Suggest 1.8K for R12 in mnats and other 1176LN Rev D DIY builds

David Kulka

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Recently I was asked to debug and calibrate several DIY 1176LN's with the mnats Rev D PCB, and discovered what I think is a fairly significant error. In the preamp section, R12 is specified as 920 ohms. I suggest that people use 1.8K instead.

The original UREI Rev D and E schematics show 910 ohms for R12, but many examples actually use 1.8K, as do the UREI Rev F version and the UA 1176LN reissues. Basically, I think the 910 ohm value was a mistake that crept into certain D's and E's. (Some D's and E's do use 1.8K, which makes me think UREI had a service bulletin or upgrade procedure that addressed this.)

If you use 910 or 920 ohms instead of 1.8K, DC bias in the Q3 / Q14 section will be incorrect, and you'll lose about 8 dB of headroom in the input section. You'll also have less limiting. (Since Q3 / Q14 precede the sidechain circuit, having less gain before clipping at this point forces you to run the input pot lower, reducing the amount of limiting available.)

The error in the value of R12 is understandable, since 910 ohms is shown on the old schematics. But these units will work and sound much better with 1.8K. I can't imagine any benefit to using 910 but if you disagree, please speak up.

(I wasn't sure whether I should start a new thread or post on the main 1176LN Rev D DIY thread, but decided on a new thread because the topic seemed deserving of attention.)
 

kazper

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I took the work out of it and posted a link back to this thread and the information you posted in the D thread.

Thanks again !

Note: removed from the A thread since it's not a issue there.
 

baadc0de

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After this replacement/upgrade, must the unit be recalibrated?

Cheers,
B.
 

mnats

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Thanks for this correction, David. I had a look at one of my builds from 2007 using an earlier version of my boards and found that I had used a 1.8k resistor there. Somehow the error crept back into the BOM sometime later. I'll fix my documentation as soon as I have some time.

I note also that the Purple schematic I have shows a 910 ohm resistor used as well so that might be something worth looking into if you own one of these units...
 

sonicwarrior

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I have two printed BOMs here, both some years old, and both are listing a 1.8 K for R12.  :-\
One is from nimbleswitch.com with Digikey/Mouser part numbers and the other looks like the original BOM.

Where then did the 910/920 Ohms came from?
 

mnats

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sonicwarrior said:
Where then did the 910/920 Ohms came from?

From the schematic in the document downloaded from the JBL Pro site for one.

Here's their Rev D schematic with some measurements I took with the 1.8k resistor in place and with another 1.8k resistor soldered in parallel to simulate the lower value:

1176_revd_r12.gif


The top underlined measurements are with the 1.8k resistor and the bottom are with 900ish Ohms. It's easy to see how this might cause the problems David mentioned earlier.
 

David Kulka

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baadc0de said:
After this replacement/upgrade, must the unit be recalibrated?

Cheers,
B.

No, you won't need to recalibrate after changing the resistor. It changes the amount of headroom in the front end, but not gain structure or anything else.

sonicwarrior said:
...Where then did the 910/920 Ohms came from?

As I wrote before, this error originates from certain early UREI builds and the fact that the D and E schematics show the early, incorrect resistor value. The change was never explained or clearly documented by UREI, and the error has lived on in many original units and clones. The first time I ran across this in a UREI Rev D I was really surprised, because it greatly compromises performance of the unit.
 

gemini86

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I wonder if it made it's way into igors f76? (If you're using the stock amps)

EDIT: It seems the f76 is based off a different rev. I doubt mayne people are using the stock amps anyway.
 

craigg

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I just checked my BOM from March 2010 and R12 was listed as 910 ohm.  I just changed it out to 1.8k.  Thanks for noticing David - I'm interested to hear the difference.
 

Davo

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Thanks David, just swapped mine out... much appreciated!
 

dmp

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Very interesting - I just changed this resistor in my units. Thanks for figuring this out Mr Kulka!
 

Alistair

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Hey all-

I just changed this resistor over in my unit and recalibrated.  It was a bit trickier recalibrating (controls were a bit more fiddly), but I did so according to the videos and now I have to crank the input gain MUCH higher to get any compression to occur. 
It sounds great, and it's nice having a little more headroom in the unit, but would the calibration need to be altered in any way to reflect the change in the unit?

 

gemini86

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From earlier in this thread:

David Kulka said:
baadc0de said:
After this replacement/upgrade, must the unit be recalibrated?

Cheers,
B.

No, you won't need to recalibrate after changing the resistor. It changes the amount of headroom in the front end, but not gain structure or anything else.

sonicwarrior said:
...Where then did the 910/920 Ohms came from?

As I wrote before, this error originates from certain early UREI builds and the fact that the D and E schematics show the early, incorrect resistor value. The change was never explained or clearly documented by UREI, and the error has lived on in many original units and clones. The first time I ran across this in a UREI Rev D I was really surprised, because it greatly compromises performance of the unit.
 

canidoit

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On my Journey Excel BOM I placed both resistors 910 and 1.8K with a note saying confirm which one to use. Hopefully I still have that 1.8k resistor somewhere????

I can't remember which one I went with but I think it was most likely 910 from reading a reply in a post.

There must be heaps of units with the 910 in there but my unit seems to be too loud anyways.

When you change the resistor to 1.8k, does the threshold of compression move higher as well sounding from Allistair's response?
 

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