[Andre Majorel]

My search engine skills must suck. I've been looking without any
success for the schematics of a DI box. The goal is to plug an
electric guitar or bass into a line input. The specs would be :
- one high impedance input (couple megohms),
- one unbalanced line output,
- unity gain,
- +/-15 V supply prefered, 9 V supply OK.
- preferably only jelly bean components. In particular, no
transformers. :-)

Would a TL072 voltage follower with a 5 M resistor to ground on
the non-inverting input do ? Or is there more to it than that ?

Thanks for your help.

--
André Majorel URL:http://www.teaser.fr/~amajorel/
"Cette supposition rappelle assez celle de ce prédicateur qui, en
pleine chaire, faisait remarquer à ses fidèles la bonté de Dieu qui
avait placé les rivières auprès des villes." -- Alexandre Dumas

[Serge Auckland[_2_]]

"Andre Majorel" wrote in message
...
My search engine skills must suck. I've been looking without any
success for the schematics of a DI box. The goal is to plug an
electric guitar or bass into a line input. The specs would be :
- one high impedance input (couple megohms),
- one unbalanced line output,
- unity gain,
- +/-15 V supply prefered, 9 V supply OK.
- preferably only jelly bean components. In particular, no
transformers. :-)

Would a TL072 voltage follower with a 5 M resistor to ground on
the non-inverting input do ? Or is there more to it than that ?

Thanks for your help.

--
André Majorel URL:http://www.teaser.fr/~amajorel/
"Cette supposition rappelle assez celle de ce prédicateur qui, en
pleine chaire, faisait remarquer à ses fidèles la bonté de Dieu qui
avait placé les rivières auprès des villes." -- Alexandre Dumas

A simple voltage follower as you describe would certainly work, but I have
to ask:- Why no transformers, why an unbalanced output, and why such a high
input impedance?

If you're taking the cable any distance, especially in an environment where
there are lights and lighting controllers, I would much prefer a balanced
connection. If so, then transformers have a great deal going for them,
mainly in providing full galvanic isolation between the instrument and the
rest of the world. A couple of megohms input impedance also seems very high,
is there a reason for it?

S.



--
http://audiopages.googlepages.com

[GregS[_3_]]

In article , Andre Majorel wrote:
My search engine skills must suck. I've been looking without any
success for the schematics of a DI box. The goal is to plug an
electric guitar or bass into a line input. The specs would be :
- one high impedance input (couple megohms),
- one unbalanced line output,
- unity gain,
- +/-15 V supply prefered, 9 V supply OK.
- preferably only jelly bean components. In particular, no
transformers. :-)

Would a TL072 voltage follower with a 5 M resistor to ground on
the non-inverting input do ? Or is there more to it than that ?

Thanks for your help.


Your going to have much more than 2 M but thats good. I would rather
try an inverting design for stability and protection. Two 10 Megs
on the inverting side. Unity gain is not going to work. Its going to be easier getting
gain with a non inverting amp. You might be able to modify my basic mic amp.
http://www.pitt.edu/~szekeres/sub/mica.gif

greg

[Eeyore]

Andre Majorel wrote:

My search engine skills must suck. I've been looking without any
success for the schematics of a DI box. The goal is to plug an
electric guitar or bass into a line input. The specs would be :
- one high impedance input (couple megohms),
- one unbalanced line output,
- unity gain,
- +/-15 V supply prefered, 9 V supply OK.
- preferably only jelly bean components. In particular, no
transformers. :-)

Would a TL072 voltage follower with a 5 M resistor to ground on
the non-inverting input do ? Or is there more to it than that ?

A TL071 might be more suitable ! ;~)

I wouldn't make the loading as high a 5M though. 1M ought to be more
than enough. Basically, you're on the right track however. Don't forget
a resistor in series with the output of around 150 ohms though. The
TL07x series tend to like to oscillate when driving capacitive loads
(i.e any length of screened cable) in voltage follower mode
particularly.

Graham

[Eeyore]

GregS wrote:

I would rather try an inverting design for stability and protection.

That will be noisy as hell.

Non-inverting is far and away the best method here.

Graham

[Mr.T]

"Andre Majorel" wrote in message
...
My search engine skills must suck. I've been looking without any
success for the schematics of a DI box. The goal is to plug an
electric guitar or bass into a line input. The specs would be :
- one high impedance input (couple megohms),
- one unbalanced line output,
- unity gain,
- +/-15 V supply prefered, 9 V supply OK.
- preferably only jelly bean components. In particular, no
transformers. :-)

Would a TL072 voltage follower with a 5 M resistor to ground on
the non-inverting input do ? Or is there more to it than that ?


Here's a tip, a Behringer DI100 is nearly as cheap as you can buy the parts
for, even assuming your time is worth nothing.
The performance is quite good, that's why they sell millions of them.
If you really don't want a transformer, the DI20 is even cheaper and has
dual I/O. However it is better used for a keyboard than a guitar IMO.
Both will work on 9V battery or 48V phantom power. Both have balanced output
which can easily be adapted to unbalanced if you really must.

MrT.

[Andre Majorel]

On 2008-05-21, Eeyore wrote:
Andre Majorel wrote:

My search engine skills must suck. I've been looking without any
success for the schematics of a DI box. The goal is to plug an
electric guitar or bass into a line input.

Would a TL072 voltage follower with a 5 M resistor to ground on
the non-inverting input do ? Or is there more to it than that ?

A TL071 might be more suitable ! ;~)

I wouldn't make the loading as high a 5M though. 1M ought to be more
than enough. Basically, you're on the right track however. Don't forget
a resistor in series with the output of around 150 ohms though. The
TL07x series tend to like to oscillate when driving capacitive loads
(i.e any length of screened cable) in voltage follower mode
particularly.

Thanks. I used a 3 M to ground and a 220 ohms in series with the
output. Left the offset nulling pins floating and didn't even
bother decoupling the op amp. It's night and day compared to
going straight into the mixer. Not bad for 0.4 EUR worth of
components.

--
André Majorel URL:http://www.teaser.fr/~amajorel/
"Cette supposition rappelle assez celle de ce prédicateur qui, en
pleine chaire, faisait remarquer à ses fidèles la bonté de Dieu qui
avait placé les rivières auprès des villes." -- Alexandre Dumas

[Eeyore]

Andre Majorel wrote:

Eeyore wrote:
Andre Majorel wrote:

My search engine skills must suck. I've been looking without any
success for the schematics of a DI box. The goal is to plug an
electric guitar or bass into a line input.

Would a TL072 voltage follower with a 5 M resistor to ground on
the non-inverting input do ? Or is there more to it than that ?

A TL071 might be more suitable ! ;~)

I wouldn't make the loading as high a 5M though. 1M ought to be more
than enough. Basically, you're on the right track however. Don't forget
a resistor in series with the output of around 150 ohms though. The
TL07x series tend to like to oscillate when driving capacitive loads
(i.e any length of screened cable) in voltage follower mode
particularly.

Thanks. I used a 3 M to ground and a 220 ohms in series with the
output. Left the offset nulling pins floating and didn't even
bother decoupling the op amp. It's night and day compared to
going straight into the mixer. Not bad for 0.4 EUR worth of
components.

Good to hear you got a satisfactory result.

Graham