This is quite the "Thread Revival" @ 12 years 
Coupling Caps for Near-Zero DC Offset Op-amps?
- Thread starter thermionic
- Start date
Help Support GroupDIY Audio Forum:
> The gain cap (the cap in series with the gain setting resistor like C7 in your schematic) stops the gain from multiplying the amp offset.
I don't know if the input in this 325 circuit is ac coupled to the outside world or not, but C3 in the following example is also responsible for unity dc gain. Still the electrolytic in series with the primary is there.
http://www.hansenaudio.se/API 325 line amp.jpg
Same goes for this schematic:
From:
https://www.steampoweredradio.com/pdf/api/manuals/api 1972 schematics.pdf
And IF the 2503 transformer in the above example could get away with the worst case dc offset of the 2520, then wouldn't it be odd if the inputs were dc coupled to the outside world? Cause then you could get away with a small film cap to block any dc from reaching the +in of the 2520 and get rid of the electrolytic in series with the primary.
I don't know if the input in this 325 circuit is ac coupled to the outside world or not, but C3 in the following example is also responsible for unity dc gain. Still the electrolytic in series with the primary is there.
http://www.hansenaudio.se/API 325 line amp.jpg
Same goes for this schematic:
From:
https://www.steampoweredradio.com/pdf/api/manuals/api 1972 schematics.pdf
And IF the 2503 transformer in the above example could get away with the worst case dc offset of the 2520, then wouldn't it be odd if the inputs were dc coupled to the outside world? Cause then you could get away with a small film cap to block any dc from reaching the +in of the 2520 and get rid of the electrolytic in series with the primary.
Last edited:
Ricardus
WILL SOLDER FOR FOOD
I have a question very similar. I saw an API 312 clone recently that had no coupling cap after the Op-Amp direction into the output transformer.
Why no coupling cap when so many other very similar API 312 clones I've seen have the coupling cap?
Why no coupling cap when so many other very similar API 312 clones I've seen have the coupling cap?
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