Old: A Direct-Coupled Input-Capacitorless Active Mic Preamp

[ricardo]

There's a simpler explanation of the LF noise with smaller caps. THAT1510 has In = 2pA/rtHz.

With 2x22u on the input, this generates In / (2 pi f C) volts = 2e-12 / (2 pi 20Hz 11u) = 1.6nV / rtHz. Starts dominating the 1nV/rtHz En of THAT1510 etc. lower down.

Using INA163, 217 or the discontinued SSM2017 will see this occur at lower frequencies cos only 0.8pA/rtHz In

In a 'conventional' THAT1510/12 circuit, bigger i/p caps have to be balanced against the risk as Wayne says. I think 2x22u is the sensible choice.

Like JR, I've doodled many capacitorless mike i/p circuits over the years but have always shot them down cos real estate & number of bits. On the performance side, IMHO, Wayne's is the nearest to 'wanna build'. This is still far from stirring me from my beach bum lethargy cos, as you know, I begrudge even 2 extra resistors.

Somewhat off topic, any chance of THAT integrating the 4x1n4004s & 2x1n914, as in Phantom Menace 2, into future generations of 1510 / 12 ?

[mediatechnology]

On the performance side, IMHO, Wayne's is the nearest to 'wanna build'.

Thanks ricardo.

...Starts dominating the 1nV/rtHz En of THAT1510 etc. lower down.

Yep, the smaller the C the larger the reactance in series with the source x current noise = more noise.

[Would THAT be adding] 2x1n914, as in Phantom Menace 2, into future generations of 1510 / 12 ?

I recall at one time there being a discussion about making the internal rev-Vbe diodes more "manly."
Geometrically speaking they are fairly small.
FWIW and OT a casual glance at the external diodes might make one think junction capacitance would be an issue but it's not since they are bootstrapped.
The AC potential at both ends are essentially the same.

Dunno if there's existing die area big enough for 4X 1N400X along with the existing ESD diodes.
I'm using a DB-104 in the preamp which is a nice commonly available bridge.
The DB-104 in a 4 pin DIP-6 does make things a little tidier.

I'm getting a new roof today. Once the hammers stop and the dust settles I'll get back on this and post the input schematic.
I also made a couple of minor mods to the CM servo.

[JR.]

While I should let go of this, I am still trying to understand completely what is going on. Electrode forming is a chemical (plating) process with internal self-limiting based on condition of the electrode so will be a much more variable mechanism than DA. I am not clear about the un-forming mechanism (rate?) that should impact how much of this we encounter in normal use.

yes, caps suck... electrolytic caps suck in obscure ways.

JR

[mediatechnology]

Electrode forming is a chemical (plating) process with internal self-limiting based on condition of the electrode so will be a much more variable mechanism than DA.

I believe that it is more variable than DA. That's been my experience.
It's leakage that we tend to see.

I am not clear about the un-forming mechanism (rate?) that should impact how much of this we encounter in normal use.

IIRC with caps out of the bin it was >>15 minutes before it really got quiet.

With phantom off overnight and then turning it back on maybe 12 hours later it was perhaps 2-3 minutes before it was "fully" quiet.

When PV tested mic preamps for noise dollars-to-donuts having phantom on was not in the test script. I doubt for many manufacturers it is.
With the active mic preamps I tested doing commissoning for SSL I always turned it on.
Curiously, the one time I had big problems with noise with a phantom on condition it was resistors.

[JR.]

Electrode forming is a chemical (plating) process with internal self-limiting based on condition of the electrode so will be a much more variable mechanism than DA.

I believe that it is more variable than DA. That's been my experience.

Yes, exactly... my point has been that DA is not very variable

It's leakage that we tend to see.

I am not clear about the un-forming mechanism (rate?) that should impact how much of this we encounter in normal use.

IIRC with caps out of the bin it was >>15 minutes before it really got quiet.

With phantom off overnight and then turning it back on maybe 12 hours later it was perhaps 2-3 minutes before it was "fully" quiet.

When PV tested mic preamps for noise dollars-to-donuts having phantom on was not in the test script. I doubt for many manufacturers it is.
With the active mic preamps I tested doing commissoning for SSL I always turned it on.
Curiously, the one time I had big problems with noise with a phantom on condition it was resistors.

Save your dollars,,, If PV didn't test with phantom on we wouldn't have rejected that entire series of caps with too much leakage noise. No phantom no leakage. I suspect Peavey QA used a phantom powered mic at QA to confirm phantom power was present at every input and everything was working properly by speaking through the channel while listening. Crude but the factory QA people get pretty good at hearing small flaws.

Reflecting on this with hindsight, those rejected noisy caps may have been incompletely formed by the manufacturer (to reduce manufacturing cost). This forming in place later in actual circuits would mainly show up as a noticeable problem in these noise sensitive circuits. If it took >>15 minutes to get quiet, the QA folks in the factory would never wait that long. Who knows maybe those parts would have eventually quieted down. We'll never know.

JR

[mediatechnology]

... my point has been that DA is not very variable

Even with it being a constant it was a PITA.

I think most manufacturers in FT just use LEDs mounted on a male XLR connected from tip/ring to ground and if they light up all is well in Phantom-ville.

[mediatechnology]

Since I worked on this project last July I've gotten a new roof, bought a truck, helped form a neighborhood website using http://www.nextdoor.com, helped sponsor two crime watch meetings and a National Night out, found a policy conflict in Dallas' 911 aggressive animal dispatch policy using Public Information Act requests and discovered a historic 18" interstate natural gas transmission line from 1927 buried in our backyards.

I've applied for a State Historical Marker for our now-abandoned "Magnolia Index 1" gas line.

See: http://www.waynekirkwood.com/Images/Mag ... rative.pdf (Pipeline Marker Application Narrative) and
http://www.waynekirkwood.com/Images/Mag ... mber_1.htm (Some fun pics of our subterranean friend and his environs.)

I think I may have gotten a little "distracted."
OK, back to this thread and the next step.

I'm very happy with the way the preamp works. There are some significant advantages that we discussed earlier.
It needs a few RFI bits added and the non-floating output stage revisited but the DC-coupled concept works and can handle large mic offsets.
I'm going to shift my focus back to the power supply and try a circuit JR posted as well as one of Doug Self's concepts to "bootleg" both 48 and 72V.

I recall JR mentioning that it would be doubtful one would need 72V (48V + 18V + VceSat = ~70V) since most mics are going to pull enough current to load 48V enough that maybe "66V" would work.
1 mA/leg - a fairly low current of 2 mA total lowers the input CM voltage by 6.8V/leg to 41.2V.
I thought about this and decided "no."

If a dynamic mic is connected to the input and phantom is accidentally left on, both inputs are going to float to 48V.
So, in order to preserve headroom we need a minimum of 48V + 15 + ~4 or ~67V.
This only allows +/-15V operation of the preamp and 4V may not be quite enough VceSat allowance. VceSat of 6V might be better. That's 69V.
72 is a good round number being 48V+24V.
Thus I think I want to see if I can still get 72V from a simpler power supply.

It's time to go back to work!

[JR.]

Maybe you can put that old gas pipeline back into service. the only pipes I found digging around in my yard (after katrina) was some plastic plumbing from an old house trailer that was in my back lot in some earlier time. Recent enough to be plastic plumbing, but long before my time.
======
On the DC-MP i haven't thought about this for a long while but I can imagine different approaches based on configuration. i.e. optimal solution for a one or two channel solution could be different than a 16+ input console.

I don't know if I have discussed this before, but I have also thought some about the actual phantom power supply, It is neither a pure voltage or a current source, but a hybrid of both due to 48V voltage source with relatively high source resistance in series. Unless we run into mic capsules that need significant DC voltage to polarize the capsule, there is little need to really source the full 48V. In fact any practical active mic that draws a major fraction of the mA available will never see that full 48V. A dynamic mic that draws zero current does not need any phantom voltage.

We do need to source a maximum of a few mA per line, but in my judgement, without the polarization voltage constraint, this modest current could be supplied from a much lower voltage rail with synthetic source impedance circuitry to manage the termination. Of course this trick phantom supply circuitry needs to be very quiet because of where it joins the gain stage.

JR

[mediatechnology]

Maybe you can put that old gas pipeline back into service.

Well they cut it, launched pigs, cleaned it, capped it and filled sections of it under streets with grout. Even if it were intact I don't think I'd want the bill just to pressurize it all the way back to Louisiana.
The good news is that I just received an e-mail that the County Historical Commission has agreed to sponsor it and the State has accepted the preliminary application.
Now I have about 3 weeks to find a landowner to agree to have the marker placed on their property. Since there's about six or so good sites just within our 'hood I may score.

On the DC Mic Preamp power supply:

I found a trick Doug Self used to make a half wave trippler and with a 36V rms CT transformer secondary can get 80-85V easily. I modded Doug's circuit to make it full-wave.
I'm still trying to grok the PSU JR sent me. It looks very clever - too clever for me to understand but I want to try it too.

When I first did the stacked "24V x 3" PSU earlier in the thread I wanted to avoid two things:

1) Avoid using doubler/tripplers unless there was some added ripple current limiting for the series caps. My service experience was that unless ripple current was kept in control these caps would fail. I changed a lot of bootstrap electrolytics in Crown DC-150 and DC-300s because they eventually oozed electrolyte.

2) Avoid using the excellent TL783 HV regulator. Why? Because like a lot of TI parts in 2007-2008 they were unavailable. When I checked yesterday Mouser had 14,500 in stock. The TL783 is back in the game.

The power supply I'm working on is tailored to a two-channel 1U version of the preamp so it will have relatively modest 48V current requirements. As JR has pointed out the solution for a console would be different than a rack mount unit or preamp 8-pack.

What I still want to avoid is an internal switching PSU. Though they would be excellent I don't want to have to deal with switching frequencies greater than 10 kHz due to FCC and other regulator approval. So we're working for now with 120 Hz and a voltage tripler.

I'll post some schematics after I have a chance to noodle around with it some more on the bench.

[JR.]

I'm still trying to grok the PSU JR sent me. It looks very clever - too clever for me to understand but I want to try it too.

I had to look at it again. While I never melted solder on it, or seen it in actual use somewhere, it "looks" like it should work.

The concept is to cap couple the AC voltage from the transformer windings. This way we can superimpose a DC voltage on top of the typical AC voltage, so the nominal +/-V is centered around some other DC voltage (presumably up to +48V).

While in the forward direction this looks similar to a common doubler circuit, where things get a little different is how the boost caps get recharged. Since this is not moving any DC current, just +/- DC potential relative to some floating input bias. The two boost caps recharge each other during the negative polarity swing of the transformer output.

On paper equal value capacitors, and symmetrical current draw, should equally recharge these two boost caps to same 1/2 Vpp of transformer output. If they don't an active clamp might be needed to force them to recharge to same 1/2 Vpp. If we pull any DC current or imbalanced draw from +/- this will need to be forced to share equally .

When I first did the stacked "24V x 3" PSU earlier in the thread I wanted to avoid two things:

1) Avoid using doubler/tripplers unless there was some added ripple current limiting for the series caps. My service experience was that unless ripple current was kept in control these caps would fail. I changed a lot of bootstrap electrolytics in Crown DC-150 and DC-300s because they eventually oozed electrolyte.

In doubler/tripler the ripple current in first boost cap increases linearly 2x or 3x for number of stages, so this becomes similar to general PS design where ripple current is sized for load (actually X times average load, since dwell angle is small fraction to total time). I made a trippler once using tiny SMD ceramic caps for a phantom supply, since tripler was driven from 100kHz or more clock and per tick current was tiny.

Electrolytic caps inside power amps also degrade more quickly due to increased operating temperatures, that accelerate loss of electrolyte.

2) Avoid using the excellent TL783 HV regulator. Why? Because like a lot of TI parts in 2007-2008 they were unavailable. When I checked yesterday Mouser had 14,500 in stock. The TL783 is back in the game.

The power supply I'm working on is tailored to a two-channel 1U version of the preamp so it will have relatively modest 48V current requirements. As JR has pointed out the solution for a console would be different than a rack mount unit or preamp 8-pack.

To KISS for a small channel count application, you could just dedicate a floating transformer winding or two, and just DC connect one end of this floated winding to the nominal Phantom voltage. Note: My trick PS is also half-wave rectified, so a transformer with dual secondary windings could be split to service two floating channels, with similar ripple, (and cheaper low voltage local reservoir caps).

What I still want to avoid is an internal switching PSU. Though they would be excellent I don't want to have to deal with switching frequencies greater than 10 kHz due to FCC and other regulator approval. So we're working for now with 120 Hz and a voltage tripler.

I'll post some schematics after I have a chance to noodle around with it some more on the bench.

I feel a little guilty about submitting un-vetted designs , but my free time is consumed by other projects (like constantly fixing stuff). While I have long been a cheerleader for this approach. While I don't expect the potential improvement to be huge, preamps are already approaching "Ivory soap" 99.9% pure. It is a great merchandising story, and as Wayne has parsed out on his bench there are some measurable benefits.

Better is always better....

JR