Looking for ideas

[EngGuy]

I have engaged in a project,,, Designing a mic preamp. I've casually browsed on-line specifications for them, seen pictures of them, and quite frankly I can't find anything like what I want to design.

So, I'm fishing for ideas on what users like and don't like, what connections you need and which ones you have never needed, adjustments, etc.

Don't be afraid to complain, and definately don't be afraid to dream, this might be the chance to get your wish. (all analog, of course)

Keep in mind I'm not a music person, so be kind with industry lingo.

Thanks !

 

[mattamatta]

How bout a plain and simple pre... XLR mic input, line input, and hi-z instrument level input.

All connections on back, plus a duplicate of the instrument input on front using a high quality 1/4" jack (the kind with the smooth-topped nut, black) like <img src="http://www.carvin.com/images/product/K31.jpg"> althought those ones are for guitars, but with the same pot size it'd be fine, but that's the style.

Gain knob, output trim knob. Small, gnurled texture like Black finish.

Subminiature toggle switches for phase invert, -20db pad, hi-pass filter, and 48V phantom power.
Full-size toggle switch for power on (power through standard IEC power connector at rear)

Two-channel, rackmount 2U chassis. Amber backlit meters.

Brushed aluminum faceplate, small size blue LED indicators for phase, pad, hi-pass, phantom power, green LED power on indicator.

On all 4 sides of the faceplate, as well as two vertical lines between channels, there are lines milled into the plate ( with rounded eges, leaving the corners connected on the face plate, otherwise the middle would fall off). Behind that is another thin plate set back about 3/8 of an inch, which will be lighted with internal LED's of the same blue as the indicator LED's, providing a graphical ou tline and channel separation indicator.

It will be a big sounding unit with massive transformers and solid state design. Plenty of headroom in all stages. Enough to crank the gain and saturate the transformer, trimming the output stage as desired. Noise is whatever is practical, as I dont find it much of a concern, but quiet for ribbon mics and low level condensers / dynamics with low level sound sources is a good thing.

 

[mattamatta]

OK, maybe that wasn't so plain and simple.

But operationally, what I care about the most is the mic connectors and a gain knob.

Instrument input is secondary, as a clean DI box can always solve the problem of not having a dedicated instrument input.

Line level input could be useful, but wouldn't factor into a decision for me at all at the moment, as I dont really have anything that's already at line level that I want to give more gain or color too.

<img src="http://www.shrinking-pants.com/matt/graphics/mic%20pre%20faceplate%20design.JPG">
[Rack ears not pictured]
I actually like some of the ideas, especially the motif of the sunken milled panel with the internal blue lighting.

I call that design for any future line I start (I actually do intend to do audio electronic design as a career).

The faceplate could also be polished chrome (I think the black knobs and toggle switches would look nice on it.)

The power indicator light could also be switched with red if the green clashes too much with the blue. Might even go amber to match the meters backlighting. That way the whole thing's a black/"white"/red/blue motif.

 

[MISTERQCUE]

Uhhhhh matta, maybe it w/b better if you design and build it instead of EngGuy!!

I LIKE your ideas!

 

[mattamatta]

Hehe, yeah, I got a little carried away there, but chassis design is the most fun part, cuz it's not somethign that actually has to WORK. Just look cool and be nicely laid out.

 

[EngGuy]

I'm dieing over here !

Oh jeeze guys, I’m rolling on the floor over here !!! Stop it !
Yikes, great input,,, but your talking about a $100 faceplate ?!
Chill. Let’s get back to function. Yup on most things,,,

But why “massive transformers” ? Kind of on that same point, does anyone need anything above 0dB to drive what ever the next piece of gear is ? I’m shooting for +6.

“Noise is what ever is practical” Cool, it will be class A input and output, and FET amps in the middle, should be very quiet, and will be able to drive very low impedance loads. Interesting, nobody mentioned headphones yet.

“Channel separation indicator” Hmmm, intriguing, very easy to do,,, That’s a neat one, never thought of having a need for it.

 

[northsiderap]

Preamp

The Neve discrete amp modules seem to be direct-coupled class A with phase-inverted feedback that increases into RF. The input xformers are probably 1:3.5 or something. Transformer isolated, of course.

The A*P*I topology seems to be a differential input pair and a 1:10 input xformer.

I think you can find schematics for the UREI 1108 online <a href="http://www.waltzingbear.com/Schematics/Urei/UA_1108.pdf"> HERE </a> if you are going to use an FET topology. Thanks to Waltzing Bear... Looks like an transformered FET input -> BJT Emitter follower -> BJT to output tranny and emitter feedback to first stage...

I'm sure you can find some 1:10 input trannys on eVilBay or something if you need a cheap way to do inputs for a test model.

Either way I'd personally prefer this in an a mic pre:
Input Jack ->
Pad & Phase Switches ->
1:10 input xformer ->
Amplifier section ->
Output transformer --

It helps to have a line in that bypasses the xformer or has its own isolation xformer...

Another way to do this is just drop $50 on a Jensen 990-c opamp and get some input and output trannies. Then just rob A*P*I of their topology... You can get DPDT switches and XLR/TRS panel jacks from MPJA.com or mouser.com for a buck or two a piece. Get 10 of each while you're at it... You're going to be building more than one!!!
You might as well get your pad & phantom 6.81k resistors while you're at it. Get 'em by the roll matched as closely as you can afford. I hand-match mine with a multi-meter after I get them for even more accuracy...

 

[Kevin Deschwazi]

Ok a real simple one but I really appreciate two outputs on a preamp.

This is valuable in computer recording as you can monitor the signal without having to go through the DAW (zero latency).

 

[northsiderap]

Two outputs

Get an output transformer with a dual secondary...

Or look up how to put an AUX send or PFL send into your circuit..

 

[HogansHiro]

My two cents worth.......freakin lights!!!! I like to see a big freakin RED light that lets me know when I'm pushing too hard.

 

[SouthSIDE Glen]

I like to see a big freakin RED light that lets me know when I'm pushing too hard.

I saw one of those in my rear-view mirror the other night when I was pushing too hard

G

 

[EngGuy]

Great feedback (pun intended)

Phase invert,,, OK. Too easy.
2 outputs,,, Another one I didn't think of since I'm not a user.

Why does everyone insist on transformers ?
That would solve a design dilemma for me, but have a bad taste in my mouth about 'em,,, Even after having serviced Audio Research tube equipment for several years, and loving their performance, I still see transformers as having certain problems of their own. Particularly being EMF sponges in super low level applications.

BIG, bright, red lights. Yup, that one came through loud and clear.

What do you need for output level? I’ve seen specs where things will put out +10 thru +20 dB, I can’t believe this is needed. Besides, all that extra gain makes it difficult to keep the noise floor under control.

Define please the function of PFL and AUX send functions, I’m guessing that one of them is an output prior to the output level adjustment.

 

[snow lizard]

PFL allows you to mute the output so you don't overload your equipment while you set up the gain. A clip light to indicate when the thing goes into distortion is handy as well.

If you want to see a preamp with a nice set of features, go look at a Great River MP2-NV.

The output dealie - are you thinking 0dBfs or 0dBu?

Having a pre with greater output allows you to have more headroom, and/or turn your recorder input down for better noise and overall gain staging characteristics. I'd shoot for around +12 minimum. The general idea is to drive the preamp as hard as you can get away with, so knocking 10dB off of the output would sort of suck.

How the pre goes into distortion is important.

The transformer thing is for that big, rock & roll "iron in the path" sound that the kids are so crazy about these days. Makes for a more interesting preamp than a completely accurate, sterile, lifeless model. Good transformers are critical. Makes it sound big.

Impedence loading switch is a good idea. Something that could go from say 300 to 1200 or so ohms input impedence would allow the user to get decent results with ribbon mics. 75dB overall gain would help too.

Oh yeah, tone. Good preamps are quiet and have nice musical character and sound well focused. The cheap ones get all sloppy once you start layering a bunch of tracks. Very important.

A pad switch and a high pass filter switch might be useful.

The "ramping" characteristic of the RNP when you turn the phantom power on sounds kind of cool.

If you're worried about headphones, design a really cool headphone amp. There aren't enough of those.


sl

 

[EngGuy]

Not enough ?

Not enough headphone amps? "Really cool"?

OK, I'm game,,, What do you want that you can't get?
Headphone amps are even easier to design than mic amps.

All class A, DC to daylight freq response, able to drive a dead short, variable input and output impedance. Anything else?

 

[snow lizard]

- loud
- clean
- cheap
- 3 band eq
- multi channel


sl

 

[EngGuy]

Uhhhhhhh

Three band EQ, or any EQ for that matter, does not fit within "clean". And certainly not cheap. A good, clean, single channel 3 band EQ would have to retail at $250+ to get my attention.

Loud and clean are easy. What do you consider "cheap" ?