G1176-FET (BF245A or 2N5457) Input HEADROOM & POT too lo

I'm starting a NEW topic about this because it's hard to find in the BIG 1176 thread & was ALSO, never resolved.

The usual complain is the input gain control has to be set VERY LOW... about 9:00. My GAIN Pot is definitly 10k Audio taper.

I just got done a full night of testing & trying the 2N5457's in place of the BF245A's... this did not change anything.

Something does not seem right. I can back this up by saying, when the unit is in BYPASS you can hear the "HEADROOM" it's capable of but with the Gain Reduction active it isn't close. I can tell that the FET's threashold is TOO low.

This is on BOTH of mine = one has the LUNDAHL LL1540 input tranny (1:1) & the other has the IC input... both do the exact same thing.

I think Jacob metioned lowering the feedback &/or sidechain signal.

What could it be... I mean, some of you say you can get the volumes up around 12:00... this is impossible on my two G1176's & I'm not alone. Either you did something different or your pumpin some weak signal into it:?

If anyone has found a solution... fess up :green:

Thanks

BTW... my 10volt power rail is only reading about 8.86 volts :?

Is this a problem?

The 30v rail is fine.

Same problem here, kinda. my -10 is -10 though.

This is crazy... all day & nite and nothing. I'm sorry I'm just venting

Could there be a wrong value on the schematic, PCB mask, or whatever that we're ALL missing? I wish it were that simple. :sad:

The first one I built was acting in the exactly same way, and now my new unit too. I get very small differences by switching BF245A's.

We are not alone.

It can't be a wrong input POT because you can hear the instant lose of HEADROOM when switching from Bypass mode... although my input pot usually sounds closest to the REAL signal with input @ 9:00, output pot @ 2:00... so it is STILL low in BYPASS too but at least you can hear the headroom this thing is capable of in Bypass.

It's now 6AM & I just cracked open an AMSTEL (beer)... I give up for now.

This was resolved by increasing the value of R26. Look here

http://www.groupdiy.com/index.php?topic=646&postdays=0&postorder=asc&highlight=threshhold+threshold+trim&start=1140

Just getting round to sorting mine after nearly a year of use...

hello,

on the original 1176 you have an input transformer in a stepdwon configuration 500/200... but the lundahl or ic-input is unity gain...
there's thread from me about exactly that problem.
do a search!

Which 'original' 1176? :?

[quote author="matthias"]hello,

on the original 1176 you have an input transformer in a stepdwon configuration 500/200... but the lundahl or ic-input is unity gain...
there's thread from me about exactly that problem.
do a search![/quote]

I did search... must have missed it.

The link above your post has some info I missed... looks good.
I Will try it ASAP.


matthias,
Could you link to your topic or just give the name :?

http://www.groupdiy.com/index.php?topic=15543

http://www.groupdiy.com/index.php?topic=14234

Here it is

http://www.groupdiy.com/index.php?topic=15543&start=0&postdays=0&postorder=asc&highlight=1176


How strange... I posted in there almost a year ago before I even built half the shit I have now :razz: I probably didn't understand at the time what it was all about.

matthias :thumb:

OK, the first link shows how to change the FEEDBACK (around the 56k)

matthias's links talk about the INPUT... which would effect the feedback :?

I will do whatever works best but I think that the #1 problem "Should" be addressed 1st = The input... but if it can't, is the added trimmer enough?

I could care less at this point I just want to fix it
It's been a LONG nite... now it's 7AM & I see the sun :shock:

I'll try whatever you guys suggest tomorrow.

THANKS

finally I couldn't find a suitable solution with the ll1540, so I decided to build all units with the electronically balanced input.
-> link 2

yeah i'm redoing mine with 5532's. i've checked over the schematic vs. the board, and the only difference, in the 5532 input version, are the transistors.

A lot of what's going on in the rev d. thread probably applies, with regard to low hfe transistors.

there is definitely a chance that some of the the bc107's, should be 107B, especially in the sidechain, are too low. the sidechain circuit didnt change very much.... i'm not sure if low hfe sidechain transistors could require one to give it too much input signal to hit threshold, or if its the other way around, but i'm been holding off on the transistors on 2 half stuffed boards until i've got something i like figured out. I may use the same Q's as in the rev d, but with updates for parts that are hard to get with enough hFe, up until the output stage, where I doubt the output stage is affecting any of this threshold and input overload type of stuff.

still floating over in my head.... but with the 5532 input g1176, as far is I've seen whilst comparing orig. schem and pcb layout, is totally the same except for Q's, so it shouldnt be a different R or Cap. I actually have the original ref g schematic up on the wall kind of large, since i found myself digging it up so often.

i dont know much, but it looks like, at least in the case of 2n3391, that transistor has a bit lower, by around half, breakdown voltages, but 5 times the continuous collector current rating 500ma, versus the bc107b's 100ma.

I actually think I'm going to stuff the one board with the same output transistor pair, but use 2n3708's in the sidechain, as my 2n3707's came with very low hFe, 2n3391's in the input stage, and 2n5088's in the 'pre-output'

and 2n5457's. i'm just going to see what happens if i follow the schem as closely as i can i guess.

I'll try the 56k mod later this week. Kevin: If you try it first, let me/us know how it went. :thumb:

[quote author="matthias"]I already tried to increase R6 (27k) which seemed to work, but when I re-calibrated the unit the behaviour is the same, as before..
[/quote]

This makes sense... there seems to be 2 problems:
Input gain is off (not as important) & the sidechain is too sensitive.

matthias,
Can you post any more voltage comparisons :?:

Like with the "SAME input voltage/ LEVEL on both" but test before R26 (56k)
& at the "Stereo interconnect (7-RED on the schematic)

This will show the circuit (sidechain) differences. Because with the inputs @ the same level the units "Should" react the same... I think all of the 1176 side chains are very close if not the same circuit.

Thanks

[quote author="khstudio"]with the inputs @ the same level the units "Should" react the same... I think all of the 1176 side chains are very close if not the same circuit[/quote]

this is true, unless the side chain is built with different parts.

@ kevin: I only see two possibilities to solve that problem:

use a suitable transormer with a 2:1 ratio at the input
maybe with a t-atten

or use the modified electr. balanced input