Lets talk opamps...questions.

[manning1]

given some of the new op amps in the past couple of years....what do the esteemed folks on here think is currently the best op amp for using in low imp mic pre front ends if one isnt using a transistor front end ??
(dont mention ...5534).
just curious on various comments....thanks.

[Tim Farrant]

I personally do not know of any opamp that can be used transformeless as a mic pre that will provide low enough noise. Most audio opamps are optimized for best noise performance with a relatively high source impedance, ie, 10k ohm give or take a few ohms. As microphones have a typical source impedance of 150 ohms, a transformer is required to make that 150 look like 10k (even for a 5534 - ooops - sorry).

What's needed is a opamp that runs high bias on the input transistors (which of course has it's problems with DC offset) and transistors that are fabricated to work with the low source impredance.

There are of course those devices with input buffering especially for mic pre applications, but there are mixed views on the sonics of these devices. Generally I suspect the slow slew rate of most of these devices is to blame for the poor sonics.

Tim.

[tINY]

If you want a low impedance mic pre-amp, you can make one using matched MF resistors and put an instrumentation-amp configuration to "probe" the resistors.

I don't know why you'd want a low-impedance topology for the op-amps.

That said, the TLE2027 looks interesting as does the LM833 and the AD8671 (though you'd need to watch out for the phase margin).



-tINY

[manning1]

sorry.....maybe i wasnt clear.
its the first stage after the 3 pin xlr input i'm interested in.
ie...rather than transistors....use an op amp.
and i was wondering if any of the newer op amps released in the past couple of years might be more suitable.
for example national is always updating their op amp line up,.... linear, analog and many others.
any comments on the use also of video op amps ??

tim....i have in fact seen a number of mic pre front ends sans transformer.
where the signal from the xlr input goes straight into a op amp as the first stage.
thats why i was confused and asked the original question.
input from someone such as yourself with such experience is highly valued.

if i remember maxim and linear have application examples on their sites.
if i remember the linear one uses a LT1115 op amp....and no transformer.
in the mag nuts and volts last year if i remember was an interesting mic pre schematic (sans transformer) using a TI op amp.

while we are on the subject.....and i bow to the many people on here who know way way more than i do on front end pre design.....if one DOES use transistor front ends...is it more advantageous to input thru the base or emitter of the transistor ??.....whats confusing me is ive seen both approaches used and would appreciate clarification on plusses of each.

tim/tiny .....i appreciate highly your input.....due to your knowledge/expertise which is way advanced than my own. i'm just a physics major whose worked in the computer industry. computers i KNOW .....but the rich world of analog electronics ive been learning for the past few years.
ive just been trying many different circuit topologies for the last 6 years for learning and fun and to use in my own little recording studio.
in fact heres a big joke on me......call it a learning experience.
after trying many fancy (expensive) op amps i started toodling around with transistors again......what i do to test a topology is i dont bother with scopes etc.
i record actual audio and listen to it back.
what i found with the fancy op amps was very pristine....but lacking "character".
then one night i got drunk.....rare for me....and went back to an old transistor circuit i had lying around ...and for laughs replaced the expensive transistors with
some trannys i found downtown....80 for a buck.....sort of thing.
well lo and behold i was amazed at the recorded track.....sounded superb.
ie...that elusive MAGIC character i'd been searching for for so long.
lots of character.....i also found lots of character with some zetex trannys i tried once.....how can this be ??
only trouble was in my stupor i ripped the circuit apart and forgot to document it. i havent LOL had better results since.....i'll never do that again.
i kick myself to this day....

[Jim Williams]

Mic pres need to have a noise spectrum that matches or is under the self noise of the mic. Most opamps will match up to a 1k source impedance ok, 150 ohms is tough. That would require an opamps's input to be 1 nv/hz sq or less. LT1028, LT1115, AD797 are in this 1 nv self noise range. Add 40 db of gain and it would be sort of dirty. This is why most pre's either use a step up transformer for extra gain or use low inpedance properly biased discrete transistors to match that 150 ohm noise spec. IC mic pre chips incorporate both the low noise opamp with a transistor pair in front. SSM2017, SSM2019, BBINA217 and a new one from That are in this catagory.

Jim Williams
Audio Upgrades

[manning1]

jim....
i keep on going back to transistors cos ive tried all the op amps.
i just seem to get a better result.

[rodabod]

I assume you have seen this before:

http://focus.ti.com/docs/prod/folders/print/ina103.html

[manning1]

rod....
thanks....yes i'm aware of the ina range...tried some awhile back.
trouble is i hate to be supplier/proprietary limited....in case discontinued in future and i need replacements.

[Helsing]

Anyone mess around with the new THAT 1510/1512 mic pre chips?

Jon Erickson

[GYMusic]

Some of the most fun I had in the early 70's, was building up an opamp mixer using Opamp Lab's 360 preamp modules. They're still in business and still making the 360. Great sounding and a lot of fun.

http://www.opamplabs.com

[Helsing]

Originally Posted by GYMusic

Some of the most fun I had in the early 70's, was building up an opamp mixer using Opamp Lab's 360 preamp modules. They're still in business and still making the 360. Great sounding and a lot of fun.

http://www.opamplabs.com

I second that. Its a really "70's" radio sort of sound. Very cool stuff down at OL.

JE