[bidoux]

Hello 

 
 
Although there are a few threads that deal with the L'espressivo (formerly known as less-pressivo) design by dsavitsk, I thought that a single thread would not be a bad idea (and I have some questions to ask, sorry ). I am not going to recall its history since dsavitsk did it on his website : ecpaudio.com (see L'espressivo Project -- Parafeed Spud Amplifiers for Headphones)
 
The schematic (click on it to see the build page)

(tube used here is 7KY6, output transformer is Speco T-7010, Q1A is 10M45)
 
 
If this was to become a semi-official build thread, I would gladly update its informations.
 
 
 
 
Now for my questions :
 
How are we supposed to understand the heater supply ? When I use triodes, I make sure that heater is around the same potential as cathode, here it doesn't seem to be the case. I would say heaters are at 20V whereas cathode is at 2V. Is it this way because the 7KY6 is a pentode ?
 
How can I modify the CCS ? I put together a BOM for this project, using as few shops as I could to cut down on shipping. I am currently planning to buy from Alliedelec, antique electronic supply and the local suppliers. But if I want to use 10M45 or DN2540, I have to order from Mouser. I saw the Torpedo schematic (you can go here : http://www.diyforums.org/Torpedo/TORPEDOschematic.php ) and thought that I could use this kind of bipolar based constant current source (with R7/R9 going to ground instead of the cathode). I can get MJE350 localy, and it adds four LEDs, so more glowing for me

 
 
 
Thats all !

 

[dsavitsk]

The Torpedo is a L'espressivo. We changed the name because I could not get in contact with Gary Dahl (who designed the Espressivo that this is all based on) to get his OK on the name. I don't believe there is any legal issue, but we just didn't want to step on any toes.

Use any CCS that you like. The advantage of the two BJT version is that it is cheaper and easier to set to a specific current without using a trimmer. The D-FETs can be all over the place as Vgs can vary from part to part. The disadvantage is that you do need to bias it externally, and theoretical performance is probably not quite as good (though the difference is minimal and probably only theoretical.)

With high Gm tubes, you may find that noise is reduced by biasing the heater up a few volts, but you can attach it to ground if you like. The tube's datasheet will tell you how high it is safe to bias it.

There are better output transformers to use than the Specos. Let me restate that -- you probably can't find worse audio transformers than the Specos Don't get caught up on the autoformer part -- it really doesn't make a difference in sound (or at least it doesn't make an improvement at this level) and just limits you. Plus transformers add an extra safety layer. One neat option is to use power toroids. A 120:7 is roughly equivalent to 10K:32, and if you put the secondaries in series it should work for high Z phones. I also have a few sets of 10K:300/32's that were made for us in the prototyping process where they screwed up the pin out. They work fine, they just don't fit our PCBs. They can be had for my cost (~$18/pair + shipping.) I also have lots of power transformer, PS PCBs, etc that will work on a project like this -- just PM me.

Oh, for the power transformer, a 120+120:120+120 wired as 120:240 will get you all the voltage you need. Use a toroid to avoid magnetic fields, then pick up a cheap 7V for making a DC heater supply.

All of that and you have recreated the Torpedo

 

[bidoux]

That is an amazing answer, thanks a lot Doug. I might eventually PM you about the transformers, but this is my 2013 summer project so I am not in a hurry.

 

[bidoux]

I have been looking at the balanced version of the l'espressivo ( as attempted by fallen angel there ) but I am not too sure what input transformer should be used. It has to be cheap from input to output, and seeing what options I have retained for the rest of the build, the input transformers need to be very cheap

 
So to be brief, would using something like a pair of sowter 4600 kill the whole point of going balanced because they are of a rather poor quality ? Although I mainly do this for fun, we wouldn't want the balanced version to sound worse than the single ended one, would we ?

 

[nikongod]

You don't have a source with a balanced output? For maximum cost savings just skip the input transformer, as cool as they are. 
 
If you still want them, or want to add some versatility to your design, a pair of Edcor WSM10K/10K or XSM10K/10K transformers should fit anyone's budget. They are less nice than the Sowters you posted, but less expensive too  

 

[bidoux]

Quote:

You don't have a source with a balanced output? For maximum cost savings just skip the input transformer, as cool as they are. 
 
If you still want them, or want to add some versatility to your design, a pair of Edcor WSM10K/10K or XSM10K/10K transformers should fit anyone's budget. They are less nice than the Sowters you posted, but less expensive too  

No I don't. It would be a balanced amp with RCA inputs and 6,35 jack output

 
I forgot to look at Edcor's, thank you very much. And they can provide for every iron needed in this project.