EMI RS124

[Murdock]

I'm interested in this compressor.
I have some nice big 10k:40k transformers but without center tap... I've read a post here saying that one can just use a "fake" center tap with resistors.
Is it really that easy?
What would be the downside of this?

 

[jazzcrisis]

I'm interested in this compressor.
I have some nice big 10k:40k transformers but without center tap... I've read a post here saying that one can just use a "fake" center tap with resistors.
Is it really that easy?
What would be the downside of this?

On the input transformer, yes.

 

[Murdock]

On the input transformer, yes.

Thanks for the answer!
But what is the benefit of using a center tap over resistors?

 

[Rob Flinn]

Thanks for the answer!
But what is the benefit of using a center tap over resistors?

On the input transformer, yes

I originally built mine with a 1:4 step up transformer & found I needed to change it to a 1:1 like the originals, to get it working nicely. My units were compressing too much. i.e they started compressing as soon as I turned the input pot from zero in the slightest. Your transformers are 1:2 so you may find they work alright but you may find you need to add a pad to the input as well as your fake centre tap.

 

[Lee_M]

I'm interested in this compressor.
I have some nice big 10k:40k transformers but without center tap... I've read a post here saying that one can just use a "fake" center tap with resistors.
Is it really that easy?
What would be the downside of this?

I don't think you'd even need to add resistors to fake the centre tap, the RS124 (and Altec 436) have a dual 50k pot connected across the input transformer secondary, the "low" side of both potentiometer decks are linked and connected to 0v, along with the transformer centre tap.

That dual pot should be enough to fake it without adding any extra resistors.

I'll let someone smarter than me answer about possible downsides to this arrangement. I'm guessing Altec didn't use a centre-tapped transformer just for shits and giggles, but I don't know enough to tell you why they used one, or what you'd be sacrificing by using an input transformer without a centre tap.

 

[Redd47vf14]

Hi!

 

[anvl]

Really beautiful!!!!

 

[mattrad]

Hi all, I built one of these point-to-point a while ago and it works great compression-wise, but I noticed that the signal loses a fair amount of high end when going through the amp. Its not terrible but clearly noticable in an A/B with the source material.

All voltages readings are spot on with the schematic. Ive pulled the signal from various places in the circuit to try to track it down and everything seems good until after V2, just before the OT. Figured it was a tube issue so swapped the 6GC7 but no dice, same problem. Pulled the 6AL5 to make sure it wasnt something goofy in the sidechain. Same thing. Double and triple checked all connections, especially around V2 and the OT, everything seems in order. Output trans is a Edcor PP1-15K-600. I have a simple output attenuator after the OT, 2 parallel 300 ohm resistors into a 1k pot. Works great but i bypassed it just in case there was some sort of loading issue. No change.


Was wondering if anyone else ran into this issue and/or has any ideas. Thanks!

 

[rock soderstrom]

Output trans is a Edcor PP1-15K-600

Maybe this is the culprit?

 

[mattrad]

Bingo! Thank you. Lashed in an Edcor XSM 15k-600 I had lying around, sounds great now. Is there any objections to using the XSM full time in this circuit? I was under the impression that these might not be ideal for push-pull situations.

 

[Rob Flinn]

Hi all, I built one of these point-to-point a while ago and it works great compression-wise, but I noticed that the signal loses a fair amount of high end when going through the amp. Its not terrible but clearly noticable in an A/B with the source material.

All voltages readings are spot on with the schematic. Ive pulled the signal from various places in the circuit to try to track it down and everything seems good until after V2, just before the OT. Figured it was a tube issue so swapped the 6GC7 but no dice, same problem. Pulled the 6AL5 to make sure it wasnt something goofy in the sidechain. Same thing. Double and triple checked all connections, especially around V2 and the OT, everything seems in order. Output trans is a Edcor PP1-15K-600. I have a simple output attenuator after the OT, 2 parallel 300 ohm resistors into a 1k pot. Works great but i bypassed it just in case there was some sort of loading issue. No change.


Was wondering if anyone else ran into this issue and/or has any ideas. Thanks!

I think my 2 boxes got better when I changed the Edcors for Sowters.

 

[rock soderstrom]

Is there any objections to using the XSM full time in this circuit? I was under the impression that these might not be ideal for push-pull situations.

The XSM 15k600 is not a push pull transformer. If I remember correctly, member DaveP used this or very similar exactly in such a circuit and was satisfied. I think there are better ones, try your luck at UTMIndustry, maybe they have something suitable on offer.

https://utmindustry.com/

 

[rock soderstrom]

Since you're on the other side of the pond, check with Cinemag to see if they have anything suitable.

 

[alphasnk]

the 3580 UTM sounds superb in the rs124! 15k600 wsm / xsm are great alternatives as well, but i prefered the utm, it sounds 'done' with the utm versus the edcors to my ears.

 

[anvl]

I never had this problem using Sowter transformers.

 

[anvl]

Regarding attack control. The original schematic has no attack control (it uses a fixed 33K resistor + 0,5uF cap) but Chandler modern version has a rotary switch with different values (different resistors).

In which range do you think resistors can be used in a rotary switch ATTACK control without inestability or strange pops?

 

[Rob Flinn]

Regarding attack control. The original schematic has no attack control (it uses a fixed 33K resistor + 0,5uF cap) but Chandler modern version has a rotary switch with different values (different resistors).

In which range do you think resistors can be used in a rotary switch ATTACK control without inestability or strange pops?

If you do some searching around here on very old Altec 436 modding threads I am sure someone said it starts to become unstable if you reduce it much further than 33k. However if you look at the "nu vari mu" project diagram (see below) they have an attack control which at it's lowest setting appears to be a 15k (R12b on its own) feeding a 1u cap which then goes through 10x 22k on a rotary switch feeding a 0.1u to step increase the time constant. The sidechain signal is the charged up 0.1u. I'm not quite sure how this works because it would appear at minimum to be much lower than the 33k that is discussed in the old threads as being the lower limit of stable. This basically a direct copy of the Chiswick reach VK1 sidechain. It may be that the combination of the 2 time constants increases the load somehow to make it stable. I didn't bother adding an attack switch to my units. Therefore I can't tell you how well it works, & you may have to experiment yourself.

 

[anvl]

Thanks a lot, Rob.

 

[emrr]

Ok, now that Chandler is rolling out this modified 436 as an “RS660”, it’s time for someone to DIY the same idea. Prob not with 6386’s though!

Bonus points for getting RS124 and 660 (weak mimic) timing into the same box

 

[anvl]

I think my new projet wil be a Redd47 preamp and a RS124 compressor together in the same box and connected in series to create an unique old school channel.. using a GZ34 rectified PSU.