Re: Altec 436 / RS124-ish Clone
« Reply #40 on: January 02, 2021, 03:22:02 PM »
I have finished the 436c, from my humble opinion it has disappointed me, I do not see it more useful than the amplification one.
It is true that I have no experience with varimu compressors but I imagine more of an optical type compression, but I see that this compressor is even more subtle, so I started by completely rectifying the 120v winding and adding the two 100k resistors , All the ground noise has gone, now it sounds without noise, I have left the 1M potentiometer, which I did not notice any use.
According to the scheme that Mr. Winston published, a hold is added (I think it's a release), an output attenuator is added, and a 3.3m potentiometer. I don't know what it is for?
I don't see that it has an attack, I don't see the threshold either.
suokngo that will be automatic like the optician la2a?


Rob Flinn

Re: Altec 436 / RS124-ish Clone
« Reply #41 on: January 02, 2021, 08:38:57 PM »
I'm not really sure why you would expect it to sound like an optical compressor when it isn't one.  If that's what you wanted maybe you should have built  an La2a.

The control you say is the hold control is actually the release control. In between the release positions it has a hold function so that you can get the unit to be compressing before the track starts so there are no surprises on the first transient after which point you switch the release control to the desired release constant.

The attack is fixed, although it is possible to add a limited amount of control.  Basically the sidechain isn't capable of driving very fast attacks, so maybe of limited use.

I can't see any 3M3 pot of the diagram, can you explain where this is in your build.  Did you use it instead of the release switch ?

It is true there is no threshold control but this isn't a big problem because you just wind up the input pot and it compresses more which has the same effect as lowering the threshold.  There are many compressors like this, including most Vari mu's & 1176's to name but a few.

I'm quite surprsied you are disappointed by the EMI version of this compressor.   I was always disappointed by the Altec versions, but the RS124 is to my ears completely much better.   Last year I built two RS124's at the same time I finished a Fairchild 670.   I'm not saying they sound anything like the same, but I like them both equally, and I built the pair of RS124's for less than 1/4 of the cost of the Fairchild.
regards Rob

Re: Altec 436 / RS124-ish Clone
« Reply #42 on: January 03, 2021, 01:57:57 PM »
Hi Rob, sorry maybe I explained wrong, I use goolge for my translations because my level of English is not fluent.
I didn't mean to say that altec 436 sounds bad, I wanted to say that it has little play due to the limited controls, it actually sounds good. regarding the emi rs124 I have now started the modifications, I just finished with the Hold and it seems that it works well, the attenuator of I don't understand the output, I don't know how it works. Could you add some stereo potentiometer as in the input?
Thank you very much and sorry for the misunderstanding.

Rob Flinn

Re: Altec 436 / RS124-ish Clone
« Reply #43 on: January 03, 2021, 02:39:24 PM »
Hi Rob, sorry maybe I explained wrong, I use goolge for my translations because my level of English is not fluent.
I didn't mean to say that altec 436 sounds bad, I wanted to say that it has little play due to the limited controls, it actually sounds good. regarding the emi rs124 I have now started the modifications, I just finished with the Hold and it seems that it works well, the attenuator of I don't understand the output, I don't know how it works. Could you add some stereo potentiometer as in the input?
Thank you very much and sorry for the misunderstanding.

You can simplify the output attenuator like the one I use.  See pic.  When the pot is turned fully down the xlr pins 2 & 3 are shorted so there is no output, but the transformers still drives into 600R.   It works really well and is simple.  It's actually one the Winston recommended to me 10+ years ago !
regards Rob

Re: Altec 436 / RS124-ish Clone
« Reply #44 on: January 06, 2021, 07:21:48 AM »
Thank you very much Rob, this solution that you have published has worked very well, now it seems that everything works as it should, now I see this device as a good tool.
I have read in an external thread to this forum that if the input volume potentiometer of 50k is changed for another of 250k it works better?
is this true?
a greeting.

Rob Flinn

Re: Altec 436 / RS124-ish Clone
« Reply #45 on: January 06, 2021, 10:50:13 AM »
I have read in an external thread to this forum that if the input volume potentiometer of 50k is changed for another of 250k it works better?
is this true?
a greeting.

I haven't tried this, so I don't know.  It would change the input impedance of the unit due to the 250k being reflected back through the input transformer.  As with a lot of internet advice you need to try it for yourself.  Can you linl to the place you read this ?
regards Rob

Re: Altec 436 / RS124-ish Clone
« Reply #46 on: January 07, 2021, 11:20:19 AM »
It's actually one the Winston recommended to me 10+ years ago !

I remember that Rob   :)

On the input, I doubt using a dual 250K attenuator over a 50K will do anything much, other than possibly under-terminate the input transformer.  This might (or might not) give frequency response changes or peaking from the transformer.   It will add noise. 
Also, you're generally looking to keep the R grid 1 value as low as possible with vari-bias comps, although a 250K pot with a single valve isn't too radical.   

What specific parameter is it that you want to change about the RS.124 Juanaco?


edited for clarity.
 
« Last Edit: January 07, 2021, 09:01:48 PM by Winston O'Boogie »
D. J. H.

Re: Altec 436 / RS124-ish Clone
« Reply #47 on: January 14, 2021, 08:19:04 PM »
...  ... Last year I built two RS124's at the same time I finished a Fairchild 670.   I'm not saying they sound anything like the same, but I like them both equally, and I built the pair of RS124's for less than 1/4 of the cost of the Fairchild.

Hey Rob,
from distant memory: was it you who had a serious, pretty elaborate & expensive chassis made up for a Fairchild type clone unit a good few years back?   I have a memory of reading that on here but?

Regardless, I'm interested in your opinion on building one.  Is it a worthwhile use of funds in your opinion? 
I've been thinking about messing around and coming up with my own Fairchildy type unit over this year, but, to do it justice, the cost can get a little daunting for sure  :o

I've had some ideas for expanding the RS.124 type comp to be able to do what a Fairchild can do but, if the F.Child type circuit kills, then so be it.  I'd go that route.

Thanks man,  D.J.H.   

Edit: Just to add, I am familiar with what 660's and 670's can do, I've used a fair few  and refurbished/rebuilt 3 X original 670's thus far.  I guess I'm more interested in your opinion of value per £ spent.   Should I bite the bullet etc   :D   

« Last Edit: January 14, 2021, 08:30:48 PM by Winston O'Boogie »
D. J. H.

Rob Flinn

Re: Altec 436 / RS124-ish Clone
« Reply #48 on: January 15, 2021, 03:50:26 PM »
Hi John, 

Yes I was doing a p2p Fairchild, & had a chassis made, but it kind of got shelved.  I had all the parts and a stack of 6386's but always seemed to get too busy to get a big block of time to finish it.  In the end I bought a used drip pcb and I finished that build about the same time as I finished the RS124's last year.     It works welland sounds good, I can't help feeling a bit ashamed of myself not completing the p2p build. 

In terms of value for money I don't think I would build one now unless I won the lottery.   It took about 5 years to get all the parts starting around 2000. The audio transformers were well over £1k.   I was getting the 6386's for around £40 a pop at the time.   I had a Parmeko vintage oil filled line up for the mains transformers & Davens etc.   It was a lot of money, I think I was about £2-3k into it & have 3 sets of 6386.    I thin a lot of people start this project & probably get 1/2 way through it to realise that it's a bit of a money pit.   Just the cost of the 6386's is eye watering.             

I did experiment with a gain cell that used 4 x 6bc8 in parallel a la Fairchild which worked pretty well. I think I tried to use an solid amp for a sidechain amp, but never really got it to work.  I never really got it further much than that, and am doing a big ATMOS install in London for the next couple of months so have little time for anything.    Funny how one seems to get busy doing everyone elses projects, giving one little time for ones own.  Maybe one day I will get some time, grow some balls & complete my p2p version ...
regards Rob

Re: Altec 436 / RS124-ish Clone
« Reply #49 on: January 16, 2021, 06:40:41 AM »
Hey Rob,
thanks for the reply and your insights.
I don't think there's any shame at all in diverting the project to a pcb version, you got it done and it works well which is what counts.  I've never once got beyond thinking about it.

Like you, I did mess with a breadboard gain cell of 4 X 6BC8 (actually, I think it was 6ES8) to see what was what.  I got it working well enough that it's certainly a viable route.

I've never fancied paying the going rate of 6386's (old or new) myself and did think about Larry Janus' (Larrchild on here) version with triode strapped 6BA6 pairs. 
Still, 6ES8 requires much less voltage from the side-chain amplifier for a given gain reduction so something along the lines of Tim de Paravicini's EAR 660 side-chain is fairly inexpensive.

Going with an RS.124 type circuit, even if we add a low output impedance side-chain amp with adjustable gain and DC threshold to get fast limiting options, we have the problem of the 6ES8 to output valve coupling caps "blocking" and recovery time.
Others (UA176) used an interstage transformer there to avoid that problem.

One other experiment I tried though was direct coupling of the gain valve into the output valve.  The big voltage swing that you get with a resistor loaded 6ES8 caused common-mode issues (actually, it played havoc) with the 6SN7 output valve so, my experiment tried a pair of low DCR chokes on the plates to hold them at a fairly constant voltage.  Still, good chokes are also expensive so I've since thought about trying a solid state alternative to chokes.

Anyway, I'm waffling.  Again, I appreciate your thoughts which pretty much confirm what I suspected.   
Thanks again, good luck with your ATMOS install, hope it goes smoothly :)


D. J. H.


Rob Flinn

Re: Altec 436 / RS124-ish Clone
« Reply #50 on: January 16, 2021, 11:28:26 AM »
John

The 4x 6bc8 gain stage I made was driving straight into an output transformer.   I was only using Edcors for i/o so I'm not sure if in the long run it would have killed the o/p due to d.c but it was sounding very nice.   I was able to control it with a bench PSU for testing.  Not wanting to reinvent the wheel, I was just using a generic solid state amp I bought as a kit off ebay when I was trying to get a sidechain going. However I seem to remember it wasn't liking driving a bridge rectifier.   I had a job come in which diverted me a way from the project.
« Last Edit: January 16, 2021, 11:31:43 AM by Rob Flinn »
regards Rob

jazzcrisis

Re: Altec 436 / RS124-ish Clone
« Reply #51 on: Today at 10:26:57 AM »
I've had some ideas for expanding the RS.124 type comp to be able to do what a Fairchild can do but, if the F.Child type circuit kills, then so be it.  I'd go that route.

I built a souped-up stereo RS-124 recently that I think comes close to a Fairchild's capabilities. It's definitely as fast, but does sound a little different. It's essentially a stock RS-124 circuit with the following additions:

1. (2) 6BC8 in parallel per channel. (still requires careful matching, but makes it much less sensitive to tube variations)
2. Regulated B+
3. Cathode Follower feeding the sidechain.
4. True stereo sidechain.
5. Sidechain HPF.

It's a monster, and I can get it faster than I'd ever need it without any hint of thumping or motorboating. Very, very good master bus compressor.

Re: Altec 436 / RS124-ish Clone
« Reply #52 on: Today at 03:11:29 PM »
I built a souped-up stereo RS-124 recently that I think comes close to a Fairchild's capabilities. It's definitely as fast, but does sound a little different. It's essentially a stock RS-124 circuit with the following additions:

1. (2) 6BC8 in parallel per channel. (still requires careful matching, but makes it much less sensitive to tube variations)
2. Regulated B+
3. Cathode Follower feeding the sidechain.
4. True stereo sidechain.
5. Sidechain HPF.

It's a monster, and I can get it faster than I'd ever need it without any hint of thumping or motorboating. Very, very good master bus compressor.

Nice one!   :)
With a lower impedance drive from your side-chain cathode follower then the attack time should be able to be pretty fast.
If I understand you correctly, then the ratio is still about the same as an RS-124 yes? About say 4:1 ? 
   
 
 
D. J. H.


 

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