TL404 Projects, what would you do with a pair of TL-404's?

[Alonzo]

As I grill dinner tonight, I'm going over the options on headphone amps for the upcoming TL404 Opts being done by Mike L.  Copying the 6C45 version of the Expressivo is one option but I'd like to use my "in stock" current transformer/tube/resistor inventory.  So I will most likely be going 2A3 or 45 or EL84, driven by 6CM7 or 12A variant or maybe a CK5755, some sort of combination that will works with the PS's I have and the Opts.  My idea is to start at the Opts and work backwards, get the simplest design possible, cleanest power possible and if that's doesn't sound good with the current parts I have then maybe try different designs or fixes. 

So a CLCRC Power supply, a good paper/oil cap at the opt, choke load with a BCP-15 is my starting point.

What are the other people who purchased TL404's thinking of doing?

[Paul Birkeland]

The obvious choice would be to squeeze them onto the Mainline.

The Espressivo, and the Bottlehead Kugelis 6C45 that came before it are designs that have been improved upon over the last few decades, and taking advantage of these improvements would be paramount to maximizing performance from the TL404.

-PB

[Alonzo]

Hi Paul,
I thought of that, but that's $1200 I don't have lying around right now. But I have a ton of parts...maybe 3 or 4 breadboard failures later I will re-consider.
 
Also this is a learning tool for me, my Crack sounds great, and not having heard the Mainline, it's hard to justify the big price.  I don't know what I don't know.

There's a headphone expo here next week or so, will BH attend?

I have 2 beautiful 2A3 mesh plates that deserve a home, they would be great heating my den while I listen to some Lake Street Dive (thanks Grainger!!!).
I think I can make a good 2A3 amp for headphones if I can limit the hum.

[Paul Birkeland]

Yeah, DC heating 2A3's is quite complicated. 

[roscoe primrose]

Yeah, DC heating 2A3's is quite complicated.

Can't be any harder than DC heating 845s or 813s, I've been doing that for years...

Roscoe

[Paul Birkeland]

I would argue that the 845 is a lot easier.  You need 10V at 3.25A, which you could get to with a 12V winding and an inefficient rectifier.

When you go for 2.5V at 2.5A, the desired winding voltage for an unregulated supply becomes pretty problematic, as low voltage simulations are notoriously inaccurate.  So, perhaps regulation is in order.  You can try something like the TI LP3876ET-2.5, but you'll be flirting with the balance between dropout voltage and excessive package dissipation (could go up to 6-7 watts with reasonable fudge factor built in, that requires a very large heatsink). 

[Alonzo]

I've tried three different (slightly) 2A3 amps, each sounds great but I could hear hum with my HD 650's.  Once the TL404's arrive I'm going to try them out in a modified Crack, then in a Smash, and finally in a Mainline.  I have a Crack I can experiment on, I need a Smash anyway and if the experiments fail then I can move up to the Mainline and give away my bedroom Crack.  So for now I give up, I'll leave the amp design to the professionals!  I didn't try DC heaters, maybe that was the route I should've taken.

[2wo]

Don't give up, look at what you learned. Managing hum in a Directly Heated Triode, is a challenge. With Headphones and high efficiency speakers, more so. Have a whack at the DC supply...John         

[Alonzo]

TL-404's have arrived, got started on the demo today.  First test bed is in a 2A3 amp, challenges are mostly controlling hum.  The size of the Opts are about the same size as the BH-5's that are coming out.  Can't post pictures, I'll have to get them on a host site until the BH forum updates are finished.