Originally Posted By: Mike Halloran
The problem with Shure dynamics is that they were designed to be 150Ω (nowadays measures around 260Ω) and the ideal load is 600Ω, the old Bell Telephone spec. (4x the input but 500–1000Ω is acceptable.). Solid state mic preamps are normally 1300–2400Ω which does not load the input directly. Shure knows this but the so-called "industry standard" has sold a gazilian units. This leads to excess proximity effect, early distortion and lousy transient response. Vintage mic pres from Shure, EV, Altec etc. are all 600Ω — even the old Shure VocalMaster PA.

There are many fixes for this including variable impedance mic pres, Cloudlifter, Cloudlifter CLZ with a variable impedance pot (turn the pot and listen to the changes). You can get an 823Ω resistor and wire it in parallel using a cable or inline barrel connector for about $10; substitute a 1KΩ variable resistor in a box and you can ape the CLZ without the volume boost. Once you've heard a cymbal or high-hat through a properly loaded 57, you'll be amazed how good it actually sounds.

At least the output of an SM7B is so low that you're forced to use a decent mic pre in front of it.

I guess it's now time for everyone to tell me how wrong I am — but I happen to be right.

You're not wrong laugh I love a good preamp with variable input impedance for this reason. It makes a night and day difference on my SM7B's, 57's, 58's, and some of my other dynamic mics. I built one of those "5 dollar preamps" - with a bit more than 5 dollars of parts to add in a pad, high pass, balanced output, toroidal transformer in the power supply, input protection, 1U rack mount with stereo pre's, and of course variable impedance. Worth every penny.


Originally Posted By: Planobilly
Good but not a lot of money. Buy an off-the-shelf regulated power supply, this is optimized for 220VDC. Do not exceed 250VDC. Use a Jensen JT- 115k input transformer ( don't buy a cheap junk input transformer) ECC82 valves or if you are on this side of the pond 12AU7 tubes...lol

https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-115k-epc.pdf



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Perhaps Video Track could comment. He is very knowledgeable about this stuff.

You may have to search around for the jensen-transformers. Everything electronic is a bit hard to find at the moment.

Most of the material cost is in the transformers. Perhaps three hundred for all the parts. Just a guess. But you would have a first-class tube mic pre.

There are only two actual tubes in this.

This could actually be point to point wired in a few hours.



Billy

Hmm, interesting circuit in the pics you attached - it's an inverted cascode compared to the one in the Jensen cascode schematic, and is two gainstages plus a single cascode vs dual cascodes. Where did that schematic come from?

At a first glance, there might be a tad more noise than the dual cascode, but it's hard to say. I should make one.....

Also, a note on input transformers - I've had good luck ripping apart old dead mixing boards for input transformers. Most of them don't have quite a high enough ratio for tube preamps, but they still tend to work fairly well for a free/cheap transformer.

I work here