How important is mic loading? In the below schematic the mike will see
150 ohm load during unpadded but see ~1300 ohms during -20 dB pad.
The circuit seems more concerned with input transformer primary
loading than mic loading. Am I missing something here?

http://www.jensen-transformers.com/as/as016.pdf

Thanks
Ed

Edvard Puskaric wrote:

How important is mic loading?

Very important. Depending on the type of microphone, the consequences of
improper loading can very well include:

[a] decreased sensitivity
[b] altered frequency response
[c] increased harmonic distortion
[d] diminished maximum SPL


In the below schematic the mike will see 150 ohm load during unpadded
but see ~1300 ohms during -20 dB pad.

Unfortunately I don't have time to view the schematic, but a "150 Ohm"
microphone should not be loaded with less than about 1000 Ohms.


The circuit seems more concerned with input transformer primary
loading than mic loading. Am I missing something here?

http://www.jensen-transformers.com/as/as016.pdf

Hmmm--Jensen's engineering is first-rate as a rule. I wish I could look
into this further at the moment. I'll check back later--probably someone
else here will have delved in by then.

Edvard Puskaric wrote:

How important is mic loading?

Important. I use the 'rule of thumb' of loading with 10 x source Z. For a
150-200 ohm source a load of 2k is appropriate and widely used by major
console manufacturers.

Proper loading ensures optimum voltage transfer to the following stage and
therefore best signal/noise ratio and also the above loading should ensure
flattest response from the mic.

In the below schematic the mike will see 150 ohm load during unpadded

No actually, the 'see other schematic' doesn't define the final load
impedance.

but see ~1300 ohms during -20 dB pad.

No again for the same reason. Certainly a higher Z though. Very wrong, the
pad should have both series and parallel resistors. Basic stuff actually.

I also see that they suggest adding series resitance for Low-Z mics. This
is idiotic since it will degrade the noise level ( higher source Z - i.e.
thermal noise ) and lose signal voltage ( worse S/N ratio ) !

The circuit seems more concerned with input transformer primary
loading than mic loading. Am I missing something here?

Don't think so.

Why do you want to use a transformer for a 150 ohm mic anyway with all its
inherent response and distortion problems ?


Graham

On Sat, 11 Oct 2003 03:41:58 -0400, Edvard
wrote:

How important is mic loading? In the below schematic the mike will see
150 ohm load during unpadded but see ~1300 ohms during -20 dB pad.
The circuit seems more concerned with input transformer primary
loading than mic loading. Am I missing something here?

Hi Ed,
The pad is out of circuit when switched to "unpadded". So the mic
sees the preamp as load (and the phantom resistors in series with
each other). Give it another look and you'll see it differently.
Good fortune,


Chris Hornbeck
new email address

On Sat, 11 Oct 2003 15:51:45 +0100, Pooh Bear
wrote:

Why do you want to use a transformer for a 150 ohm mic anyway with all its
inherent response and distortion problems ?

I have built a pair of fixed gain tube mic pre's and was looking to
pad the inputs. I ran across the jensen info while doing some design
research.

Ed

How important is mic loading? In the below schematic the mike will see
150 ohm load during unpadded but see ~1300 ohms during -20 dB pad.
The circuit seems more concerned with input transformer primary
loading than mic loading. Am I missing something here?

Yes: typically a Jensen input transformer, when properly terminated, presents a
load of 1k or greater to the microphone. For example, the JT115K-E is a 1:10
transformer, nominally 150 ohms : 15k. But those are the equivalent *source*
impedances; the recommended loading is 150k, which transforms down into a 1.5k
load, at least at most audio frequencies (we won't get into that).

As for the series resistors...as you note, the designers are concerned with
input primary loading. Some transformers, when presented with a too-low source
impedance, ring. As Graham notes, that will degrade input noise performance.
Whether that matters or not depends on the source; if it's a low-Z ribbon
microphone it'll make a real mess, whereas if it's a low-Z high-output
condenser, like a TLM-103 or a Schoeps with loud stuff in front of it, the
extra resistance won't matter at all, as it'll be swamped by the microphone's
self-noise.

Peace,
Paul

Edvard Puskaric wrote:

On Sat, 11 Oct 2003 15:51:45 +0100, Pooh Bear
wrote:

Why do you want to use a transformer for a 150 ohm mic anyway with all its
inherent response and distortion problems ?

I have built a pair of fixed gain tube mic pre's and was looking to
pad the inputs. I ran across the jensen info while doing some design
research.

OK, so you want the 'valve sound' hence a transformer is a must as a voltage
step up device.

Do you know what load the primary of the transformer presents to the mic when
directly connected ( will depend on your tube stage values ) ?

Given that It'll take a couple of mins to give you a 'proper' pad design.


Graham