SSM2019 monolithic pre-amp chip

[steve]

Common Mode Rejection is a function of the current match between the two sides of the balanced line.

Absolutely. With a balanced input signal, you have a voltage differential - the voltage on each line is referenced to the voltage on the other - a kind of push/pull relationship.
Ohms law I=V/R
The same thing applies to AC. The current is inversely proportional to the impedance, so for a given voltage differential the current through each wire will be inversely proportional to the impedance of its input. As you say, they must be balanced.

Regarding the overall input impedance, SM57's (and 58's and quite a lot of other dynamic microphones) tend to "overshoot" in their high frequency response unless there is a sufficiently low impedance load (around 500 - 600 Ohms) producing a harsh peak in the high frequency response. If the impedance is taken too low, then the low frequencies begin to roll off. The "sweet spot" for SM57s is around 500 to 600 Ohms.

[bgravato]

Just to let you know that I finally got myself an SSM2019 chip and now I just need to find some time and patience to do something interesting with it

[kozikowski]

Two problems you'll probably hit right away are power supply rejection and environment noise.

Microphone signals are reeeely tiny and the power supplies are probably ideally +/- 15 volts, filtered very well. If there's -60dB power supply ripple and trash and the chip has a power supply rejection ratio of -60, then the internal trash is at -120. If the microphone signal is -70dB (not uncommon with somebody playing classical guitar in a stair well) then the trash in the show is only -50. The numbers add up the wrong way and they do it quickly.

The other thing that will kill you is the inability to test unless the cabinet is closed. Microphone amplifiers will not work outside of their metal shell. I have a pix of one I built here somewhere...
Koz

[bgravato]

Power supply is probably the most important piece of equipment in any signal sensitive application such as audio... That's why I also got a bunch of high quality panasonic fc series capacitors...

I still need to get a decent transformer... (which will probably be the most expensive part of the party)

[kozikowski]

I still need to get a decent transformer... (which will probably be the most expensive part of the party)

I know.

http://www.kozco.com/tech/pix/MicAmp2.jpg

The big silver transformer in the back isn't the power transformer. That's the output transformer. It will do +24dBm into 600 ohms at any audible frequency. With suitable cables, you can probably weld metal with it.

The power transformer is the little orange thing hiding under the regulators up-left. It's a toroid, chosen because they don't radiate. They just don't have external 60Hz fields. I put the mains transformer inside the cabinet for which I should probably be locked up, but I don't have hum problems.

This is just before I finished the final wiring and I hadn't tested it yet. It turned out to have terrible frequency response due to some poor choices in the preamp stage.

The two red lights were supposed to be stage clipping indicators. Never designed or connected. The green power light was a surprisingly difficult problem. Remember the two supplies are +/- 15 volts. You can't put the light between them because if you lose one supply, the leakage in the dead one will keep the light lit. Two green lights would be admitting defeat.

I put a constant current device and a zener diode in with the LED. The LED will not light at 20 volts, but is perfectly happy at 30.

It did have one unexpected attribute. It turns off exactly like a tube amplifier. Pull the plug and the class-A FET preamplifier gain slowly fades and the sound finally turns crunchy and stops. If I had thought of it, I would totally have designed that in.

Koz

[bgravato]

I was thinking about getting a +/- 18V power supply, but I need to read the datasheets again, to be sure of the tensions I will need...

I made my components list a few months ago and I've completely forgot why I chose it that way then...

I think the +/- 18V was to be able to achieve 48Vdc (18Vac * 2 * sqrt(2) ~= 51Vdc after rectification).

[kozikowski]

I think the +/- 18V was to be able to achieve 48Vdc (18Vac * 2 * sqrt(2) ~= 51Vdc after rectification).

Three volts is right on the edge of not having enough difference for a voltage regulator -- assuming you use a packaged one, and it may have problems with normal mains variations -- it works between breakfast and lunch, but not between dinner and bedtime. Maybe a passive regulator, a resistor feeding a shunt zener, heavily filtered. It doesn't need to be much and it doesn't have to be perfectly stable. It's re-regulated inside the microphone. It can't be super critical because it has to survive any length of microphone cable.

Phantom is an interesting design problem. You will need carefully matched resistors to inject the voltage and that may be expensive. You also can't turn the voltage on and off without special consideration. That collapsing tension will go through the electronics like a nuclear blast. There is a protection circuit I need to find...

My supplies are both pre-packaged 15 volt units. Bypassed with mylar film .1ufd in and out, plus the large aluminum filter cap at the input. I put the diode front to back to absolutely avoid any reverse voltage. One of the two regulators, I think the negative one, demands the mylar caps for stability, so I put them on both. I did a sound check on the voltages and the ripple is in the -65 or better range.

The transformers are both Jensen. The input is a JT-115K-E60 with double shielding.

http://www.jensen-transformers.com/mic_in.html

If you can't be compulsive, life just isn't worth living. I assume your chip matches the output impedance of the microphone with no transformer. That's the holy grail of microphone amplifiers. So no input coil for you.

What are you going to follow the chip with? Generally, you can only get about 60dB of gain and the sound channel needs at least another 10 or 15 to be comfortable. Also, you may not want to run the chip at maximum gain all the time.

I have two portable mixers that only have 60dB gain, total, and they all run out. The knob hits maximum and I would kill to get a little more.

http://forum.audacityteam.org/viewtopic ... =20#p81588

This is a write-up on phantom power regulation. They bring up a good point. The phantom power supply can't be connected to anything else.....

http://sound.westhost.com/project96.htm

Koz

[kozikowski]

Thanks guys. I'm using hand-matched .1% 6k8 resistors...

That's for the phantom injection into the microphone line. An mismatch kills the microphone's ability to reject cable generated interference. You don't want to start listening to Terra Nova on your mic amplifier.

Koz

[steve]

Three volts is right on the edge of not having enough difference for a voltage regulator

assuming that you actually need 48 V.
If Bruno is building this for a specific microphone he may not need the full 48V, (and certainly will not need 200mA for a single microphone. (AKG C1000S is specified for phantom power in the range of 9 to 52 V, 2 mA)

[kozikowski]

assuming that you actually need 48 V.

Depends on what the design center is. If he wants it for any microphone, then he needs to at least get close.

Koz