How: adapting a line level signal for mic level input

[Farshad]

Hi everyone,

I need a box or circuit to adapt a line level signal to input a mic pre. Could it be a simple resistor like a potentiometer or this circuit actually exists? I also appreciate a DIY design.

Cheers,

-Farshad

[TonyBelmont]

Originally Posted by Farshad

Hi everyone,

I need a box or circuit to adapt a line level signal to input a mic pre. Could it be a simple resistor like a potentiometer or this circuit actually exists? I also appreciate a DIY design.

Cheers,

-Farshad

It's called a DI box!

http://www.countryman.com/html_data_sheets/t85data.html

[Kiwiburger]

Depends on what you mean by Line level. If you want to go from instrument or semipro unbalanced, an active DI box with variable output level should be fine.

Or are you already at +4 balanced pro line level? You basically need an attenuator. A reamp box would get you almost there - they are basically a transformer with some resistors, but with an unbalanced output.

Attenuators seem to be expensive for what they do. Especially if you want control over the amount of attenuation. You can use multi-tap transformers and rotary switches. Possibly a passive resistor device would work, at the expense of noise.

If you have them already, a reamp box into a DI should work.

Why do want to do this? Can your mic preamp not accept line level? Boosting a signal, then attenuating, then boosting again is not usually a great idea.

[Farshad]

Originally Posted by Kiwiburger

Depends on what you mean by Line level. If you want to go from instrument or semipro unbalanced, an active DI box with variable output level should be fine.

Or are you already at +4 balanced pro line level? You basically need an attenuator. A reamp box would get you almost there - they are basically a transformer with some resistors, but with an unbalanced output.

Attenuators seem to be expensive for what they do. Especially if you want control over the amount of attenuation. You can use multi-tap transformers and rotary switches. Possibly a passive resistor device would work, at the expense of noise.

If you have them already, a reamp box into a DI should work.

Why do want to do this? Can your mic preamp not accept line level? Boosting a signal, then attenuating, then boosting again is not usually a great idea.

Hi and thanks for the reply,

My line output is +4 from my soundcard. I am interested in a transformaer-based DIY circuit. Do you know of any design?

Thanks a bunch,

-Farshad

[tINY]

Look on Jensen Transformers web page. They have a lot of information there.



-tINY

[brian_delizza]

Sounds like you need a reamp box to me....

http://www.reamp.com/

[brianroth]

If going from a balanced +4 dBu to a typical mic preamp balanced input, all you need is a pad. My good friend Rick Chinn wrote this paper describing the design, and at the bottom of the page are the values for a 40 dB pad that properly matches impedances (bridging on the line source, around 150 Ohms to the mic preamp's input):

http://www.uneeda-audio.com/pads/

Bri

[Sean01]

Hi, just resurrecting this thread some, um, 4 years later.

I need to make an attenuation pad for a mono out line from a basic PA system to record to the mic in of a camcorder and a mini-disk recorder.

Making up the circuit etc is OK, but how should I (easily) determine whether the mono out line is a -10dB or a +4dB line? The unit is an old 'Redback' distributed by Altronics in Australia, they may be able to provide some assistance, but the unit is probably over 10 years old with no doco of course. It's one of those combined head unit/speaker and passive speaker combos. The mono line out is just a single RCA socket.

If it's a -10dB line, the consumer electronics standard, I can just use a 100 ohm and 10k resistor as a voltage divider, if it's +4 I need to use 33k apparently. I'm assuming it's -10dB for now for no real reason, just that it's likely intended to go to a consumer recording device. (Although the unit has a built-in tape recorder as well, remember 1/4 inch tapes?)

[Thumper]

If it's a standard RCA Jack, I'd bet good money on -10.

[Sean01]

Originally Posted by Thumper

If it's a standard RCA Jack, I'd bet good money on -10.

thanks for that. there's 3 bonus questions for the lucky answerer:

1) I don't know if there's a simple test I can do to measure the line level, or do you need to set up a CRO on input and output levels with a test signal to be absolutely sure, in the absence of a technical spec? e.g. gain could even be 0dB, who knows...

2) I'm thinking I'm just going to make a simple L-pad (K=100), another common reference on the web suggests using 10k/100 ohm values, but the Uneeda pages suggest 15k/150 ohm to try to match input and output impedances a bit better.

Apparently for a line out to mic in application 'the input impedance is more important' presumably meaning it should match as closely as possible, so I should choose R1 and R2 so that R1 is ~ the output impedance of the PA?

3) How can I measure input and output impedances on the gear? Can I just stick an ohmeter across the mic terminals of the camcorder, and similarly for the output of the PA RCA jack?

Pretend that I'm a bit rusty on this stuff not having studied it for 20 years, but have a bunch of gear available at work in the tech-elec's lab...

[Sean01]

Ground loop problem with an L-pad?

Hi all,

Not sure if this thread should be in this forum or another forum like extremely low cost theory, but just a continuation on L-Pads:

I just built an L-Pad as discussed above and put it inline on a line level output cable which is then split to 2 mic level stereo inputs (L+R joined together), one going to a JVC HDD camcorder mic input, and the other to a Sony mini-disk recorder mic input. The Sony minidisk has both line level and mic level inputs available, the camcorder only has mic level. Hence, I decided to take the single line and split between two devices at mic level. (Apparently that's OK to do.)

The problem is that now the mic in recording on the camcorder is doing really good with the L-pad attenuated voltage, but the minidisk recorder is now recording a low hum all of the time on its mic input -- not sure if this is a ground loop effect caused by the L-Pad -- but doesn't seem to be a problem on the camcorder recorded signal. (Why?) The minidisk did fine before with its line level input and no L-Pad, but to save hassle with cables I'm now trying to use mic level on both. The PA is some 12m away on a different power point from the 2 recording devices, in case that is the likely cause of ground loop effects.

Some designs call for a capacitor to be placed in the line also to 'block DC' -- I'm not sure if that's the problem here, or whether it's a ground loop effect where the ground level on the PA is different from the other devices etc.

A commonly repeated design on the web appears at, for instance, Line level signal to microphone input adapter which is what I've done -- -40dB atten circuit. The author points to possible hum problems and suggests a further ground isolating circuit as a solution.

Fixes that occur to me are:

1) just insert the L-pad on 1 line only out of 2 after the split -- send a mic level signal to the camcorder, and a line level to the minidisk, as these both seem to work without problems -- will this work or will the 2 devices somehow interact? I can easily cut my existing dual cable on one plug and insert my L-Pad on it inline, which then lets the cable be used for line/line or mic/line as options by just plugging in the L-Pad. This would be my preferred option for elegance at present.

2) use this capacitor thingummy as a DC uncoupler on top of the basic voltage divider, but it may not resolve the problem. thoughts?

3) use a ground isolating circuit which is an isolating 1:1 transformer or something by the looks, only $20 or so, but yet another device on the line -- is this really necessary or the best option?

The line so far looks like circuit 1 on the web link above, where the mono mic level output then gets split to 2 mono lines each terminating in a 3.5mm stereo plug with L+R joined together on each.

I'm doing this on the cheap for a community organisation without much money, so far I've built these custom cables for under $20 total. Each week there's a new wrinkle on this problem which I have to try to solve for the next week...

thanks in advance for any help