Firstly, there's no impedance matching on audio. The days of a 600Ω output driving a 600Ω input with the associated 6dB attenuation are long gone.
Microphone have a source impedance between 50Ω and 1k, and microphone input impedances are generally 10k or 22k

Except for dynamic microphones. For them, the preamp input impedance is the load on a current generator (often through a transformer), and has a direct effect on the diaphragm's transient response and the microphone's overall tonal characteristics. Changing the preamp input impedance affects the microphone's spectral response in ways that are very difficult to achieve with standard equalization controls. A common input impedance value for "classic" dynamic mics is 200 ohms.

Some rackmount preamps have an input impedance selector switch on the front panel, while some big consoles have a switch on the rear panel next to each mic input. I once worked with a small Neve console in a remote truck; it had switches near the input connectors, but this might have been a modification by the engineers.

"Some" this, "some" that, "a common value"... Lotsa qualifiers, because there are lotsa variations out in the world.

ak

 

Except for dynamic microphones. For them, the preamp input impedance is the load on a current generator (often through a transformer), and has a direct effect on the diaphragm's transient response and the microphone's overall tonal characteristics. A common input impedance value for "classic" dynamic mics is 200 ohms.
ak

A dynamic microphone is a voltage source. Just look at data sheets for something like an SM58 - output is quoted as a voltage - for the SM58 it is -54.5dBV/Pa, and a SM58 is always used with a standard mixing desk input impedance of 10k.

 

All the preamp circuits from THAT have a fairly old and VERY expensive condenser mic powered from a phantom 48VDC. The wiring is differential.

THAT does not show any preamp circuit that uses a modern electret mic with unbalanced wiring.

The THAT preamps all use a fairly old but good and still available NE5532 opamp. 30 to 45 years ago all my audio circuits used TL07x opamps.

 

Professional audio doesn’t use unbalanced wiring for microphones. Professional electret condenser microphones still have a balanced output and run off 48V phantom power.

 

the preamp input impedance is the load on a current generator

A dynamic microphone is a voltage source.

The only reason I wrote it that way was because I knew someone would take a swing at it.

To the TS: The change in tonal quality is caused by damping the diaphragm movement, and this is a direct consequence of Lenz's Law. This is the same reason a power generator is harder to turn under heavy electrical load than with a light load.

https://en.wikipedia.org/wiki/Lenz's_law

ak

 

This question about building an audio console is an exciting one!

24 years ago I embarked on a project with similar aims. I ended up building something that borrowed a lot (90%) from generic audio consoles of the time... Basic analog ones haven't changed a lot since then. And the operational workflow of the digital consoles still follows the same concepts. Mic preamp and line amplifier design is worthwhile learning about. Whether it is admitted or not, these building blocks are still relevant.

My project lead me into a career in professional audio and broadcast audio for major industry players. I have a suggestion for you... Talk to the service or maintenance manager of one of the larger live sound or professional audio companies if you are lucky enough to have one near your location. Or maybe a broadcaster if they have a maintenance manager who is willing to share some information with you. I say the larger ones because they often have their own in house maintenance department. In these times they may be interested in someone with your curiosity of sound equipment and hopefully will be willing to show you examples of professional audio consoles. Even if you have a look at digital consoles, you will get an insight into the operational workflow and technical requirements of a console. There are oodles of schematics out there but some patience may be required to sift through them and try to understand how they work together. Start with the block diagram and decide on what features you ultimately want. Learn about the basic configurations of operational amplifiers in audio circuits. Try to learn a bit about mic pre-amps. I based my first console on an Electronics Australia Magazine design.

 

I based my first console on an Electronics Australia Magazine design.

Schematic or link?

ak

 

I'd have to dig for that. But most 8-12 channel analog mixers are similar and there are several free online schematic repositories these days. Maybe start by looking at a basic Mackie.

 

hey guys
back after a bit of time
i have a question
i've seen many balanced mic pre amplifiers but most of them take the Hot(+) and Cold(-) signal and combine them into one output thus making the signal unbalanced
i've thought of making 2 circuits each for each signal and then feed that to the main mix (Male XLR Connection) output of the console (which will go to an amplifier) but my question is how to adjust the CMRR
even if i make the signal unbalanced and then balanced again i will need to readjust the CMRR
do professional sound consoles make the signal from balanced to unbalanced and then the opposite or keep the signal balanced and amplify each signal separately?
If they do the first how do they reajdust the CMRR when it goes to the main mix output?

P.S.
The Mic pre amplifier i am talking about

 

Normally, all the mixing is done single-ended, because otherwise you need twice as many components.
The the outputs are converted back to balanced using either the SSM2142 or the equivalent made out of a couple of op-amps.

Often, the outputs are balanced but NOT differential. i.e. both hot and cold have output impedance of approx 600Ω, but only hot has the signal. Cold is Simply 600Ω to ground.
Provided that it is connected into a balanced input the interference rejection is just as good.

By the way, something seems to have gone seriously awry with your circuit. Each input should look like a Sziklai pair. As you have drawn it, the PNP transistors are biased permanently off, and there should be some connection down to -9V.

 

Agree with Ian - that is a strange input stage. It is a combination of dangle-biasing through the 100 K resistor and a low-end inverting amplifier with strange feedback from the collector to stabilize the operating point and set the gain. So it is two independent amplifiers rather than a differential pair, and all of the common mode rejection is done in the opamp. From the collectors to the bases, is that supposed to be a 0.88 mA current source rather than a resistor? If so, why?

Whatever, the input stage does not need a -9 V connection to operate. And since the opamp stage is AC-coupled in and out, it doesn't not need a -9 V supply either, unless the signal amplitude is too high for a single 9 V supply.

Please post a link to the original circuit/article.

Also, the balance (common-mode rejection) adjustment (the 100K pot with no reference designator) could be improved. If the goal for the gain-set resistors is a total of 89 K, I suggest replacing the negative feedback loop with a single 91 K part (which is well within the 10% tolerance of the original components), and replacing the positive feedback loop with an 82 K fixed and 20 K variable resistor. This puts the balance point near the center of the pot's rotation, with way better adjustability because the pot is a much smaller percentage of the desired resistor value (22% rather than 300%).

ak

 

I think it’s meant to be this one, but something went wrong in the copying. . . .

 

Agree with Ian - that is a strange input stage. It is a combination of dangle-biasing through the 100 K resistor and a low-end inverting amplifier with strange feedback from the collector to stabilize the operating point and set the gain. So it is two independent amplifiers rather than a differential pair, and all of the common mode rejection is done in the opamp. From the collectors to the bases, is that supposed to be a 0.88 mA current source rather than a resistor? If so, why?

Whatever, the input stage does not need a -9 V connection to operate. And since the opamp stage is AC-coupled in and out, it doesn't not need a -9 V supply either, unless the signal amplitude is too high for a single 9 V supply.

Please post a link to the original circuit/article.

Also, the balance (common-mode rejection) adjustment (the 100K pot with no reference designator) could be improved. If the goal for the gain-set resistors is a total of 89 K, I suggest replacing the negative feedback loop with a single 91 K part (which is well within the 10% tolerance of the original components), and replacing the positive feedback loop with an 82 K fixed and 20 K variable resistor. This puts the balance point near the center of the pot's rotation, with way better adjustability because the pot is a much smaller percentage of the desired resistor value (22% rather than 300%).

ak

this is the video that i got the schematic from, it is shown at minute 20:47 but now that i rewatched the video i saw that there is an other circuit (probably the correct one) shown at minute 20:18

 

this is the video that i got the schematic from, it is shown at minute 20:47 but now that i rewatched the video i saw that there is an other circuit (probably the correct one) shown at minute 20:18

The correct circuit is shown first - the well-known Sziklai pair circuit. Then he tries to make his own version, and makes a complete horlicks of it. Stick with the original - it works, you'll find it everywhere including here.

 

The correct circuit is shown first - the well-known Sziklai pair circuit. Then he tries to make his own version, and makes a complete horlicks of it. Stick with the original - it works, you'll find it everywhere including here.

Elliott sound products
Great site and forum
i had come across this circuit in the past too (but i don't know why i had ignored it ) and and was looking for a parametric equalizer there yesterday

 

Elliott sound products
Great site and forum
i had come across this circuit in the past too (but i don't know why i had ignored it ) and and was looking for a parametric equalizer there yesterday

The secret of the circuit is the input transistor. If you examine the circuit, you'll see that the current rating of that input transistor is MUCH higher than it needs to be (in fact, it's higher than the transistor that it drives). The much larger transistor has lower resistance in the base region, and resistance generates noise (√4kTfR), so a larger transistor gives a quieter preamp, provided that your source resistance is low. You can do even better for noise with some of the Zetex parts from Diodes.com.
If you have the latest edition of Horowitz & Hill, there is a section with a series of noise measurements on various transistors.

The only drawback on this circuit is its distortion. Because there is no feedback around the transistors, the distortion isn't as good as it could be (it's still quite impressive). The Sziklai pair is about the most linear you can get without feedback - better than a single transistor and better than a darlington.

 

Hello again guys!

I was looking for equalizers and i came across this circuit in Elliott sound products (ESP)
https://sound-au.com/project150.htm (see figure 4)
And i was wondering if there is a way that i could combine the above mentioned circuit with this one
https://sound-au.com/articles/state-variable.htm (see figure 6)

In what way will they need to be combined in order to have an adjustable Q as well and be able to control cut/boost and the desired frequency?
Thanks in advance

 

Hello again guys!

I was looking for equalizers and i came across this circuit in Elliott sound products (ESP)
https://sound-au.com/project150.htm (see figure 4)
And i was wondering if there is a way that i could combine the above mentioned circuit with this one
https://sound-au.com/articles/state-variable.htm (see figure 6)

In what way will they need to be combined in order to have an adjustable Q as well and be able to control cut/boost and the desired frequency?
Thanks in advance

You need Doug Self's book. It's all in there.

 

Most audio equalizers do not have adjustable Q. Instead they have many narrow (high Q) frequency bands (1/3rd octave) or only a few wide (low Q) frequency bands (bass, mids and highs). They all have adjustable boost and cut for each frequency band. The settings you make on each frequency band sets the frequency you are changing its level.