Dual Opamp MicPre

I kludged this thing together to make recordings with two differnt inputs, right gets a Markeley acoustic piezo guitar pickup, the left gets an SM-57.

I want to record sounds of falling objects down a mine shaft that has a steel ladder all the way down the side, which generates cool sounds when boulders bounce off on their way down.
The piezo gets bolted to the steel ladder, the 57 dangles by a 30 foot cord down the shaft.

This is a rough draft, I can tweak gain resistors, so don't worry about that.
This needs to be done in a hurry, and nothing fancy as far as fidelity.
I am going to run the outputs into L and R line inputs on a ghetto blaster tape recorder.

1) How do I bias that transformer input opamp for gain?
2) Do I need a cap between opamps.

I think thats really a 4324a quad version of the famous single version. that (no longer made)


Thanks!

cj

[quote author="CJ"]
2) Do I need a cap between opamps.[/quote]

If so, could someone be kind enough to explain why? Would be good to understand what a cap in that position would be doing to better come up with strategy in chosing the cap type.

dave

Let us know what sound is made, when a bolder bounces off a SM-57.

"Deep Listening Band"

=FB=

I believe the cap would be needed so as to not amplify DC offset with the second amp. if you had a 0.1V offset on the output and your gain was 20dB (voltage gain = 10) then wouldn't you get a volt of offset at the output and the reduced headroom that the DC volt would eat up? Also, I may be mistaken, but I usually think I see large value resistors shunting the input of non-inverting stages so the input bias current has somewhere to go?

Take what I say with a grain of salt: stupid school teaches me loads of math I hate and skimps on circuits which I love. Wish I'd know that 4 years ago...

Ian

I believe that the mic pre is not going to work as shown. For inspiration you can check a schematic I posted a few month back: Shared_Gain_PreAmp_r1.gif

Very simple but excellent performance.

Samuel

OPA4334 gets one hit on google (a good thing for that game people were playing for a while) and it's not about an opamp. Are you sure of that number? Could you intend the OPA4134?

Samuel's circuit is a good one. For your piezo you could even make the input resistor R1 a bit larger. OPA4134 would work for the four amps as well.

Opamp x3 could use some feedback, and the 1M pot in the sm57 channel should be tied to -input of x4, might just be a drawing error though...
Considering the HUGE amount of gain (as it is now) in the piezo channel, i would put a dc blocking capacitor in there.

x1 non inverting input needs a resistor to ground. did anyone mention that already?

i might be mistaken but i believe a piezo pickup will not provide the dc path for that input.

input offsets should be small enough that you dont need cap between stages... with only two stages... but cap at output = definitely a good idea.

CJ

I did not find the opamp spec but if it is a "regular" opamp maybe the following might be of use.

what do you want to use for a power supply? single voltage like say 12VDC?

If so make a divider with say two 10Ks in series + to ground and a 220uf (or what ever you got)cap from the center node to ground. connect the end of the transformer to this node and say a 10meg resistor from the + input of X1 to this same node, that should bias the rest of the circuit up.

Cap couple to ground R2 R4 R6

The X3 part you might want to connect the - input to the output pin for a buffer setup.

edit Depending on the input currents and the 10meg for the piezo you might want to connect R4 to the "center" node.

Sorry!
4134, yes.
Thanks to all, I will compile all the info, do a new schemo and we can knock it off from there I bet.
cj

Edited

Gus, plus and minus from two 9 volts or better, if ma draw is too high.

What kind of piezo are you using, a guitar-bridge thingy?

[quote author="peterc"]

I believe that the mic pre is not going to work as shown. For inspiration you can check a schematic I posted a few month back: Shared_Gain_PreAmp_r1.gif

Click to expand...

I cant open the link, is anyone else also having this problem?

Thanks
Peter[/quote]

Opened fine for me Peter. :thumb:

OK, thanks again, and here is the latest.
I have a lot better feeling about this one.
Only change was temporary increase in gain pot, if too
much, will work my way back down to 22K.

How does the trasansformer hookup look?

Should I do differential input instead?

I increased input to 1 meg per Brad.


Also, I am using battereies since this is for field recording.
This means I can drop the minus signal out protection cap?

This could make a cool setup, after I am done at the hole, for a
one man acoustical band. Take your acoustic guitar and ghetto
blaster down to the record store and plug in.
Acoustic uses the piezo, vocals or second mic on the soundhole for the 57 input.
Or stuff it inside a small power amp.
Thanks!

Looks good. :thumb:
Whats the reason for R14?

Thats what I want to know.
I just stole that out of the Engineers mini opamp book.
I was hopin you could tell me!

The original has the signal going into the minus input of the opamp.

cj

> Whats the reason for R14?

Boulder hits SM57.
SM57 makes 3V peak signal.
Transformer steps this up to 30V.
Chip lives on 9V.
Chips sees input 21V outside its supply rail.
Normally reverse-biased chip junctions become forward-bias.
Forward-bias junctions attempt to eat all the current they can get.
Chip fries.
Game over.

With the 10K, current in this made-up scenario can only go to 21V/10K= 2mA, which is generally safe.

It isn't boulder proof. CJ can find a bigger boulder and aim better.

But it is more boulder resistant than no resistor at all.

And the effect on noise, in series with the stepped-up 30K mike impedance, is negligible.


I think this design is way more than you need. Going to a portable tape deck, you only need 2V signal and 300uV noise. The maximum gain you could need on the SM57 is 1,000. Assuming 1:10 transformer, gain of 100 in the amp. Your requirement for musical fidelity is essentially zero. You could work a TL071 at gain of 100. Doing A/B studio checks, you'd hear a difference between live and hi-gain TL071. With chums dropping boulders down the hole, you won't survive the "live in-studio" audition, so nobody can prove your TL071 was not perfectly faithful.

I'm not sure how much gain the piezo could need. Maybe a little more.

Gain requirement could be a lot less. What you really want is fast-acting AGC, to drop the gain fast on the first BANG, and recover slowly as the boulder falls out of hearing. But you need it quick and AGC is not a hasty design chore.

I vote for TL072, 2x9V battry, 100K audio-taper pot for feedback, 1K resistor from "-" to ground. Wire the pot as a rheostat: 100K at one end, 10K in the middle, zero at the other end.

Guys;

Quit knocking CJ.
The sound it makes is very cool.
I have heard that mine shaft before!
I will find the link to the Canada silo!

Notice that the 57 still puts out after getting hit. :thumb:
Spoken from experience, I can tell.

I am mostly interested in picking up the faint signals down below, so maybe a zener limiter would be the ticket. After the first few hits, the signal drops quite a bit, due to the porous walls of the tunnel. But when they hit the steel ladder, you can pick it up from 3000 feet down.

Well, I already pulled the kit, so I better stert with this one.
I can always scale down the gain with a quick resistor change.

I wonder if the Piezo will protest the metal ladder as a mounting surface?
Should I mount it on a small piece of wood and bolt that to the ladder?
Thinking the metal will mess up the capacitance or something.
I will try and see on something metal around here.

Well, I just pulled the kit for this job, so I think I will solder up Mr. Groner's circuit and plug it in.


http://vacuumbrain.com/The_Lab/TA/The_Hole!/dual_pre.jpg