API-ish Mic Pre Project

[synthetic]

What do you mean by "bypass C6"? Put an additional cap in parallel, or add a cap going to ground after it? Thanks again, -jl

 

[chrissugar]

Additional cap in parallel with the electrolyt condenser. It is for low Z at high freq. It has to be polyprop.

chrissugar

 

[synthetic]

Thanks Chris. I had actually bought a bag of 10pf caps the last time you suggested them, but forgot all about it. The RF noise now seems to be gone, phantom power is working, and the 100nf polyprop caps are on order. Cheers. :guinness: :guinness:

 

[Flatpicker]

[quote author="chrissugar"]I have an API topology preamp with almost the same components, LL1636 input, LL5402 output...[/quote]Chris, how do you have the LL5402 connected? Primary-parallel, secondary-series? Also, are you using the "mixed feed-back" method?

 

[chrissugar]

The 5402 is a 2+2 : 1+1 transformer. I use it to ballance the output, no step up or down, so I connected parallel the 2+2 part and in series the 1+1 part. This way you have a 2:2 transformer.
No mixed feedback method.

chrissugar

 

[daArry]

synthetic, good to hear ya probs are gone did you change the connections around the LL1636 as Flatpicker suggested or just do what chris mentioned?

Cheers,

 

[chrissugar]

[quote author="Flatpicker"]...you have the transformer primary wired 1:10 and center-tapped. Wire it like the non-center-tapped version in diagram ?C? of the 1636 data sheet (the diagram on the left-hand side) since you are not using the center tap.
[/quote]

I didn't notice this post.
Flatpicker is right, you should use the non-center-tapped version.
Page two, C, left:
http://www.lundahl.se/pdfs/datash/1636.pdf
This is the way I wired my transformers and they work great.

chrissugar

 

[synthetic]

What effect would rewiring the transformer have? I'm only hesitant because a) it was tough to connect all of those jumpers on my perf-board system, b) it sounds great as it is and c) this system seems to be more similar to the original API 312 gain block, shown in the link below.

http://www.jefflaity.com/misc_pix/api312_gain_block.pdf

I realize that the Lundahl is not the same transformer, but it has a similar configuration.

 

[chrissugar]

Are you talking about the input or output transformer now?

chrissugar

 

[synthetic]

Input. I have no output transformer ? I'm running unbalanced directly out of the output cap.

 

[chrissugar]

If you like how it works leave it like it is, just don't use the center tap or you will distroy the transformer with phantom power.
Although I don't see the similarity you are talking about. :?:

chrissugar

 

[daArry]

Hey,

Was just checkin that schemo you posted again...have you got the polarity of C6 connected the right way? Shouldn't + be facing the pot side?

 

[chrissugar]

[quote author="daArry"]Hey,

Was just checkin that schemo you posted again...have you got the polarity of C6 connected the right way? Shouldn't + be facing the pot side?[/quote]

No, it is not OK. Minus to ground, plus to pot.

chrissugar

 

[Flatpicker]

[quote author="chrissugar"]The 5402 is...[/quote]Thanks. Right now I using Fred's servo circuit and running unballanced with no coupling cap, but I'm going to order a 5402 and try it out.

 

[Flatpicker]

[quote author="synthetic"]What effect would rewiring the transformer have?...it sounds great as it is...[/quote]It might sound greater. :wink:

[quote author="synthetic"]...I'm only hesitant because it was tough to connect all of those jumpers on my perf-board system...[/quote]I can relate to that. Maybe a good project for a boring rainy day. Oh well, glad it's working - have fun. :thumb:

 

[synthetic]

Here's the final schematic:

Here's a photo of the completed unit:

Many more photos here:

http://www.jefflaity.com/gallery/micpre

Thanks everyone for your help, -jl

 

[louder]

alo
very nice.how does it sound?
just one note:100nfd=0.1mfd(c6).
best regards
pedro