1:10 step up Mic input transformer

[analag]

If a mic outputs 1V into a 1:10 ratio transformer like the JT115K-E wouldn't that overload the tube if no pot is used as is most often the case. Current feedback is not as effective as voltage feeedback at preventing overload in this type of situation. So it is fair to say the input stage overloads before the transformer saturates and what you thought was the sound of your iron is the sound of your tubes clipping?

Analag

 

[gyraf]

You'll need to have "real" variable gain at the first amplifier stage in order to prevent input stage from clipping at high input levels. Look at e.g. the G9 project for an example of how it can be done.

An attenuator pot after the input transformer interfers with the need for a low-noise amplification stage.

Transformer saturation and tube clipping are very different entities indeed.

Jakob E.

 

[tk@halmi]

Wouldn't 1V into most 1:10 transformers overdrive the transformer already?

 

[SSLtech]

...if a mic outputs 1V, maybe you don't need a mic pre!

That kind of number is line-level stuff. if you were hell-bent on using a pre for a paricular flavour, I'd recommend a male-Female XLR pad, perhaps 20dB or so...

Keith

 

[CJ]

That Jensen will handle 1 volt at most frequencies. You will see problems with that level starting at about 40 hz and below. My guess at least.

How many volts does a U-67 put out if placed in front of an stack?
Anybody?

 

[SSLtech]

240...

...in the UK!

Anarchy everywhere else!

Not a volt... I still use at least 20dB gain on a U67 in front of a loudish stack.

Keith

 

[PRR]

> If a mic outputs 1V

Then it is a very hot mike too close to a very loud source.

For the hottest large condensers, it is about 128dB SPL. This is roughly the nearfield of an overpowered 12" guitar speaker. It is a few dB hotter than you find in front of a large orchestra. I scale my AKG 414s for about 0.5V max, but have never hit it.

For dynamic mikes, it is about 148dB SPL. You do not have any 140+dB SPL sources around the studio.

In The Olden Days, US and UK mike inputs overloaded about 30mV, and we were usually happy. With the advent of rock-n-roll, we paid more for 100mV transformers in a portable Langevin. On dynamics and ribbons, 30mV is usually ample and 100mV is always plenty.

The really-hot German condenser mikes were not made for such inputs. In essense, these have the first preamp inside the mike, and the board input is a high-gain line input. Such mikes (when sold commercially) always have a -10dB or -20db PAD for use with ordinary mike inputs. In my other room, the 414s run with the -10dB pad engaged so they can work with a simple preamp that was not scaled for their hot output. Most better general-purpose mike amps have a PAD switch. And Shure sells inline pads.

There is NO sin in padding a mike when it is as hot as 1V. I dislike it on principle, but there is nothing wrong with it in practice.

 

[pstamler]

[quote author="PRR"]For dynamic mikes, it is about 148dB SPL. You do not have any 140+dB SPL sources around the studio. [/quote]

Probably not, but if you record classical music...

A friend of mine is the percussionist with the local symphony orchestra, and he's also the Occupational Safety guy for the union. He measured 140dB aqt his position by the tympani during a loud orchestral climax (I believe it was either a Mahler or Stravinsky piece), which would get into your mikes if you were multimiking the orchestra.

Oh, and he's lost most of his hearing below 100Hz; he started using ear protection too late.

Peace,
Paul

 

[SSLtech]

[quote author="pstamler"]He measured 140dB aqt his position by the tympani during a loud orchestral climax (I believe it was either a Mahler or Stravinsky piece), which would get into your mikes if you were multimiking the orchestra.[/quote] If there's that much noise coming from that section of the orchestra, I personally would not be using the spot mic for that section, because it would be too hot acoustically!

-If you have the measurement mic about an inch above the tymp head that number doesn't sound unreasonable, but if it's that loud unweighted 3 feet away (at ear level or mic-height) I think there might be some other noise sources close by... like perhaps a howitzer or a space shuttle! :wink:

This weekend I recorded a 34-piece brass section plus two regular stickmen who had drafted in three additional percussion players from the nearby university (which has a halfway decent music department and is known for concentrating on percussion). The 5 players were mainly lined up left-right, but for the end of their arrangement of Ravel's 'Bolero', the conductor also grabbed a pair of sticks and made it six percussion players. I was using the soundfield un-padded and used about 11dB of gain, on the preamp, the front end of which you laid out for me here, last week! -Thanks by the way! -The unit was checked on the A2 (Neutrik) for gain, distortion and noise, which is in fact the only reason that I know the gain used on that occasion was about 11dB... :grin:

-A pic of the ending of Bolereo, Conductor on the right.

Keith

 

[pstamler]

[quote author="SSLtech"][quote author="pstamler"]He measured 140dB aqt his position by the tympani during a loud orchestral climax (I believe it was either a Mahler or Stravinsky piece), which would get into your mikes if you were multimiking the orchestra.[/quote] If there's that much noise coming from that section of the orchestra, I personally would not be using the spot mic for that section, because it would be too hot acoustically!

-If you have the measurement mic about an inch above the tymp head that number doesn't sound unreasonable, but if it's that loud unweighted 3 feet away (at ear level or mic-height) I think there might be some other noise sources close by... like perhaps a howitzer or a space shuttle! :wink: [/quote]

Horns + tymp, I suspect. He did have the measurement mic at ear level.

Peace,
Paul

 

[toby]

i'm going to agree with keith and say i wouldn't mic the section. i've always found that orchestral percussion doesn't sound right miced from less than 20 meters away from a recording point of view.


toby