Crosstalk in the preamp

[tomrasdf]

Anyone have any specific examples of causes of this? Jelle, I'm curious if you were able to find exactly what it was in the build you referenced in another thread here that caused the crosstalk. Is it ghost capacitance between two in phase leads? What specific areas are the serious problem spots?

I had a little bit of a buzzing problem in my recent build (ODS 101 non hrm) which sounds similar to what was described in the "'Buzzing' sound behind the note" thread. Not enough to ruin the amp for me, but just enough to really tick me off. I did everything to try and fix it short of rebuilding it to no avail. Now, all this bluesmaster talk has me inspired and my old build is currently under the knife destined to become a hrm bluesmaster based on several of the recent threads on the subject. I'm hoping that with some insight on the topic from other people's experiences and a little luck, I can avoid the headach of trying to track down a paracitic oscillation by getting the layout right the first time.

I've read through all Gerald Weber's thoughts on lead dress (short grid wires, long plate wires), as well as everything else I could find on the topic and I feel I understand the theory behind it (though a little review never hurts), but I have to admit that this area still seems a bit like voodoo to me.

[jelle]

Hi,

I have learned to make a good layout before I start building an amp. I have had the buzzing sound/ lack of sustain before and I remember a post by Novosibir on the topic that was posted about a week ago. Another more recent amp of mine had a bad buss to chassis connection and that amp was unstable and sensitive to mechanical vibrations. Very annoying in a combo.... Please post a pic of your build as this is the only way for me to help you.

Jelle

[novosibir]

... and I remember a post by Novosibir on the topic that was posted about a week ago.

You're certainly speaking about this post:

http://www.brownnote.net/forum/viewtopi ... =3691#3691

Larry

[tomrasdf]

I'd post pics, but there's not much more than an empty chassis yet. The layout is still on the drafting table. I've been inspired by some of Kendrick's layouts where the tone caps are right next to the pots leaving a very short grid wire. Gerald seems to know what he's doing, so I thought I'd try it in a HAD type amp; not going for a clone here. Though I've taken in everything I could possibly find about preamp layout, I admit to my novice standing on the subject. I'll probably put it together, realize that it sucks, and rebuild with a tried and true layouts.

The brownnote thread was actually one I hadn't read. Good to read through, informative. Here's the post I was refering to:

Hi,

FWIW, I build a 97 hrm that also had a buzz after the notes in both clean and OD mode. I had to rebuild the thing to find out what it was....crosstalk in the preamp...Now I use a strict layout and shielded wire.
I hope you find out what it is in your amp.

Good luck,
Jelle

Did you ever find out what specifically was causing the problems?

[jelle]

I did not really try to find the specific problem of this amp as I was trying to learn to make a problem free layout/building technique. I did learn how to do this and it has learned me to make a good drawing first where all the wires that could interact with eachother (plate and grid input wires..) are separated physically or shielded.

Please post your layout drawing and we may be able to give advice. You can also pm it to me if you prefer that.

Jelle

[tomrasdf]

sent

[jaysg]

Is it ghost capacitance between two in phase leads?

Plate wiring in long runs creates a very nice broadcast antenna situation. Low level inputs anywhere in the amp can pick up the signal from parallel wires. Solutions are coax or serpentine plate runs if you can't find a way to make them shorter or get the offending wire pair more perpendicular.

[tomrasdf]

Is it ghost capacitance between two in phase leads?

Plate wiring in long runs creates a very nice broadcast antenna situation. Low level inputs anywhere in the amp can pick up the signal from parallel wires. Solutions are coax or serpentine plate runs if you can't find a way to make them shorter or get the offending wire pair more perpendicular.

Interesting. You just gave me a serious "well, DUH" moment. I had looked at long grid wires as the cause of all the problems. Never really dawned on me that longrt plate wires would add to the problem. (duh). Thanks for the clarity.