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the CTH (Compact Tube Hybrid) Rev A thread... - Page 34

post #496 of 577

Hm, that all makes sense, especially since I ended up using a 10k pot - couldn't find the 50k one. I'll see what I have left in terms of resistors - or get a few at 200 or so. And when next I feel brave enough to take the whole thing apart again... Thanks Mullet!

post #497 of 577
Quote:
Originally Posted by mrhk13 View Post

Hm, that all makes sense, especially since I ended up using a 10k pot - couldn't find the 50k one. I'll see what I have left in terms of resistors - or get a few at 200 or so. And when next I feel brave enough to take the whole thing apart again... Thanks Mullet!

Changing from a 10k to a 50k pot without adding a series resistor will not change the input attenuation of the signal into the amp. The 50k will only make the amp an easier load on the source components.
post #498 of 577
Quote:
Originally Posted by Mullet View Post

Otherwise, you could up your pot to 100k and see if that helps. I'm not sure what the input impedance of the CTH is though. I'd assume 50k was picked for a reason. From what I know with other amps like "The Wire" I had to up the input impedance to 500k ohms to match a 50k pot. It was advised to not use the pot to correct gain problems. Hopefully, someone else will chime in with more info on if changing the pot to a higher value will help or is correct in this situation.

With your "Wire" build, you did not up you input impedance to 500k, rather you made it 50k when you put the 50k pot in front of it.

The "Wire" circuit featured a fixed attenuation circuit at it's front end that made the amp look like a 10k load to the source components. When you added an attenuator without "retuning" this circuit, an excessive amount of attenuation was introduced thereby causing your gain to appear to be an issue when it was the attenuation that was causing the apparent lack of amplification. There was never a gain problem per se.

The choice of a 500k (or a 1M) resistor tied to ground at the amp input provides a safety in case the pot wipers or wiring fails. This insures the input finds a path to ground and does not go full rail and toast your cans with a spectacular burst of power. Take a look at the CTH schematic , you will see a similar safety.

If you wish to add additional attenuation in front of the amp, a appropriate series resistor after the pot and before the 1M safety will form a simple voltage divider that will further attenuate the signal prior to the amp doing it's thing. IMHO, you would be better served adjusting the gain of the amp at that point, but, it will most likely change the sound signature and might even make the amp unstable without resizing the compensation capacitors. Unlike the "Wire", which is mainly a current buffer by design, I doubt the CTH can be made to run unity gain stable without significant effort.
post #499 of 577

Guys - I'm working out the last bits of the BOM, and the MPSA14 seems to be obsolete at Mouser. There's plenty of replacements that match up fine - except for maximum DC collector current. The 512-MPSA14 was 1.2 a, most of the replacements are 500ma. Is this critical?

 

post #500 of 577
Quote:
Originally Posted by Misterrogers View Post

Guys - I'm working out the last bits of the BOM, and the MPSA14 seems to be obsolete at Mouser. There's plenty of replacements that match up fine - except for maximum DC collector current. The 512-MPSA14 was 1.2 a, most of the replacements are 500ma. Is this critical?

 



The bulk pack are out of stock, but the reel and ammo pack are still very much available.

http://www.mouser.com/Search/Refine.aspx?Keyword=512-mpsa14

Might want to buy them soon though, they are EOL as well.
post #501 of 577

how are the kits coming? ive gotta say, im very exited biggrin.gif

post #502 of 577

not sure if i am stealing anyones thunder on this, but if anyone else is as exited about these kits as i am, they should know that images of the cases have been posted at joeaudiophile! and they look absolutely stunning! 

post #503 of 577

Yea, they kinda do - don't they? wink_face.gif

post #504 of 577

Woha! Those look amazing! Now I want to build a CTH just to have this sitting on my desk. tongue_smile.gif Is this brushed aluminum? <3

 

Can't wait for a price now. Being able to buy a full kit makes this build even more interesting. It's fun sometimes not having to spend days sourcing parts from different suppliers. Hoping the full kit will be reasonably priced, but those enclosures alone are hinting at a high price tag.

post #505 of 577

any news on the kits Misterrogers?

post #506 of 577

I'll be posting an update on the site tomorrow :-)

post #507 of 577

great! cant wait!

post #508 of 577

Misterrogers, any chance of the kits being available before christmas? i dont mean to be pushy, and i really appreciate how much effort goes into an endeavor like this, but i want to have a cth up and running before christmas, and i need to know if i can wait on ordering my own parts or not.

 

Also, if you need help testing the kits, or are just looking for some early builders, id be happy to purchase an early kit.

post #509 of 577
CTH kit? Ok, subbed and you have my attention.
post #510 of 577

Same here. Would be great to have a kit anytime soon.

Do you think I can build it as a second project? My first was Project Sunrise 2.
 

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