U73b from scratch

[DaveP]

This is what Ruairi sent me privately from an initial test:-

I've spent the whole evening here listening to the U73B and measuring it in every way possible with the dScope.  The short review is that there's a lot to like and with a few tweaks it will be very strong.

Listening impressions
- The overriding impression is colored by the frequency response (we're losing top and bottom)
- The action/knee of the compression is very nice
- The threshold point of the compression and limiting needs to be adjustable for modern studio use (downward mainly)
- The coloration caused by the THD is not objectionable at all, it's sweet in a classic tube way
- A variable attack time would be very useful, the current attack time is very fast which works wells for some things but we use a lot of slower attack times in production
- The release times are very slow, at anything longer than the 0.3 sec they are too slow for modern music and even that is slow.
- At a certain point in the compression and limiting curves the output level starts going below input

The U73b is a vintage compressor now and I decided to recreate it as it was because it was used for decades in Europe for mastering.  The timing can easily be modified but it would need an extra hole drilled somewhere to accommodate an attack switch.  The values of the release resistors can be easily changed.

This is a feed forward compressor which makes it immune to oscillation but it's tricky to match the twin amps.  At overload the matching is not exact and as Ruairi says. output can go below input.

The frequency response of the original was limited to 15kHz and 40Hz for FM radio which did not bother people in the 20th century, but for today's music with sub-sonics in films and dance music it may not be suitable.

Best
DaveP

 

[etheory]

It's the footer on my posts

Thanks
DaveP

Typically, I missed the obvious right in front of me ;-)
Really liking your music, I just started to have a listen. Thanks.

 

[shoonbassjp]

thank you DaveP !!
for great idea that split Edcor CT

 

[ChrioN]

Best build-thread in quite a while if you ask me. Bravo Dave!
Did you save your p2p layout in any form or did you do it all in your head? I live to build p2p after already-made-layouts. I feel so badass, but its still painting by numbers  ;D

 

[DaveP]

Did you save your p2p layout in any form or did you do it all in your head? I live to build p2p after already-made-layouts. I feel so badass, but its still painting by numbers.

It's just the application of logic using well established principles.  I keep photos of the stages.

Centres of tubes are usually a couple of inches apart,  high current stages as far away from inputs as possible.

The schematic is an excellent starting point.  Add up how many stand alone connections there are and put in more tag strips than you need, if you are lucky it will all fit in your case.

Use toroid transformers wherever possible for power supplies and mount them on edge with the wire outlets pointing in a harmless direction.

You have to think at least one stage ahead when starting to solder in components, I try to never do this when I'm tired to avoid foul-ups.  Keep all wiring as short as possible and avoid hum loops.

I hope this helps, good luck

DaveP

 

[flatresponse]

Hi Dave,
thank you so much for that extended build thread with loads of practical advice.
I´m not sure if it´s doing your project justice being tested as a mono device in a nowadays mastering suite. For my M49 replica the plan is to have a recording producer compare them to his real life M49´s. Something similar should be done with your beautiful creation, maybe. Just my two cents.

Since I was working as a music producer (bigband and symphonic) for a german radio station from 74 to when I retired in 2015, I started during the last days of the valve aera. I have to admit, I wasn´t so enthusiatic about valve technology at that time, but things have changed.

Especially 2 U73b with a Varicom is still on my list and your build from the scratch is a big step towards it.
Your thoughts about planning ahead when building P2P is the practical side of view - like real life !
I might be going with the original sheme since the valves - though expensive - are still at hand.
And I  go and ask the (very friendly) guys at Haufe, who built many of the historic transformers and might still have the original BV (Bauvorschrift = build plan).
As there obviously is no plan for the Varicom, the only chance is TAB, resp. Mr. Brüning.

I share your OT  thoughts, regarding what is happening in our countries. If anyhow comparable, Germany is worse than the UK.

Stay healthy in body and mind, which is - fair play - the most important thing of all and stick to your skills
Best
Manfred.

 

[DaveP]

Thanks for your kind comments Manfred and others that have replied too.

You are fortunate to have contacts with the transformer suppliers, it will make life easier for you.

I like the concept of the feed-forward compressor, but the matching of the two audio streams can be tricky (and interesting) but the result when set-up is a totally rock solid performance with no oscillation possible.

I miss the country of my youth more than I can say, but it has gone forever.  It was a country of opportunity where things that used to be impossible for our Fathers and Mothers, were suddenly opened up. (The Beatles. Stones, Geoff Emerick case in point)  The 50's and 60's were a great time to be alive.  The places I grew up in, now only exist in my head.  They have been taken over by other cultures and are lost to me.  There are places in the UK where they still exist but I am priced out of those areas, but rural France is a very close second.  It appears that anyone who does not like what has happened to their countries is automatically branded a racist, which is a sick excuse  for an argument  IMO.  The whole immigration/globalisation scene has been dumped on people to keep wages low for business, but we never saw that coming.  Rant over!

DaveP

 

[abbey road d enfer]

I miss the country of my youth more than I can say, but it has gone forever.  It was a country of opportunity where things that used to be impossible for our Fathers and Mothers, were suddenly opened up. (The Beatles. Stones, Geoff Emerick case in point)  The 50's and 60's were a great time to be alive.  The places I grew up in, now only exist in my head.  They have been taken over by other cultures and are lost to me.  There are places in the UK where they still exist but I am priced out of those areas, but rural France is a very close second. 

France of my youth does not exist anymore either. And the grass is always greener somewhere. That's what Yats* say about Nawlins, which is my second homeland.

*Yats are New Orleans citizens; it comes from the way they salute each other: Where y'at?

 

[DaveP]

France of my youth does not exist anymore either

Aaah "les trente glorieuses"
Tu sais ce que je veux dire!

DaveP

 

[Michael Tibes]

And I  go and ask the (very friendly) guys at Haufe, who built many of the historic transformers and might still have the original BV (Bauvorschrift = build plan).
As there obviously is no plan for the Varicom, the only chance is TAB, resp. Mr. Brüning.

Hello Manfred,

Do you mean plans for the Varicom or it's transformers? And did you work with the U73b / Varicom combination? If so, I'd be grateful to get to know more about it. I have 2 U73b with varicom, but I have no information on how to set things up as originally intended.

Michael

 

[flatresponse]

Hello Michael,
regarding to Haufe, I was talking about the BV for the historic T188.
But they´re no longer able to build the T188, because of lack of the prefabricated material.
And I have to stand corrected, as I found the schematic for the Varicom combined with the calibration instruction.
Kubarth has the file called: "TAB VARICOM Data-Schematic". It will tell you, how to modify the compressors to work with the Varicom, which indeed is some kind of a sidechain  control. Will help setting sustain, decay and bass sensitivity (yes, something very contemporary sounding) !
Hopefully you read and understand German ?
Description is only for connection with U73, I assume it might meat the requirements for the U73b as well.
There is a position "80" (= 5M in the U73) (= 10k in the U73b)  and a position "55" (= 5M in the U73) (= 800k in the U73b).
Just replace these with 10M.
Bridge position 38 in the U73, which isn´t present in the U73b, so leave it as it is.
And connect position 23, just in between the 25uF and the T188 (which is already marked "Z" in the U73b schematic) with position Tuchel 5a. In case there is a silver plating between 5a and 5b, you have to remove it.

And no, I haven´t worked with this special piece of gear.
I´m just fascinated by good old german radio design, setting steps by making precious gear like that - with the typical lack of material after WWII.
How lucky you are to have two U73b AND a Varicom !!
Johannes Brüning at
http://www.funkstunde.com/de/menue/kontakt.html
will be of help if you go and ask him.

Best, Manfred.

 

[Michael Tibes]

Hello Manfred,

thank you for the info, this is basically what I had found as well. I am very keen on a more detailed description of the varicom, what the intended use and settings were. But there seems to be nothing available publically and Mr. Brüning didn't seem to have any more information either. The time constants in the U73b are a bit different than in the original U73, so I wonder how this comes together with the varicom. Oliver Archut wrote an article about the U73 and Varicom history here http://www.tab-funkenwerk.com/id6.html and he mentioned that the combination was quite popular as a compressor for vinyl mastering, but no one I know seems to have experience with it. I'll keep digging... And yes, I'm german, so I can read the descriptions ;-)

Michael

 

[wmarden]

Great thread DaveP!

 

[opacheco]

Thanks Ruairi,
I've been working on the mock up I've made to check that the parts I'm using actually work.  I've not shown a pic of this because its lethal and a bad example to set.

I have set it up using the instructions I translated and it does work ok.  ..........
best
DaveP

DaveP could you provide me a copy of this "Instructions" for adjust the U73 that you translated??
Thanks anticipated,
Opacheco.

 

[DaveP]

DaveP could you provide me a copy of this "Instructions" for adjust the U73 that you translated??
Thanks anticipated,

I have attached the instructions Opacheco
DaveP

 

[DaveP]

Here are the main instructions again.

DaveP

 

[opacheco]

Here are the main instructions again.

DaveP

DaveP,
Thanks a lot. I have some question about this design and it’s mods  for you but this will be later.

Opacheco

 

[opacheco]

........I then realised that it was safe to use a WSM15k:15k because in the U73 there is only 1-2V max  on it and its rated for 10V.....

........There is 160V DC on it but this makes no difference as it is totally balanced in push-pull.  I am still using one XSM15k:15k for the  CV -ve voltage as the output from the CF drive can exceed 10V......

DaveP,
could you give us more information about this 1-2V max?...where specifically does this happens in the circuit?

What transformer is supporting these 160V DC?......does not need to have this transformer any GAP for DC current flow?

Thanks anticipated,
Opacheco.

 

[ruairioflaherty]

Hi all,

I have Dave's U73B build here and it's just back from a demo with a friend and great engineer who will likely be buying it if I can make some mods to make it more useful for him (he really liked the box overall).  Some of these mods I discussed with Dave back in the day based on my own listening tests.

The plan right now
1) Characterize the unit stock (distortion plots, hum, gain reduction curves etc, frequency response)
2) Make mods
3) Characterize again and share data here
4) Sell unit and send Dave some dough.

This was all to happen a few years ago but 2 adopted kids got in the way and bench activity has been restricted mostly to repairs for PMC / Prism / Maselec.  My apologies to Dave and the community for dragging my feet on this.

Some notes from my friend

"Just did some pre-session testing as an insert in Pro Tools.
Gain seems to be about +4dB when inserted in compression mode, but -4dB when inserted in limit mode.
It's susceptible to vibration, tubes V1 and V2 seem to be the culprits. Could they be shock mounted?
XLR-M and IEC connectors are mounted upside-down.
Needs input gain or threshold control. Would be nice to have an output gain control as well.
Release times are a little long. Fastest three options were useful. Would like to have something even faster.
Sonically, it seems pretty dark. Going to try it tracking some vocals today."

He went on to use it on several sessions including some tracking sessions with a great artist and really got to like it.  It has a hum problem that I also need to get to the bottom of.

I'm slammed with mastering now before NAMM but I'll be back to document this process and ask questions of Dave once I get home.

 

[opacheco]

Hi all,

I have Dave's U73B build here and it's just back from a demo with a friend and great engineer who will likely be buying it if I can make some mods to make it more useful for him (he really liked the box overall).  Some of these mods I discussed with Dave back in the day based on my own listening tests.

The plan right now
1) Characterize the unit stock (distortion plots, hum, gain reduction curves etc, frequency response)
2) Make mods
3) Characterize again and share data here
4) Sell unit and send Dave some dough.

This was all to happen a few years ago but 2 adopted kids got in the way and bench activity has been restricted mostly to repairs for PMC / Prism / Maselec.  My apologies to Dave and the community for dragging my feet on this.

Some notes from my friend

"Just did some pre-session testing as an insert in Pro Tools.
Gain seems to be about +4dB when inserted in compression mode, but -4dB when inserted in limit mode.
It's susceptible to vibration, tubes V1 and V2 seem to be the culprits. Could they be shock mounted?
XLR-M and IEC connectors are mounted upside-down.
Needs input gain or threshold control. Would be nice to have an output gain control as well.
Release times are a little long. Fastest three options were useful. Would like to have something even faster.
Sonically, it seems pretty dark. Going to try it tracking some vocals today."

He went on to use it on several sessions including some tracking sessions with a great artist and really got to like it.  It has a hum problem that I also need to get to the bottom of.

I'm slammed with mastering now before NAMM but I'll be back to document this process and ask questions of Dave once I get home.

ruairioflaherty,

Fantastics notes!
We will wait for results soon.
Opacheco.