[Spencerleehorton]

Hi Guys,

some may see as a dumb question but I have long struggled with the perceived difference between line level and hi-Z.
I have several Neve 1290 channels i've built and wanted to incorporate a dedicated D.I. input for guitar and bass to give best S/N.
I have been inputing guitar and bass by means of a passive D.I. box up until now and also wondered whether its best to compress before the D.I. or after the preamp?

But back to my main question, would it be best to build another gain pot (lorlin could work?) into another transformer then into DPDT switch to switch between mic and line/hi-z.

is there anything else to think about to get it to hi-z rather than line level?

[ruffrecords]

Neve and nearly all pro line level inputs present an input impedance of around 10K to the source. This represents a negligible load to an pro output that is most likely capable of driving a 600 ohm load. For a DI you really need an input impedance of around 1Megohm in order so as not to load the output of a guitar and alter its tone (guitar amps typically have 1Megohm input impedances. Most active DI boxes use a FET or FET input op amp to achieve this high input impedance. You could do something similar inside your Neve preamp.

Cheers

Ian

[Spencerleehorton]

everyone seems to rave about the Bo Hansen

http://www.hansenaudio.se/techpage.htm#active%20DI-box,%20my%20work%20horse%20from%201975

i'll try and make a couple and see how they sound

[ruffrecords]

everyone seems to rave about the Bo Hansen

http://www.hansenaudio.se/techpage.htm#active%20DI-box,%20my%20work%20horse%20from%201975

i'll try and make a couple and see how they sound

That will do for a start. It is a very basic circuit, just a couple of cascaded emitter followers. You don't need the Lundhal transformer - just go straight from the 10U output cap into the Neve line input transformer.

Cheers

Ian

[Spencerleehorton]

At the moment I only have mic input txf and ideally wanted to be able to switch the primaries from series parallel and be able to switch from mic to line.
Ideally I’d just like one gain control but it seems I can’t get round that?
I’ve got a switched 11 position gain switch.

[ruffrecords]

The big question is do you want to isolate the DI I.e. pass it thru the transformer. A lot of DIs on mic pres do not do this - I did not on my Classic mic pre design. I just fed the DI in straight after the input transformer. This works well on tube design because there is usually a 10 to 1 step up transformer at the input so the expected signal level at ths point is already 20dB greater than mic level. In your case it is not so easy as you only have a 6dB step up mic transformer. Never mic press usually feed the line input in after the first gain stage for this reason.

What is possible for you depends on the gain switch arrangement you have. Do you have the full blown Neve original or a simpler version.

Cheers

Ian

[Spencerleehorton]

Mine is much simpler, as in 1290 style, 3 gang, 11 position gain switch.
I was thinking i could add he line level switch with a lorlin and then tap into the other gain switch, not ideally what i want to do but it could work.
Be good to keep it simple as I only want one channel for guitar and one for bass.

[Spencerleehorton]

Could definitely use your advice on how to implement this correctly please Ian as I’ve never really been happy with di boxing the bass

[ruffrecords]

Could definitely use your advice on how to implement this correctly please Ian as I’ve never really been happy with di boxing the bass

Can you post a schematic of what you have please?

Cheers

Ian

[Spencerleehorton]

Hi Ian,

This is the link to the pdf I’ve used.

https://s3.amazonaws.com/EZ1290/assembly6.pdf

Regards

Spence.

[ruffrecords]

Hi Ian,

This is the link to the pdf I’ve used.

https://s3.amazonaws.com/EZ1290/assembly6.pdf

Regards

Spence.

OK I will take a look at it and get back to you.

Cheers

Ian

[Spencerleehorton]

Found this

https://groupdiy.com/index.php?topic=40894.0

But don’t really get it yet?

And this

https://www.jlmaudio.com/shop/opamp-di-kit.html?display_tax_prices=1

[ruffrecords]

Found this

https://groupdiy.com/index.php?topic=40894.0

But don’t really get it yet?

And this

https://www.jlmaudio.com/shop/opamp-di-kit.html?display_tax_prices=1

OK, there are several basic ways of making a DI input. The BAE method uses a series resistor from the DI input into the Neve gain switch at the line input position. So this forms a pot divider of 100K with about 4K8 which will have a loss of around 20 times which is about 26dB and the input impedance is just over 100K. If this is fed in place of the transformer secondary (which I think is what the switched jack socket does)you can boost this level by 20 to 40dB. As you have 20dB loss to start with then the input level can be from -20dBu to 0dBu which will cover most guitars and synths.

The downside of this method is the input impedance is only 100K of so which may be too low for some guitars. The JLM device is a FET op amp buffer that provides a very high input impedance (1 Megohm) so any guitar or synth will work with it. It has a very low output impedance so it can be fed directly to the mic input. The only disadvantage of this method is the minimum gain is 20dB which may be too high with some synths.

Cheers

Ian

[Spencerleehorton]

Is there a better way?

I do have all my mic pre’s at the moment into line ins on my desk through patch bay so patching anything in to line in is no problem, so I don’t need a line in txf,  but this way I’m using the A&H GS3000 line input txf rather than the Neve.
I’m wanting to sell GS3000 and do away with my digidesign 96i interface and upgrade to a UA Apollo 16 with thunderbolt 2 or 3, I’m pretty sure the Apollo has all line ins on d-sub which I can attach to the patch bay and have all my pre’s patches In the same and use the virtual mixer which gives me much more flexibility for effects etc.
I need to allow for when I upgrade to this system, so is it best to have a dedicated di which goes straight into interface or put the di through a Neve or api-312?

[ruffrecords]

I would just have a dedicated DI box and wire it to a point on the patchbay. Then just patch it in to an interface input when it is needed.

Cheers

Ian

[Spencerleehorton]

I have several passive  di boxes, does an active one like the bo Hansen improve it a lot?

[ruffrecords]

I have several passive  di boxes, does an active one like the bo Hansen improve it a lot?

THe thing about active DI boxes is they can have a much higher input impedance than a passive one so they can work better with some guitars. Also they have a low ratio output transformer rather than a high ratio input transformer. This means they are less likely to curtail the frequency response.

In other words they should be cleaner and faster.

Cheers

Ian

[Spencerleehorton]

My next question to try and cover all bases is:
Api-312 which I have 8 of, would it be easier putting an active di in this circuit?

[ruffrecords]

My next question to try and cover all bases is:
Api-312 which I have 8 of, would it be easier putting an active di in this circuit?

Probably yes. The 2520 discrete op amp has a pretty high input impedance so it would would well as an active DI. You would need to add a switch to route the DI input to the + input of the 2520 and a 1Meg resistor from the + input to 0V to ensure it is still biased when switched to DI. The switch could be part of the jack socket as in other mic pre DI designs.

Cheers

Ian

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