external mic pre into an m-308 mixer

[j_u__s___t__i_n]

im not shure where i should plug my mic pre into my mixer (tascam m-308) in order to get the best results? should i plug it into the xlr mic input, and just turn the trim down? should i go into the line in? my mic pre has a balanced out, as far as i know the mixer's line ins are unbalanced. if i got one of those impedance converters and went into the line in would that be better than going into the mic input?

thanks

-justin

[Beck]

What model mic pre?

You should normally use the line-in or bypass the mixer altogether and go direct from pre to recorder. Otherwise you'll be going through the M-308 mic pre as well.

[j_u__s___t__i_n]

i'm very lucky to have a blue robbie mic pre, and a trident s80 producer box.

im recording onto a tascam 38 that has rca ins and outs, which means it's unbalanced right? so the best way would be to use an impedance converter to go to the line in of the mixer, or better yet strait to the recorder. am i right?

thanks for your help.

-justin

[Beck]

It’s very easy to make a balanced XLR-to-Unbalanced RCA adaptor. All you need are:

- Female XLR connector
- Male RCA connector,
- ¼ watt 4.7k ohm and ¼ watt 1.5k ohm metal film resistors
- Short length 2-conductor, shielded cable

All the parts are available at Radio Shack. This takes balanced +4 dBm (Pin 2 hot) to unbalanced –10 dBv RCA. You can put the resistors inside the XLR connector. Excuse my sloppy drawing.

The Robbie is pin 2 hot. I don’t know about the Trident. The adaptor will work with the line-in of the M-308 or an input of the 38.

[cjacek]

Originally Posted by Beck

This takes balanced +4 dBm (Pin 2 hot) to unbalanced –10 dBv RCA. You can put the resistors inside the XLR connector.

Excuse my total ignorance on this subject, Tim, but wouldn't the above also solve problems connecting a -10db mixer to a +4db open reel recorder, without the use of potentially expensive -10/+4dB converter boxes?

[Dr ZEE]

Originally Posted by cjacek

... wouldn't the above also solve problems connecting a -10db mixer to a +4db open reel recorder, ...

A taker takes. He does not give (Even if you spell his name backward)
The answer is: "No"

[Beck]

All you have to do to make a connection is reverse the above method, but minus the resistors. However, to go from -10 dB unbalanced to +4 dB balanced can be a little more complicated because you have to add 12 dB of gain somehow. Depending on the mixer or deck, that could be as simple as adjusting input or output controls, but you may have to build an active circuit.

So on a mixer with a balanced +4 dB nominal balanced input the -10 dB nominal output of something like the TASCAM 38 would look rather anemic… but you could raise it high enough with the first stage Trim Control. Again it depends on the console… some have mucho gain and some are marginal.

If you have enough gain all you have to do is connect the center conductor of the RCA connector (or ¼”… whatever your system uses) to pin 2 of the XLR connector, and then ground pin 3 to the shield, which connects to the outer conductor of the RCA. This assuming that pin 2 is hot for the device in question.

You can also build an active circuit around a single op-amp to make up for the 12 dB difference between -10 dBv and +4 dBm. I have some DIY circuits around here somewhere… if I can find the links I’ll post them in the next couple days.

You can also buy ready-made XLR to RCA cables or adaptors from HOSA and others, but I usually make my own cables. They don’t have the internal resistors, but many consoles have a pad that you can use to attenuate hot signals coming in.

[Dr ZEE]

Originally Posted by Beck

..some DIY...

Originally Posted by Beck

..in the next couple days

for that time. ...here is a couple of devices to "investigate":
http://www.thatcorp.com/datashts/1600data.pdf
http://www.analog.com/UploadedFiles/...ts/SSM2142.pdf
*********
at eBay:
http://cgi.ebay.com/THAT1646-Bal-Out...mZ120179445595, that is for 5 buck a line plus your body-shop labour & material, psu parts, connectors, wires and what not

[j_u__s___t__i_n]

thanks for the help guys. i work at a music store, so i went ahead and picked up an Ebtech 2-Channel Line Level Shifter, and an Audio-Technica CP8201 In-Line Transformer, and spent a late night hooking every thing up in every possible way. i got the best results using neither of them. the Ebtech just made it a bit quieter and added a faint hum which didn't seem to help, and the A/T boosted the signal a lot, i think too much. i wonder if thats the way it's supposed to be and i'm just not use to my mic pres having so much gain?

-justin

[The Ghost of FM]

If it were me, I'd just plug the pre's into the mixer's balanced Mic input and turn the trim all the way down which will effectively and quietly become a balanced +4 db input and then all the issues of making or buying level adapters is negated.

Cheers!

[Dr ZEE]

Originally Posted by j_u__s___t__i_n

...hooking every thing up in every possible way.

Justin, there should be only one way to hook it up and I would imagine it should work well. Looked into Ebtech manual and it may be a bit confusing. If I've got it right , you must make sure that you use correct cables. I've made a picture (attached) rather than typing words in. If I'm not mistaken , that's the way you should do it.
The AT gadget is cool but not applicable in your situation
/later

[Beck]

Originally Posted by j_u__s___t__i_n

thanks for the help guys. i work at a music store, so i went ahead and picked up an Ebtech 2-Channel Line Level Shifter, and an Audio-Technica CP8201 In-Line Transformer, and spent a late night hooking every thing up in every possible way. i got the best results using neither of them. the Ebtech just made it a bit quieter and added a faint hum which didn't seem to help, and the A/T boosted the signal a lot, i think too much. i wonder if thats the way it's supposed to be and i'm just not use to my mic pres having so much gain?

-justin

Nice graphic from ZEE.

Yeah, get rid of the Audio-Technica CP8201... it's not the right adaptor to go from a pre. If you want to use the Ebtech all you need is an XLR-to-TRS balanced cable like this to go from the Robbie to the +4 input of the Ebtech .

http://www.buy.com/prod/stereo-phone...203463478.html

Then just use a standard 1/4"-to-RCA cable to go from the -10 dB output of the Ebtech to the line-in, or Tape-in of the M-308.

When going to the trouble of an external mic pre you should normally bypass as many stages as possible on the mixer.

You could go through a XLR mic input of the M-308, but you’ll be stacking mic pres, so to speak, because the mic input on the mixer is a pre. Using a Line-in or Tape-in on the mixer bypasses the mic preamp circuitry. You can bypass even more clutter by plugging into the Insert point, but you won’t have use of the EQ on the channel because the Insert Receive comes in after the EQ, but before the main fader.

Finally, since the Robbie has its own output level control you can bypass the entire mixer and go from the Ebtech -10 dB out to your tape deck with the same cable in the above link. In many cases you can eliminate the need for active EQ through proper mic selection and placement. IMO, on-board EQ is a last resort for micing and should be used judiciously if needed.

You may even try plugging your mic directly into the mic input of the M-308 without the Robbie or anything else. The on-board pres are pretty decent.

[j_u__s___t__i_n]

very clear diagram, thanks dr. zee.
i did some more testing. i switched back and forth between: (1) what is shone in the diagram (except i'm going into a patchbay which is then going to the recorder) and (2) going strait from the mic pre (using the xlr to trs cable) into the patchbay to the recorder.
using the first method i noticed a decrease in volume which was expected since it's going from +4 to -10 and also a slight hum which i think is coming from the ebtech because i can hear it when i turn the mic pre down. and also when i tap or slightly shake the mic (a rode nt1 and a blueberry) it seems to distort where as using the second method it just makes a clicking sound but doesn't realy distort. it might be my imagination but when i talk into the mic using method 1 it might have a little more low end.
conclusion, going from +4 to -10 doesn't seem to be a good thing in this situation since it isn't clipping without it, in fact it seems to be the opposite when i tap the mic. and when using the ebtech there is a hum, althought very slight it is a hum. so going from the mic pre directly into the recorder seems to be the best way so far.

the only question i still have is:
from what i understand a balanced cable (xlr or trs) splits the signal coming from (lets say my mic pre) in two, reverses the phase of one signal and then changes it back which cancels out any noise picked up. and if you go from a balanced signal to an unbalanced input (my mic pre xlr-trs to my patchbay then ts-rca to my recorder) it only takes one of the split signals witch would be a significant drop in volume. do i have this right, or am i completely wrong? and would there be any difference in sound or just volume?

you guys have been very helpful.
thank you,
-justin

[Beck]

Sounds like there could be a problem with the cable or the Ebtech converter. Going from +4 to -10 is normally pretty straightforward with properly functioning devices. One thing about the hum… if you can open the XLR connector and separate pin 3 from the shield it may eliminate it. Unfortunately there are competing standards and different ways manufacturers ground their products, so experimentation is often necessary. Also make sure your cable is wired pin-2 hot. Some are wired pin-3 hot, which will throw everything off.

And yes taking just the hot pin (in this case pin 2) from a balanced device will result in around 3 dB of loss compared to the balanced method because the 180 degree out of phase signal is not restored and combined… but this can be a good thing when converting down to semi-pro equipment with -10 dB interfaces.

Being that the Blue Robbie has an output level control the way you’ve managed it can work just fine. One concern is that you don’t exceed the rated headroom of the recorder or mixer input (that's why I use pads like in my drawing or a pad on the console). So it’s all about gain-structure… making sure the output from your pre is not too hot, but not too low either. Since it’s made to interface with balanced +4 equipment I’m sure the slightest nudge of the output level knob will go farther than you might think from the perspective of the -10 dB interface that's receiving it.

As for difference in sound between balanced and unbalanced it’s negligible, but every device in your signal path is going to impact the character of the sound.

[cjacek]

Excellent info you guys! (... and thanks, Tim, for your reply ). I was 'bout to ask about the ebtech level shifter thingie and I'm glad it was brought up and discussed. Great graphic by Dr. Zee indeed! It's nice 'cause I'm a visual sorta person.

BTW, I was kinda hopin' for a nice sound out of the ebtech product, 'cause it looks real good, but I'm kinda disappointed about justin's experiment, the hum and such... I hope it's not the product.

[SteveM]

I'm using a UA solo 610. Going direct in and out on the line in on the M-35 yields great results, also gives me use of the inserts for other things and the eq section. After reading Tim's cable instructions I decided maybe I'd make a cable and try going direct into the back of the 38 . BUT, I went into a well know music store today the "kid" working there says that an xlr to rca cable is already a +4 to -10dbv cable, without the need of resistors. Is this false?

[Dr ZEE]

Originally Posted by SteveM

...says: "an xlr to rca cable is already a +4 to -10dbv cable"

People say things all the time
In the form of typing here's my 'saying':
A cable is a cable. A cable connects, and does nothing else (well, ideally, of course )
A cable with built in attenuator may be called as "+4 to -10 cable".
Here's a nice short article from ESP:
Voltage Dividers & Attenuators
and the GADGET to play with

[Beck]

He was mistaken if he said any XLR-to-RCA cable turns +4 dBm into –10 dBv. There are adaptors and cables out there that pad the level down, but they are clearly marked. The most common cables simply solve the interconnection issue, but do nothing about levels.

[Dr ZEE]

btw, speaking of chasing hum. Don't forget that metal racks and rack ears got a devil in it. If your setup is complex with racks, patchbays etc then trouble is everywhere. Connecting everything and keeping it that way all the time whether or not you are using all the gear at the time is not very good idea (arguably so, of course). But it sure looks and feels cool
At home studio one have a Privilege to keep them short and plug only into things what are in use.

[SteveM]

Thanks for that link Mike.

Originally Posted by Beck

He was mistaken if he said any XLR-to-RCA cable turns +4 dBm into –10 dBv. There are adaptors and cables out there that pad the level down, but they are clearly marked. The most common cables simply solve the interconnection issue, but do nothing about levels.

Tim, I guess for some reason he thought if that the conversion from balanced to unbalanced made it -10. I knew it couldn't have been that simple!

[SteveM]

Cable works great Tim!

[Beck]

Originally Posted by SteveM

Cable works great Tim!

Awesome!