bcarso

trafoless microphone preamp wish list
« on: June 05, 2005, 03:07:37 PM »
Any design is an exercise in tradeoffs and constraints, and mic pre design as much so as any.  Some parameters are mutually almost exclusive.

Having said that, in what would a reasonable wish list consist, for a transformerless balanced design?  Let's start with one where the power supply is external so at least we're not tied down to phantom power restrictions.  If in the end phantom is enough, all the better.

The device can OTOH provide good clean phantom power for the particular mic---I just mean don't expect the preamp to be powered from the console.

Some of the things that come to mind:  low noise, wide dynamic range with polite overload if possible, variable input loading, low distortion with special attention paid to making higher-order products negligible.  Excellent common-mode rejection, excellent  match of + and - outputs' impedance to common, low susceptibility to EMI/RFI and power line noise/fluctuations.  Stability with time and ambient temperature.  Flat or at least adjustable frequency response, like some low frequency rolloff adjustments. Good transient response. Fairly bulletproof against abuse.

Remote control of some of the adjustable parameters?

No use of unavailable parts, within reason.  Obsolete but supposedly readily available NOS tubes considered but discouraged if production units will work.

Low cost ;^o !

Your inputs are respectfully solicited.


Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
trafoless microphone preamp wish list
« Reply #1 on: June 05, 2005, 03:38:40 PM »
Quote
Remote control of some of the adjustable parameters?


Remote controlled gain is handy for many situations. The gain range should get down or close to 0 dB, without any interruptions or large steps from a pad.

If gain switching could be made silent, I'd be very happy!

Samuel

bcarso

trafoless microphone preamp wish list
« Reply #2 on: June 05, 2005, 03:49:03 PM »
Well that's another one of those interesting constraints.  Silent gain changes are only going to happen if the gain is adjusted continuously and with no control signal feedthrough, but this will tend to conflict with low noise and low distortion .  At some point you start to ask your mic preamp to become your compressor-limiter!

But that's a great input Samuel and exactly what I'm looking for.

tmbg

trafoless microphone preamp wish list
« Reply #3 on: June 05, 2005, 04:36:38 PM »
well generally it seems like you could at least make switched gain in such a manner that it goes to unity rather than ~infinity :)

fum

trafoless microphone preamp wish list
« Reply #4 on: June 05, 2005, 04:53:49 PM »
I recently started down this path, and started with just a simple opamp design.

The parameters I am primarily after are:

variable loading ( may not be needed in the long run, but I wanted to build this in so that I can see what effect it may have on different sources)

Gain on the order of 70 dB.

Simple parts.

Low noise would be good. =)

The thought was to create a very simple design that meets these parameters, to compare against a swiss army knife mixer.

I got sidetracked by other work of course, but that's the beauty of this place, it reminds you that you still have work to complete =)

ju

bcarso

trafoless microphone preamp wish list
« Reply #5 on: June 05, 2005, 05:13:49 PM »
"well generally it seems like you could at least make switched gain in such a manner that it goes to unity rather than ~infinity :)"

Holes in the audio are better than dumping the control room monitor woofer, that's for sure ;-).

fum, what range of variable loading would you think would cover all the bases?

As far as simplicity, it is always a virtue if the same performance can be achieved, or at least very close to the same performance.  I like a simple signal path but often it's one supported by a lot of other stuff.  It gets a bit controversial as to what is in the path and what is not.  For example there are people who think components that shunt signals to ground rather than being in series with the signal are somehow not in the signal path, which I think is a trifle absurd.

fum

trafoless microphone preamp wish list
« Reply #6 on: June 05, 2005, 06:07:32 PM »
Quote from: "bcarso"

fum, what range of variable loading would you think would cover all the bases?


Well, I can't speak for all bases, but I've had ribbon mics on the brain a lot lately, so I was looking at varying more on the the higher end.  I think I'd planned somwhere between 600 ohm and 3K.

Anyone have opinions on how much lower/higher you might want to take into account for other purposes?

Regards

ju

rafafredd

trafoless microphone preamp wish list
« Reply #7 on: June 05, 2005, 07:10:53 PM »
I think 600 to 3k a good range for most mics...

[email protected]

Re: trafoless microphone preamp wish list
« Reply #8 on: June 05, 2005, 07:13:48 PM »
Quote from: "bcarso"
No use of unavailable parts, within reason.  Obselete but supposedly readily available NOS tubes considered but discouraged if production units will work. Low cost ;^o !


"Low cost" is relative. For a studio worthy transformerless preamp, I think $40 per channel is very reasonable, but some poor DIY-er in Guatemala might get heartburn.

I would say that if you can't get the parts at places like Digikey or Mouser you are using exotic parts.

I would exclude the use of reverse log potentiometers beacuse it is almost always an hassle to get them.

Cheers,
Tamas

[email protected]

trafoless microphone preamp wish list
« Reply #9 on: June 05, 2005, 07:16:32 PM »
And here is the most difficult criteria to satisfy: it has to sound good.  :green:  :thumb:


bcarso

trafoless microphone preamp wish list
« Reply #10 on: June 05, 2005, 08:15:10 PM »
If we knew how to do that...

And then there is the question: sounds good to whom?

But there is probably a little more consensus on what sounds bad at least.  I don't think even Peter Aczel would praise an amp with only 13th harmonic distortion, however low ;-)

Sir Rupert tells us that he went to great pains to reduce 7th at least, iirc.

Hiraga thinks everything should have a certain structure otoh.

If you sacrifice good transient response to get another movement to the right of the decimal point, once you're dealing with a bit of low order THD and small IM, clearly that's a poor tradeoff.

And then there's the thought that the presence of second makes the third easier to swallow.

(my 1000th post!---time for a drink)

BYacey

    Where dogs wear thermal underwear, Alberta, Canada
  • Posts: 769
trafoless microphone preamp wish list
« Reply #11 on: June 05, 2005, 08:27:21 PM »
Transformerless in the post Title and tube preamps in the text contradict each other; unless you're going to use crystal microphones with this preamp. After reading your text, I may be wrong, but it sounds like you want to power this preamp with phantom power??!!?
Bill Yacey
"Adjust R116 for least smoke"

bcarso

trafoless microphone preamp wish list
« Reply #12 on: June 05, 2005, 08:36:44 PM »
No---what I am trying to say is if a design could be phantom-powered it would be fine, but I DON'T want that restriction of phantom power.  And if tubes are used (definitely NOT with phantom power as the power source) they would probably be in conjunction with sand state.

If one wanted to do tubes directly it would be possible but not small or cheap for low noise with low output low Z mics---see that funny thread where PRR and others go back and forth on designs with a bunch of paralleled tubes---I forget the subject title.

analag

trafoless microphone preamp wish list
« Reply #13 on: June 05, 2005, 09:12:26 PM »
Schematic anyone? let's all get together collectively and build a mic pre, featuring the greatest minds we have here at our disposal. Is this wishful thinking?

Analag
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.

trafoless microphone preamp wish list
« Reply #14 on: June 05, 2005, 11:54:38 PM »
...
If you want to sound crazy please tell me about THE DEEP STATE.

fum

trafoless microphone preamp wish list
« Reply #15 on: June 06, 2005, 12:26:01 AM »
Here would be some additional requirements I'd place on the design:

I'd lean towards sandstate, as tubes, although lovely as hell, tend to take up lots of real estate.  Think a project whereby you could fit 4 channels in a 1RU case.

Power supply:  Preferably something that either runs single ended 24V, or at least not greater than +/-24V.  Reasoning here is ease of building power supply and sourcing a proper tranformer.

I'd love to help work on a group project of this type.  I'm probably not the one to do the design, as you guys have more chops in this area than I ( I'd be throwing out datasheet type opamp examples, rather than something based on years of design experience), but I'm certainly willing to contribute from a making it happen perspective.  So perhaps I'm stating that my skill are more applicable to The Lab board than the Drawing Board  
:grin:

My additonal contraints above are of course debatable, I'm just trying to bring more clarity to the type of project we're talking.  

ju

rafafredd

trafoless microphone preamp wish list
« Reply #16 on: June 06, 2005, 12:38:12 AM »
Quote
have you played with Jean's 'Le Monstre' circuit at all


Anyone gota a link?

fum


bcarso

trafoless microphone preamp wish list
« Reply #18 on: June 06, 2005, 12:46:08 AM »
Quote from: "Winston O'Boogie"
Quote from: "bcarso"
Hiraga thinks everything should have a certain structure


I have to say, I do like this approach myself.   Brad, have you played with Jean's 'Le Monstre' circuit at all?  I have a couple of small-signal line amp circuits I built that were 'inspired' by this. One is single-ended but has the same elements as Monstre - BJT cascoding J-Fet's (// array in this case) that is fed into the so called 'Darlingnot' - and it is sonically the most tube-like squalid-state circuit I've cobbled together thus far.  Harmonic Analysis is almost spot on that of a good triode valve.


That sounds interesting, and no, I'm not familiar with it.

Perhaps there would be a way to have one's paté and save the goose as well ;-)  We could have the harmonic structure variable.

fum

trafoless microphone preamp wish list
« Reply #19 on: June 06, 2005, 12:58:11 AM »
Quote from: "Winston O'Boogie"

I have a couple of small-signal line amp circuits I built that were 'inspired' by this. One is single-ended but has the same elements as Monstre - BJT cascoding J-Fet's (// array in this case) that is fed into the so called 'Darlingnot' - and it is sonically the most tube-like squalid-state circuit I've cobbled together thus far.  Harmonic Analysis is almost spot on that of a good triode valve.


Winston,

After reading a couple articles on this, the commentary seems to be concerned that matching of transistors is very important in this design.  Is that something you found to be the case, or was it written at a point when transistor consistency still left something to be desired?

If it is critical to the circuit, it becomes tougher for the average diy'er to get from here to disco.

Trying to make sure this fits into the "parts easy to source" category =)

Regards

ju


 

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