EMI RS124

[tubemonkey35]

Anyone have a part number for the Hold function Relays (H1, H2) on the schematic?

 

[mich]

can someone sketch out the output attenuator connection to the output terminal?
i dont get the resistor R17(160R).
and isn't an H attenuator better here?

 

[emrr]

Almost no one ever used an H attenuator; huge and expensive.  You see them on long lines (miles - telephone); receiving or to a transmitter.  You will find a T feeding a balancing transformer more commonly than an H. 

 

[EarBud]

Hello,

I see that no one's commented here for over a month, so I hope it's okay to bump this - it seems to be the definitive thread for the RS124.

After MANY years of Googling "RS124" in the hopes that some (genuine) information would find its way to the web, I was overjoyed to finally get a hit leading me here.  I'm very happy that John decided to post that holy grail of compressor schematics.

But could someone please help me with a few things?

1) I second the request mich made for someone to complete the attenuator circuitry missing from the right side of the schematic.  I'm a pretty good builder, but an engineer I'm not...

2) I don't understand the relay installation at all.  If it's supposed to stay energized in any of the "HOLD" positions, why is the relay coil going to TS-3?  And where is its 24-volt source supposed to come from?

3) The schematic posted by rotheu before John put up his EMI original turned out to be impressively close to the real deal.  It would be great if he would make all the corrections and repost - it's much easier to read than the original.  Just a wish on my part.

Thanks.

 

[EarBud]

Hello,

I'm going to attempt to attach a .jpg file of the RS-124 output circuit which has my best guesses regarding the missing area of the schematic.  Could someone please clarify this for me?

I'm also still in the dark about how the relay works.  I'd understand if the RECOVERY switch activated the relay coil - but according to the schematic it doesn't - it's activated by an external 24v source.

I'd really appreciate a reply from someone who understands this design.

Thanks.

 

[DaveP]

Earbud,

Don't get hung up about the output side, the Abbey Road studio was set up on 200 ohms and most outputs on vintage gear have. accepted a 600 ohm standard.

Look up pads on Google to learn the basics, its all based on the voltage divider principle.

The sound of the RS124 is based on the circuit, the tubes and the transformers, what happens in the pad will not have any effect on the sound, the impedance of the next bit of gear will have more of an effect, it should be 600 ohms or more, 10k is the current standard

best
daveP

 

[EarBud]

Dave P,

Thanks.  I may end up just using a Daven 600 ohm T.  But I'd still like to know how the original RS-124 schematic looked.  After all, we waited almost 50 years to see it!    I guess the reason I don't know which leads go where is because I don't understand the role of R-17.  Then I'd know which lead was the DIRECT output (but I still wouldn't know where the remaining lead went).

My main quandary is the relay.  Could you please tell me what the deal is?  As I posted earlier, I'd understand it if the "HOLD" positions of the RECOVERY switch activated a relay which "HELD" the previous setting in place.  But according to the schematic, this isn't what's happening.  The relay coil is being powered by an external 24v source.  This just doesn't make sense to me.

I'd really like to understand how the original design worked.

Thanks.

PS: 'Really like your BA-6A build!

 

[MagnetoSound]

If you look at the RECOVERY switch, as it is rotated every second contact is left open. These are the HOLD positions.

The HOLD function works by placing the RECOVERY switch in a position where the wiper is disconnected from all of the release resistors, so that the timing cap cannot discharge.

The relay just provides a remote control for the HOLD function, by disconnecting the wiper of the switch from the timing cap, thereby over-riding whichever position the RECOVERY switch happens to be in.

The source of the relay energising voltage doesn't matter at all.

 

[EarBud]

MagnetoSound,

Okay - so the relay isn't even necessary for the RECOVERY function to work.  Now THAT makes sense to me.

Thanks!

 

[EarBud]

If someone will answer my questions about the role of R17 and the destinations of the output leads in the .JPG I posted, I promise to go away for a while  .

Thanks.

 

[DaveP]

Earbud,

If you look at the circuit on the output TX secondary you will see that R17 is in series with the EMI home-made attenuator.  The output windings are in parallel so they are 150 ohms.  The direct output is between the top wire BLK/White and the Black wire.  The middle wire is a fixed proportion of the output as it forms a potential (voltage) divider with the attenuator.  I don't have the time or the inclination to work out exactly what the proportion is, but I guess it was for connection to another piece of gear so that it never overloaded.

The engineering set-up at Abbey Road was very regimented, and there were strict rules about who did what (demarcation).  Forms had to be filled in and work signed off by bosses etc.  If some senior guy wanted you to put in R17 as a belt and braces fix for a special bit of kit then you did it!  You might say "there is already an attenuator there" but it would do no good!  It was this kind of attitude that the Beatles gradually broke down, but you had to live through those days to know what it was like in Britain in the 60's.  This is my considered guess about R17, but let's not let it become an urban myth, it has no bearing on how you might use an RS124 today.  Just connect the output windings in series and stick 600 ohms across it and it will be fine!
best
DaveP

 

[EarBud]

Dave,

Okay, thanks.  'Sorry to be such a squeaky wheel.  It just bothered me not knowing what the remaining lead was and whether R17 was considered to be external to the basic circuitry or not.

I guess I'll get over it in time  - thanks again.

 

[DaveP]

Earbud,
A certain degree of Asperger's Syndrome in the DNA is prerequisite for this forum, full marks for persistence!

Best
DaveP

 

[Jean Clochet]

Post removed.

 

[DaveP]

Thanks Jean,

I think your explanation of the role of R17 is correct, because the feedback from the 100k resistors R26/R27 will have reduced the output Z from the normal 150 ohms by the feedback factor, to about 40 ohms.  R17 would not have been necessary on a stock 436B but would have been on the RS124.

So there we are Earbud, it was required for correct matching to 200 ohm gear to obtain the correct frequency response not the overload application that I had surmised, that is the forum in action.

best
DaveP

 

[Jean Clochet]

Post removed.

 

[DaveP]

Hi Jean,

Well spotted on the R17 change.  But if you are correct that R16 was across the output, then the output resistance was formally 90 ohms not 68 ohms.  90//276=68, requiring a 132 to make 200.

I am aware that the output Z is lower than the 150/600 published, but I always like to have a terminating resistor for safety, because the tubes need to see a reflected impedance of roughly twice their internal resistance to work into.

best
DaveP

 

[Jean Clochet]

Post removed.

 

[emrr]

There's an Altec tech note that addresses build out resistances for specific amps, and the requirements for correct interface with various types of Altec EQs.  One of the best primers I've seen. 

 

[Jean Clochet]

Post removed.