed to work just fine. 

post #1594 of 6029
Quote:
Originally Posted by elwappo99 View Post

Has anyone ever tried putting an adapter on the headphone out to make this work better with a lower impedance headphone (like the D7000)?  Something like a 70Ohm resistor ?

 

Just what are you expecting that resistor to do?

 

The problem with an OTL for low impedance headphones is the small size of the output cap causing bass cutoff, and poor damping due to the high output impedance. Adding a resistor will marginally improve the first issue, while simultaneously making the second worse.


Edited by Beefy - 3/12/13 at 4:57am
post #1595 of 6029
One significant issue with a series resistor is that the added resistor creates a voltage divider with the headphone impedance and thus the headphone sees much less signal voltage, i.e., it won't play as loud. If the cans are very sensitive it might work OK. This is the kind of experiment that is simple and inexpensive to try - and share your results! You might try about 120 ohms as a starting point with sensitive 32 ohm cans. Resistor should be conservatively rated for about a watt of power dissipation, half watt is marginal but probably OK.
post #1596 of 6029
Quote:
Originally Posted by Armaegis View Post

I tried polishing the top plate once... gave up shortly thereafter and went with paint instead rolleyes.gif

 

Although now that I think of it, a mirror finish paint might work... 


What did you use for polish? I'm going to give Mothers Mag & Aluminum Polish a try. I've never used it; hope it works.

post #1597 of 6029
Quote:
Originally Posted by stephenbou View Post


What did you use for polish? I'm going to give Mothers Mag & Aluminum Polish a try. I've never used it; hope it works.


If youre going for a mirror finish I think youre gonna need something a lot more abrasive than that.

post #1598 of 6029

The one time I tried for a mirror finish, I started with some wet sanding with sandpaper from 400 to 1200, which took more time than I care to admit. I then started with a dremel polishing kit, did one pass across the surface... and gave up. If I had a bigger polishing attachment for a drill, maybe, but that's hours of polishing in total. It might be more time/cost/sanity effective to go with a reflective paint, several layers of clear coat, and polish the top coat.

post #1599 of 6029

Or get a cheap random orbital sander and use it as a buffer too: LINK

post #1600 of 6029
Quote:
Originally Posted by Doc B. View Post

One significant issue with a series resistor is that the added resistor creates a voltage divider with the headphone impedance and thus the headphone sees much less signal voltage, i.e., it won't play as loud. If the cans are very sensitive it might work OK. This is the kind of experiment that is simple and inexpensive to try - and share your results! You might try about 120 ohms as a starting point with sensitive 32 ohm cans. Resistor should be conservatively rated for about a watt of power dissipation, half watt is marginal but probably OK.

 

Quote:
Originally Posted by Beefy View Post

 

Just what are you expecting that resistor to do?

 

The problem with an OTL for low impedance headphones is the small size of the output cap causing bass cutoff, and poor damping due to the high output impedance. Adding a resistor will marginally improve the first issue, while simultaneously making the second worse.

 

Awesome to hear! It wouldn't be for like an orthodynamic that needs a lot of power, but being able to swing the oddity in my collection (Denon D7000) without having a whole separate amp would be great! thanks for the info.

post #1601 of 6029

As far as polishing aluminum goes, sorry for the photos of my motorcycle, but this is how I did it cheaply.  In the case of the Crack panel, you can probably start at 800, and work your way to 2000 or 3000 grit sandpaper.   At that point you need some sort of high speed buffing technique using buffing compounds of finer and finer grit.     It can be done cheaply with an electric drill using cloth buffing wheels, but the higher speed you attain, the more mirror-like your results will be.

 

This is what 1000 grit sandpaper does to aluminum

 

 

It doesn't look much different from what ships from the Bottlehead company in terms of the way it catches and reflects light.

 

I used cheap Harbor Freight buffing wheels that cost about 8 dollars for a three pack.  They look like this:

 

http://www.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/i/m/image_17472.jpg

 

Then you buy an assortment of buffing compound sticks that look like this.

 

http://atomictoasters.com/wp-content/gallery/sharpening101-7/buffing-compounds-4oz-bars.jpg

 

You use grittier or finer compounds on your three wheels.  And don't ever mix them up.  That way you get a more mirror-like finish with each finer buffing wheel you use.  

 

This is what my engine looked like after getting after it with the drill and buffing compounds.

 

 

 

Sorry for the off topic photos, but I wanted to let you know that no sand paper, even 3000 grit auto body stuff will give you a mirror finish.  It'll look like 'brushed nickel.'  You need to use some sort of higher speed buffer with compounds to pull out a reflective finish. 

post #1602 of 6029

Nice work! How long did that take from sandpaper and up?

post #1603 of 6029

About three hours and a lot of elbow grease.  Mostly, I was learning as I went though.  The Crack panel would go more quickly.  The trick is to sand in one direction so you don't have swirl marks the catch the light.  You can buff in multiple directions though.  

 

The trick with the amp plate is keeping it secure.  I'd probably screw it to a board so there was no chance of bending it because you have to push hard when sanding and buffing.  I'd secure it with flat or domed screws so something solid before pushing on it much.

post #1604 of 6029

Usually when I polish metal samples in the lab, I alternate sanding/polishing directions and criss cross them to sand out the previous lines with each pass. But that's for tiny specimens where I'm going down to nanometer finishes. Large surfaces probably take better or don't care about unidirectional rough sanding.

post #1605 of 6029

You might be able to 3M foam double sided tape it down to something to work on it and then use some Goo Gone afterwards to remove the stickiness from the foam tape.

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