See the fruit of my labor here... Quad 1272

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Svart

Well-known member
Joined
Jun 4, 2004
Messages
5,134
Location
Atlanta GA USA
Ok I have been hard at work finishing the cosmetic aspects of 3 pieces of DIY. I have just about finished one of them, the Prodigy Chimera 1272. I just have to machine some knobs for it and I'm done! I'll be posting more pics of the other gear soon when I get them more complete.

but for now:


http://groupdiy.twin-x.com/displayimage.php?pos=-311

http://groupdiy.twin-x.com/displayimage.php?pos=-312

http://groupdiy.twin-x.com/displayimage.php?pos=-313

Let me know what you think, and I'll get more pics when I get the knobs made and the jewel light hooked up.

:thumb:
 
Svart

What pcb`s are you using ?

I literally just bench tested a version of the 1272 using Fabio`s BA283`s wired as Joes Malones "cheap" version. It is using Carnhill out & the 31267 reverse wired as in iron.

When I switch into high gain mode with the 39R resistor in, & crank the gain up the amp goes into orbit with a high frequency oscillation.

Did you use a Zobel network on the input transformer ? If so what value ?
 
I'm using the Chimera version sold by Chae here on the forum, I also have some of Fabio's versions that I'll be trying out shortly too.

The Chimera is similar to the hot rod Malone version. I'm using the sowter 8751 output tx's I have two of the 31267s and two Lundahl 1538xl as inputs.

the board has 100pf across the output windings of the input tx, but not a full zobel. I had trouble with this also as the noise level was insane. I tried different values and full zobels of different values which made things better but not tolerable. the channels that were using the Lundahls where quiet and clean though which puzzled me.

I got around to looking at DC resistance for both of the input TXs and found that the lundahl had twice the input and twice the output resistance of the 31267 wired like JLM suggested. I tried different wirings and found that if I wired both the input and the output of the 31267 in series, the noise practically went away and the audio was just as clean and clear (for a neve..) as the lundahls. I left it as such.

I plan on replacing the 31267s with lundahls in this unit just to keep everything the same and using the 31267s for my Fabio based boards so I'm quite interested in your findings!
 
I have no problems with the 31267, until I switch into high gain mode. Even then it only bursts into oscillation at high levels settings. I`m using JLM`s wiring method with just the pot. I`ve noticed that when using the input pot it affects the input signal when monitored on the scope.

In low gain mode it is hum free & sounds really nice. It has a good square wave response on the scope as well.
 
Hello Chae,

Yeah the trafos were definately the problem. Only wiring the input and output in series cured the problem. the lundahls never exhibited the same problems at all. Moving the 31267s to other channels moved the problem also and using the lundahls near the toroid did not introduce AC noise or hiss nto the system. Seemed to be some kind of impedence issue, but they work very well now!

thanks!
 
i've been using them since before the fronts were done! I've tried them on everything, but they rock on guitar/bass! not too sure about them on overheads though..
 
It looks really good. I was thinking of doing a FPE gray with white lettering front panel with some big Daka knobs for a rack mounted tube amp.
 
To connect the Marinair 31267 up I found the following to work well.

Connect the input from XLR +ive to pin 7 -ive to pin 9.
Take the output to the 5K pot using screened cable positive from pin 4 & the screen from pin 2.

Using this method the Marinair is perfectly stable with no HF oscillation. I don`t even have the internal screen pin earthed or a compensation cap across the secondary. The square wave output is quite good as well.
 
[quote author="Rob Flinn"]To connect the Marinair 31267 up I found the following to work well.

Connect the input from XLR +ive to pin 7 -ive to pin 9.
Take the output to the 5K pot using screened cable positive from pin 4 & the screen from pin 2.[/quote]
Rob, this is very interesting. So you're flipping the polarity? And are you saying that using the screen as return yeilds less noise than a twisted pair for for + and return with the screen connected to chassis ground? I know that the Neve documents specify using screen as a return path - fully unbalanced - for almost all connections, but this has never made any sense to me from a noise standpoint. Maybe they did the same tests as you and Svart to arrive at this conclusion?

for reference
 
So you're flipping the polarity?

I still have the whole preamp wired so it is in phase.

And are you saying that using the screen as return yeilds less noise than a twisted pair for for + and return with the screen connected to chassis ground?

I can`t categorically state this, since I never tried it with a twisted pair in the first place. The reason I used screened wire is that if you wire it up as in the JLM `cheap `n` cheerful` diagram one side of the pot is earthed anyway so I took advantage of this for my screen connection.
 
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