Microphone boosters?

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emrr

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Ya know, even the Mackie 8 Bus preamp has a 5K input Z. True of a lot of modern preamps. The assumption that everything is sub-2K is exactly that.
 

moamps

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Most often, DOA's are considered low noise when their EIN density is about 3-5 nV/sqrtHz and their OSI about 5-10kohms. It seems very much the case here (although the OSI might well be much higher).
For a mic booster, the targetted noise voltage density is about 1nV/sqrtHz.
The JFET 2SK58 from the above circuit has En of about 2nV/sqrtHz, the JFET uPA68H from the lower circuit has En approximately 1.3nV/sqrtHz so they are obviously not far from your set target.

Also, the JFET pair 2SK163/2SJ44 used frequently (several in parallel) for MC head preamplifiers from the late 70's have En=1.5nV/sqrtHz.

I believe the first commonly available FET's capable of such performance were the 2SK170 and its complementary (2SJ74?).

The JFET pair 2SK147/2SJ73 (NF=1dB), which began production in 1981, is even mentioned in Horrowitz-Hill - The Art of Electronics, second edition from 1989, page 442 as very low noise transistors (En=0.7nV/sqrtHz for Id=10mA). They have better characteristics than 2SK170/2SJ74 pairs as far as noise is concerned.

1637359854392.png1637359884770.png

Around the same time, the 2SK240/2SJ75 (En=0.95nV/sqrtHz) pair also appeared which also had slightly better characteristics than the 2SK170/2SJ74.

1637359957512.png

In the mid-1980s, the very popular 2SK389/2SJ109 (NF=0.5dB) pair appeared, which has approximately the same characteristics as the 2SK170/2SJ74 pair.

1637360086597.png

All of these transistors were readily available at the time Sony, NEC and Toshiba produced them.
 

abbey road d enfer

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All of these transistors were readily available at the time Sony, NEC and Toshiba produced them.
You're probably right surmising these were available at a time mic boosters were not deemed necessary/useful.
I suspect most of them were EOL before anybody began to think about boosting low level mics.
One may wonder why super low-noise devices such as LM394 and 2SB737 have been discontinued without replacement.
 

Newmarket

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One may wonder why super low-noise devices such as LM394 and 2SB737 have been discontinued without replacement.

My understanding is that it was down to the fall of demand for phono' preamplification etc Add in that re-tooling for some devices as semiconductor fabrication moves to smaller geometries is not considered financially viable.
 

JohnRoberts

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You're probably right surmising these were available at a time mic boosters were not deemed necessary/useful.
I suspect most of them were EOL before anybody began to think about boosting low level mics.
One may wonder why super low-noise devices such as LM394 and 2SB737 have been discontinued without replacement.
They went EOL because customers stopped buying them in enough quantity to justify making and selling them.

The 2sb737 and 2sd786 complementary pair were designed for MC head amps and lower Rbb than optimal for mic preamps, but they were widely used in console mic preamps. Slightly more optimal discrete bipolars for low Z mics have been developed since then, but the real killer was mic preamp ICs that didn't suck. Now junior engineers can buy canned solutions off the shelf.

The LM394 was low noise for its time but importantly optimized for precision log conversions (current to voltage conversion using Vbe). Again we can now purchase off the shelf IC op amps with ein comparable to the old LM394. Similarly dedicated ICs can now rectify and log convert audio signals for use in dynamics control side chains.

The dedicated logging ICs may go EOL right around the same time people stop using VCAs but they aren't done just yet.

JR
 

gsbe

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Very informative discussion here. Has anyone ever seen a dissection of the various boosters commercially available? Most of them have been mentioned here but I was wondering about a comparison of circuit design and resulting performance.

Cloud, Royer, Mogaine, Cathedral, etc. They all seem to have slightly different designs which would suggest they may all be more or less effective and providing the low noise gain that has been discussed here. Thanks for all your insights on this topic.
 

moamps

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Using a booster is not such a new idea. The picture shows a Neutrik booster at least 30 years old, which was delivered with a 333x measuring set.

1637518270209.png

One may wonder why super low-noise devices such as LM394 and 2SB737 have been discontinued without replacement.

Recently, original Toshiba JFETs 2SK369 can still be purchased on eBay
which are IMO better transistors for microphone boosters than 2Sk170.


In the Mouser you can buy IFN146 dual JFETs (new versions 2SK146) which I believe are better than the new LSK389 transistors. But the price is around 20Eur/pcs.

As far as bipolar transistors are concerned, nowadays the low Rbb/low noise pair ZXTN2018/ZXTP2027 and the low noise pair 2SA1162/2SC2712 which has a higher Rbb can often be seen in microphone preamps.
 

abbey road d enfer

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Using a booster is not such a new idea. The picture shows a Neutrik booster at least 30 years old, which was delivered with a 333x measuring set.
I don't remember this one. I should, since I've been the neutrik distributor in the 80's and 90's...
Was it bipolar or FET?
As far as bipolar transistors are concerned, nowadays the low Rbb/low noise pair ZXTN2018/ZXTP2027 and the low noise pair 2SA1162/2SC2712 which has a higher Rbb can often be seen in microphone preamps.
They seem to be in the same category as ZTX851/951. Medium power transistors where low noise is a collateral of large geometry. Clearly, the manufacturer is not interested in attracting the "low-noise crowd". Compare to Toshiba who proudly bragged about the 2 ohm Rbb' of 2SB737.
 

JohnRoberts

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They seem to be in the same category as ZTX851/951. Medium power transistors where low noise is a collateral of large geometry. Clearly, the manufacturer is not interested in attracting the "low-noise crowd". Compare to Toshiba who proudly bragged about the 2 ohm Rbb' of 2SB737.
I used to buy my 2sb737s from ROHM, and they bought the tiny Japanese company that originally developed them.

I don't recall them ever being second sourced and as I recall ROHM issued EOL lifetime buy around the turn of the century.

JR

PS: There are several hot newer parts but I stopped paying close attention a couple decades ago.
 

moamps

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Darlingtons are not generally low-noise. Not a factor for most acoustic measurements.
For sure. I used this booster a few times to amplify the signal from the selfpowered accelerometer when I was doing some measurements on the speakers. And IIRC, we only used it once in the recording studio to amplify the signal from an 8" speaker playing the mic in front of a big kick drum.
 

amplexus

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I’ve got a pair of the cathedral boosters. I remember cracking them open and if memory serves (and it might not) seeing not much more than a couple smd components on a very empty pcb.
I should crack them again and see what’s what.
 

abbey road d enfer

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I should crack them again and see what’s what.
That would be nice.
I tried to do that with my Triton FetHead (1st gen) but it's stuck with glue. Anyway, the schematics are commonly available on the web.
IIRC, the 1st-gen Fethead is just two FET"s; two source resistors and two gate resistors. Main issue was that the drain voltage was excessive, which resulted in fried FET's, and the elevated drain-gate voltage made it noisy. 2nd-gen uses cascodes to solve both issues.
 

amplexus

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Well, Cathedral Pipes definitely doesn't want you getting inside- 8 rivets need to be drilled out to get the case open.IMG_0493.jpeg
IMG_0494.jpeg



Once inside? As expected- not much. A pair of LSK389 dual JFETs and a smattering of supporting components.

IMG_0495.jpeg

At least the Cathedral Pipes units are very reasonably priced. $65 USD doesn't offend me for something like this the way Cloudlifter prices do.
 
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