Portable mic preamp project

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
User avatar
mediatechnology
Posts: 5444
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Portable mic preamp project

Post by mediatechnology »

What exactly would it mean to make a solid V/2? Just one well filtered divider to take care of all V/2? Buffered or unbuffered?
It's gotta be buffered. Use this for V/2 and input bias.

Image
THAT1510 1512 Single Supply Preamp

Pin 5, the 1510 Reference connection must see a low impedance V/2 source or the CM rejection will degrade.
Note that the input capacitors are required to be bipolar or bipolar-connected since the polarity under phantom on/off conditions reverses.
The good news is the right-hand pair are continuously formed.

I think you'll find that 9V will be the minimum the 1510 will operate. 10V is the spec'd minimum. Your 10V min/11V alarm should be OK.
The MC33178 (SMT only now) pulls very little current.
You could also use a MC33078 which is still available in DIP but it pulls more current.

IIRC the 1510's p-p output level is about 4V less than the supply.
If you were to use balanced output drive you could pickup - at least with balanced loads - +6 dB more output level.
You could then run the 1510 with 6 dB less overall gain to conserve headroom with low battery voltage.
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Portable mic preamp project

Post by carlmart »

I have three suggestions for implementing an analog supply on several chips: SSM2019, INA217 and THAT 1510.

What I wonder is they will work on all those chips with the mods suggested, particularly the one I called SSM2019_single2
Attachments
SSM2019_single2.jpg
SSM2019_single2.jpg (113.94 KiB) Viewed 25164 times
This arrangement I plan to implement on the boards I already have with split-supply and SSM2017 chips.
This arrangement I plan to implement on the boards I already have with split-supply and SSM2017 chips.
SSM2019_single1.jpg (113.74 KiB) Viewed 25164 times
SSM2019_AD.jpg
SSM2019_AD.jpg (86.64 KiB) Viewed 25164 times
User avatar
mediatechnology
Posts: 5444
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Portable mic preamp project

Post by mediatechnology »

Based on a quick glance they all will work across the THAT1500-series, INA 217 and SSM2019.

I don't recommend the Zener clamps because under phantom fault conditions they will likely short.
I'd use the diode bridge to rails circuit I posted with 10R series resistors. (R5 and R6).
R5 and R6, at 49R9 add 100 additional Ohms of source impedance. With 10R/leg it drops to 20 Ohms.
The reason can be found here in a paper Rosalfonso Bortoni and I co-authored: http://www.thatcorp.com/datashts/AES790 ... eturns.pdf

C1 and C2 in the first two drawings may be too small.
The differential impedance in the first example is approx 2K1 making the cutoff 160 Hz.
The second drawing has about 20K Zin-diff making the cutoff 16 Hz.

In the second example, the Zeners are also already pre-biased to 1/2 the supply which may cause them to limit too early.

The first example, which has T-bias, provides a common mode input impedance on the right-hand side of C1 and C2 that is roughly 10 times higher than what it would be if the 22K were 0R.
This improves the LF common mode rejection considerably because it lessens capacitor matching requirements.

If you do switch phantom I'd put the switch after C3.
It's just a personal preference but the peak turn-on current can be quite large if the 48V supply is fairly stiff.
If the 48V supply was capable of it the peak turn-on current could be as high as about 480 mA.
You might also want to back-ground R3 and R4 when phantom is off to discharge C1 and C2.

C1 and C2 cannot be polarized in the first or second examples.
When phantom is switched off the polarity is one direction: The right-hand side is more positive.
When phantom is on the left-hand side is more positive.

"single 2,"with a few adjustments, should work with the THAT1510,THAT1512, INA217 or SSM2019.
ricardo
Posts: 262
Joined: Sun Apr 10, 2011 7:24 am

Re: Portable mic preamp project

Post by ricardo »

mediatechnology wrote:I don't recommend the Zener clamps because under phantom fault conditions they will likely short.
I'd use the diode bridge to rails circuit I posted with 10R series resistors. (R5 and R6).
R5 and R6, at 49R9 add 100 additional Ohms of source impedance. With 10R/leg it drops to 20 Ohms.
The reason can be found here in a paper Rosalfonso Bortoni and I co-authored: http://www.thatcorp.com/datashts/AES790 ... eturns.pdf
I strongly second the THAT diode bridge protection with 1n4004s even if you use 2019 or the Texas equivalents.

My experience of this was not even a P48V application but Factory Test gear with the 2017/9 and their recommended circuit. They died with depressing regularity. :( Switching to the THAT bridge protection made the problem go away.
Last edited by ricardo on Thu May 22, 2014 8:21 pm, edited 1 time in total.
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Portable mic preamp project

Post by carlmart »

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
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Portable mic preamp project

Post by carlmart »

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.
Attachments
SSM2019_single3.jpg
SSM2019_single3.jpg (170.68 KiB) Viewed 21997 times
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Portable mic preamp project

Post by carlmart »

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.
User avatar
mediatechnology
Posts: 5444
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Portable mic preamp project

Post by mediatechnology »

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.
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Portable mic preamp project

Post by carlmart »

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.
User avatar
mediatechnology
Posts: 5444
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Portable mic preamp project

Post by mediatechnology »

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.
Post Reply