G1176 - LL1540 input transformer - level calibration ???

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matthias

Well-known member
Joined
Jun 4, 2004
Messages
768
Location
germany / frankfurt
hello,

I just finished another G1176...
this time with lundahl in- and output transformers


I just made some measurements and found out that the input signal after the 10k input pot is still to loud and when I turn the input pot to 65% of its range the signal sounds distorted..

I measured a test signal at the G1176 and the original 1176 at the input of R6

the result is: G1176: 592 mV AC
orig. 1176 : 304mV AC


so what do you think I should do to get the ll1540 input work properly??


increasing R6 ??

change the 1540 wireing to 2:1 ??

use another input pot or the original input attenuator?



thanks,
mat


11761540.gif
 
Hey Matthias, dont worry about the level before the pot, that is if you
are not exceeding +30dBu. The LL1540 is quiet capable of handling alot
of level. Of course the calibration of your input is just at the first 25%
of the pot movement. On the other hand it´s a very sensible unit, that is if the input level are somewhere at -20 dBu.. then its still a compressor.
Its the same situation when you use the "active" input. 1:1
Cheers Bo
 
thanks for your reply...

the problem is not the transformer, but the signal preamp, which gets overloaded and causes distortion...

as I write I compared the levels to an original 1176 with utc input and there
it is only half the level compared to the ll1540 input...

mat
 
.
Think PRR mentioned somewhere the original UTC input was set up 2:1, to step down gain on input. So a 1:1 or 1:2 input setup would increase the signal.

http://www.groupdiy.com/index.php?topic=4080&postdays=0&postorder=asc&highlight=utc+o12&start=15

http://www.groupdiy.com/index.php?topic=7715&highlight=1176+utc

hope it helps, Paul
 
[quote author="matthias"]
but how should I deal with the 12k resistors??
[/quote]

Yes, and why are they there in the first place.....I really don't get these things yet.
Sorry, no answer, only more questions...
 
Hey Matthias, I cant see the real problem? If you overload the line amp I suppose your out of compressor mode otherwise there should be a heavy compression. I can´t remember the set level of the FET but its certainly in tenth of mV region.
If you reed the 1540 "its suppose to be wired series-series"
Make Lundahl a call and ask him.
The 12kohm is helping the overshot a bit.
Try to check your levels backwards,.. what do you expect to bring out of this unit? I hope not more then +20dBu and so forth.
The 2:1 input and 1:2 Tx as output is more convenient with the pots.
Cheers Bo
 
the problem is that half of the range of the input pot is useless..

with my original 1176 I don't have this problem, because the signal that enters the "signal preamp" after the fet has only half of the level... (measured in ac) compared to the clone...

I did all tests in bypass mode, so we're only talking about the standard signal path...
compression comes later :) I haven't finished all the wireing to the switch banks...
 
Hey Matthias, I understand your problem clearly. If you look at the scheme.. at top you see the base of the first transistor is sitting 1,02 V above 0V.
Which meens that anything over 0dBu at that point will clip.
This construction was men´t originally to deal with levels around +4(+8)dBu. If you place +18, as I assume, it dont have much headroom left, just as an amp. So the Tx trick 2:1 and 1:2 gives you headroom.But..
If you see the input control as a threshold as well( which it is) it gives
you the possibillity to compress(when you´ve soldered it) signals at
-18dBu as well. Thats one of the beaty´s with this amongst others.
I have at home 2 original´s and 6 ch of diy´s and I love them all.
Cheers Bo :grin:
 
[quote author="matthias"]the problem is that half of the range of the input pot is useless..

with my original 1176 I don't have this problem, because the signal that enters the "signal preamp" after the fet has only half of the level... (measured in ac) compared to the clone...

I did all tests in bypass mode, so we're only talking about the standard signal path...
compression comes later :) I haven't finished all the wireing to the switch banks...[/quote]

I'm getting ready to build... is this a general problem & did you (or anyone) find a solution?

Kevin
 
I'm Bumpin this thread - 7 MONTHS LATER :razz:

I can reverse my tranny input but is that all... is this the fix?

matthias,
Thanks for the detailed info & your efforts :thumb:
 
Like Pink Floyd said, "is there anybody out there"? :roll:

I've been searching but could only find fragments & incomplete answers.

I'm going to reverse the LL1540 in my G1176 to 2:1 ratio -

My questions are:
- How does this affect the IMPEDENCE RATIO?
(I'll be using them the most on 10k un-balaned inserts)

- The 10k pot after the tranny reflects the impedence also right?... so how does it affect things WHEN SET UP 2:1 & should the 10K pots value be changed too?

- Lastly, what about the 12k (or 10K) loading resistors for the overshoot when changing from 1:1 to 2:1?

Yes, you should leave these damping resistors out.
_________________
Kevin Carter
K&K Audio

I see this response... but is this a general statement or does it apply to the 2:1 Ratio ONLY???


Thanks
 
Hey, dont worry about the impedance. You are not matching, like 600 to 600 ohms to get right freq resp. If you are setting the 1540 in 2:1 mode and the 10kohm pot is there you have about 40kohm at the primary side.And you get what you asked primarly for, a 6 dB lower level at the pot.
The 12kohms that Per Lundahl suggests is for taking overshots down.
You can change the 10k pot to a 5k pot if you want to be surely on the safe side with overshots. Cheers Bo
 
Bovox,
Thanks for helping :thumb:

OK, I see that you did the 2:1 trick on yours & like it... Did you add the 10k or 12k resistors to yours? And what, if any, other changes did you make?

matthias said he didn't have much luck with flipping the input tranny... this makes me wonder if you did anything different to yours. Having 2 Originals & liking these as much with the TX mod is killer.

I'm finishing another 1272 & my patch bay so I'll be getting to this very soon.
 
[quote author="bovox"]Hey, yes keep the 12k at the sec. /cheers Bo[/quote]

You mean 12K = R* and R*1 on the Mnats boards?

Does anyone know about the C*, C*1 and C*2 caps on these boards... I can't find any info on these :roll:
 
What? Mnats says you should skip the R* resistors if you turn the transformer 2:1. I did, and mine works great, infact the frequency response is so flat and accurate it scares me.
 
Hey, keep them,the 12k at the Tx, if you are not able to make a scope measure. Otherwise,test with or without,use your ears.
Cheers Bo
 

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