Phase Switches and Pads

[emrr]

[quote author="JohnRoberts"]

If you desire some euphonious non-linearity, put that in a separate black box with properly labeled bypass switch.

JR[/quote]

That would be a stand-alone preamp!

Do I want the 1947 Collins 6Q-1, or the 1961 RCA BA-71A sound today?

 

[TomWaterman]

On my calrec re-design which is still evolving when I have the time..I have moved the pad from the front of the input TX primary to the secondary.

For a number of reasons, one it should not interfere with the coupling between mic and transformer...an effect heard by many on designs such as the API312.

Its also much easier to specify low levels (6-10dB) of attenuation with a standard voltage divider as part of the TX secondary load than using a U-pad on the input, which has a minimum loss if you wish to obtain reasonable reflected impedances to the mic and TX.

I'm planning to do something with gain staging to achieve mic and line ins and placing the pad there, splitting it into two attenuator stages meant it could be active at different attenuation levels for mic and line in respectively to tailor the gain ranges for each input.

For example the pad is 20dB on mic in and 10dB on line in/DI...

You can also make one leg of the voltage divider a pot to create a variable pad - thats a tip from JLM Joe and very useful if you don't have fine gain switching...

One issue I found with the polarity flip on a preamp is if you want both TX and impedance balanced transformerless outputs...its much easier to flip at the in instead of the out because the outputs can end up out of polarity with each other...

-Tom

 

[adamasd]

I'm planning to do something with gain staging to achieve mic and line ins and placing the pad there, splitting it into two attenuator stages meant it could be active at different attenuation levels for mic and line in respectively to tailor the gain ranges for each input.

I was thinking of doing something similar, but using a DPDT switch for going between mic and line/di and using the other half of the DPDT to change the pad to a more suitable value.

One issue I found with the polarity flip on a preamp is if you want both TX and impedance balanced transformerless outputs...its much easier to flip at the in instead of the out because the outputs can end up out of polarity with each other...

I can not imagine a reason I would ever do both a transformer and impedance balanced out, but if I ever do that is a mistake I am sure I would have made if I did not just read that. I tend to make those oversights quite easily.

I am coming to the conclusion that it is pretty much a toss up on which is better for the pad. The trade offs are becoming pretty equal for me.

 

[Samuel Groner]

I'm not sure I follow. There may be subtle inter winding capacitance differences with one end of the output winding shorted or the other, but should not be a distortion issue with any adequate driver IMO.

They seem not to be subtle enough to make B. Whitlock not mentioning them in one of his JAES articles (Vol. 43, No. 6, 1995).

I have moved the pad from the front of the input TX primary to the secondary.

Only with the best transformer you will not run into overload problems.

Samuel

 

[JohnRoberts]

I knew there was a reason I try to avoid transformers in quality audio paths... :grin:

Perhaps ironic I have done a lot of design for install market where they want transformers on both ends of audio path, often inside the same chassis. You want some transformer fun put a 100V constant voltage step up output transformer inside the same box with mic pre step ups... The 15W or 35W version has the same voltage ratio mic-to-speaker, just in a smaller box... arghh.

JR

PS: One of my install related patents was for a bass boost clamp to mitigate against output transformers saturating from too much high level LF when bass was cranked.

 

[TomWaterman]

[quote author="Samuel Groner"]They seem not to be subtle enough to make B. Whitlock not mentioning them in one of his JAES articles (Vol. 43, No. 6, 1995).[/quote]

Thanks for the article number Samuel...

[quote author="Samuel Groner"]Only with the best transformer you will not run into overload problems.[/quote]

Admittedly I am still waiting for a bunch of high quality low turns ratio transformers to compare the distortion on such things like snare drum etc...

Am going to compare many Lundahls including the LL7903 to the original iron and also want to get a Cinemag CMMI-3.5C.

Looking at Cinemags distortion plots and Lundahl datasheets it appears that some models (most I'm looking at) start saturating badly at around 0dBu upto about +10dBu. Then its full on grime.
Surely the mic would have to be hellishly hot to reach that kind of level?

Lots of these classic pres didn't have pads fitted to them...although there are pads on some mics. Maybe letting the transformer saturate a little more isn't so bad?

-Tom