2254c (IGOR's) Mods, Changes, etc...

[khstudio]

Has anyone tried (LISTENED TO) setting up the "Gain" stages to be more like the original Neve 2254a/e?
I have & I like it... it is a little more "Colored" but in a good way IMHO.

The way it is now, the output (183.110) isn't driving nearly as hard as the original (version A/E)... this "could be" the reason some say they hear a difference... it is "close" to the the older 2254 (no suffix) but missing the input pad.

NOTE: it is NOT BAD the way it is now so if you're happy, don't sweat it.

The mod is fairly simple to try... but not perfected yet (I'll explain later) this is why I'm starting this thread.
I know of a least one other "Clone-commercial product" that does it like I'm trying to. 

The MOD:
Set the output gain resistor in 2nd gain stage (183.110) to 91ohm OR just turn the switch to +20dB...  this = 100ohm on Igors.
Next, remove the 2x 5k6 resistors (voltage divider) R27 & R64. Put a 470R resistor in the R64 pad that goes to GND.
Connect pot (5k-10k...  original is 5K but read below) top of pot goes to "P" on the schematic, bottom goes to "V" which is GND & the wiper goes to  "L" (next gain stage, 183.110)
That's about it except there's about +4dB of gain that I just can't trim out with the "GAIN Trimmer"... even after changing the Trimmers surrounding components back to stock.
NOTE: Study the original schematic & docs to understand & see what I'm trying to do here.
You must have some knowledge of the original circuit or you won't get it. IF you'd like to try the MOD & don't understand what you're doing... PLEASE, wait until it's completely "Debugged"


OK... so I don't have to retype what happened after the mod - Here's an email I sent to a friend that is trying to do the same thing:

Here's the rundown:
As you know, when I installed the 10k pot, my available GAIN was WAY above 20dB! (about 6dB)
I have been doing nothing but searching & reading to find the answer & after reading several of Geoff's posts, he ALWAYS says the pot (which should be a "Switch") is "5K" & is for 20dB... no more!!!
so it got me thinking... the 10k pot must be out of spec for the original design?

OK, so Neve had a 5K pot, 470R to GND & that's paralleled to a 51k
Igor has: about 11.2k (divider-pot) that's paralleled to about 54k... not too far off.
So, as you know I used a 10k pot (with a 1k on top to pin "P") to stay close to Igors setup

Now, for a quick test I paralleled a 10k resistor across my 10k pot to get it to 5k & WHAMO!!!
A PERFECT SWEEP from 0 - 20dB.  HAAAAAAA!
But here's the catch... the 10k POT doesn't allow more gain per say... it allows a larger "range" of about 26dB.
The extra 6dB, when the pot is down "OFF" (with just the 470R in line) pushes the gain 6dB lower than it should!!!
This in turn causes you to drive the "Unity Gain TRIMMER" 6dB higher than it should be.

NOW, I'm confident with moving forward but Oh, one last thing...
How does the new 5k pot affect IGORs signal being sent to the "SC"... It doesn't seem to but that's not a problem as that "Voltage divider" (47k-6k8) can be easily modified.

In a nutshell... the 10k pot makes the circuit more "Lossy" than it already is... by 6dB + you're driving the 1st gain stage 6dB more than it should be.

NOW the signal path a much closer to "Stock" & the noise floor did not rise much... IF AT ALL!

RECALIBRATION "Will" be needed I'm sure.

EMAIL #2

well, I'm tired. everything was looking good & all I thought I had left to do was adjust Igors V-Divider to the SC but I can't seem to adjust the (Unity-GAIN) Trimmer... it's a little too loud. (aprox = 4dB)
I've tried using the real Neve values in that spot but no go... not sure what's going on. Only thing I can think is that the GAIN coming INTO the 1st stage is hotter than normal the way Igor has it set up.
I think _____ pulls his SC prior to the 1st amp stage similar to the original... maybe a slight drop in gain where that divides? Not sure without seeing a schematic marked up with VOLTAGES or dBu NOT dBm.

I have notes & voltages documented... especially the ones going to the SC =measured with unity input (.774v), IGORs stock circuit  - just to make sure I can get it back to that spot AFTER these mods.

BTW... with the 10k pot installed & can easily get UNITY thru the unit but it goes back to the "26dB" Range.
Setting the 183.110 Gain stage resistor to aprox. 220ohm get VERY CLOSE & is probably what I may end up doing.
As you told me, Geoff Tanner recommends not going under 220ohm anyway because it can limit frequency range... BUT I do like the "Sound" of it "Driving" more!!! (using the 91 or 100 ohm)

I wish you could get into your unit for me sometime soon to take some measurements.  )))))

I'm not sweating it that much.... I can see my options, just would like your input &/or voltages.


Hey, 1 more thing - What is R60, 18k* doing on Igors schematic?
I do have it installed & even lifted one side to see if it would help my gain issue but nothing happened... not on any Neve schematic as far as I can find.

DISCLAIMER: I think IGOR did a GREAT job putting this together... I know he invested a lot of his time & energy into this & just want to say "Thank you". This thread is NOT meant to discourage other builders OR disrespect Igors efforts.