Two channel portable balanced mic preamp

[carlmart]

I'm starting a new project for a portable mic preamp, to be used in video location recordings, interfacing DSLR cameras.

In 2000 I did design and built about fifty similar preamps, based on the SSM2017. At the time I used the basic two chip schematic designed by Walt Jung for AD.

This time there are some further limitations, one of them being I want this preamp to feed from a single supply, probably +12v, instead of a split type.

The trick I know is for that you bias the input at Vsupply/2, but if I'm not wrong you will also be biasing the output like that. How do you interface that with other parts of the system which are biased to the negative side of the single battery as a ground? Would a blocking capacitor just solve it?

 

[LeeYoo]

I'm also thinking of designing a multi-channel battery powered mic pre.
One way is to make the negative supply with a DC/DC converter.
Maybe you also need a boost SMPS for the +48v phantom.
Leo..

 

[ricardo]

I'm starting a new project for a portable mic preamp, to be used in video location recordings, interfacing DSLR cameras.

In 2000 I did design and built about fifty similar preamps, based on the SSM2017. At the time I used the basic two chip designed by Walt Jung for AD.

How did you do power for your old design?

As your DSLR is almost certainly unbalanced input, you can get away with 1 chip.  THAT1510/12 are superior versions.  even if you use AD or TI parts, use the THAT circuits

 

[ricardo]

I'm starting a new project for a portable mic preamp, to be used in video location recordings, interfacing DSLR cameras.

If long battery life is important, a good mike transformer feeding a LN OPA which can be single supply is hard to beat for sheer performance & simplicity.  This is how Sound Devices do their excellent portable stuff.

I've got a suitable circuit in my Yahoo MicBuilders Files.  You have to join.

Read the Jensen App. Notes for choice of transformer.

A stack of 5xPP3s will have MUCH better battery life than ANY SMPS arrangement for P48V

Another trick that Sound Device use for long battery life is P12V.  Many modern mikes work well with much less than 48V.

My circuit works well on a single PP3 so that might save you the size & weight of your 12V lithium.

 

[carlmart]

It's quite likely I will make this into a commercial product, that I will add to a box I'm working with additional functions.

As such I am entering most of the limitations I mentioned at the beginning.

1) Battery would be external, between 12v and 14.4v.

2) Supply should be single instead of split supply type.

3) The problem with transformers, as you may know, is price if they are good quality. 

4) The output will quite likely be unbalanced, but it would quite easy later on to balance it.

5) As the external batteries to be used should be relatively large in capacity, I think a DC-DC 48v supply might be a better idea than the PP3. In fact, if I can I'm not planning to put any battery inside.

 

[ricardo]

1) Battery would be external, between 12v and 14.4v.

2) Supply should be single instead of split supply type.

3) The problem with transformers, as you may know, is price if they are good quality. 

4) The output will quite likely be unbalanced, but it would quite easy later on to balance it.

5) As the external batteries to be used should be relatively large in capacity, I think a DC-DC 48v supply might be a better idea than the PP3. In fact, if I can I'm not planning to put any battery inside.

Have a look at my MicBuilders LNprimer.  It'll at least give you some ideas for good balanced out without more current consumption.

Worth asking THAT if they have any circuits for single supply THAT1510/12.  Some of their new chips may be single supply.  If you do use single supply, be careful of the input electrolytics cos they can be reversed if P48V is switched off.

If you use SMPS for 48V, you might do -ve supply with the same supply .. with a current consumption penalty.  Also seriously consider a switch for P12V.  This might increase your battery life by 3x with a hungry mike.

 

[ricardo]

In 2000 I did design and built about fifty similar preamps, based on the SSM2017. At the time I used the basic two chip schematic designed by Walt Jung for AD.

Can you tell us how you did the power supply for these?

 

[carlmart]

Have a look at my MicBuilders LNprimer.  It'll at least give you some ideas for good balanced out without more current consumption.

Where is that?

Worth asking THAT if they have any circuits for single supply THAT1510/12.  Some of their new chips may be single supply.  If you do use single supply, be careful of the input electrolytics cos they can be reversed if P48V is switched off.

Have a look at how THAT implements a single supply on their 1510/12. Very simple indeed.

If you use SMPS for 48V, you might do -ve supply with the same supply .. with a current consumption penalty.  Also seriously consider a switch for P12V.  This might increase your battery life by 3x with a hungry mike.

The only DC-DC circuit on this project should be for the 48v, and it will be 48v. Current shouldn't be a problem, because the battery will always be external and large one.

 

[carlmart]

Can you tell us how you did the power supply for these?

Nothing complicated: +/-9v from two batteries. One of the batteries fed the DC-DC 48v booster, using a MAX chip

There was an alarm LED warning if the battery feeding the preamp and the DC-DC supply got to 7v, which is where the 9v battery curve went down quickly.

On that design I also did consider going single supply. Many designs do. The only price, besides the bias generators, is the obligatory blocking capacitors.

 

[carlmart]

One thing I would like to know more about is implementing a limiter on the first stage.

That is the way it should be done, and it's so on the mixers and preamps I have seen.

One way is using a Vactrol, which is a four-pin optoisolator. Two pins are for the encased LED, two pins are for the LDR on the same case. The LED is driven by a circuit  that follows the output on the first stage. The LDR is connected at the input, between the + and - pins. According to the output, the LDR controls the resistance + and - pins, limiting the peaks.

On other designs I have seen, the limiter is after the first stage, so when the peak happens you can't do anything to control it, the distortion is already there, even if the limiter attenuates it. The key is to really limit the distortion at the input. That's how I see it.

I wonder if someone has tried limiters here, particularly those I describe. Vactrol and Silonex used to make these parts.

 

[ricardo]

Have a look at my MicBuilders LNprimer.  It'll at least give you some ideas for good balanced out without more current consumption.

Where is that?

https://groups.yahoo.com/neo/groups/micbuilders/info

You have to join.  It's in my Files directory

Worth asking THAT if they have any circuits for single supply THAT1510/12.  Some of their new chips may be single supply.  If you do use single supply, be careful of the input electrolytics cos they can be reversed if P48V is switched off.

Have a look at how THAT implements a single supply on their 1510/12. Very simple indeed.

Yes.  You might get away with that.  The noise of the OPA appears as Common Mode which the THAT rejects.  In fact, for your simple stereo circuit with only 2 chips, you could probably get rid of the OPA & its noise and adjust the 2x10k divider.
___________________

There's at least one 'Vactrol on input' thread on this forum.  There are caveats which I can't remember.

What are the "mixers and preamps I have seen" that have this scheme?

 

[carlmart]

Sorry, Ricardo, I gave up on joining the Yahoo club.

I am a Yahoo member, and it was a pain to recover the password.

Then I filled up I don't know how many captcha words, even the audio numbers, and was always wrong. I forgot how bad Yahoo is for interfacing.

What is the OPA you are talking about?

About the Vactrol, Sonosax used one. I think Audio Developments did too. Never had the chance to see a Sound Devices to see how they do it.

 

[ricardo]

What is the OPA you are talking about?

The one forming 1/2V supply in THAT_single.jpg

 

[carlmart]

The one forming 1/2V supply in THAT_single.jpg

Oh, you must mean the ICs buffering the dividers. The idea was to make the bias generator as good as possible.

I'm also not sure how do you do to use that many as I will have to.

About the Vactrol, I have two implementations, one that I don't know who sent it to me and looks the most interesting because it limits the first stage, which can be a 2017 or a 1510.

 

[carlmart]

This is the second Vactrol that I devised, though never built it, to put the limiter on the second stage.

I believe the first one should be more effective.

Please tell me what do you think of both or if someone uses/used any of these implementations.

 

[ricardo]

Oh, you must mean the ICs buffering the dividers. The idea was to make the bias generator as good as possible.

I'm also not sure how do you do to use that many as I will have to.

You've only got stereo.  LNprimer.doc is a 4 channel preamp for a tetrahedral mike using a simple voltage divider.  Using the OPA draws more current and has slightly worse noise.  You don't need a stiff rail for this .. but I don't know your final circuit.

About the Vactrol, I have two implementations, one that I don't know who sent it to me and looks the most interesting because it limits the first stage, which can be a 2017 or a 1510.

I've never built such a circuit.  Limiter/Compressor circuits are VERY subjective and need field testing.  At Calrec, Clem, my mike mentor was considered the guru.

But at first glance, you need some input series resistance to form a potential divider with the Vactrol which will degrade noise.  There's a separate thread on just this approach.  I don't think it came up with a good circuit but at least it will tell you the CONS.  ;D

For limiting after the preamp, THAT have some very nice & flexible chips designed for exactly this application including ENG where low consumption is important.

 

[carlmart]

Limiting after the input, on a second stage, is what Tascam does on their digital portables, and I don't think it helps much. The damage is done.

The THAT solution involves too many parts, IMHO, to solve a problem I think might be better dealt off with a good barmeter.

I wonder what thread is that dealing with Vactrol at the input, I would certainly like to read it. On my original circuit I had put 50R series resistance, which didn't add too much noise, but I wonder if that would be enough. If you have a look at the hand-drawn design, there's no series resistance though.

 

[ricardo]

I wonder what thread is that dealing with Vactrol at the input, I would certainly like to read it. On my original circuit I had put 50R series resistance, which didn't add too much noise, but I wonder if that would be enough. If you have a look at the hand-drawn design, there's no series resistance though.

You're already reading ricothe troll's thread.  That has loadsa good stuff but I don't think its the one I remember.

Your hand drawn circuit w/o series resistance is poor.  Maybe OK for certain dynamic mikes but not high quality

 

[carlmart]

I will look for the ones I had seen using a Vactrol (Sonosax and Shure at least), but the principle was certainly different from the one I had found for the 2017.

The circuits used a transformer and a single opamp as first stage. The Vactrol was applied on the large gain resistor, between + and output pins. It worked very effectively. I haven't seen it, but it's quite likely Sound Devices does the same.

I always thought the LDR should be between the gain pins in the 2017 or 1510, in parallel with Rg. But I never saw any circuit doing that. You would need a large capacitor there too or a DC offset servo, which would increase the number of parts.

During my tests time I never tried a servo, but I did try a pot between the gain pins, in series with a large bipolar capacitor (2200uF 5v). But I ended up using a three position switch for the gain, and the cap was not necessary. In my case space was a problem, and the large cap and the pot would be difficult to include.

One idea that I do find quite effective is to use a dual pot: one side handling the gain and the other the output. That's they way Shure does. But the pot has to be special, combining linear and log elements, which wouldn't be too cheap to get. The Shure pots got noisy too.

Going the transformer way is very expensive, at least for this present project. You can build 4 transformerless channels for the price of one transformer.

 

[ricardo]

The circuits used a transformer and a single opamp as first stage. The Vactrol was applied on the large gain resistor, between + and output pins. It worked very effectively. I haven't seen it, but it's quite likely Sound Devices does the same.

I always thought the LDR should be between the gain pins in the 2017 or 1510, in parallel with Rg. But I never saw any circuit doing that. You would need a large capacitor there too or a DC offset servo, which would increase the number of parts.

Do you mean output & -ve input?

Transformer, LN OPA & Vactrol across Rf is most straightforward, easiest & best.  Sound Devices are all ex-Shure though their stuff are the new Nagras.

The problem with Vactrol on Rg is that the LDR goes to LoR very quickly but takes a loo.oong time to recover.