[Tynan AgviÐr]

John Byrns wrote in
:

In article ,
Ian Thompson-Bell wrote:

Ian Iveson wrote:
Ian wrote

The hump is indeed caused by that cap but it is made worse
by the fact that one end of the transformer is not
connected to ground but directly to that cap. I can see
the transformer zero would probably cancel it out in the
overall response and maybe that is what the designers
intended. However, the bass hump in the closed loop
response will still give rise to an unnecessary increase
in 1/f noise, so whilst a zero at the input fixes the
response it is treating the symptoms rather than curing
the disease.

Simulating with a 0.22uF series cap at the input removes
the hump.

Yes, but a cap won't get rid of the noise hump any more than
the transformer will.

A cap won't give you the 20dB gain that the transformer
will.

Why use a cap in series when you already have an inductor in
shunt? A cap is a bad idea IMO.


I know, I just put it there to check that the zero worked!

The noise hump is cut away by the output transformer.


Indeed, but if the hump was not there, the noise would be even
better. The hump is completely removed simply by connecting the lower
leg of the transformer to ground. To do that, and keep the first
stage bias the same you need to add a series cap to the input
transformer secondary.

Also, I am considering a variant of this for the RATMIXER mic pre and
as this would feed the mix bus there would be no need for a
transformer at this point in the circuit. Another reason for getting
rid of the hump and its added noise.

My version of the "RATMIXER" also uses a very similar microphone
amplifier circuit, however I don't understand your comment that "this
would feed the mix bus"? If the microphone amplifier feeds the mix
bus, where do you put the channel fader and pan pot, or is the point
simply that no transformer is needed?


Regards,

John Byrns


The piece below has served as the catalyst for the desire to acquire my
own mixer. All of my favorite recordings were produced with it. One of my
favorite engineers, Jan-Eric Persson, designed it in tandem with Kjell
Malmberg and has used it on virtually all Opus 3 Records releases(please
check the label's recordings out..simply stunning). Jan's recording
approach is simple, just mics(most of the time two, sometimes an extra
mic for double bass)mixertape machine. No EQ, Compression, or
anything of the sort. Just good mics, good amps, good venue, good
musicians. (for the record, *I am not posting this in hopes that yall
will come up with a copy of it for me*..just posting because I thought
you may find the pictures interesting)..so please, no comments like
"well, you said you wanted faders and those are rotaries"


http://farm3.static.flickr.com/2128/...c38eeb.jpg?v=0

http://farm3.static.flickr.com/2262/...2e720f.jpg?v=0

http://farm3.static.flickr.com/2187/...ffd675.jpg?v=0

The red knobs are bass-cut filter selectors for the 4 inputs and with
5 positions.
150 & 60Hz at 12dB/oct and 450, 225 & 75 Hz at 6dB/oct.
The yellow knobs are the input-level controls

The red one to the the far right is the Master output control.

The green one is the listening volume to the monitor amp.
The yellow button is before and after tape.

The actual meters are VU, plus that from -5dB and up to +5 there are peak
LED´s
The 0-level with the Telefunken M-28 is 514 nW/m.

Total max gain is 54 dB


it is not that heavy, and the power supply is a separate unit and that is
connected to the 8-pole flat pin connector on the right. (to avoid
hum).

-Tynan