DI box with +4dBU balanced Line Level output instead of Mic Level

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Whoops

Well-known member
GDIY Supporter
Joined
May 9, 2008
Messages
8,186
Location
Portugal
Hi,
I have quite a few nice DI boxes already that I use to send instrument signal into different Mic preamps.

Lately I've been having the need to send Instrument  signal directly into non-amplified line inputs. (without using any Mic Preamps)

Is there any DIY circuit around or that you know that used the same concept of the DI but with a +4dBU balanced Line Level output instead of Mic Level?

The circuit could be passive or active (it would need it's own PSU)

I'm interested in:
- cheap circuit using Opamps (with output level control)

- more expensive circuit using Discreet circuit or discreet opamp and output transformer (with output level control)

Instrument signal voltage is closer to line level than to a Dynamic Mic output voltage. In a normal DI the instrument level is attenuated to Mic level and them amplified again in the Mic Pre.
It could make sense to not attenuated it and just amplify it one time and enough to achieve +4dBU line level.



 
line level DI boxes were trendy in LA back in the 1980's.
for a tube DI, simply copy the LA-2a circuit, minus the input transformer, opto and side-chain.
you can use a higher value gain control (500k or even 1 Meg) for less instrument loading.
of course, there are other suitable amp designs such as the Pultec MEQ.

for cheap:
TL071 with an high value input pot, 2:1 output transformer,  powered from a bank of 9V batteries.



 
Something like this maybe?

We have made several of these for our studios and for live use. Unbalanced, but you can impedance-balance if you think you really need to..

Jakob E.
 
gyraf said:
Something like this maybe?

We have made several of these for our studios and for live use. Unbalanced, but you can impedance-balance if you think you really need to..

Jakob E.

Thanks Jakob.

In your example the signal is instrument singnal in and buffered instrument signal out, isnt it?

For line level I would just have to amplify the signal with the TL074, instead of zero gain.

I  need to have Balanced Line Level +4dBu output, how could I impedance balanced?
I never done that
 
gridcurrent said:
line level DI boxes were trendy in LA back in the 1980's.

Really? didn't know that. What units where popular back then?

gridcurrent said:
for a tube DI, simply copy the LA-2a circuit, minus the input transformer, opto and side-chain.
you can use a higher value gain control (500k or even 1 Meg) for less instrument loading.
of course,

Thanks.
Using a tube make up gain circuit is a great idea also.


gridcurrent said:
there are other suitable amp designs such as the Pultec MEQ.

Something like these circuits and going straigh into the 12AX7 or 12AY7?
I guess I would need a 1 Meg grid leak resistor at the input to set the 1 Mega impedance

http://img.photobucket.com/albums/v713/jdr555/meq-5.jpg~original

https://www.gearslutz.com/board/attachments/high-end/443293d1421605987-pultec-makeup-gain-eqp1.jpg

gridcurrent said:
for cheap:
TL071 with an high value input pot, 2:1 output transformer,  powered from a bank of 9V batteries.

Wouldn't be better to use the transformer as 1:2?
 
On the Discreet solid state circuit end,
could something Neve and API inspired work?
I'm thinking of:

- half of BA283 card and carnhill output transformer

- 2520 opamp and output transformer

I just don't know in this cases how to prepare the inputs of  half BA283 and 2520 opamp for Instrument Signal that wants to be loaded by 1 meg impedance

Thanks
 
..for instrument-level-in and line-level-out, you basically just need an impedance buffer like the above. We use them for e.g. running remote GTR amps..

Jakob E.
 
Whoops said:
Lately I've been having the need to send Instrument  signal directly into non-amplified line inputs. (without using any Mic Preamps)

Is there any DIY circuit around or that you know that used the same concept of the DI but with a Line Level output instead of Mic Level?

The circuit could be passive or active (it would need it's own PSU)

I'm interested in:
- cheap circuit using Opamps (with output level control)

Basically, you're building a preamp, albeit one with an unbalanced high-impedance input (to match with an instrument) and a line-level (+4 dBu or 1.23 Vrms) low-impedance output.

Easy. Use a good FET-input op-amp to buffer the input.  You might wish to do some tests to see if it is necessary to make this a gain stage.  If you make the input stage with variable gain, you can drive a THAT1646 differential output stage directly. If you hang a pot on the input op-amp's output, you need to buffer the wiper with another op-amp (the other unit in the dual TL072 will work) before driving the 1646.
 
Back last century I designed an inexpensive active DI at Peavey (EDB-1).  I provided a unity gain buffered thru jack on the input side as well as padded down low z mic level feed. Op amp inside that was TL07x bi-fet. 

As Andy said you seem to be describing an instrument pre-amp, or a DI with mission creep. You can't add too many features before it becomes something else.

JR
 
gyraf said:
..for instrument-level-in and line-level-out, you basically just need an impedance buffer like the above. We use them for e.g. running remote GTR amps..

Jakob E.

Thanks Jakob I was thinking in +4dBU Balanced Line Level.

I have to re-write my first post and be more clear,
_______________

Instrument In (guitar/bass passive or active pickups)

+4dBU Balanced Line Level Output

With Volume control

_____________________________

 
Andy Peters said:
Basically, you're building a preamp, albeit one with an unbalanced high-impedance input (to match with an instrument) and a line-level (+4 dBu or 1.23 Vrms) low-impedance output.
before driving the 1646.

Exactely.
Forgot to say in the first post "Balanced +4dBU Line Level low-impedance output"


Andy Peters said:
Easy. Use a good FET-input op-amp to buffer the input.  You might wish to do some tests to see if it is necessary to make this a gain stage.  If you make the input stage with variable gain, you can drive a THAT1646 differential output stage directly. If you hang a pot on the input op-amp's output, you need to buffer the wiper with another op-amp (the other unit in the dual TL072 will work) before driving the 1646.

What do you consider a good fet-input op-amp?
TL071, TL081, OPA134, OPA604?

Did you wanted to say "If you make the input stage with fixed gain, you can drive a THAT1646 differential output stage directly" ?


 
Whoops said:
Exactely.
Forgot to say in the first post "Balanced +4dBU Line Level low-impedance output"


What do you consider a good fet-input op-amp?
TL071, TL081, OPA134, OPA604?

Did you wanted to say "If you make the input stage with fixed gain, you can drive a THAT1646 differential output stage directly" ?

OPA134 can drive 600ohm.

If you are keen on a transformer balanced output, you could go with OPA2134 which is the dual version of the OPA134 - that will give you fet input to allow a nice high impedance input for the instrument, then follow that with a pot for volume, then onto the 2nd stage to drive your output transformer.

Having the 2 opamp gain stages gives you a good range for gain as well.
 
Hi Hummer, I don't want to mix IC Opamps and transformers.

I want to do 2 versions:

- one using just IC Opamps based. Like Andy Peter's suggested

- A much more expensive one using Class A discreet circuitry and output transformer.  Thinking of API 2520 Opamp and Neve BA283

Thanks
 
Whoops said:
What do you consider a good fet-input op-amp?
TL071, TL081, OPA134, OPA604?

I used an OPA1652 in a recent thing, and it's perfectly fine. Quieter than the TL072, less expensive and better distortion spec than the OPA2134. I didn't compare the two directly. Of course it's available only as dual or a quad, and OPA134 and 604 are singles.

Did you wanted to say "If you make the input stage with fixed gain, you can drive a THAT1646 differential output stage directly" ?
No, I said what I meant to say. You mentioned "output level control." That implies that this box doesn't have fixed gain. And in a simple circuit like this, you can vary the gain (level) in one of three places:

a) right at the input, meaning you have a 1 Megohm pot right at the input (high Z pot to bridge a pickup's impedance),
b) you buffer the input with an amplifer whose gain you can vary,
c) you buffer the input with a fixed-gain amplifer and put a pot after that amp, just like in a fader circuit.

Option a) definitely require an op-amp to buffer the pot wiper before driving the THAT1646.
Since it is always best to put the gain stage up front, option b) is probably what you want.

If your requirement is fixed gain, you can use a FET input opamp configured for whatever gain you desire, and connect the buffer op-amp's output to the THAT1646 input directly. (Is a DC blocking cap needed?)
 
Whoops said:
Hi Hummer, I don't want to mix IC Opamps and transformers.

I want to do 2 versions:

- one using just IC Opamps based. Like Andy Peter's suggested

- A much more expensive one using Class A discreet circuitry and output transformer.  Thinking of API 2520 Opamp and Neve BA283

Thanks

i see - in that case here is a 2520 discrete option by Bo Hansen to consider

http://www.hansenaudio.se/techpage.htm#API%20325%20line%20amp%20with%20DI%20input%20modification

http://www.hansenaudio.se/API%20325%20DI%20mod..jpg

 
Andy Peters said:
b) you buffer the input with an amplifer whose gain you can vary,
Since it is always best to put the gain stage up front, option b) is probably what you want.

So that't it.
Basically I just want to be able to vary the gain.
Having around 20 dBs boost at the Max Pot setting.

So would this work?
1) Good FET-input op-amp to buffer the input with 20dBs variable gain
2) drive a THAT1646 differential output stage directly from the input opamp output

 
Humner said:
i see - in that case here is a 2520 discrete option by Bo Hansen to consider
http://www.hansenaudio.se/API%20325%20DI%20mod..jpg

Thank Hummer, I was checking that circuit yesterday, yes I could do that one.

Do you think it provides a good input for guitar and bass with passive pickups? How about piezo pickups?

Thank you
 
Here are examples of some commercial products that do the same thing I'm trying to achieve:

WAVES PRS Guitar Interface (11 or 15 dBs of Gain I'm not sure)
http://www.waves.com/1lib/pdf/hardware/gtr-interface.pdf

Phoenix Audio "Nice DI" - Class A DOA (30dBs Gain)
http://phoenixaudio.net/products/nice-di/

Avalon U5 DI (MIC or LINE Level Output) (30dBs Gain)
http://www.avalondesign.com/instrudi.html

API 505-DI (55dBs Gain)
https://apiaudio.com/product.php?id=102

 
I’m going to potentially try and revive this extremely old thread! I’m actually looking to build this exactly but a I’m little further behind in my DIY journey, so please forgive my ignorance.

My ideal scenario is to be reamping into my space echo, and have my space echo out signal come directly into my patch bay. so it needs to be converted from instrument level to line level, with a gain control (as OP was looking for above). as it’s going directly into a balanced input, it would ideally be transformer balanced no? If so I have a JLM 1:4 transformer lying around I could use or is that completely wrong? I would be curious to know where the OP landed on this, all those years ago!!

All the best!
 

Latest posts

Back
Top