Input gain on 1176 kit way too hot

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Nik Kotzev

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Sep 18, 2020
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Hello gentlemen,

I recently purchased two 1176 kits, which use a board of an unknown origin (to me at least).

Both compressors work fine and are calibrated. The seller said they might be an early mnats version, but there is no documentation with the kits, so it's impossible to tell. Mike Havok from this group said the boards look like Gyraf, but they are not.

The compressors don't have input transformers.

I am having a problem with the input level - both units start compressing at a way too low setting of the input pot.

I am reading the Groupdiy forum since a few days, but can't seem to find a solution.

I stumbled onto talks about the  R26 mod and the R1-3k7 mod, but cannot find proper information on which one does what and how should I perform them.

Of course, as an alternative I would be fine with a mod for rising the compression Threshold.

Would someone kindly point me to a place or link/s, where I can get the proper information, please?

My input pots are 10K, the output pots are 100K. The boards have Distortion trimmers, so they might be a D or F Revision.

The gain reduction and signal level FETs on both units are 245A.
I changed the gain reduction FET on one of the units to 2N5457 - no change.


Thanx for your time:

Nikolo Kotzev

nk@nostramusic.com
 
 

shabtek

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Did you find mnats schematic

Is this board with opamp balanced input and footprint for optional transformer input
 

Nik Kotzev

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shabtek said:
Did you find mnats schematic

Is this board with opamp balanced input and footprint for optional transformer input

Hello and thank you for the reply,

There is no footprint for optional transformer. The input is opamp balanced.

I have no documentation with the kits. I am looking at mnats and Gyraf sites, checking schematics etc.

You could actually see a picture of the board here:
https://groupdiy.com/index.php?topic=76249.0

Thanx again:
Nik
 

JMan

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You could compare your board to the schematic on Hairball‘s website.  They use an optional resistor (~100r, if I’m remembering correctly) to tame the input, but I can’t quite recall where in the circuit it goes.  At any rate, you could take a look at their documents and see if you could implement something similar.
 

Nik Kotzev

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JMan said:
You could compare your board to the schematic on Hairball‘s website.  They use an optional resistor (~100r, if I’m remembering correctly) to tame the input, but I can’t quite recall where in the circuit it goes.  At any rate, you could take a look at their documents and see if you could implement something similar.

Thanx, Jman

I will check that out.

Cheers:
Nik
 

Nik Kotzev

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Sep 18, 2020
Messages
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JMan said:
You could compare your board to the schematic on Hairball‘s website.  They use an optional resistor (~100r, if I’m remembering correctly) to tame the input, but I can’t quite recall where in the circuit it goes.  At any rate, you could take a look at their documents and see if you could implement something similar.

Hm... I can’t find any schematics on the Hairball Audio site.
Tried to find info on that 100r, to no avail...  Actually that’s the info I need most, as there are schematics on the mnats and Gyraf sites...

Regards:
Nik
 

JMan

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Nik Kotzev said:
Hm... I can’t find any schematics on the Hairball Audio site.
Tried to find info on that 100r, to no avail...  Actually that’s the info I need most, as there are schematics on the mnats and Gyraf sites...

Regards:
Nik

Ah, yes, I forgot that they are rather buried on the website.  The link below takes you to the first part of the build guide for their 1176 -- just scroll down a little until you see the "FET/Rack Documentation" section.  Links to schematics/BOMs/etc for the various revisions are there.  Also, if you go to step 2 of the build guide (scroll all the way to the bottom of the linked page to find that link), it discusses the input resistor mod.

https://www.hairballaudio.com/blog/resources/build-guides/fetrack-v2-build-and-calibration-guide
 

Nik Kotzev

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JMan said:
Ah, yes, I forgot that they are rather buried on the website.  The link below takes you to the first part of the build guide for their 1176 -- just scroll down a little until you see the "FET/Rack Documentation" section.  Links to schematics/BOMs/etc for the various revisions are there.  Also, if you go to step 2 of the build guide (scroll all the way to the bottom of the linked page to find that link), it discusses the input resistor mod.

https://www.hairballaudio.com/blog/resources/build-guides/fetrack-v2-build-and-calibration-guide

Hi JMan and thanx for the link. I found the mod. It seems to be relevant for builds with an input transformer.

My boards are with electronically balanced inputs and their assembly somehow reminds the Gyraf builds (after looking at the Gyraf schematics). However, I found the 3k7 mod instructions - the original R1A needs to be replaced with a 3k7 and this is for the electronically balanced builds. Thus the input gain will be lowered.

I finally  found a discussion in the G1176 threads, which is about the R26 mod and it probably is the one I need. The mod is done by adding a 470K resistor in series to the R26 - 56K, which will modify the side chain. Thus the gain reduction will kick in after a higher threshold.

All other mods I found are for builds with an input transformer.

I will try that R26 mod, hopefully it will be the solution I’m after.
There was a  DIYer in the forum, who did both mods and seems to be happy.

Thanks again for your help and best regards:
Nik
 

JMan

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Nik Kotzev said:
Hi JMan and thanx for the link. I found the mod. It seems to be relevant for builds with an input transformer.

My boards are with electronically balanced inputs and their assembly somehow reminds the Gyraf builds (after looking at the Gyraf schematics). However, I found the 3k7 mod instructions - the original R1A needs to be replaced with a 3k7 and this is for the electronically balanced builds. Thus the input gain will be lowered.

I finally  found a discussion in the G1176 threads, which is about the R26 mod and it probably is the one I need. The mod is done by adding a 470K resistor in series to the R26 - 56K, which will modify the side chain. Thus the gain reduction will kick in after a higher threshold.

All other mods I found are for builds with an input transformer.

I will try that R26 mod, hopefully it will be the solution I’m after.
There was a  DIYer in the forum, who did both mods and seems to be happy.

Thanks again for your help and best regards:
Nik

Ahhh I totally missed the earlier post where you said that yours does not have a transformer!  At any rate, glad you found a potential solution.  Good luck!
 

Nik Kotzev

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JMan said:
Ahhh I totally missed the earlier post where you said that yours does not have a transformer!  At any rate, glad you found a potential solution.  Good luck!

Cheers!
Nik
 

Nik Kotzev

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JMan said:
Ahhh I totally missed the earlier post where you said that yours does not have a transformer!  At any rate, glad you found a potential solution.  Good luck!

OK, I did the R26 mod - I added a 560K resistor in series to R26 and the sidechain now is perfect. I can do a light and heavy compression on all ratios, without having to keep my preamp (feeding the 1176) at a crazy low level. I figure I also improved my S/N ratio by a bunch, since now I can use the preamp at a normal output level. Also, before the mod, my 1176 output was barely showing any movement even with a full output volume ( of course that was when I tried to apply a very light compression). Now things are nice.

One very big question popped up though, since after the mod I could not perform the standard calibration with 0DBu (0.775VAC), cause now my sidechain was not strong enough  to drop compression down to -10db and reached only only to -7db.
After some time of head scratching I decided to try and calibrate with +4dbu at the input, since that's the working level in my studio.  It worked out really nicely. Signals are clean, I can see a healthy output level, the calibration worked out well and the compression is perfectly controlable.

The question is: did I do the right thing, or am I just babbling?

I guess some expert must chip in on that one....

Cheers and thanx:

Nik
 

RuudNL

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Mmmm.... I probably see a little problem here.
By increasing the value of R26 you lower the signal level going to the side chain.
This means that you will get a much higher level over the FET before compression sets in.
Normally distortion will increase when the AC voltage between drain and source gets higher.
Just to be safe, I would check the distortion at the point the compression sets in now.
It might be higher than before...

A better way would be to reduce the signal level at the input.
This would also shift the onset level of compression up, while maintaining the desired level over the FET attenuator stage.
Just a thought...
 

Nik Kotzev

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Sep 18, 2020
Messages
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RuudNL said:
Mmmm.... I probably see a little problem here.
By increasing the value of R26 you lower the signal level going to the side chain.
This means that you will get a much higher level over the FET before compression sets in.
Normally distortion will increase when the AC voltage between drain and source gets higher.
Just to be safe, I would check the distortion at the point the compression sets in now.
It might be higher than before...

A better way would be to reduce the signal level at the input.


This would also shift the onset level of compression up, while maintaining the desired level over the FET attenuator stage.
Just a thought...

Hi there and thanx for your input.

Thinking logically, I have not tempered with the operating level of the schematics, so the FET should be able to accomodate these levels. The useful amplitude of "heavy compression" vs "non compression"  varies in the range of 20-30db, which should be well within the capacity of the FET.

On the other hand, if I don't lower the signal of the side chain, during light compression I am sending a way too low level to the output, which makes my compressor unusable in my environment. If I lower the input level, I will even worsen the situation.

Also, at the moment there is no audible distortion during normal operation, and I am able to operate at much higher levels than before I did the mod.

So I still wonder...
 
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