Hi, i'm planning on building a custom passive -10 dBV to +4 dBU line level reference point transformer unit, and I have a couple of questions.
Let me explain the background first. I own a Tascam M35 mixer and a Tascam 38 1/2" reel to reel tape machine (both are -10 dBV). My plan is to use the board normally, and record 8 tracks which will go to the reel to reel Inputs by Direct Outs. Then, i would go out of the line level Outputs of the Tascam 38 into the Fosucrite Saffire 40 for A/D, which has non switchable +4 Line level inputs and Outputs. After that perhaps I would return that signal to the M35 mixer for monitoring (to avoid monitoring through a digital stereo out).
Now obviously the difference in level needs to be compensated, and I found a schematics for that kind of unit.
Here is the schematics with the use of 1:1 transformer (4:1 is not available in my vicinity): http://www.engineeringradio.us/blog/wp-content/uploads/2011/03/balun3.jpg
Here is the whole article: http://www.engineeringradio.us/blog/2011/03/unbalanced-to-balanced-audio/
My main question would be if someone could explain to me the schematics of the transformer, since I don't get how to connect it. Or if you could point me to the literature where I could find explanations on how to connect a transformer properly according to this schematics.
This is my transformer:
http://www.kelco.rs/katalog/images/17954_bg.jpg
My second question is: in this schematics they are using an XLR connector as a connection to the +4 side, however wouldn't it be better for it to be a TRS, so it could be plugged into a line input? What i mean is it better to plug into a microphone input to amplify the signal (because it's a passive unit) or into a line input? Or it doesn't make any difference?
Thank you for reading this extra long text, any help would be appreciated
Let me explain the background first. I own a Tascam M35 mixer and a Tascam 38 1/2" reel to reel tape machine (both are -10 dBV). My plan is to use the board normally, and record 8 tracks which will go to the reel to reel Inputs by Direct Outs. Then, i would go out of the line level Outputs of the Tascam 38 into the Fosucrite Saffire 40 for A/D, which has non switchable +4 Line level inputs and Outputs. After that perhaps I would return that signal to the M35 mixer for monitoring (to avoid monitoring through a digital stereo out).
Now obviously the difference in level needs to be compensated, and I found a schematics for that kind of unit.
Here is the schematics with the use of 1:1 transformer (4:1 is not available in my vicinity): http://www.engineeringradio.us/blog/wp-content/uploads/2011/03/balun3.jpg
Here is the whole article: http://www.engineeringradio.us/blog/2011/03/unbalanced-to-balanced-audio/
My main question would be if someone could explain to me the schematics of the transformer, since I don't get how to connect it. Or if you could point me to the literature where I could find explanations on how to connect a transformer properly according to this schematics.
This is my transformer:
http://www.kelco.rs/katalog/images/17954_bg.jpg
My second question is: in this schematics they are using an XLR connector as a connection to the +4 side, however wouldn't it be better for it to be a TRS, so it could be plugged into a line input? What i mean is it better to plug into a microphone input to amplify the signal (because it's a passive unit) or into a line input? Or it doesn't make any difference?
Thank you for reading this extra long text, any help would be appreciated
Not quite clear on the setup but you want to convert between -10dBV and +4dBu. Possibly both ways ?
But you won't change the signal level with a 1:1 transformer and you say you can't get a 4:1 type ?
On the connector: presumably the XLR shown is an output from the level changing 'box'. So you simply use an XLR to TRS cable if you want TRS at the other end.
Or you can build it with a TRS connector and use TRS/TRS cables. It's a choice depending on personal preference. At +4dBu level you'll almost cetainly need to use a line input unless your actual levels are lowish. assuming the mic input has a minimum gain of approx 10dB ?
But you won't change the signal level with a 1:1 transformer and you say you can't get a 4:1 type ?
On the connector: presumably the XLR shown is an output from the level changing 'box'. So you simply use an XLR to TRS cable if you want TRS at the other end.
Or you can build it with a TRS connector and use TRS/TRS cables. It's a choice depending on personal preference. At +4dBu level you'll almost cetainly need to use a line input unless your actual levels are lowish. assuming the mic input has a minimum gain of approx 10dB ?
Both ways, but i would build another unit based on the same schematics
Yup, i can only find 1:1 transformers.
I would like to use the TRS connector, to avoid buying additional cables, and my question was is it better to have it plugged into a line input or a mic input of the FF Saffire interface, since the signal drop will occur due to the passive circuitry.
So the signal will be low, and the minimun gain of the mic input of the saffire is +10 dB.
Yup, i can only find 1:1 transformers.
I would like to use the TRS connector, to avoid buying additional cables, and my question was is it better to have it plugged into a line input or a mic input of the FF Saffire interface, since the signal drop will occur due to the passive circuitry.
So the signal will be low, and the minimun gain of the mic input of the saffire is +10 dB.
Okay. If you have only 1:1 transformers then you can't change level by transformer action. In that case what is the transformer for ? Just Unbalanced to Balanced conversion ?
In that case then yes I think you could use the mic input on the Saffire to make up the gain. Bear in mind that it's peak levels which really count in terms of headroom. A '+4dBu' input doesn't tell you what the max input level is but it's usually around +21dBu or so.
( I thought you were talking about the mic/line input on the desk initially and that the signal level was '+4' out of the Saffire).
The passive circuity won't significantly attenuate the level itself assuming the Unbalanced signal has a reasonably low impedance and ignoring transformer losses.
In that case then yes I think you could use the mic input on the Saffire to make up the gain. Bear in mind that it's peak levels which really count in terms of headroom. A '+4dBu' input doesn't tell you what the max input level is but it's usually around +21dBu or so.
( I thought you were talking about the mic/line input on the desk initially and that the signal level was '+4' out of the Saffire).
The passive circuity won't significantly attenuate the level itself assuming the Unbalanced signal has a reasonably low impedance and ignoring transformer losses.
Good Luck. fwiw you may be better off thinking about a 'non-transformer' solution to balancing. Transformer costs for all the channels adds up !
In fact, you can try running unbalanced first but make sure to join R+S (of TRS cable) at unbalanced connector only.
That should give you some benefit from the balanced input. Depends on the detail of how the gear is designed - Pin1 problem etc...
If hum / buzz is a problem try breaking the screen (at the balanced TRS/XLR end if possible).
Depending on result you might want to put some combination of 100 Ohm Resistor / 100n capacitor in the screen connection in place of complete break eg. if hum / buzz goes away but you start to pickup the local radio station !
In fact, you can try running unbalanced first but make sure to join R+S (of TRS cable) at unbalanced connector only.
That should give you some benefit from the balanced input. Depends on the detail of how the gear is designed - Pin1 problem etc...
If hum / buzz is a problem try breaking the screen (at the balanced TRS/XLR end if possible).
Depending on result you might want to put some combination of 100 Ohm Resistor / 100n capacitor in the screen connection in place of complete break eg. if hum / buzz goes away but you start to pickup the local radio station !
Why?
The "-10dBv" TASCAMs will deliver 8V or +20dBu at any output into >10K loads.
The S'40 line inputs are not well specified. I would be sure they are >10K impedance. Being 'balanced', they should accept the TASCAM's unbalanced signal without problem.
However the S'40 also has "instrument" input, unbalanced, of high impedance. At min gain and with pad, they will accept +18dBu. Or +8dBu without pad. Which is 15dB headroom over the TASCAM's intended working level of -10dBV. All you need is an RCA-1/4" adapter.
I think you leave the S'40 Inst input at low gain, adjust mix levels so the S'40's meters are happy, and make your music.
And without the real audio compromises of little teeny transformers.
Coming back: I do not know if the S'40's "balanced" outputs work happy with unbalanced loads. I would hope so. Use clip-leads to try various combinations of 3-to-2 wires. It may be valid to ignore XLR pin 3.
Yeah, I actually tried going into the instrument input of the FF, and it sounded nice, however only the first two channels have instruments inputs, so the rest need to be line level or mic.
One of the solutions suggested to me was wiring the pin 3 of the XLR to the shield of the unbalanced connector, which supposedly can damage the equipment in some instances. That's why I would like to avoid messing with the cables.
And I'm too broke to buy something like this
Product: LA-80MKII | TASCAM
But i need the second model, LA-81MkII, too, since this unit is a one way converter. So I need to buy two of these
or this
Model 898B 8 Channel Balanced 8 Channel Unbalanced Line Level Translator
I like it since it is a one unit, and it costs the same as one of the Tascam's
One of the solutions suggested to me was wiring the pin 3 of the XLR to the shield of the unbalanced connector, which supposedly can damage the equipment in some instances. That's why I would like to avoid messing with the cables.
And I'm too broke to buy something like this
Product: LA-80MKII | TASCAM
But i need the second model, LA-81MkII, too, since this unit is a one way converter. So I need to buy two of these
or this
Model 898B 8 Channel Balanced 8 Channel Unbalanced Line Level Translator
I like it since it is a one unit, and it costs the same as one of the Tascam's
Pasted from the Focusrite S'40 manual. (Just Googled it):
See 2. and 3. accepts unbalanced jacks so should be no danger of pin 3 connection causing damage.
Try 'wiring' solutions first before spending on extra hardware boxes.
Pasted Detail:
Hardware
Back Panel
The Back Panel provides the majority of input and output connections on the Saffire PRO 40.
1. 6 Combo XLR input sockets for Mic / Line. – Each socket will accept an XLR, or 1/4 inch TRS (balanced) or TS (unbalanced)
Jack connector.
2. 8 TRS jack sockets for balanced outputs 3-10. – will accept either balanced or
unbalanced jacks.
3. 2 TRS jack sockets for balanced outputs main monitor mix. – will accept either balanced or unbalanced jacks.
4. 2 Optical ADAT input and output sockets - also available as additional S/PDIF optical i/o.
5. 2 IEEE 1394 6 pin Firewire sockets.
6. 2 Din5 MIDI input and output sockets.
7. 2 RCA S/PDIF input and output sockets.
8. 1 IEC Power inlet socket
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