Line to mic conversion

[nabberigu]

My tube microphone preamps have only mic inputs but a lovely tube sound and a great limiter.
Would it be possible to convert a stereo line level signal (main mix) into a microphone level and so use the mic inputs on my preamp to record the signal again using the tube sound of my preamps?
Have been reading about using a di box in reverse but I am not technical and don't want to ruin my nice mic preamps.
To reduce the much higher line level signal to a lower mic level signal will surely introduce deterioration. Is it worth buying a good di box and more importantly is it safe for my preamps?

Harrie

[resistorman]

If you wanted to try it, there are these available:

https://www.amazon.com/Shure-A15LA-Adap ... B0006NMSXS

It's not something I've ever done, and my gut reaction is that you would get noise etc., but who knows? I've done many "wrong" things with fabulous results. I think it would depend largely on the style of music and what kind of sound you're after. Alternatively: is the whole unit tube? Is there an insert after the pre that would allow a line level input where you can reap the benefits of a tube and transformer path through the limiters?

[nabberigu]

There is indeed an insert point that seems to include the tube stage as well the limiter before the signal hits the AD converter.
The first stage is a class “A” PNP ultra low noise variable gain differential amplifier having a gain ranging from -3 to 41dB. The second stage, directly coupled to the first stage, is a patented “Reflected Plate Amplifier1” differential tube stage with a fixed gain of 21dB. The combination of these two stages provides a novel and unique front end yielding from 18 to 62dB of gain.
I am almost sure the impedence will play a role as the signal reduction in order to bring it to the mic level. Surprising the preamps is extremely quite for it having a tube stage.
I am trying to find a safe way to to satisfy my curiosity without ruining my expensive preamps.

[ef37a]

That Shure "slug" will do the job but 50dB is a lot of attenuation and might make the line signal so low that noise DOES become an issue. You will in any case likely have to get an adaptor cable as the line source is unlikely to be on XLR?

Warning! Hobby Horse rampant! Once again a post on a subject that could be solved by a bit of very simple, passive, electrical work. Such an attenuator can be made with just 3 resistors (actually 4 is technically better) and of course the correct connectors and OP already has that problem with the Shure device anyway!

What is the make and model number of the pre amp (or has this old fool missed that?) The circuit "description" suggests it does not have a valve as the first stage and so some caution as to input signal level is in order. Mind you, if it has phantom power the input stage SHOULD be well protected. In fact, whatever you plug in to convert line level be sure to have phantom power OFF. If you did go for a DIY box it would be worth incorporating a pair of spook juice isolating capacitors.

Dave.

[Sam Spoons]

My tube microphone preamps have only mic inputs but a lovely tube sound and a great limiter.
Would it be possible to convert a stereo line level signal (main mix) into a microphone level and so use the mic inputs on my preamp to record the signal again using the tube sound of my preamps?
Have been reading about using a di box in reverse but I am not technical and don't want to ruin my nice mic preamps.
To reduce the much higher line level signal to a lower mic level signal will surely introduce deterioration. Is it worth buying a good di box and more importantly is it safe for my preamps?

Harrie

You would not be using the DI box in reverse but the usual way round. A DI box takes a (usually unbalanced) instrument or line level signal, reduces it to mic level ant converts it to balanced at the appropriate impedance. A couple of DI boxes would seem the least technical method of achieving what you want.

Dave's method or the Shure attenuator will work fine too (agreed that 50dB seems a lot but there are plenty of cheap attenuators with different levels of attenuation, I have -10dB and -20dB ones). However Orchid Electronics Micro DIs are a bit cheaper than those Shure attenuators at £25 each inc VAT and postage.

edit :- I guess the OP is in the US so price comparisons are moot.

[ef37a]

Excellent though they are, the Orchid boxes may not suit the OP's purpose so well? They are active boxes, not transformer types and require phantom power. I am also not sure they are available in the States?

This, https://www.sweetwater.com/store/detail/ZdirectDual would seem a decent alternative?

Dave.

[Sam Spoons]

Yes you're right a transformer DI would be the optimum solution. That Art box look perfect.

John Godsland (Orchid Electronics) does post worldwide but doesn't do (as standard) a tranny DI only a 1:1 isolator.

[Mike Stranks]

The Orchid boxes will be ideal for this application.

John will post to anywhere in the world.

There is a two-channel/stereo version which would suit:

http://www.orchid-electronics.co.uk/Dual_Micro_DI.htm

[ef37a]

The Orchid boxes will be ideal for this application.

John will post to anywhere in the world.

There is a two-channel/stereo version which would suit:

http://www.orchid-electronics.co.uk/Dual_Micro_DI.htm

Agreed Mike if the pre amp has phantom power but I will admit that that is almost certainly the case.

Dave.

[Hugh Robjohns]

Would it be possible to convert a stereo line level signal (main mix) into a microphone level and so use the mic inputs on my preamp to record the signal again using the tube sound of my preamps?

Yes. You just need to attenuate the line level signal by 20-30dB. You could use a simple in-line attenuator to do that, either ready made or DIY if you can solder. Or you could use a DI box.

Have been reading about using a di box in reverse but I am not technical and don't want to ruin my nice mic preamps.

Nothing 'reverse involved. A DI box is designed to accept an unbalanced line or instrument level signal and convert it to a mic level signal. If your line source is balanced you might need to be a little careful about how you connect it to the DI unbalanced input, but that's not rocket science.

To reduce the much higher line level signal to a lower mic level signal will surely introduce deterioration. Is it worth buying a good di box and more importantly is it safe for my preamps?

You will potentially introduce some very low level of extra noise and distortion, but since the whole point ofte exercise is to add 'nice distortions' from your valve preamp, who cares?

If you go the DI route, then passive units will be a better choice than active. I'd make (or buy) some in-line balanced attenuators -- but if doing so make sure the input impedance is high enough not to upset the line source equipment!

H

[Wonks]

Otherwise there are a huge range of DI boxes available. Radial boxes have a very good reputation http://www.radialeng.com/index.php and will certainly be available in the US.

If you're not afraid of spending even more cash, then the Rupert Neve DI box could be worth a look. http://rupertneve.com/products/rndi/. It's active, not passive, but got a good SoS review and again can add 'character' to the sound. http://www.soundonsound.com/reviews/rup ... signs-rndi

[resistorman]

Since you have an insert after the transistor first stage, you should be able to inject signal in at that point with no fuss, though it's likely to be unbalanced. It would help if you gave us the make and model of the device

[nabberigu]

Great responses. Thanks everybody for the input. I found a switchable (10-20-30 dB) Audio Technica attenuator and probably go that way.
I completely forgot to mention the make of the preamps. We are talking about Aphex 1100 MKII and btw these things are fantastic.
I just recorded today with them. Electric guitar through a Fender Rumble 100 with a SM 57 and a electric violin through a Fishman with a CAD E-200 on it.
There is very little for me to do here.
Both instruments sounded stunningly natural. I would say that many will be surprised to hear that the violin is actually electric. Just striking how much fullness and natural tone these preamps produce not to mention the level of detail. The word sophistication comes to mind.
Recorded the with the same guitar setup into not to bad and modified (Burr Brown OPA 627 and Jensen transformers) Presonus MP 20 preamps. That recording seems lifeless almost boring now. Of course there is a considerable price and technical difference so comparison is really not realistic and fair. The Presonus are quite good for the money.

[ef37a]

I have checked the very comprehensive user manual for the Aphex pre amp and had a look at their description of the "Reflected Plate" amplifier circuit. The latter is ingenious but I am not sure it is really revolutionary? Better valve techs than I could comment?

The Burr Brown+ Jensen transformer pre amp is going to give very clean results whereas the Aphex has both a THD and IMD specification at least an order worse than a top line, fully solid state pre amp. Yes! Even one with those horrible chips in it! It is no surprise to me therefore that the two pre amps sound differently under critical listening conditions.

I am rather surprised that such a complex and expensive pre amp does not have a high Z input. Almost any guitar pedal, NOT of the daft "true bypass" stamp and using an op amp, not FET buffer stage will be more "transparent" than the Aphex IMHO.

Dave.

[Hugh Robjohns]

I found a switchable (10-20-30 dB) Audio Technica attenuator and probably go that way.

It it's the AT8202 that has an input impedance of around 1K Ohms which is enough to keep most modern line output devices happy. Higher would be better... but it should be fine. Listen out for any hints of transient distortion or reduced bass...

I completely forgot to mention the make of the preamps. We are talking about Aphex 1100 MKII and btw these things are fantastic.

It's a good preamp. And -- as someone enquired/suggested earlier -- it does have a line level insert point with the return signal passing through one of the valve stages in the preamp before hitting the converter. So that is another option which is open to you to get some valve colouration in the preamp without having to smash the signal down to mic level and back again. Just adjust the output level of your source carefully to avoid overloading the preamp converter.

H

[nabberigu]

Thanks for all the great responses. I will try both the attenuation and the insert route.
And to clarify I am not leaning towards tube or solid state although the combination makes a lot of sense. As I use to co-own and sell High End audio equipment this combination in pre-amp and power-amp often resulted in great audiophile experiences.
For me the only thing matters is what my ears like to hear and I think other people like to hear and the output of the Amphex is in a great way very natural. Actually so natural that I am already on the lookout for another uni

[ef37a]

Hugh, the signal diagram for the pre amp shows that insert is post the front end and one of the valve stages that give this "reflected" plate malarkey.

Presumably this is what the OP is after?

Dave.

[Hugh Robjohns]

Hugh, the signal diagram for the pre amp shows that insert is post the front end and one of the valve stages that give this "reflected" plate malarkey. Presumably this is what the OP is after?

Presumably so, and the block diagram also shows that there is a second reflected plate stage in the signal path from the insert return (shaded in pink in the diagram below).

I have no idea whether this single stage will provide the colouration the OP seeks, but it is probably worth some experimentation to see...

H