U47 fet project

[Cirrus]

A year ago I built a couple of U87 clones, which I've used on a bunch of things over the course of the album recording my band is doing - most notably drum overheads, on guitar cabs in a blend with a more peaky dynamic, about half the lead vocals, all the backing vocals & acoustic instruments. They're just good utility mics that'll always come in handy, so having a pair feels warm and fuzzy in my loins.

It turns out that about a year is long enough for me to forget what a pain in the tits soldering things is, so I'm ready for the next one. Decided after a bit of umming and ahhing that I'll go for a U47 fet - decent choice of transformer and capsule suppliers, and the same level of complexity as the U87s. I was considering a U47 valve build, but between the valve gubbins in the mic and the need for an extra external power supply I think it'd end up costing more than I can afford just now. The U47 fet should come in around the £400 mark;

- cheap mic to gut and use case
- PCBs
- Transformer
- Capsule
- resistors, caps etc

I'll update this thread as and when anything happens. For the moment, I've ordered the PCBs and some polystyrene caps from here;

PCB supplier

Objectively, including shipping £50 is a lot for a couple of tiny PCBs and a jiffy bag of caps and I should be ashamed, but since I just don't have the facility, knowhow or sense to make my own, I'm just paying it and doing it with a smile.

 

[FuzzyAce]

Cool project. Look forward to see/hear the results!
I've contemplated a mic build too. Would be fun I think.

 

[Cirrus]

Capsule ordered. The maker of the U87 capsules I used also makes a K-47 style capsule that's a bit more flat EQ than the traditional one. It's flat rather than having a broad EQ lift of above 5k. She had some b-stock ones where only one side meets spec, and that's absolutely fine - the U47 fet only uses one side, it doesn't have the polar pattern selector of its valve big brother.

Arienne Audio flat K-47

Here's a U87 capsule plot - see the big boost up top, which is pretty much 5dB up at 10k vs 1k. It gets tamed down by the circuitry, so the result is much more flat with a small high frequency lift and then drop off above that. I've taken both these graphs from this thread

The K47 is more flat anyway, so no high frequency circuit correction is needed. Apparently the original neumann ones are more variable than the U87s, some have more of a 5k boost.

 

[Cirrus]

Well. The capsule is already here, which has surprised me! Looks like the Atlantic is back in business.

Donor mic arrived this morning too. A valve condenser mic with power supply, shock mount and 7 pin cable for £80, no wonder the world economy is fucked . Since the mic and capsule are here, I might do an A/B with the stock capsule and Arienne Audio K47 with the stock valve circuit, before I start gutting things.

 

[Cirrus]

And this is the transformer I ordered yesterday (for £120 from eBay) - wound by a guy based around Manchester by the looks of it. Seems like it'll do the job.

I was a bit surprised by the price of this style of transformer. The two U87 transformers cost me £80 each from the USA. The equivalent quality U47fet transformer from the same manufacturer is £200. They are physically bigger, about twice the size, and they're humbucking so have more wires/ coils.

Cinemag do one that'd be £100 + shipping to the UK, then there was this one and a small winder from Serbia who charges around the same. I figured for £120 might as well keep the money in country.

 

[Cirrus]

For some perspective, I had a small UK winder based on the south coast wind me a Vox AC30 power transformer with my own custom secondaries for £120. You're definitely not paying for this iron by the kilogram.

 

[Cirrus]

Last night I put the new capsule in the existing mic & circuit. Didn't get much chance to try it because;

a; 2 year old sleeping.
b; forgot to check what the stock circuit polarises the capsule to before I closed it all back up. I think the U47 fet polarises to about 45v (but need to do more reading) and K47s are good up to 60-70v, but for all I know this cheap circuit could be higher than that and I didn't want to keep the mic on for too long.

Here's the old capsule. Like many Chinese K67 clones, it's 32 rather than 34mm diameter. The black rubber capsule holder is part of the shock mounting strategy. It's pretty solid but flexible enough that you can twist and bend the capsule a bit. Luckily the bigger 34mm capsule came with its own plastic mounting bracket, but I had to file the metal bit at the top of the rubber to get it to fit.

Verrrrry unscientific, but some clips that at least give a ballpark idea of the sound differences; Last week I recorded acoustic and talking with the stock cheap valve mic and a u87 clone side by side, last night I recorded acoustic & talking with the new capsule in the stock circuit.

The stock & U87 clone takes really show how much brittle HF lift there is in Chinese capsules, and when it matters - It makes the acoustic sound really thin, but isn't terrible on speaking voice. In years past I've spent so much time trying to get good acoustic recordings out of cheap mics, fighting with EQ endlessly.

To my ears, the K47 capsule in the stock body is better on acoustic than the U87 clone, because though neither mic has a harsh treble peak, the 87 circuit has a bit of a midrange emphasis (which is part of what makes or breaks the real thing, and why the U87 is often great for sitting in busy mixes, so I'm not worried about it), whereas the k47 capsule & stock circuit sounds more relaxed in the chesty mids and warmer down at the bass end.

I'll do a more scientific side by side with a U87 later in the week. Hopefully next weekend I'll have the bits to get started on the new circuit.

 

[Cirrus]

Oh. Forgot the link.

SCT 700 comparison

Shared with Dropbox

www.dropbox.com

 

[Cirrus]

And wife says the transformer has arrived.

Still waiting on the PCBs from the USA.

 

[SillyOctpuss]

And wife says the transformer has arrived.

Still waiting on the PCBs from the USA.

Pcbs from the USA to UK suck at the minute. My last few orders from pedalpcb and Madbean sat in customs for ages before moving on.

 

[Cirrus]

Well everything was delivered on Friday, and I just got on and built the mic, forgetting to take a single picture.

So, no belaboured build thread. It works, at least at low volume. Gonna take it to record something louder hopefully this week for a proper test. In the meantime, did a comparison against the 87 style mic I made last year, with the two mic capsules as close as I could get them.

U87 acoustic clip
U47 Fet acoustic clip

First impression? It's not a hundred miles away from the 87 in tone. It's more relaxed, softer on top. I think I'd be happy with either on acoustic depending on my mood and the context. The 87 has some more mid push and pick attack. But will be interesting to put it on front of something loud - a belting singer, a guitar amp or kick drum - and see what happens when the two circuits are pushed.

 

[Cirrus]

I did take two pictures.

Higher component count on the Fet 47 vs an 87. They both have a FET as the gain stage, but the 87 just sends that straight to the output transformer, while the 47 uses another 6 transistors to basically make a descrete op-amp 2nd gain stage before the signal hits the transformer. It also self biases the 2n3819, so no need for a trimming resistor, test probes and oscilloscope.

The two mics around the same output level, which is pretty low - on a par with an SM57. I therefore have literally no idea why one has a tiny transformer and the other is so big it barely fits in the oversized body.

There are several components that need to be floating - where the capsule connects is a very high impedance part of the circuit, with a 1 gigaohm resistor, and even a stray fingerprint will be like a resistor loading the input down. So... A bunch of the parts are soldered to a little isolating pin, and a capacitor is simply installed upside down.