GroupDIY Bridge compressor.

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DaveP

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These are the final pics before packing tonight.

Overall.jpg


Commended into the tender care of FedEx tomorrow

Front-1.jpg


Then en route to EMMR in the US for audition.

Back-1.jpg


Lettering done and conversion to 115V

This project is now out of my hands and into the Forum's.

Be gentle with her, she was 9 months in the making!

all the best
DaveP
 

DaveP

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dryman said:
What about the report from Jay. It would be interesting.

Dryman,

He went on holiday the next day so it would have been difficult to do from the beach!

I'm hoping for the report on his return.

best
DaveP
 

emrr

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In da house, passing signals to ears, comparing against Collins 26C, taking notes. 
 

shabtek

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this is the coolest. DaveP, I love you.
from the start this thread was fresh and inspirational--this project has become the embodiment of this place at its finest;
cheers to you
 

DaveP

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Thanks Shabtek,

But we better wait til Doug's report until we crack open the champagne, his next post might say it sucks!

The people who have inspired me have been CJ, PRR and EMRR in no particular order.  I used to think it was cool to be cynical as a young man, but that just leads to Bittersville.  Now I've learned that its not all about money, its about putting something back.  These guys put it better.......

"And in the end, the love you take, is equal to the love you make."

best
DaveP
 

emrr

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The sound quality is so different from the Collins, that I am hampered by the comparison, attempting to see past that part and evaluate the compression characteristics.  It sounds most like the Collins with max input, and minimum output, which is really related to distortion.  More listening to do, only various program so far, and use it for tracking.  More soon. 
 

DaveP

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Maybe it needs a vintage transformer on the front end?  That will roll off the treble and boost the gain.
best
DaveP
 

lassoharp

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What I will be listening for specifically when I go up first of next week is the "squeeze quality" so to speak.  That's what stood out to me most about the original - the sonic characteristics under GR.  I imagine that quality would have stayed relatively the same with a different set of iron.
 

shabtek

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right on Dave--he did not say it sucks---better frequency response maybe...the result is not so much the crux as the process you've let us in on...
The people who have inspired me have been CJ, PRR and EMRR in no particular order.  I used to think it was cool to be cynical as a young man, but that just leads to Bittersville.  Now I've learned that its not all about money, its about putting something back.  These guys put it better.......

"And in the end, the love you take, is equal to the love you make."

best
DaveP
I agree absolutely, big respect to all you
 

DaveP

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Just to let you know what's going on at present.

Jay has been very ill and has been in hospital but is ok now, he will post his report in a day or two.

Doug has been testing the GroupDIY26C against a Collins 26C and making some improvements; he will report soon, then it's off to some other studios.  When its done the rounds, the aim is to put it up for auction in the autumn/fall.

best
DaveP
 

lassoharp

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I'll preview my impressions by saying that the tube compressor fan who wins it is likely going to be very happy.

;)
 

emrr

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Finishing up some tweaks, will file a tech report soon.  About to start new listening with the tweaks.  Going well.
 

DaveP

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I thought it about time for an update.

EMRR has done some extensive testing along with lassoharp and they will be reporting separately.

This testing has been on the dynamic characteristics as well as electronic, and it showed up some  flaws which have now been corrected.

The first of these was the need for a balanced input and a boost to the sensitivity, this was accomplished by fitting an Edcor 600:15k input transformer.  The response was also made more linear by fitting a 470 ohm resistor to increase the source resistance of the WCF driver.

These changes are incorporated in the attached revised schematic .

Next post will describe a change to Stage 2.

best
DaveP
 

Attachments

  • 26C Stage 1.pdf
    27.4 KB · Views: 59

DaveP

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Continuing from the last post:-

The gain of the output stage was way too high and that made the control very tricky.  We decided to apply 10dB of feedback to cure this and this also improved the S:N ratio as a bonus.

The revised schematic is attached.

From the feedback that we have had so far, and EMRR and Lassoharp will give more detail, the main problem area is hum.  These pre-war designs only had S:N ratios of 45~50dB and this could make their universal use problematic.  Having said that, it appears that this compressor does do some things better than a vari-mu and that it is completely thump free.

The major cause of hum is the 32kHz coil which needs to be enclosed in a screened box.  If you look at the Pultec threads you will see that they potted their coils in a box to avoid the hum pick-up.  An unscreened coil is like a guitar pick-up remember.  With hindsight I should have fitted a screened box during the build ......hey ho. :'(

best
DaveP
 

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  • 26C Stage 2.pdf
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emrr

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Sorry for the delay folks, it's been balls to the wall around here.  Lassoharp and I spent the better part of a day listening and evaluating, after I'd already listened and run some preliminary tests. 

The unit spent 3 days with Mitch Easter who said "It's really good!"  He tracked bass, banjo, and drum room mic with it, and found all the control ranges agreeable. 

Thanks for the schematic updates Dave, they look correct. 

What's interesting about hum with regard to early pro audio designs is the mechanical approach taken.  At the beginning of the 1930's it was customary to run everything off of batteries, and later from large centralized rectifier units.  By the mid '30's, AC rectification stages in program amps became normal but the power transformer was frequently an external piece of equipment.  Preamp B+ was generally taken from taps on the program amp, and filament from batteries or distant external filament transformers.  Manuals of the age warn about keeping power transformers at least 3 feet away from low level audio transformers and circuits, and this with incredibly heavily shielded parts.  The Collins 26C uses an external power transformer as was custom, along with 2 stages of LC filtering in the amp.  This makes for a dead quiet AC noise floor, but is clearly impractical from a modern view.  In those days pro audio was an industrial pursuit, with industrial level facility build-outs. 

My next exploration will be to remove the inductor based 32kHz trap and evaluate, also to see about fitting it earlier in circuit at higher level as pre-emphasis.  The hum pickup is undoubtably related to the operating level where it it fitted, the lowest of the entire amp and therefore most susceptible to interference and amplification. 

The compression quality is really extraordinary, particularly the release.  Piano and bass release can be very fast, smooth, and clean sounding, which is a strong 'quality challenge' for a lot of comps.  It is super effective for single source compression, release smoothness and speed is really the hallmark of this and the Collins.  I think I would personally shy away from use on kick or snare, but it does sound good on a drum submix or room mic.  You can push 10+ dB on a high tempo mix with fully silent breaks and watch it fall almost immediately to 0 GR and right back to 10 dB GR as the break passes, not a bit of pumping or artifacts as found in the typical vari-mu. 

Actually it's an Edcor WSM600/10K that I put up front.  I found that with slower attacks and faster releases there was not enough gain available in the side chain to push much gain reduction.  It feels better with the transformer boost, and it also tamed a bit of the frequency response extremities, top and bottom, for the better.  I tried a 500:50K at one point, and it was really too much input gain for typical levels here.  A 600:10K provides enough input gain that one can overload the input stage, much like the original Collins.  After the mod I found with the Audio Precision THD+N test that I could get into high levels of THD (9%) with the input knob below noon, when fed a hot signal, and the input clipping was nowhere near as apparent as the measurement would suggest. 

The sweet spot so far really seems to be compression in the 6 dB range.  There is a gentle top end boost under gain reduction, that increases with more compression.  This concerned me at first, but it always stayed smooth, even with a small DPA condenser inside a piano being beaten to death on a rock tune. 

I'll sign off now and see about getting some plots posted next. 
 

dissonantstring

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this is really great work and a very nice chronicle of the GDIY bridge compressor. 
i really like the generosity of this thread and the collaboration. 
thank you daveP for donating the time, funds/materials, energy and knowledge for this project and to emrr and lassoharp for testing and circuit modding.  one of the best examples i've seen of the gdiy forum spirit. 
great stuff guys!
kindest regards,
grant
 
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