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DI inputs on Mic Preamps Instrument Preamps
Old 11th October 2015
  #1
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CJ1973's Avatar
 

DI inputs on Mic Preamps

I'm looking at purchasing a couple of racks from either VKLA or elsewhere for my v72s and other preamps. Some of these racks come with DI units for extra $.

Someone told me that the "DI inputs on mic preamps are usually not isolated, but typically just patch point inserts after the mic gain stages"

Is this true? If so, is the sound quality not the best? Is it good to then get a DI unit separately and send that signal into the Mic Input of these units? Is there a compromise in sound on the DIs of these units or other preamps even with DIs in the front?

Thx
Old 11th October 2015
  #2
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BrianVengeance's Avatar
 

A DI will take a signal to mic level, so patching it in after the mic preamp would be rather worthless. On the other hand if the input were line level, that would make more sense.
Old 12th October 2015
  #3
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DarkSky Media's Avatar
If you have a DI that inserts after the initial pre-amplification stage, it won't sound the same as a pad (and transformer) on the mic input. But that is a good thing, since it gives you an option you wouldn't otherwise have.
Old 12th October 2015
  #4
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Hi
The definition of a 'DI' has changed the original premise.
One of the earlier purposes was to allow a high impedance instrument or mic, often unbalanced to be used with balanced mic cabling at a LOW impedance so that longer cables could be used with reduced loss.
The DI may be a transformer or an 'active' circuit (amplifier with high input impedance and low output impedance).
Originally the DI would be placed close to the instrument so the unbalanced high impedance cable is as short as possible.
Where a mic amp uses a transformer some manufacturers 'add' a DI facility where the mic transformer is bypassed and the DI socket connects into the amplifier stage following the transformer either with or without additional amplifier stages.
Most of the issue is down to exactly what is being done and where it happens and there is no definitive answer.
As a generalisation, a valve mic amp will have a very high input impedance (after the mic transformer) so makes a very good 'DI' input. Some transistor stages might have a 'medium' input impedance so may then need an additional amplifier stage to 'boost' the impedance.
You HAVE to look at the specifics for any piece of gear under review to find out exactly what is going on.
In this situation, ask VKLA EXACTLY what is on offer.
Matt S
Old 12th October 2015
  #5
Gear Nut
 
Silvan's Avatar
 

The impedance is indeed the main issue. A mic input is typically low impedance, a DI is high impedance, irrespective of the levels or amount of gain required.
Old 13th October 2015
  #6
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NoEgo's Avatar
Quote:
Originally Posted by CJ1973 View Post
I'm looking at purchasing a couple of racks from either VKLA or elsewhere for my v72s and other preamps. Some of these racks come with DI units for extra $.

Someone told me that the "DI inputs on mic preamps are usually not isolated, but typically just patch point inserts after the mic gain stages"

Is this true? If so, is the sound quality not the best? Is it good to then get a DI unit separately and send that signal into the Mic Input of these units? Is there a compromise in sound on the DIs of these units or other preamps even with DIs in the front?

Thx
To answer your question in another way. On all my pres even though some have a good DI like Manley for Bass on a VOXBOX, a killer bass pre if I can promote it... ...but I normally always use a Radial or Martin DI box and run it through the mic input. My rationalization on this is that the unit is designed mainly as a mic pre so what happens after the pre is also designed to be in tandem. I have just had the best results this way. The box is fed next to the talent and the Low Impedance output to the pre.

Lou
Old 14th October 2015
  #7
Gear Maniac
 
Tommyswami's Avatar
I would try the Reddi by a designs it's great on everything . The bass and acoustic gtrs sounds really great through it .
Old 14th October 2015
  #8
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Geoff_T's Avatar
 

Quote:
Originally Posted by CJ1973 View Post
I'm looking at purchasing a couple of racks from either VKLA or elsewhere for my v72s and other preamps. Some of these racks come with DI units for extra $.

Someone told me that the "DI inputs on mic preamps are usually not isolated, but typically just patch point inserts after the mic gain stages"

Is this true? If so, is the sound quality not the best? Is it good to then get a DI unit separately and send that signal into the Mic Input of these units? Is there a compromise in sound on the DIs of these units or other preamps even with DIs in the front?

Thx
Hi

I have looked at a number of units DI inputs, particularly on Neve/Neve clones where I can claim some experience and they vary from cheap bodges to intricate separate buffer amplifiers.

The former are much as you discussed... the original mic amps were never designed nor intended to have a DI input so you aren't purchasing a Neve DI input... you are buying an OEM frig to get a DI input and it uses the first stage of the actual post transformer/pre amplifier input.

One recent thread here had DI's added to Neve 1081 inputs. This was achieved by taking the low level, unbalanced output of the mic transformer, out of the module to a Switchcraft switch jack socket that then routed the mic or DI input back to the first B338 in the 1081 via the attenuator in the gain switch. This is not a particularly high impedance path as the gain switch wants to match the input transformer's 4.8Kohm matching impedance and the B338's input impedance is only 47Kohm if it found it's way there directly.

This is high enough impedance for synthesisers but not enough for the coils of an electric guitar that really ought to be 100Kohm and up.

Another alternative is the simple fix of a series 100Kohm resistor into the secondary of the mic transformer... also the input to the gain switch. This works, in a fashion, but there are better alternatives.

Better alternatives are to use a separate buffer amplifier for the DI that provides a very high impedance and better matches the coils of a guitar amp.

Two ways of achieving this are possible... either field effect transistors (FETs) or boot strapped silicon transistors.

For the former Dan Kennedy of Great River uses a FET to get the very high input impedance and also a beefed up output stage so that it can drive the mic input transformer and include that "iron" in the DI circuit path.

I use the boot strapped system on my pre's that gives a 10 million ohm input impedance along with 10dB additional gain that can help get the overdrive sound at higher gains. Once one gets into the 10Mohm range the guitar is barely aware that anything is connected to it and every nuance of sound on the pick ups is captured.

Lots of different approaches!

Old 14th October 2015
  #9
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burns46824's Avatar
In my experience with synthesizers and drum machines, using passive DIs going into mic preamps is not the best idea. Best to use a preamp that has a DI built-in. You don't want to bring your signal down to mic level if you don't have to, especially if it is going through more transformers. This will give you a "softened" sound. You may want this, however...
Old 14th October 2015
  #10
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Geoff_T's Avatar
 

Quote:
Originally Posted by burns46824 View Post
In my experience with synthesizers and drum machines, using passive DIs going into mic preamps is not the best idea. Best to use a preamp that has a DI built-in. You don't want to bring your signal down to mic level if you don't have to, especially if it is going through more transformers. This will give you a "softened" sound. You may want this, however...
Hi

The problem with using synths and keyboards into unbalanced inputs on mic pres is the possibility of ground loops depending on how the synth/keyboard connects to the power.

The fix is to wire the unbalanced cable from the synth/keyboard to the mic input xlr (hopefully greater than 1Kohms input impedance) wiring hot to pin 2 and shield to pin 3, leaving pin 1 high. That way the input transformer (if fitted) floats the source from the ground of the preamp.

Old 14th October 2015
  #11
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burns46824's Avatar
Quote:
Originally Posted by Geoff_T View Post
Hi

The problem with using synths and keyboards into unbalanced inputs on mic pres is the possibility of ground loops depending on how the synth/keyboard connects to the power.

The fix is to wire the unbalanced cable from the synth/keyboard to the mic input xlr (hopefully greater than 1Kohms input impedance) wiring hot to pin 2 and shield to pin 3, leaving pin 1 high. That way the input transformer (if fitted) floats the source from the ground of the preamp.

Yeah, I have some cables wired for that, as well. Can't say I've done a direct comparison between the two methods. I generally don't experience ground loops running into unbalanced DI inputs on mic pres.
Old 14th October 2015
  #12
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funkycam's Avatar
 

you just have to use your ears.
My best sounding DI is on a BAE 312 mic pre with 70s components.
I also have an Avalon U5, & on guitar the BAE is noticeably better.
Old 17th October 2015
  #13
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CJ1973's Avatar
 

Quote:
Originally Posted by Geoff_T View Post
Hi

I have looked at a number of units DI inputs, particularly on Neve/Neve clones where I can claim some experience and they vary from cheap bodges to intricate separate buffer amplifiers.

Lots of different approaches!

Thanks, Geoff! Thank you for time here and feedback, which is priceless. I know you have a lot going on, so to give experienced thoughts is very much appreciated.
It's all starting to make sense. As you said, there are various approaches. It comes down to if it works for the end user, for the price-point.

I am considering having a rack purchased for 2 x V72 units and to me, it now makes sense to request a separate transformer installed for the DI purely to know I have my bases covered. As you mentioned, include the 'Iron' in there is probably the route i am looking to take, so it's all optimized.......
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