Help me Identify these circuits? :)

[danrhmg]

Hi guys and girls.

Apologies, i'm new to these parts and not sure if this is the right place to be.

A few months back I acquired a box of old audio gear that was hand made by a chap that has now sadly passed away.

I've been routing through the box and have found good use for most of the bits and bobs however there are these two boxes that I don't have a clue what they are for.

They are both very similar, one has 2 inputs and an output the other 1 input and 2 outputs. Both have 9v batteries a few risistors and other stuff (electronics isn't my strong point) One of them has an LF351 and the other an LF353 op amp.

Ive attached some pictures but does anyone have any idea what the hell they do!!??

Thanks
Dan

View attachment 66376
View attachment 66377

 

[ronv]

Can't tell for sure from the picture, but if it is an LF353 it might be a preamp.

 

[danrhmg]

A mic pre amp? In theory if I put an XLR input on it and plugged the output into a Digidesign AD would i get signal?

What pictures would you need to determine what the circuit is?

Thanks for the info!

Cheers!
Dan

 

[unclejed613]

actually we would need a schematic. if we had good pictures of both sides of the circuit board, many of us here could probably draw a pretty close approximation.

taking a wild guess from what i can see, i would say it's a preamp. if it were a fuzz box, it would normally have two pots, one for gain, and one for output level. this only has a pot for gain, and judging by the resistors on the board, and the 100k pot, probably a gain of 100 maximum. something about the flash spectrum and the paint used on the resistors makes it darn difficult to tell the difference between orange, red, and brown.

also, this is a vero-board, and the lines go horizontally from the chip outwards, just like on a plastic breadboard.

 

[danrhmg]

Thats fantastic, thank you so much for the info.

I will see if I could get some better pictures for you. If it is a mic pre what sort of mics would it support? Would i be able to add phantom to it somehow?

Cheers
Dan

 

[unclejed613]

iirc phantom power is usually something like 36 or 48 volts, so you're not going to get phantom power for a mic from a 9V battery easily or efficiently. you're getting into the realm of "the box needs a line powered supply" at that point. as is, with the rail splitter circuit (on the left side of the chip), you have +/-4.5V, barely enough for the op amp to operate properly to begin with. if your battery drains to 7.5 to 8 volts, it won't work so well.

 

[KJ6EAD]

It's possible that one circuit is a summing preamp such as one might use to combine the left and right channels of a stereo source in order to feed a monophonic power amplifier like a P.A. system.

The other circuit may be somewhat complementary to the first. It could be a distribution amplifier to convert a monophonic source into the two channels of a tape recorder with control of the recording level.

 

[unclejed613]

the left side is a "rail splitter" to convert 9V into +/-4.5V with a ground. the right side is a preamp, and i think it's fixed gain (at 10 or 100)and the pot is an input level control from the way it's wired. the jack on the right is the input jack, the middle one is the output jack, and the left one is who-knows-what??????????