Capacitorless Mic Pre from AML - Neve-like?

4x 5W power resistors? Wow.

It's a Langevin AM16 clone. Looks neat, but have no idea how it sounds.

www.technicalaudio.com/pdf/Langevin/Langevin_AM16_schm_by_Uneeda.jpg

 +24V circuit, they're using those to drop 8 volts from the 500 spec.  I dunno, is that more reliable than a diode string for dropping?  Or separate regulation/dropping?  

It can not load a mic properly.

It's one thing to think highly of the AM-16, it's another to use that info to bet on a clone with an incorrect attenuation plan.  

That shows how much I know about preamp circuit design (so far!). I see carnhill transformers, a preamp design, lacking DI/trim knob and circuit but having hi/lo Z, 48v and phase and think a barebones Neve style preamp that can be sold cheaper! I know AML's support personally has been top shelf, but what makes you think the Pre can't load a Microphone properly? The lack of capacitors keeping the voltage steady? What's the point of this if it can't amplify a Microphone correctly?

This product is so close to being right it pisses me off. I stated a fact (or two) that apparently isn't being grasped. I wouldn't bet on rep here, everyone screws up sometime. I'm sure it works, but it ain't right.  If you were taking a class in preamp design, this decision would get a failing grade from any professor.  You assume correctly, they are fixed gain preamps, the originals came without gain control. He's stuck an H pad in front, which is used between matching impedances (which hasn't been done with mics in decades - like 6-8 decades). It can't load the mic right, it can't load the input transformer right either. It has different gain steps with both input impedance settings. It creates a LPF with dynamic mics. It should be a bridging U pad, period....even for line bridging use up to 50 dB. I'd personally be ashamed to trot something like that out into the marketplace (maybe someone would be brazen enough to call it a 'vintage feature of 1935'), and would fully expect to get called on it. But ya know, no one's an engineer anymore, or knows anything technical, and if it makes sound people are happy. So whatever. 60 years of AES papers and audio science can't possibly be right. Buyer beware, know what you're buying. If I wanted an AM-16 that acted correctly with microphones, I would have to take this product off the list because of that gain control, that bit totally screws the pooch to me. I spelled it out in more detail in my earlier link, in which I challenged the design and have so far been met with 7 months of silence. GroupDIY has no build thread for it so far. If I were 100% certain of a non-standard design decision like that you could be certain I'd defend it. Maybe the first batch is quietly going away and version 2 gets a different gain control. You could always just leave the gain control at max gain (no pad) and stick a proper 20dB U pad barrel in front of it, but that would make you have to use an outboard phantom supply. Maybe it works perfectly while being technically (wildly) non-standard, then the lack of defense is even more mystifying.  Fixed gain preamps are what they are, and sticking a stepped gain control up front like this is an attempt to make it palatable to the average user who expects fully variable gain control.  

realgearonline.com/post/79278/thread

The next best hack possibility is to make the 150/600 switch a standard -20dB mic pad. Maybe cut traces and put that H pad on the output side, maybe re-resistor it for 1, 1.5, or 2 dB steps. Stick your own labels over the things you've changed. Or a giant DPDT pad toggle in the hole where the gain control is! Simplest ignores the gain control and wires past it, anything else gets into board/trace cuts, which could be easy or nasty.

I have never had to fix an AM-16, but I've repaired a bunch of Altec 9470/9475 over the years, and they are very particular about the transistors you put in them. If there's any balance mismatch in the output pair the low end goes away, if the HFE is too high (you need vintage transistors!) they oscillate uncontrollably. John Hall designed the Langevin and then the Altec in a span of a year or so.