Impedance switching box with mic booster

[Nen'O]

Hi community,
I expanded mobile recording rig with Metric Halo 2882 interface, since mic preamps on this interface are struggling with gain (only 25db of clean gain, total 40gb), I'm planning to build 8 channel mic impedance switching box and use it in front of interface.
It should be fairly easy task according to this tutorial. Is there something else I should do, or is there better way to do it?



Thanks

 

[Khron]

And what exactly are you hoping to achieve by "mic impedance switching"..?

 

[Nen'O]

Boost mic output, i already own 2 channels mic impedance/booster and it works great with ribbon and dynamic mics.
The problem with MH 2882 is that preamps get noisy above 25db gain, which sucks for clasical/ethnic music recordings session.

 

[Khron]

I only briefly scrolled through the video, but that thing is only a potentiometer wired between XLR pins 2 and 3, is it not?

Assuming that is indeed the case, how do you expect to boost a microphone's output, by (current-)loading down its output even more?

Adding any resistance in parallel with the preamp's own input impedance will only make the total impedance LOWER. Which will require more (signal) current than normally, which will only serve to ATTENUATE the signal coming from your mic.

https://www.electronics-tutorials.ws/resistor/res_4.html

https://www.electronics-tutorials.ws/dccircuits/current-divider.html

https://www.electronics-tutorials.ws/dccircuits/dcp_2.html

Apart from a transformer (with its associated limitations / compromises), you CAN NOT amplify signals in any way, using only passive components. Or will there also be other (active) circuitry involved, that you haven't yet (clearly) mentioned?

 

[Nen'O]

Indeed, looking at my Mic Booster there seems to be more than a passive components.
I definitely need some more input about this project.
Maybe I should call it impedance switching box with mic booster

 

[Khron]

Theeeeeeen why bother with the impedance-altering part? Or is that supposed to be just a "tone" thing?

Just to make it clear - the video indeed only deals with the things i had an issue with, and explained / questioned in my previous post.

 

[Nen'O]

Apart from a transformer (with its associated limitations / compromises), you CAN NOT amplify signals in any way, using only passive components. Or will there also be other (active) circuitry involved, that you haven't yet (clearly) mentioned?

What transformer could be used for this project? Maybe it's even better solution.

 

[abbey road d enfer]

In most cases, reducing the impedance the mic sees results in deterioration of sound.
As the posted video says, it is shocking how it changes the sound, but he neglects to say that reducing the impedance always results in worse sound.
It has been clearly demonstrated that dynamic and ribbon mics benefit from the largest possible impedance; this has been known since the inception of ribbon mics in the 1930's.
Condenser mics may not be as sensitive to loading than other types, but in any case, there is a common consequence: the level drops, thus reducing S/N ratio.
This kind of variable load acts only in one direction, it does not "open" the microphone. It is only capable of choking it.
I find it pathetic this trend to sabotage sound with that kind of device or introducing distortion, when generations of engineers have pulled hairs to improve objectively the performance of equipment.

 

[GJB17]

Hi... I'm not entirely sure what is needed here... but I have recently designed and built some low-noise, high gain Mic Amps that can provide up to 80dB of gain... quietly! They are most suited (as mentioned) to Dynamic and (moreso) Ribbon Mics... some of which have outputs at -60dBV... or lower. Picking up another thread of the post... these amps also incorporate an input impedance switch... nominally in the range of 2k to 200 ohms... the adjustment of which (I am informed by people using them) makes a noticeable difference. Some up-market Mic makers even specify a preferred loading level for their products. Proper Impedance matching of the transformers contained therein could be a likely suspect!

 

[abbey road d enfer]

Hi... I'm not entirely sure what is needed here... but I have recently designed and built some low-noise, high gain Mic Amps that can provide up to 80dB of gain... quietly!

How quietly? You know there's a limit that's given by the Brownian noise. -129.7 dBu for a 200 ohms resistance. So even if your preamp had no noise (which is imposiible) the max S/N ratio with an input signal of -60dBu would be 69.7 dB.
A mic pre with 80dB gain can achieve at best a S/N of 49.7 dB.
Many solid-state mic preamps have a noise factor of less than 2dB, resulting in a S/N atio of 68dB with -60dBu input.

Picking up another thread of the post... these amps also incorporate an input impedance switch... nominally in the range of 2k to 200 ohms... the adjustment of which (I am informed by people using them) makes a noticeable difference.

Well, as a designer, you should have formed an opinion about this. Sure it makes a difference, but I've consistently noticed that increasing the load impedance results in a "clearer" sound and better noise performance.

Some up-market Mic makers even specify a preferred loading level for their products.

And most have ceased doing so, either because they were wrong -(like Shure with the SM7b, for which they recommand "19 to 300 ohms" Have you tried an SM7b on a 19 ohms load?) -or simply because there is no proper objective definition for preferred load impedance.

 

[abbey road d enfer]

Hi community,
I expanded mobile recording rig with Metric Halo 2882 interface, since mic preamps on this interface are struggling with gain (only 25db of clean gain, total 40gb), I'm planning to build 8 channel mic impedance switching box and use it in front of interface.
It should be fairly easy task according to this tutorial. Is there something else I should do, or is there better way to do it?



Thanks

What do you mean "only 25db of clean gain"? I must admit the 2882 brochure is rather cryptic, but I believe it offers plenty of gain.

 

[Nen'O]

What do you mean "only 25db of clean gain"? I must admit the 2882 brochure is rather cryptic, but I believe it offers plenty of gain.

2882 mic preamp has 40db gain, anything above 25db is unusable because it's too noisy. I have MH ULN and that interface mic pre has 80db gain. ULN stands for Ultra Low Noise.

 

[GJB17]

Hi Abbey... some good comments thanks... but I am not sure that I entirely agree with your thoughts on Mic loading impedances.
Firstly... and as we well know... if a transformer is terminated in a relatively high impedance it produces overshoot (ringing) and a low impedance will (conversely) produce sag. There is (or should be) a point where the transfer characteristic is (near) flat... in other words, and if it was possible to run a square wave, it also would be flat.
While I definately do not have a "Musos ear" a number of those who do have told me that optimising the termination impedance with the 4 way switch I have provided on my mixers provides a not huge, but nonetheless noticeable, "sweet spot".
The comment you make re Ribbon Mics running in to as high as possible impedance could well be correct, but, primarily in instances where the Mic uses an internal transformer to (significantly) boost its output voltage... but of course also providing a large boost in its effective output impedance at the same time. As you have commented that was likely the only way to get usable output level back in the day... but things have changed... i.e. amps with much lower S/N.
Speaking of which the Mic Amps I have done... based on a design by Douglas Self... but with some "enhancement"... have an S/N normally around 65dB) when running at 60dB gain... and better than 60db when at 80dB of gain.
One of the main motivators of building these was for those who wanted to use Ribbon Mics... but without the need for (excessively) high step up transformers.
I will be interested in your further comments. Cheers.

 

[abbey road d enfer]

Hi Abbey... some good comments thanks... but I am not sure that I entirely agree with your thoughts on Mic loading impedances.
Firstly... and as we well know... if a transformer is terminated in a relatively high impedance it produces overshoot (ringing) and a low impedance will (conversely) produce sag. There is (or should be) a point where the transfer characteristic is (near) flat... in other words, and if it was possible to run a square wave, it also would be flat.

Dynamic and ribbon mics are not flat.
In particular, most ribbons have a deficit at HF, so adding a little resonance there tends to help. RCA mics litterature recommands loading their mics with an unterminated step-up ytansformer, for this very reason.

While I definately do not have a "Musos ear" a number of those who do have told me that optimising the termination impedance with the 4 way switch I have provided on my mixers provides a not huge, but nonetheless noticeable, "sweet spot".

One's sweet spot can often be another's hell.

The comment you make re Ribbon Mics running in to as high as possible impedance could well be correct, but, primarily in instances where the Mic uses an internal transformer to (significantly) boost its output voltage... but of course also providing a large b, which is an improvvement of onlyoost in its effective output impedance at the same time. As you have commented that was likely the only way to get usable output level back in the day... but things have change noise factor ofd... i.e. amps with much lower S/N.Vintage tube mic pres had a noise factor of about 6-8dB

Things have not changed that much. Vintage tube ùic pres had a noise factor of about 6-8 dB. The best SS pres better thqt by only 4-6dB<

Speaking of which the Mic Amps I have done... based on a design by Douglas Self... but with some "enhancement"... have an S/N normally around 65dB) when running at 60dB gain... and better than 60db when at 80dB of gain. dBu.

This is simply impossible; that would mean an equvalent input noise of -140. The absolute minimum is -129.7.

 

[Matt Syson]

There had not been any mention of the capacitive loading of the cable, or whether the mic or preamp is resistive impedance or reactive (transformer probably) ALL of which have a very real impact on the overall sound whether it is liked or not and althopugh many things are changing around the planet, the basic physics that goive a noise figure of real world amplifiers, for typical microphones is still around 129.7 as Abbey road says, for which a consistent 128.3 was the goal of most mixing desk manufacturers in the 1980's when transistors were made reliably quiet (sometimes assisted by a transformer which also provided decent balancing to reduce interference pickup AND isolation of ground gurrents AND reliability in hostile environmentsequipment in other areas with different mains supplies). The reasons WHY some things were designed as they were years ago is often ignored by the young 'internet age'.

 

[GJB17]

Hi Abbey... my apologies... you are correct. I didn't bother to look at my notes and simply tried to recall them from my (ageing) memory. While I have still not found those notes I do recall that they were very much in line with those that Douglas Self published... which I have attached below FYI.
I also recall that I was in fact able to achieve slightly better results as I had used Toshiba (original) low-noise FETs (2SK117's)... in lieu of the BJT's that Self had used and also used LM4562 Opamps.
I also agree with Matt's comment about... "impact on the overall sound whether it is liked or not".
It seems to me that often times Micamps are judged on "how they sound" which, to me, is not quite right. In my view the Micamp should NOT have a sound... it should be as transparent as possible... (to use something of a cliche) Let the Mic do the talking!
For that reason my Amps are also fully electronically balanced... no transformers... so that whatever coloration or other "effects" that might be present are (90%) down to the Mic.
I'll be interested in any further thoughts you might have.
Cheers,

 

[RuudNL]

I think there is a good reason why most microphone manufacturers recommend a load impedance of (minimal) 10 x the impedance of the microphone itself.

 

[abbey road d enfer]

Most of the frequency response deviations attributed to loading one can encounter in normal situations can be easily EQ'd if deemed necessary.
Within limits.
A typical 200 ohm source loaded with 100m of cable sees a loss of 0.5dB at 20kHz, even less if the impedance is slightly inductive.
It would take an enormous length of cable, or an abnormally high impedance to create a large HF attenuation, which woukd result in high noise if EQ'd.

 

[Nen'O]

Thanks all for the input.
Exchanged few emails with Igor from UTM audio, he recommends transformer for up to 10db gain boost, which might be perfect for my application. As he said, It's hard to get more, coz the hi-end rolls down.

https://utmindustry.com/utm2572/

I own Phoenix Audio preamp, it has transformerless input stage with very high mic impedance of 10K and transformer at output, works great with just about any mics, especially ribbons.

 

[abbey road d enfer]

Thanks all for the input.
Exchanged few emails with Igor from UTM audio, he recommends transformer for up to 10db gain boost, which might be perfect for my application.

10dB boost means that the impedance ratio is 1:10. In other words, your preamp with 10k input impedance will appear as 1kohm at the primary of the transformer. 1kohm is half the recommended impedance of most mics. It may or may not be a problem. I wouldn't do it.

https://utmindustry.com/utm2572/

This transformer has a ratio of 1:10. The impedance ratio is 1:100.
Your 10k preamp would appear as 100 ohms to your mic.
Something does not fit.