Is Summit TLA100 optical, or what?

[strangeandbouncy]

Johan,


  Many thanks for OTA explanation. I am certain it is a transistor array on my TLA50. No sign of vactrols anywhere!



  AndyP

 

[etheory]

I know this thread is old, but.... after looking at the schematic for some time, I actually think the gain element is most likely to be a discrete version of the MFC6040 electronic attenuator used in the EMT262. It's the only part I can find from the era that this thing was designed that would account for the dual 100R potentiometers that are connected to the TLA compression cell. If this is indeed the case, then the time constants this device is using to emulate an optical cell are all entirely present in the side-chain, which is, extremely interesting....

Has anyone actually got one to measure the response curve between the control voltage input and the output attentuation? I'd be super curious to see if the response was:

1.) Linear or exponential.
2.) Directly proportional, or subject to some other time-constants hidden inside the "compression cell", to determine if the cell itself adds extra time-constants, or if it's just a direct VCA.

 

[gyraf]

Aahhh - good thinking there...!

Attached MFC6040 data sheet for reference. Can't figure out when it was discontinued?

Jakob E.

 

[etheory]

Yeah, I accidentally found that datasheet on the oldcrow website a few days ago and my brain almost immediately matched it to the TLA-100a schematic I'd coincidentally also recently been studying. The telltales are the single rail power, capacitive input coupling and dual-to-ground resistors with a need to balance them for control feedthrough rejection. I also feel the 20K pot to ground in the TLA-100a schematic is probably an intentional schematic red-herring or actually always directly connected to +15V. Alternatively there could also be dual rail -15v,+15v connections to the compression cell that simply aren't drawn on the schematic to confuse people. The distortion stats of the MFC6040 also seem a tad high (3-5%) compared to the claimed TLA-100a stats of <1% though....

If someone with a TLA-100a could measure the compression cell response to cv we could either put this to bed or come up with a that2180 version with better specs. Either way it'd be a fun project....

 

[gyraf]

The distortion stats of the MFC6040 also seem a tad high (3-5%) compared to the claimed TLA-100a stats of <1% though....

I read the data sheet as around half-a-percent until some 12dB gain reduction, which is consistent with claimed stats..

Jakob E.

 

[cyrano]

I seem to remember the MFC6040 being replaced by the MC3340. Quite an expensive IC, between 10 and 15$ in small quantities...

 

[etheory]

Funny, I was just about to post the same thing, not to mention that the  MC3340 datasheet also has all the resistor values.

However, 10 to 15 dollars in small quantities is quite cheap compared to the $150 Summit are charging for their replacement compression cells.... assuming of course I'm even vaguely correct. I could still be way off.

 

[scott2000]

I was looking around for related info and saw this little comp using that chip....Pretty cool......

I've only used a DCL200 briefly so don't have any thought on the TLA but I remember really liking that 200... Real classy sound....

Interesting revival of thread....thanks!!

 

[abbey road d enfer]

There must be some active stuff inside that gain cell: note the very high input impedance (0.1uF input capacitor) - and the low output impedance (drives a 10K potentiometer from it's output)

However, there is no power connection (the 15V via a 20k pot cannot be a power connection), so either the schemo is wrong, or the circuit is fully passive,which leaves not many possibilities, FET or optocoupler. In both cases, I don't see what the two 100r trimmers are for... The plot thickens.

 

[abbey road d enfer]

I know this thread is old, but.... after looking at the schematic for some time, I actually think the gain element is most likely to be a discrete version of the MFC6040 electronic attenuator used in the EMT262. It's the only part I can find from the era that this thing was designed that would account for the dual 100R potentiometers that are connected to the TLA compression cell.

Very good find! Still it doesn't fit with the absence of power connection.

 

[abbey road d enfer]

If someone with a TLA-100a could measure the compression cell response to cv we could either put this to bed or come up with a that2180 version with better specs. Either way it'd be a fun project....

It looks like the timing is entirely around the dual-time constant network atttached to the rectifier, which would suggest an instantaneous response of the gain cell.

 

[etheory]

Very good find! Still it doesn't fit with the absence of power connection.

But there is one. Isn't there a 20K trimmer between pin F and +15V? In theory that could supply power?

 

[etheory]

It looks like the timing is entirely around the dual-time constant network atttached to the rectifier, which would suggest an instantaneous response of the gain cell.

Is the response to CV linear or exponential? Very curious about that one....

 

[etheory]

It looks like the timing is entirely around the dual-time constant network atttached to the rectifier, which would suggest an instantaneous response of the gain cell.

I almost forgot to also say thank you abbey road d enfer!! (did you measure this on a real unit or is this a hypothesis by schematic inspection?)

Since that proves we can finally answer the original question this thread posed.

No. The TLA-100a is not an optical compressor. If it was, the compression cell would not be instantaneous.

 

[abbey road d enfer]

But there is one. Isn't there a 20K trimmer between pin F and +15V? In theory that could supply power?

It would not be capable of delivering enough power for anything sensible. And what would be the purpose of making the voltage variable?

 

[abbey road d enfer]

I almost forgot to also say thank you abbey road d enfer!! (did you measure this on a real unit or is this a hypothesis by schematic inspection?)

Since that proves we can finally answer the original question this thread posed.

No. The TLA-100a is not an optical compressor. If it was, the compression cell would not be instantaneous.

Don't make me say what I didn't write. I concluded from very incomplete evidence that the only timing is the dual time-constant network. Typically opto compressors do not require timing in the side-chain.
However, I know of one example of an opto compressor (by Ted Fletcher) that uses some clever circuitry to force the opto in almost instantaneous response, the actual timing being generated in the side-chain.
The more I look at the schemo, the more I think it is full of red herrings. I really would not be surprized if there were actual power rails connected to the gain cell. The two 100r trimmers suggest some kind of symmetry adjustment of a differential pair, which in turn suggests a variable transconductance amp. That is pure conjecture! 

 

[scott2000]

Since that proves we can finally answer the original question this thread posed.

No. The TLA-100a is not an optical compressor. If it was, the compression cell would not be instantaneous.

DH : Well, the Summit thing happened because I had a 1inch Ampex 8 track and I think it was called a VTR1000 or MM something, it was a 2inch video tape transporter and I wanted to see if I could find the stuff to see if I could turn it into a 16 track or 24 track for my studio. So I called around and that’s how I met Mike Papp and we got chatting and he said " if you can design a compressor that sounds like this I can sell it " and it was an LA2A. So he sent it over.

So I did. It got taken into a Hall & Oates session and it was in some scrap yellow box, and they wouldn’t let it go away!..and that was the first TLA-100. The other thing is, it doesn’t use an Optical device

https://www.kmraudio.com/news/designer-talk-dave-hill-crane-song/

 

[Recording Engineer]

Huh! I’ve got a VTR1000 full manual and transport, and a NOS custom Norelco 24-track headstack for it sitting in my garage right now! What can I do with them?