Portable mic preamp project

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carlmart
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Re: Portable mic preamp project

Post by carlmart » Wed May 21, 2014 5:37 am

OK, yours are very interesting comments. Thanks.

One detail I should have corrected was the voltage for the zener diodes, which should be lower for my supply. Zeners were (and still are) the recommended types for the protection. But I think I'll switch to the diodes bridge.

Your also bought me on using 10R series resistors, instead of 49.9, as long as I am protected, and the diodes seem to do so.

The one I call "Single 2" is the one I would like to use, as long as the chip can be swapped and be able to listen for eventual differences, which I'm not sure if anyone has tried or found.

Let's see if we can get to a compromise with the C1 and C2 capacitors, at least on the cutoff, which goes from 160Hz to 16Hz. The first is a bit high, but it's effective with dialogue on location recordings.

The first thing I would do, sticking to film cap types, is to go to 2.2uF Siemens pol, which are small enough and reasonably priced for a film. That should reduce the cut to about 80Hz, right? Sounds like a nice improvement.

I would like to go to about 50Hz, but no increasing the cap or going for 'lytic type. Perhaps using 2.2K for R1-R2? I don't think noise would be much higher.

On my actual preamp, using the DC-DC supply I showed above, the switch is after the capacitor. BTW, THAT recommended me that the phantom switch went to V/2 instead of chassis ground.

I think I will draw two more examples, introducing the improvements, to see what you think. BTW, two things were not included on my design, and are on my preamp: the gain is through a three position switch and there is an output cap, which should be essential with rail-splitter.

carlmart
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Re: Portable mic preamp project

Post by carlmart » Wed May 21, 2014 11:11 am

Well, I re-designed my schematic more or less following the alerts and recommendations, and I think it might work well.

As I think I explained, what I will be doing is modifying some pre-assembled boards I already had and see what happens.

This time I added the filter I had before the input caps (why you used higher values would be nice to know), the gain switch and the output cap.

There's an Alps log pot after that set the level, which I prefer to a gain pot (potential noise problems due to dirt and all that). After that there's a bass cut switch and the 10:1 output stage, using an OP-275.

Except for the bass cut, very important on film/video location audio, the project and final preamp had followed Jung/Garcia's paper.
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carlmart
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Re: Portable mic preamp project

Post by carlmart » Wed May 21, 2014 1:32 pm

About the battery.

The reason for working with 12v as my standard voltage, is because this project is to have several powering option, meaning the battery types you will be able to choose from should be different types.

But it should designed for film & video applications, where there's not just one battery standard. It could be a single 14.4v large brick battery, or 12v li-ion or NimH type, or two 7.2v batteries from Sony or Canon in series.

My idea is to provide several top covers for the box, depending on the battery choice, to make it a compact package. That's all I can mention for now.

And that is why I'm basing the design on a single supply.

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mediatechnology
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Re: Portable mic preamp project

Post by mediatechnology » Thu May 22, 2014 6:58 am

Zeners were (and still are) the recommended types for the protection.
By whom?
TI and THAT have both moved away from it. (INA217, INA163, PGA2500, PGA2505, THAT1510/1512 datasheets all show steering diodes).
TI changed many of their datasheets after Menace II was published.
ADI's work is almost 20 years old. They got away using Zeners due to the large 49R9 resistors and did not test for differential faults.
When we wrote Phantom Menace II we tested everyone's parts: THAT1510, INA217 and SSM2019.

EDIT: BTW those protection zeners in "single1" will have DC bias on them and even though they are not in conduction will be very noisy.

Also add D5 and D6 as shown here to assist the internal Rev Vbe diodes.
The previous schematic I posted predates them being added.

Image

Some notes about the last schematic (single 3).

If you switch phantom off - and "off" is V/2 - it isn't really off is it?
Though you may not anticipate it would you want to plug a ribbon mic into that?
If the capacitors are non-polarized by virtue of being film there's no advantage to switching it to V/2.

Add a large-value electrolytic to the supply rails, one that's at least as big or bigger than the input caps.
You need a capacitor to absorb the charges in C1 and C2 that are transferred to the supply rail.
During fault conditions the input stored charge gets transferred to the bypass capacitor through the bridge diodes. (Don't use 1N4148s.)

From your previous post you mention a balanced output stage.
You may be able to eliminate the output coupling capacitor and use the V/2 potential on the output of the preamp IC to bias the final output.
If you later decide to use the preamp IC output to drive loads directly it is limited to 2K loads and should have a 49 to 100R build-out.

I used larger value capacitors in the input since most people don't need or want a permanent input HPF/bass cut.
The penalty for having small value input capacitors is greatly increased 1/f noise since the reactance of the capacitors at LF adds to the overall source impedance. The IC's noise current sees a larger value source impedance.
If you want to add a HPF to limit LF gain you can also use a series capacitor in the Rgain leads.
The cutoff frequency varies with gain however.
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carlmart
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Re: Portable mic preamp project

Post by carlmart » Thu May 22, 2014 8:24 am

The suggestion to switch off the phantom to V/2 came from THAT themselves. But I do have to prevent anyone might plug a ribbon into it. So what should I do?

Don't worry that I got your point on the protection diodes, and I will not use the zeners. Will also add the 1N4148s as you suggest.

The reason for going for film types, is because when I made my tests, the HFQs recommended by AD made the amp noisy. There were some Siemens 'lytic caps that stopped the noise, but the Siemens film types were much better sounding.

Let's clear up one thing: I am first of all what they call a user. My experience is as location sound recordist, coming from the Nagra 4.2 times. So please do respect my gray hairs. ;)

My electronics knowledge is limited and hopefully you will acknowledge that as a not too serious disadvantage.

So instead of using instruments to measure what I build, I rely mostly on my ear. Of course I get help from my friends to measure things up, but listening comes first. On my listenings film caps did sound cleaner in all aspects, or at least much better than all 'lytics types I tried. I must confess I didn't try the "series inverted caps with resistors in the middle" arrangement, mostly because of space reasons. I did consider going to 2.2uF film types, but they were a bit larger, more expensive and not that easy to find. Things might be different now, also for 'lytic types. The one you call "permanent HPF" at the input I wish it to be at 40-50Hz, so I think I'd the choice of going for 2.2uF at the input or 2K2 for R1-R2, right. Not sure which might be better.

Safety wise, if switching my phantom off to chassis ground will prevent from harming ribbon mics, and that is fine with film input caps, then that's the way I want to go.

The supply cap is not shown, but it should be at least 120uF, but going higher or choosing specific types is something I am very much open to suggestions on. Same thing for going with a star grounding scheme, if it proves useful.

About the output, I will probably need balanced and unbalanced options, due to the units it will be connected to. Some will be unbalanced, some balanced. I'm already making up my mind on the output arrangement I will use, as it will be a thick cable carrying several things.

The output will have another stage, because I wish to put a level setting pot in between. Also a HPF passive filter is essential, so it should to before the output chip.

On my previous preamp I used an OP275 with 10x gain, which allowed me a fake balanced option and XLR outputs, which could also be used unbalanced. This time the main output will go through the thick cable mentioned above.

Besides all that there will be a headphone amp, a 1KHz generator and VU-meter driver. Undecided yet is whether I will also add a mid-stage limiter.

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mediatechnology
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Re: Portable mic preamp project

Post by mediatechnology » Thu May 22, 2014 8:54 am

Let's clear up one thing: I am first of all what they call a user. My experience is as location sound recordist, coming from the Nagra 4.2 times. So please do respect my gray hairs. ;)
We do. My neighbor has a Nagra museum down the street. It's quite a collection. He does a lot of re-furbs for tape collectors including spending a huge amount of time polishing the covers. He is also a location recordist. You both sit on the line of fire when you're on the set.

Film input caps are the best option where size, value, cost and low-frequency cut-off permit. It's quite likely that the noisy electrolytic caps you experienced had high leakage current or had not had time to re-form the electrolyte. It's always better to trust your ears and I'm not surprised the film sounded better. Too bad we can't get a 100uF/63V polypropylene in the same can as an electrolytic. I think your electronics knowledge gets you out of jams on location when others might not be able to.

Since you will be using film caps they are inherently non-polarized so you can simply back-ground the phantom switch when off.
Another alternative is to back-ground the off position with a a high-value discharge resistor. (10-100K)
The first option discharges the caps through the 6K81 resistors.
This discharges them faster than the second option of discharging them through a shared resistor.

The 1.1 uF/4400 Ohm cutoff (2.2uF/2 and 2K2x2) will be about 33 Hz which in itself shouldn't be a problem for your application.
I didn't initially realize that this was for non-music recording.
The IC preamp input, looking backwards into the microphone, is not going to be able to "see" it's 150 Ohm typical source impedance because the capacitors will add about 7.2K Ohm (at 20 Hz) source impedance.
If LF gain is not being limited by a Cgain capacitor - the HPF filter is only at the input - then there will be quite a bit of gain applied to the 1/f input current noise.
That may make it noisier overall than the electrolytics' leakage currents.
What you have is a balancing act...

Regarding the protection bridge diodes.
Some have recommended Schottky diodes there but what I've found is that the reverse leakage characteristics aren't that good or well-specified.
This can make them noisy compared to the glass-passsivated 1N4004GP diodes.

The first schematic I posted is a reference design I did for THAT.
We spent quite a bit of time discussing the polarization of the electrolytics.
They have that prototype and used it for the T-bias tests.
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JR.
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Re: Portable mic preamp project

Post by JR. » Thu May 22, 2014 9:35 am

+1 to keeping impedance low for input blocking caps and putting the HPF pole in the gain leg. With a gain switch, you can switch the pole frequency with the Rs. It is an old trick to let the LF response fall off at highest gain to make the preamp appear quieter than it is. This way the preamp makes less noise (input noise current X source impedance) and then that lower noise level gets HPF.

I would not quibble with Wayne about input protection, he has done a lot of work in that area.

I am still not enthusiastic about single supply but it's your call... A simple switching supply (charge pump) could turn a single battery voltage into split supply with no magnetic field, and probably run a doubler/tripler off the switching square wave to generate the +48V too. I've generated +48V for almost free, off a high frequency dc to dc switcher using small caps for boost and signal diodes for steering.

Whatever... good luck.

JR
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mediatechnology
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Re: Portable mic preamp project

Post by mediatechnology » Thu May 22, 2014 9:48 am

+1 to keeping impedance low for input blocking caps and putting the HPF pole in the gain leg. With a gain switch, you can switch the pole frequency with the Rs. It is an old trick to let the LF response fall off at highest gain to make the preamp appear quieter than it is. This way the preamp makes less noise (input noise current X source impedance) and then that lower noise level gets HPF.
If I may "amplify" (pun intended) on what John said about the HPF pole in the gain leg: Since this project is going to run off 12-14.4V supplies (rather than 30V) the maximum gain is limited to begin with. Rather than the 60-66dB gain of conventional preamps this one is about 50 dB. This makes the minimum gain resistor significantly larger in value and as a result the Cgain capacitor value (and size) smaller for a given cut-off.

I have a schematic of a Rane preamp where they used a small-ish Cgain and also reduced the first stage gain to make the minimum Rg larger in value. This kept the pot from being as "twitchy" at high gain, the Cgain smaller, and made the preamp appear quieter.

For a single-supply preamp this one worked pretty well.
In 2008 we didn't know to add the external RevVbe diodes and to have a big electrolytic on the rail to absorb the stored charge.

Image
THAT1510 1512 Single Supply Preamp
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carlmart
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Re: Portable mic preamp project

Post by carlmart » Thu May 22, 2014 1:24 pm

With a gain switch, you can switch the pole frequency with the Rs. It is an old trick to let the LF response fall off at highest gain to make the preamp appear quieter than it is. This way the preamp makes less noise
Please elaborate on how to do this.
The 1.1 uF/4400 Ohm cutoff (2.2uF/2 and 2K2x2) will be about 33 Hz which in itself shouldn't be a problem for your application. I didn't initially realize that this was for non-music recording.
Location recording is a mess on what refers to reverberation and voice intelligibility. Nagra had an excellent filter for that, which I would love to adapt here. But I lost the data.

What if I leave 1uF and say go for 4K7 or 3K3 for the resistors?

This equipment is being designed to help pros and semipros using video cameras to get out of jams, and still have top quality. I would love to use transformers, but that would increase cost a lot.

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JR.
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Re: Portable mic preamp project

Post by JR. » Thu May 22, 2014 3:23 pm

carlmart wrote:
With a gain switch, you can switch the pole frequency with the Rs. It is an old trick to let the LF response fall off at highest gain to make the preamp appear quieter than it is. This way the preamp makes less noise
Please elaborate on how to do this.
You can put Cs in series with the Rs for your gain switch so each gain has it's own unique HPF cut-off... Rolling off the LF at high gain will "appear" quieter.

The 1.1 uF/4400 Ohm cutoff (2.2uF/2 and 2K2x2) will be about 33 Hz which in itself shouldn't be a problem for your application. I didn't initially realize that this was for non-music recording.
Location recording is a mess on what refers to reverberation and voice intelligibility. Nagra had an excellent filter for that, which I would love to adapt here. But I lost the data.
I am not aware of a filter that selectively suppresses reverberation. Perhaps a variation on mid-side stereo pair recording where you favor the L+R which is mostly direct sound and trim back L-R which has more ambience or reverberant content.
What if I leave 1uF and say go for 4K7 or 3K3 for the resistors?

This equipment is being designed to help pros and semipros using video cameras to get out of jams, and still have top quality. I would love to use transformers, but that would increase cost a lot.
Back a decade or two ago I designed a small battery powered mixer for Peavey (RQ200) that ran on 2x 9V batteries.. Phantom was only 18V. But the whole mixer was only $200 full retail...

JR

PS: I learned long ago to not rely just on my ears... they tricked me several times, and my bench equipment could measure things that I could not hear, not the other way way around.
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