Adding impedance pot on Mic preamp

Hi folks

I just came across this nice video from Peterson Goodwyn: https://www.youtube.com/watch?v=r1yPMDHiRuc

I was asked to add an impedance pot just like in the video into a access 312 board. So I would like to know if my design is ok?

And I would like to know if I'm better to strap the input tranny in 150 or 600? For what I understand we can toggle the pot between really low imp. to around 1K (in the mic preamp world, 1k5 is more of a standard, isn't it?).  150ohm recall for standard condenser microphone and 600 for ribbon and more dynamic microphone?

The RC resistor is 150K (is it a load resistor??). For what I know, the input of a mic preamp should be 10x the output impedance of a microphone.

Thanks for your time

http://groupdiy.com/index.php?topic=55464.msg708029#msg708029

Cheers

Ian

Thanks Ian

Looks like it could work but with certain compromise such as lesser gain, depending on the impedance selected. For what I understand, I should strap the transformer in 1:7 (150ohm) and put this pot with a resistor in serie so the minimum impedance load is 150ohm. Is that it?

It looks to not like the best idea for ribbon microphone, thought

Deepdark said:

Thanks Ian

Looks like it could work but with certain compromise such as lesser gain, depending on the impedance selected. For what I understand, I should strap the transformer in 1:7 (150ohm) and put this pot with a resistor in serie so the minimum impedance load is 150ohm. Is that it?

Click to expand...

That is correct.

It looks to not like the best idea for ribbon microphone, thought

Click to expand...

Agreed.

Cheers

Ian

Thanks Ian

Deepdark said:

Thanks Ian

Looks like it could work but with certain compromise such as lesser gain, depending on the impedance selected. For what I understand, I should strap the transformer in 1:7 (150ohm) and put this pot with a resistor in serie so the minimum impedance load is 150ohm. Is that it?

It looks to not like the best idea for ribbon microphone, thought

Click to expand...

All the fuss about variable-Z is giving access to higher impedance, not unduly-low. Making the impedance variable between 150 and 1k is a receipe in desillusion.
I would connect the primaries in series, make R6 about 1 Meg, strap a 22k log pot across the primaries with a 1k padding resistor.
Then you can really hear some mics open up.
BTW, alternatively, you can install the pot on the secondary; that would be a 1 Meg pot with a 47k padding.

abbey road d enfer said:

Deepdark said:

Thanks Ian

Looks like it could work but with certain compromise such as lesser gain, depending on the impedance selected. For what I understand, I should strap the transformer in 1:7 (150ohm) and put this pot with a resistor in serie so the minimum impedance load is 150ohm. Is that it?

It looks to not like the best idea for ribbon microphone, thought

Click to expand...

All the fuss about variable-Z is giving access to higher impedance, not unduly-low. Making the impedance variable between 150 and 1k is a receipe in desillusion.
I would connect the primaries in series, make R6 about 1 Meg, strap a 22k log pot across the primaries with a 1k padding resistor.
Then you can really hear some mics open up.
BTW, alternatively, you can install the pot on the secondary; that would be a 1 Meg pot with a 47k padding.

Click to expand...

Thanks for your reply. I'll simply put a DPDT to select between the 2 transformer configuration. But I'll probably give it a try later

I haven't posted this to this thread (yet). Most mic designers anticipate a nominal bridging termination (1.5-2k ohm). For some mics this will not matter only changing output level but not the frequency response. For others it may alter the response or even damping. Be aware that you are operating the mic in a way different than it was designed for. It may be different, but if universally better the mic makers would specify different (high impedance) preamps for improved performance.

Note: A less than nominal termination impedance can be easily built into the mic itself by the manufacturer if desirable. 

So perhaps an interesting studio trick to make an old mic sound slightly different . Have fun. 

JR

JohnRoberts said:

I haven't posted this to this thread (yet). Most mic designers anticipate a nominal bridging termination (1.5-2k ohm). For some mics this will not matter only changing output level but not the frequency response. For others it may alter the response or even damping. Be aware that you are operating the mic in a way different than it was designed for. It may be different, but if universally better the mic makers would specify different (high impedance) preamps for improved performance.

Click to expand...

The "optimum" load impedance for the SM range of mics has been defined nearly 50 years ago, at a time where the idea was more to define a minimum, because at the time, making Hi-Z inputs for mics was a real challenge. I don't think the "inventor" of the SM57 ever considered that his creations would be loaded by more than 5k. IIRC, several mic mfgrs recommended a transformer input with unloaded secondary, establishing clearly that they had understood the benefits of maintaining a higher Z at high frequencies. One of the issues with modern mic pres is that most would establish their input Z by a hard-wired resistor, which maintains a constant impedance all over the spectrum, which would be ok at LF and MF, but not "optimum" at HF.

There's a whole world of difference between a xfmr-coupled mic driving a xfmr-coupled mic pre and a TL mic drive a TL preamp, when the interaction with the capacitance of the mic cable is taken into account.

While I was working at Peavey I visited the transducer engineering department and asked a senior mic design engineer about this.  I was paraphrasing the answer he gave me.  IIRC he may have worked for Shure before Peavey but I don't recall his name and did not ask about his former work. He was older than me.

Back in the (not so) good old days, mics and mic preamps were actually designed as combination units so mics were designed for very specific preamps. It was only a later market development that de-facto termination standards emerged as equipment became more mass produced and used interchangeably .

I do not doubt that the SM 57 creator never anticipated modern transformer less preamps or that he would ever enjoy that much sales success.  ;D

YMMV amuse yourselves.

JR

Data sheet (27A2634) for the Shure SM7 specifies the output level into an open circuit.

Manual for the Neumann U47 dated May 1958, paragraph 9.
"The U47 and U48 microphones are designed to work into a load impedance of 250 respectively 1000 ohms, or even higher". 

The only quality microphone system that I am aware of that is designed to be terminated to any degree is the Altec M11.  Quoting from the manual,
"The output of the M11 system is designed to connect directly into either a resistive load equal to the nominal output impedance or an input transformer whose secondary side is terminated in a resistance of proper value.  When the microphone system is connected to an input transformer whose secondary is not terminated in a resistor, is is necessary to either place a resistor of proper value across the input transformer secondary or to terminate the microphone system output as shown in Figure B."

Why are we  discussing a SM57 ?

gridcurrent said:

The only quality microphone system that I am aware of that is designed to be terminated to any degree is the Altec M11.  Quoting from the manual,
"The output of the M11 system is designed to connect directly into either a resistive load equal to the nominal output impedance or an input transformer whose secondary side is terminated in a resistance of proper value.  When the microphone system is connected to an input transformer whose secondary is not terminated in a resistor, is is necessary to either place a resistor of proper value across the input transformer secondary or to terminate the microphone system output as shown in Figure B."

Click to expand...

That's because it's a measurement microphone, where frequency response linearity is the paramount performance; it's not intended to "sound good" (or bad).

Why are we  discussing a SM57 ?

Click to expand...

Because the effects of increasing the input impedance are most noticeable on this affordable and readily accessible mic. If I had used the Microtech Gefell MD100 as an example, I think it would not trigger the same familiarity .