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  #1  
Old 01-30-2009, 12:43 PM
Pern Pern is offline
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Have anyone done this mod to the 1620 ?

Add a effect bypass switch connecting it from the summing amp capacitors (c14,c18) directly to the tone control PCB input. ?

If you don't use the effect loop, you would still have Tape out but remove 4 OPs etc from the signal path.


Like this :


Summing amp Cap
>--O
O---------------> Tone control PCB input
>--O
Effect inp buffer out

Last edited by Pern : 01-30-2009 at 05:18 PM.
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  #2  
Old 01-30-2009, 02:53 PM
thermionic thermionic is offline
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IC2 has a gain of just over 10dB.

The 5532s are the quietest op-amps in the 1620 – far quieter than any TL07-series. In fact, 5532s are used in early models of Audio Precision analysers – that’s how quiet they are.

edit - if the 5532s were there as impedance amps for the tape outs, they'd be wired for Unity. If IC1 is near its maximum GBW, IC2 could well have a useful purpose.

Personally, If I had to make a circuit from 074s and 5532s - and had a choice - I'd get the gain from the 5532s and use the 074s for Unity / low gain applications.


Justin
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Last edited by thermionic : 01-30-2009 at 02:57 PM.
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  #3  
Old 01-30-2009, 03:04 PM
Pern Pern is offline
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EDIT: My first thought was that they added the 5532 because of how the balance function affect the signal level.


I know that the 5532 is the quietest OP in the 1620. I have use that a lot before in my own stuff. And always as output buffers. I think it's strange UREI use it here and not for the BOOTH output also.

Normally i would never think about using a TL in the main signal path, i only use it in VU Rectifiers, CUE functions and maybe in headphone amps. If i had designed the 1620 back then i had used 5532 almost everywhere.

I also have some other "reflections" about the design. But i guess i read Walt Jung's "IC Op Amp Cookbook", "Audio IC Op Amp Applications" and other by him to many times 20 years ago ;-) The idea that noise etc could be a good thing in a mixer is new to me, but why not ....

But every component that could be removed from the signal path when not used for anything is good. Well, that 's how i think anyway. I think it's to easy to just add stuff to be safe, like RANE does.

Last edited by Pern : 01-30-2009 at 05:09 PM.
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  #4  
Old 01-31-2009, 02:28 AM
Digitalis Digitalis is offline
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I think the TL07x was chosen for it's very high input impedance. It makes certain aspects of the circuit design much easier to get away with... For example, I think additional input buffers would be required if using a 5532 for the summing amp. If you are doing this much redesign, you could also consider THAT 1510 chips... anyway....

I would either leave the design completely alone or design the whole thing anew. The redesign route is lined with unexpected pitfalls... it always is.
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  #5  
Old 01-31-2009, 04:58 AM
Pern Pern is offline
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Quote:
Originally Posted by Digitalis
I would either leave the design completely alone or design the whole thing anew. The redesign route is lined with unexpected pitfalls... it always is.

I agree. Doing major modifications with hi-grade components to the schematics just for the "fun" of it is not worth it. It is easier and better to do a new design based on them then.
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