Svart

FETbloke for summing results
« on: January 20, 2007, 11:45:50 PM »
So I've been away for a while but I wanted to share one of my recent projects.

I've been itching to get rid of the stock summing amps in my console and today while the crew was finishing work on a new isolation booth in the studio I thought I would take the time to work some FETblokes into the summing network.

I figured out the buss impedence and then went from there.  I removed all the parts related to the summing amp including the BJT frontend, the opamps and resistors and replaced them with a FETbloke and it's feedback network and some pots for trimming the gain.

Needless to say it works great.

In fact it works so great that my noise floor dropped 15db.

15db!

even with the faders fully up and inputs idle, my noise floor is now below 70 when it was previously at -40 with the faders up.

I'm pretty sure I hear a small difference in the sound but not much.

I mentioned removing this section before but many told me that it was fine.  I suggest that anyone thinking about this give it a try after they do their math and see if it works for them.

 :thumb:
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.


Sorr

FETbloke for summing results
« Reply #1 on: January 21, 2007, 01:49:19 AM »
Whats a Fetbloke?

Svart

FETbloke for summing results
« Reply #2 on: January 21, 2007, 02:20:20 AM »
it's a discrete opamp designed by TK, one of the forum's members.

There were a few different types of Bloke opamp, this one has a JFET front end and power darlingtons on the output.  

It's a 5 transistor CFB opamp.

One of the best I have come across and yet so very simple.
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

Sorr

FETbloke for summing results
« Reply #3 on: January 21, 2007, 03:14:00 AM »
Can you post a link to your schematic? :razz:

chrissugar

FETbloke for summing results
« Reply #4 on: January 21, 2007, 03:26:29 AM »
Christian Mike Sugar
        CMS-LAB

haima

FETbloke for summing results
« Reply #5 on: January 21, 2007, 09:11:48 AM »
nice svart  :thumb:  - what's your console?

Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
FETbloke for summing results
« Reply #6 on: January 21, 2007, 09:55:32 AM »
15 dB is indeed a very impressive improvement! Any chance to see the before and after schematics? From your description it is impossible to follow the reason why such a large improvement was possible.

Samuel

Svart

FETbloke for summing results
« Reply #7 on: January 21, 2007, 11:21:32 AM »
I will try to post some this week.

The best I can figure is that the summing amp design was not ideally matched to the bussing impedences?  I have a feeling that the JFET frontend had something to do with it as well.

I am studying it more as I have time today.

The 15db improvement was totally by suprise, I did not expect this at all!

and it's an Alesis x2
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

bcarso

FETbloke for summing results
« Reply #8 on: January 21, 2007, 01:06:21 PM »
That is a shocking improvement.  But I've been cleaning up other people's bad designs lately so much that it's a plausible possibility.

One thing that occurs:  perhaps the propensity of JFETs to handle spuriae like RF banging around and not self-rectify as much could be part of it, depending on your overall system situation.

Does your bloak use the 2SK170 as the FET?  What were the amplifiers before?

Svart

FETbloke for summing results
« Reply #9 on: January 21, 2007, 01:49:25 PM »
yes the Jfets are now sk170B.  the frontend BJTs were unmatched 2n4403s, one base to signal, the other base to ground, differentially driving a 5532.

An interesting note, the schematics for this console show a coupling cap between the buss and the BJT but the version of board I have does not have this cap at all.  I added BP caps before the Blokes for safety as I didn't measure the DC on the system when I installed them.

I expect the base/ground part was allowing noise to flow from the ground rails?

I need to post a schematic I'm sure but think of a simple preamp with BJTs driving an opamp.  It's identical but with a fixed gain.

In the new section I used 15k feedback and a 10k pot to ground so I could play with the gain.  I was able to feed signals into the channels and find their clipping points.  I backed off the gain a few db and then fed the same signal into the new section and then tweaked the pot to find that clipping point and back it off a few db.  I did this to ensure that the channels would clip before the master section would even with all channels engaged.

Works pretty well for sure.  I'm able to trim the gain up on the blokes to the point where they will clip the next section out before they even start to run out of headroom.

Some time ago I started a master buss section for a mixer project and it got put on the shelf due to work and other projects.  I think it's time to bring it back out and lay out a section based on these opamps.

I know that TK has been extremely busy with his life and wish him well.  He really did an outstanding job on these opamps.
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.


Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
FETbloke for summing results
« Reply #10 on: January 21, 2007, 02:14:19 PM »
BTW, with which bandwidth did you measure? I remember the original schematic and I think the summing amp had some funny compensation which made it have pretty high high-frequency noise gain. I guess you made sure that signal gain is still the same so that the measurements are comparable?

Quote
Perhaps the propensity of JFETs to handle spuriae like RF banging around and not self-rectify as much could be part of it, depending on your overall system situation.

That sounds like a plausible explanation.

Samuel

Svart

FETbloke for summing results
« Reply #11 on: January 21, 2007, 03:42:35 PM »
Quote
I guess you made sure that signal gain is still the same so that the measurements are comparable?


yessir.  I used the channel strip circuit as the comparison and once I found the clipping point for that circuit I kept the signal level the same.

I was more concerned with keeping the buss channels from clipping when all the main channels are already clipping than anything else.  this would inherently ensure headroom and bandwidth.

 :thumb:
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.


 

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