Mic Pre-amps

There are a range of ways of making microphones. Presumably the broad-cast industry has standardised on one - and Phil Allison will probably tell us what it is - but without that information it's pretty silly question.

What's "broadcast quality"? does that mean really good or like not as good as "studio quality' or...?

I don't think any "jellybean" op-amp has the combination of noise, open loop gain and distortion performance to make a really good dynamic mic pre by itself. An NE5534 will work OK for some applications in something like:

but the high-quality solid-state pres tend to use discrete transistors or transistor/hybrid designs. Just because it's audio doesn't mean an op-amp can slum it, a good dynamic mic pre should have up to 70 dB of low-distortion gain out to like 15kHz around where the -3dB point of say the SM57 is, that's a tall order for a jellybean.

A jellybean amp would be better with an input transformer, but then you could afford a better op amp anyway...

I should qualify that as high-quality, but still built to a reasonable budget, there are all sorts of amazing op amps out there that might work on their own but they're not priced so you could build a whole mixer input section with them and end up with something you could sell.

One of the few remaining areas in small-signal audio where if you need cheap and good an op amp isn't the way to go, all the budget analog mixers by e.g. Behringer use discrete transistor mic pres AFAIK.

A number of studio-types swear by 'em for micing stuff like guitar cabs with dynamic mics, they're not bad! They stole the design from Mackie, after all lol

Fair point. I suppose a close approximation would be something the BBC Radio 4 station would use for studio interviews on spots such as Today and The World Tonight:

Discretes eh? Well, that's a surprise. That would be better still from my PoV; having less of the work done for you affords more scope for learning something new.

Some jfets get down to 1 nv/rthz noise levels. A few fairly expensive opamps do too, but their current noise can be bad.

Current noise into an inductive transducer will make high frequency noise, hiss.

You can parallel up op amps to get lower equivalent input-refereed voltage noise but as it only goes down as the square root of the sum-of-squares I think that exercise applied to jellybeans soon becomes silly and you destroy your current noise performance well before you get down to the performance of the best low noise transistors.

In olden times a step-up transformer of say 1:10 was the only way to get

70 dB of gain across the full audio band with reasonably low noise, out of a couple tubes. I don't think most tube mic pres used global feedback, or any feedback at all for the cheaper ones, but if there was feedback I think it was usually around the driver and output stage, putting it around the input triode or pentode was likely hard to stabilize, didn't add much in the way of performance anyway, and leaves the question of where do you put the input gain control if there is one, after the input transformer and first voltage amp is a logical place for it.

Using an input transformer with a solid state pre I think is silly unless you need the isolation; a low-noise discrete diff pair to dominate the noise performance of the chain, followed by a FET op amp is a good compromise. It's OK without global feedback also, wrapping global feedback around the whole chain is also a bit tricky and is explained more in the book "Small Signal Audio Design" by Doug Self.

Paralleling opamps improves voltage noise and wrecks current noise. Large-geometry bipolar transistors with bias current cancellation can have a lot of current noise.

Paralleling jfets works better.

There was some triode, three characters like 6xx, that got down to around 1 nv/rthz. It was a populat mic and tape head amp.

Diff pairs mostly add noise, another reason to not use opamps.

The "GE Circuit" started with an NPN that had very low impedance feedback into its emitter. Single-ended of course.

In my NMR amps, I used a low noise opamp and a brute follower, to let me use absurdly low values of feedback resistors that the opamp couldn't drive.

70 dB gain across ~20kHz with low distortion and low noise with cheap op amps alone is one of those very tricky carpets that are 1 square foot too big for the room I think, each way you come at it to tack three corners down the fourth pops up.

Going discrete or at least hybrid shifts the whole burden onto the designer in the first case, and takes some of the pressure off the cheap op amp in the second.

A bridge-T type peaking circuit in the feedback loop can also work to bump up the closed-loop gain at high frequency where the op amp output Z starts going up as open-loop gain drops, the downside is the size of the cap in the bridge-T goes up in proportion to how low value of feedback resistors you use and for anything very critical you probably wouldn't want to use electrolytics.

<snip>

Keeps the mic balanced too, good if you have a long cable.

It worked OK for my isolated NTSC video buffer though, a $1 voltage-feedback op amp can't really do "DC" to a 10 MHz -3dB into a 75 ohm feedback network impedance but you can fake it.

Not single supply, but a couple of possibilities come to mind:

They're not cheap though.

Page 539 of AOE shows a simple 70 picovolt per-root-hertz preamp design using paralleled bipolars.

See Figure 8.42. Ribbon microphone preamp congurations.

"But it requires a huge input blocking capacitor (150,000F to preserve the signal's very low source impedance down to a few hertz)"

I designed a DC-coupled low noise preamp to overcome this problem. I posted the TinyUrl link here, but it has already gone 404. I'll post a new link when I get my new web site up and running.

Multiple sections can be added in parallel to reduce noise, but this quickly runs into practical limits of 1/sqrt(N). The exponential increase in number of devices soon limits the amount of noise reduction you can achieve. Gerhard also makes very low noise preamps,and I'm sure he could elucidate further.

Steve Wilson snipped-for-privacy@not.com wrote:

Here is the ASC file:

Version 4 SHEET 1 1888 724 WIRE 544 32 480 32 WIRE 672 32 608 32 WIRE 480 112 480 32 WIRE 528 112 480 112 WIRE 672 112 672 32 WIRE 672 112 608 112 WIRE 720 160 576 160 WIRE 864 160 720 160 WIRE 576 176 576 160 WIRE 720 176 720 160 WIRE 864 176 864 160 WIRE 480 192 480 112 WIRE 512 192 480 192 WIRE 544 192 512 192 WIRE 640 208 608 208 WIRE 672 208 672 112 WIRE 672 208 640 208 WIRE 400 224 384 224 WIRE 544 224 400 224 WIRE 384 240 384 224 WIRE 480 256 480 192 WIRE 576 256 576 240 WIRE 784 256 576 256 WIRE 784 272 784 256 WIRE 864 272 864 256 WIRE 384 336 384 320 WIRE 480 368 480 336 WIRE 528 368 480 368 WIRE 576 368 528 368 WIRE 672 368 576 368 WIRE 720 368 720 240 WIRE 720 368 672 368 WIRE 784 368 784 352 WIRE 480 384 480 368 WIRE 576 384 576 368 WIRE 672 384 672 368 WIRE 480 464 480 448 WIRE 576 464 576 448 WIRE 672 480 672 464 FLAG 384 336 0 FLAG 400 224 Vin FLAG 640 208 U1O FLAG 480 464 0 FLAG 528 368 R1C1 FLAG 784 368 0 FLAG 864 272 0 FLAG 512 192 U1N FLAG 576 464 0 FLAG 672 480 0 SYMBOL voltage 384 224 R0 WINDOW 0 33 22 Left 2 WINDOW 3 37 77 Left 2 WINDOW 123 35 103 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 10 SYMATTR Value2 AC 1 SYMBOL opamps\\lt1028 576 144 R0 SYMATTR InstName U1 SYMBOL res 624 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 4950 SYMBOL res 464 240 R0 SYMATTR InstName R1 SYMATTR Value 50 SYMBOL cap 464 384 R0 SYMATTR InstName C1 SYMATTR Value 10mf SYMATTR SpiceLine Rser=25m Lser=2n SYMBOL voltage 784 368 M180 WINDOW 0 33 22 Left 2 WINDOW 3 33 44 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 15 SYMBOL voltage 864 160 R0 WINDOW 0 33 22 Left 2 WINDOW 3 33 44 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value 15 SYMBOL cap 608 48 M270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName C2 SYMATTR Value 100p SYMBOL cap 560 384 R0 SYMATTR InstName C3 SYMATTR Value 10mf SYMATTR SpiceLine Rser=25m Lser=2n SYMBOL res 656 368 R0 SYMATTR InstName Rleak SYMATTR Value 1e6 SYMBOL diode 704 240 M180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value 1N4001 TEXT 368 -96 Left 2 ;'Measure Power Supply, Zener, LED Noise TEXT 368 -64 Left 2 !.ac dec 500 1m 1e8\n;.tran 2

Ya but you don't need a transformer for that when you have an op amp or op amp + diff pair with 90dB+ of CMRR.

Just use a SSM2015 or SSM2016 mic preamp IC if they are still made... Analog Devices I think bought the old SSM products line.

Here is an example of the circuit used in the old Symetrix Audio broadcast microphone preamp as well as the Symetrix SX202 mic preamp...

If you don't need the 48V phantom ciruit, leave out the 4K99 resistors and even the capacitors inline with that and the preamp IC.

They are still around evidently. Might be something newer of course.

Clive Arthur wrote: ==============

** Dynamic mics all have a "floating" output, so can be used with unbalanced cable & pre-amps. They actually work better that way and can use a longer cable.

A strange but true audio heresy.

..... Phil