mic pre #7

[freespace377]

Greetings, all.
I noticed this bbd about a week ago when a friend turned me on to the shure 57 xfmrless thread.
I decided to join just now, so I could post this, to disagree with a person I came to have great respect for in reading the '57 thread, namely, CJ.

So jumping in with both left feet,
CJ said:
"So a positive grid signal turns into a negative plate signal, which is applied to the cathode. Since the cathode and grid are, in effect, moving further apart, the voltage between them is increasing.
This means the input signal has become bigger because of the feedback.
What kind of feedback is this?
Positive or negative?"


Here's how I look at it:
The signal goes through the "common cathode" stage and is inverted. It is then applied to the cathode, where it goes through the tube in "grounded grid" mode, being amplified, but not inverted, and summed with the inverted signal at the anode.
Seems to me that has a degenerative, not regenerative effect, que no?
Variations of this are used in phase inverters for SE to P/P.

Of course all the talk is resovled by IO telling us what happens to the amplitude of the signal when the feedback is applied.

Be gentle, its my first time.

Dan

 

[NewYorkDave]

The tube amplifies the grid-cathode differential. When you apply the same signal to the cathode as seen at the grid, but in opposite polarity, that increases the differential at any given point in the signal swing. It has the effect of increasing the input signal and is therefore positive feedback.

 

[freespace377]

Yeah, Mr New York, your right, and that was what CJ said, but it wasn't until I made my post and stepped away that I realised thatI had goofed.
Putting it in the terms I used to make my case, the feedback signal may transit the second time through the tube with out being inverted, unlike the grid signal, but it's still an inverted signal, and at the anode they are now both inverted, and sum, rather than being an inverted and an non inverted signal.
My Duh.
So this is a great introduction to Me, Gents. Solder is a natural medium for me because its so easy to repair mistakes.
Thanks.
Dan

 

[ioaudio]

important: update first post, some of the shown values(feedback path!) are now corrected. sorry for this...

samples of a squarewave can be found here: http://groupdiy.twin-x.com/thumbnails.php?album=133

 

[ioaudio]

did somebody have a look at the soundfiles?
what do you think?
if i should check something else/do other samples please let me know.

 

[mattmoogus]

Dont want to sidetrack this thread, but ioaudio, does the bolt thats holding the toroid down touch the lid of the metal box when its closed? If it does and it ever wears thru the paint and touches the bare metal, you will have major problems.

Just wanted to check this!


M

 

[ioaudio]

hi m,

thanks, i´m aware of the possibility of a short, but there´s a 5mm air gap.

max

 

[mattmoogus]

Cool! It was hard to tell from the pic, I just wanted to make sure you knew.


M

 

[ioaudio]

sound sample of a fender precision bass via di :
http://groupdiy.twin-x.com/displayimage.php?album=68&pos=5
no dynamics or eq added, pure signal.

 

[rotation]

Hi Max!
You told me that two 12v lines are used for the heathers and fantom. How do you get 260 for anodes? Do i need another 12-220 v?
And how is about line level input? With pentode triode switch or i should have another switch somwhere else?

 

[ioaudio]

yes, you need 2 transformers. the 12 v secondaries are tied together, so the second transformer goes back to ~ 220vac. full wave rectification 220*1,4 minus iron and diode losses give you about 280 v dc.

no, you should pad the signal before the input transformer.
something like

xlr 2------3,9k-----input transformer
......................|
...................470R
......................|
xlr 3------3,9k-----input transformer

 

[mattmoogus]

I think you might need to move the 470 to the other side of the 3k9s...



M@

 

[ioaudio]

the spaces where not recogniced in the post, i had to put dots instead.

thanks m@

 

[mattmoogus]

:thumb:

 

[rotation]

Ok,i understand now. What does the pentode/triode switch? Only changes sound or also gain?

 

[CJ]

both.
and impedance too.

 

[mik]

I can lissent to your files... doenst works
M.

 

[rotation]

So pentode/triode switch is not pad. Pad is switch in feedback loop..
I found Ultron EF86. I never heard for this company before. Is it worth paying 25 euro for it?
Max, do you suggest buying new tubes and components? I can't find any opinion on pre's sound qualitys. Can you tell me more about it? This would help me very much.
Miha

 

[ioaudio]

the gain switch in the feedback loop is not really a pad. it affects gain, like the pentode/triode switch affects gein. if you need a pad, put it before the input transformer.
i dont know ultron ef86 either, 25 euro is not very cheap. i tried telefunken, phillips/miniwatt, valvo and tesla ef86. i could find the tesla for very cheap (~ 7 euro) on ebay. you can buy used one´s of course.
there arent more opinions about that mic pre because it seems that i´m the only one who build it yet.

mik, try to download the bass recording:

http://groupdiy.twin-x.com/albums/userpics/10055/bass.wav

 

[mik]

Nice sound, witch Bass did you play through it ?

M.