[dokebi]

I was wondering if you can use a cmoy as a microphone preamp for recording with an MD player. If so, what gain would be appropriate?

 

[CaptBubba]

You would need to modify it slightly to charge the microphone element. Basically you need a certain amount of voltage across the microphone for it to work properly. I'm not sure of the specifics, however.

 

[rickcr42]

the short answer is yes.

all the Cmoy amp is :

a simple non-inverting op-amp gain stage with an output current high enough to drive headphones

nothing more.

A ll a mic preamp is (assuming non-balanced operation) :

a simple non-inverting op-amp gain stage

high output current being of no importance unless you are driving long cables or a difficult load.


So what you need to build is a simple battery box / mic preamp.

The microphones need a bias voltage in order to operate and this is usually provided at the mic input of an MD deck.

If you build the external device you will now be using the line input or you will overload the mic in and increase the noise level

Build :

the mic bias supply consists of a 10K resistor from EACH mic element to the +9V battery terminal and a 1uF DC blocking capacitor/LP filter to the preamp input .

this is essential or you will be passing 9 volts directly into the op-amp input !

Build a straight X10 op-amp gainstage (cmoy) with a single +9V supply and either a passive RC artificial ground/ dual polarity supply circuit or use a TLE as the network

Plug it into the MD line input ,adjust the gain control and

HAVE AT IT !

rickamundo

 

[tangent]

You might need a gain of more than 10x, depending on the type of capsule you get. If you need to go much higher than 20x, be sure you get a low-noise op-amp. The AD797 is perfect for this purpose. You may have to go as high as 1000x with some setups. Try 10x first to be conservative, though, and raise it until you get a decent line level output from the loudest sound you'll ever record. Remember, if the amp clips, you can't recover the information. Much better to have too little gain than too much.

 

[rickcr42]

Quote:

be sure you get a low-noise op-amp. The AD797

sorry man but that is not a good idea.The 797 will not function well off a single 9 volt supply AND is overkill for an MD player.there simply is no need for a proffesional level chip in this application.

for a good low noise chip that will opeate well on -4.5/+4.5 volts the opa2134/2 or OP275 are fine.

The gain setting is arbitrary and can be adjusted to the application