links to a couple of interesting mic pre schematics.

[manning1]

1. linear technology (the op amp manufacturer) have a pdf for the LT1115 op amp. a few pages into the pdf is a nice ultra low noise mic pre schematic. i subbed a op37 type op amp and found i like the sound a bit better for some strange reason.
2. the december issue of nuts and volts last year had a nice simple mic pre schematic using a TI op amp.
3. maxim semiconductors and analog devices have parts that look like they are aimed at microphone signal conditioning.
but they are new parts and not had time to check them out.
if you build circuits 1 or 2 mentioned above there is no volume control so what i do is take the output of the circuit into a line
mixer with volume controls that lets me control the level going to the sound card line input or into a comp/limiter.
hope you find these interesting.

[Tim Farrant]

Hi Manning, can you post some links to these schematics please?

Thanks
Tim.

[manning1]

linear.com i think the other is nutsandvolts.com
if i remember.

[DanV]

OT - Tim - check your pm's!

[Flatpicker]

Originally posted by The Buzz
Hi Manning, can you post some links to these schematics please?

LT1115
OP37
MAXIN AUDIO ICs

I checked the Dec 2003 issue of Nuts & Volts but didn’t see a preamp circuit.

[manning1]

flatpicker - i'm terribly sorry you didnt find it in the dec issue.
i'm getting old i guess!! i KNOW it was towards the end of last year.
try oct or nov issue. i KNOW it was one of them.
also if your interested in this crazy stuff i do ,
jensen transformers have some nice schematics as well.
analog.com have the i hope i got this right ssm2019
as a front end chip for a mic pre and thatcorp.com
have some schematics and chips for diy mic pre's.
hope this helps further.

[tINY]

I thought the SSM 2019 was obsoleted about 6 years ago...

Is Maxim making it again?


-tINY

[Flatpicker]

Originally posted by manning1
flatpicker - i'm terribly sorry you didnt find it in the dec issue...

Hey no problem, Manning. I didn’t mean to sound like I was complaining. I just thought I’d help you out and post the links. I checked some other issues as well, but again, no biggie.

I’m familiar with Jensen and the others, but I appreciate you mentioning the OP37 in your first post. I had forgotten about that one and your remark reminded me to give it a try!

Nice to make your acquaintance, btw. Seems we have a similar passion.

[manning1]

nice to talk to you as well flatpicker. just some tek stuff ive found that might save you some hassle. ...for mic pre's and other audio circuits.
having tried every op amp there is.
i keep on returning to the op37 for mic pre's.
different vendors op37's do sound a bit different. for some reason. for some reason as well - and i'm talking sound quality
here in particular the maxim max437 is one of the best i ever tried.
(they will send you a free sample if your nice to them).
also the motorola 33171 op amp seemed to work well in some app circuits (not mic pre's).
also what ive found is the lowest voltage noise op amps seem to loose "something" i cant put my finger on. very clinical i find.
one TIP is ive find through trying tons of op amps is .....
look at the current noise rather than the voltage noise.
ive found low curreent noise really in an op amp makes a big difference in the mic pre.
overall i prefer the op37 varieties to the old 5532.
what i do is i have a standard "mic front end pre circuit"
i can plug new op amps into. a test bed as it were.
the only downfall to the op37 is the "juice" they need in terms of ma's. which has given me a problem as ive been trying to design a portable mic pre. so i used high quality transistors for this.
using a 9 v battery. i would like to use a micropower op amp. but as yet have failed to find one i like. one of the best audio transistors ive found is a mpsa18. from motorola.
very sweet sound to it. also some of the nte newer low noise transistor parts. only draw about a milliamp so a 9 v batt lasts
quite a time.

[tINY]

Manning - What are your opinions of the LM833? I was using the quad version (LM834) for some audio-frequency measurement equipment and they seemed pretty accurate. Hard to tell how they sounded when frequencies of interest are 150 - 5kHz.

I did run into a few baffling stability problems in one or two sub-circuits, though.


-tINY

[manning1]

tiny - never had a chance to try 833. though ive heard its good. prolly the stability problems are caused by people not following spec and putting the right caps off/close to the supply pins. ive found this with other op amps that are sensitive to layout.
really tho i hear that funnily enough there is a new tiny smt series from national (forgotten the number) (not close to my op amps books). apparently good for audio circuits.

[Flatpicker]

Great info, Manning - thanks! I'm putting together a mic pre test board too. I'll definitely give those op-amps a try.

[tINY]

Well, I was using SSR's for gain switching and signal routing. Then I ran out of space and had to use a daughterboard piggybacked on the main board.....


There was definately a 1 MHz-ish oscilation, but it was small (about .3 p-p) and consistent from unit to unit. It didn't affect my measurements, so I stopped looking and went to fry bigger fish.


-tINY

[manning1]

flatpicker. this is difficult to get hold of.
but a few years back i got hold of a raytheon op amp book,
and in there hidden away is an interesting circuit.
it shows a jensen transformer(if you got the bucks try a LLUNDAHL as an alternative) handling the diff in from the 3 pin mic. then single ended into the op37. the reason this is IMPORTANT is the transformer gives one free gain. the ADVANTAGE being then one can lower the value of the feedback resistor around the op37(or another op amp if you wish).
thus lowering THERMAL NOISE significantly.
very important in mic pre's. (note ; use low esr caps and low noise metal film resistors).
my suspicion is the venerable sm57 (from my tests) does not seem to like coming into an op amp direct but through a transformer. this is why some people dont find it performs as well
using cheapie consumer mixers with no transformer inputs.
anyway - i'm afraid i lost the contact but get hold of someone in raytheon if you can (i think i called them up). and get a hold of their op amp book. its a nice circuit.
also you can use low noise siliconix type fets as front ends for mic pre's. (advantage - high input impedence).
analog devices (analog.com) have a big fat book showing all sorts of circuits and will send it - if you ask them nicely.
hope this helps.

tiny - by ssr - do you mean single supply rails ?
ive yet to do a successfull single supply mic pre unless using
transistors. did you bias half supply voltage ?

[tINY]

SSR meaning Solid state relay - a bilateral FET switch.

I think that the SM57 has a line-balancing x-former in it....

How much termal noise is there in a jensen transformer? It seems that the DCR is close to 1kOhm so you wouldn't be getting much thermal noise benefit over a low-Z transformerless design.


-tINY

[manning1]

re: SSR . tiny my mistake terribly sorry,
i wondered about that after leaving the forum.
re: thermal noise.
well. maybe its just me . a bit of history.
it seems to me the original 57 and its predecessors
were designed in an era when transormers were more widely used on mic pre inputs. i really dont know.
i can only go from my experience and listening and with friends that a 57 seems to LIKE a transformer front end instead of a transformerless front end. maybe i'm wrong.
but its certainly sensitive to loading and the circuit impedences it "looks into", you know the old "low into hi" imp thing.
or maybe i'm out to lunch and its the darn iron in the mic pre xformer that adds some mojo.
i'm always willing to learn if you have any more input on this.
in transformerless designs one thing i HAVE found is low current noise figures in the op amp seem to improve things.
some burr brown op amps have very very low current noise
figs if i remember. but the op37/varaieties does it for me.

[tINY]

There is certainly something apealing about X-former based mic pre-amps. It certainly improves CMRR and cross-talk performance over all but the most carefully designed direct coupled amplifiers.

I think there is some Mojo going on in the transformers. I suspect it is 2nd order harmonics from hysterisis. But, they don't cover that much in universities any more.


-tINY

[Flatpicker]

Originally posted by manning1
...it shows a jensen transformer(if you got the bucks try a LLUNDAHL as an alternative) handling the diff in from the 3 pin mic. then single ended into the op37...
...analog devices (analog.com) have a big fat book showing all sorts of circuits and will send it - if you ask them nicely.

Yes, I'm using Lundahls now and that is exactly the circuit I was going to use the OP37 in.

Hmmmm. Seems like I remember a book that AD was selling... Maybe I'm thinking about National Semi, though. I'll check on that - do you remember the name of the book?

Thanks again.

[manning1]

flatpicker you dont have to pay anything for the ADI
op amp book. it contains detailed specs on all their op amps together with thousands of applications examples including audio.
ive forgotten their tel number, but go to their web site.
its toll free in the good ole USA, at least it was last time i checked.
and in canada too. and if your nice to them they will even send
the occassional sample. i found them very helpfull.
just ask for the big fat op amp spec book with all the detailed data sheets. burr brown have a similar book.
i would be really interested to hear how those llundahls
work out for you with the op37 following.

tiny - i think your are smack dab correct. about transformers.
that hysteresis comment i find very interesting.
if either of you guys (we discussed this on another part of
gslutz) get something close to patsy clines wonderfull vocals on "after midnight" and many of her other songs.
tell me how you did it. i marvel at those 50's songs,
and sure can build a nice pre - but not with that texture
they obtained in the 50's. someone on GS suggested they used
altecs if i remember.