Tore Seem Preamp

[erik_DK]

looks really nice -
how to go about adjusting Threshold

I don't know. I was never interested i this device as a comp/limiter, only the high gain of the mic-amp.

/Erik

 

[gyraf]

..perhaps make R7 in the limiter circuit (9K1) variable - that'll be your threshold (i.e. sidechain-gain-before-rectifier)

 

[snö]

I don't know. I was never interested i this device as a comp/limiter, only the high gail of the mic-amp.

/Erik

yeah I know - its just cause it already there so why not try and use it to my advantage

 

[snö]

..perhaps make R7 in the limiter circuit (9K1) variable - that'll be your threshold (i.e. sidechain-gain-before-rectifier)

Thanks - ill by trying that

 

[erik_DK]

..perhaps make R7 in the limiter circuit (9K1) variable - that'll be your threshold (i.e. sidechain-gain-before-rectifier)

In fact that is exactly what is written in norwegian in the schematic: "R7 justeres til korrekt terskelverdi" means "adjust R7 to correct threshold".

Maybe I don't fully understand what happens if you change R7, but isn't it correct, that this could lower the sidechain gain (higher value of R7) and thus give a higher threshold (which is what you will want, because the limiter cuts in very abruptly at relatively low levels). And won't you then at some point run into the very problem, that started this whole thread: that the high gain in the second audio stage is not attenuated enough by the FET, so that you run out of head-room. Maybe you still need to make the mod, I described changing R3 to a somewhat higher value.

I am not at all sure, that this was intended as a limiter. When I listen to an unmodified module it sounds more like an automatic gain control.

/Erik

 

[erik_DK]

Thanks - ill by trying that

I look forward to learn about your results.

/Erik

 

[erik_DK]

And BTW: Before you design in too many extra switches and pots, bear in mind the very limited space behind the frontplate where the output transformer resides.

/Erik

 

[gyraf]

With that type of output transformer, don't expect too much of it regarding low-frequency response or drive levels (after all this was probably made for speech) - this is probably where I'd look first if I wanted to upgrade the circuit. Otoh, it may be all good, those guys knew what they were doing.

As for limiter/AGC, my feeling is that that terminology was always f*cked, at least around here (DR etc.)

 

[erik_DK]

Sure it was made for speech - so that the mixing engineer could use the talk-back microphone without worrying too much about gain and talking distance to the mic etc.

I don't know if the output transformer is "all good", but after I lowered the gain of the second stage, it doesn't sound bad - though I have not done any measurements.

The 1:5 input transformer is one of those small relatively high quality transformers from BeyerDynamic, so it makes no sense using a completely crappy output transformer.
However in other modules from Tore Seem that are intended for the broadcast signal - mic-amps, eq's and line-amps, you find transformers from JS and Lundahl.

OTOH I had the same thought when I first opened up the module, and someday I will experiment with different transformers from JS, OEP and Haufe, that I have somewhere in my drawers. Then I can do an A-B blind test.

/Erik

 

[gyraf]

..a simple sanity check would be to compare maximum output level before distortion at e.g. 1000Hz and 40Hz..

/Jakob E.