Portable mic preamp project

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carlmart
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Re: Portable mic preamp project

Post by carlmart » Thu May 22, 2014 5:57 pm

JR. wrote: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.
How do I compute the LF roll-off?
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.
There's not such a thing, AFAIK. you mostly filter LF. But Nagra had designed a three position filter that had a roll that worked very well for film. I wonder where the response curves might be, and of course the parts they used.

Of course I didn't rely just on my ears, particularly for the highs. They were thoroughly tested on a scope. I just said they sounded cleaner with the film caps, but then they were scope checked. My concern had been with one of the DC-DC 48v generators I had tried, with an LT chip, to see how much RFI they irrradiated. The one I used in the end, with the Max chip, was excellent.

ricardo
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Re: Portable mic preamp project

Post by ricardo » Thu May 22, 2014 8:49 pm

mediatechnology wrote: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.
DON'T use Schottkys.

Angelo Farina bought me a Mk2 MOTU Traveler. It becamy noise on 2 mike channels after 3 mths. The replacement took 12 mths before one channel went noisy. MOTU refuse to acknowledge there is a problem but have a $200 replacement PCB policy on non-warranty stuff ... but it take 3 mths for us in Oz.

The Traveler uses TI PGA25000 and has excellent performance .. when working. The noise when wonky is loud enough to flash all the LEDs.

I changed the SK12 Schottkys for 1n4004s and the problem went away. It was also my first attempt to repair a SMD PCB so it was a real botch job :(

From my own experience, friends & the various forums, Traveler Mk2s and Mk3s have this problem. Mk1s appear immune. If anyone here has a MOTU Traveler, I'd appreciate their experiences. Looking at my 'replacement' PCB, its obvious the SK12s have been swapped in the past. My guess is that if you are lucky, you have 12 mths before your Traveler Mk2 plays up .. and it WILL play up.

I noticed a slight improvement in noise on the 1n4004 channel but I only changed the offending channel cos my ham-fisted efforts with SMD. The other channels appear OK another 12 mths on.

The Glass Passivated 1n4004GP are the best devices to use but common 1n4004s are usually fine.
_______________________

If you can use an ISOLATED P48V supply with your floating battery, it might be nicer all round to use Wayne's MC33178 virtual ground as the REAL ground so you have +/-6V supplies. Less & smaller caps and better earthing & layout too.

You can use the circuits & layouts directly from THAT datasheets & ANs

carlmart
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Re: Portable mic preamp project

Post by carlmart » Fri May 23, 2014 7:29 am

Some last minute questions:

1) What better value for pF capacitors at input, and why using 47pF as a leg to ground?

2) How high can I go with R8/R9 before noise becomes a problem. I mean 2K2 + 2K2 and 3K3 +3K3. I am looking for higher capacitance 5mm pitch film types, like 2.2uF and 3.3uF, and they are expensive or hard to find. So maybe I should increase the resistors, at least within limits.

3) Any rule for choosing the rail splitter resistors values? Can I use one single splitter for the V/2 of the whole preamp? This may seem obvious, but there's little info about it, and I did look around. Any section in Horowitz/Hill where that is?

4) How to compute the capacitor + resistor combo on the gain switch? That one looks like a neat trick.

5) Ricardo: which specific "ISOLATED P48V supply" are you referring to?

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mediatechnology
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Re: Portable mic preamp project

Post by mediatechnology » Fri May 23, 2014 8:54 am

DON'T use Schottkys.
Well put.
Everyone I've ever known that's tried them eventually had Schottky diodes come back and bite them in the a$$.

ricardo - A post for another day but I've found a way to reduce the junction capacitance variation from one diode to the next that can potentially upset CM balance. I've also found an interesting inductive resonance to be avoided from capacitors bridged across tip/ring.
da clues be here: http://www.waynekirkwood.com/DC_Preamp/ ... k_1_BW.JPG
Might be best to post that discussion in it's own thread...
JR. wrote: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.

How do I compute the LF roll-off?
The LF rolloff is the inverse product of the gain resistor and the size of the capacitor.
With higher gains Rg is lower in value.
Thus, for a given size capacitor, higher gains (lower Rg) will have a higher cutoff.
The formula is 1/(2*pi*f*R*C) or 1/(wRC) where "w" is lower case omega.
So a 10 Ohm Rg and a 1000 uF Cg is 15.9 Hz
You can also use this handy web-based calculator: http://www.muzique.com/schem/filter.htm
What if I leave 1uF and say go for 4K7 or 3K3 for the resistors?
That direction makes the noise issue worse.
The ideal, within limits, are small-value bias resistors and big value capacitors.

Some lower limits for bias resistors might be 1K each.
Too low and you load the mic.
Too high, bias and noise currents become a problem.

The higher limit for input capacitors is about 47-100 uF.
Set too high and there will be issues with stored charge, turn-on time and leakage.
Too low noise and CM rejection suffer in addition to reduced LF response.
The single supply design I posted, with 4X 100 uF is actually 50 uF/leg due to series-connection.
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carlmart
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Re: Portable mic preamp project

Post by carlmart » Fri May 23, 2014 9:07 am

mediatechnology wrote: The LF rolloff is the inverse product of the gain resistor and the size of the capacitor.
With higher gains Rg is lower in value.
Thus, for a given size capacitor, higher gains (lower Rg) will have a higher cutoff.
The formula is 1/(2*pi*f*R*C) or 1/(wRC) where "w" is lower case omega.
So a 10 Ohm Rg and a 1000 uF Cg is 15.9 Hz
You can also use this handy web-based calculator: http://www.muzique.com/schem/filter.htm
Well, what comes to bit me in the ass are the electrolytics caps I wish to stay away from for that arrangement.

One thing I do not like in using a pot as Rg is because you need a BIG cap, which has to be 'lytic. That would also increase space used by those caps, which would have to be x2 because of both channels.

OTOS I do use 'lytics to isolate stages, even more on this single supply design. :(

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mediatechnology
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Re: Portable mic preamp project

Post by mediatechnology » Fri May 23, 2014 9:22 am

Even if you switch gain (instead of a pot) you will get clicks without a Cgain capacitor or servo.
I realize that you probably won't do that on the fly within a take but do realize it will be there.

If you absolutely insist on using input film caps in the sub 10uF range you might be better-off with a FET input design.
The noise current will be many orders of magnitude less allowing you to use high value bias resistors and small input capacitors.
There's one in this application note from Linear Systems: http://www.linearsystems.com/assets/med ... n_Note.pdf
I suspect it will be voltage-challenged at 12V.
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carlmart
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Re: Portable mic preamp project

Post by carlmart » Fri May 23, 2014 9:37 am

mediatechnology wrote:Even if you switch gain (instead of a pot) you will get clicks without a Cgain capacitor or servo.
I realize that you probably won't do that on the fly within a take but do realize it will be there.
Yes, I did have slight clicks on my other arrangement, which of course were never switched during the take. This time I might have to make a sacrifice and put a large capt there.
If you absolutely insist on using input film caps in the sub 10uF range you might be better-off with a FET input design.
The noise current will be many orders of magnitude less allowing you to use high value bias resistors and small input capacitors.
There's one in this application note from Linear Systems: http://www.linearsystems.com/assets/med ... n_Note.pdf
I suspect it will be voltage-challenged at 12V.
No, discrete parts are out for that application. Or I'd go with that other arrangement using transistors in LTP as first stage feeding a low noise IC. Like Tascam or other mixers do.

Either do I sacrifice the low response using 1K bias resistors, and perhaps a larger film cap, or definitely go for a 'lytic cap you can recommend. Preferably just one, not the four caps plus resistors you use on your circuit.

What about the resistors values on the V/2 rail splitter?

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JR.
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Re: Portable mic preamp project

Post by JR. » Fri May 23, 2014 11:56 am

You can still get clicks with a gain switch even with caps if you switch while signal is playing, from the sudden gain step multiplied times the audio signal.

Good luck, decent electrolytic caps are unlikely to be your weakest link.

JR
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carlmart
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Re: Portable mic preamp project

Post by carlmart » Fri May 23, 2014 12:39 pm

The clicks I had got, using no cap on the Rg switch, were not that loud. So if I will get them anyway, better leave the cap off.

Let's do all the changes and see what happens.

Which would you consider decent eletrolytics?

Which would be my weakest link?

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JR.
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Re: Portable mic preamp project

Post by JR. » Fri May 23, 2014 1:02 pm

carlmart wrote:The clicks I had got, using no cap on the Rg switch, were not that loud. So if I will get them anyway, better leave the cap off.

Let's do all the changes and see what happens.

Which would you consider decent eletrolytics?
There are a number of good brands (panasonic etc). I would avoid dodgy unrecognizable brands or possible old stock.
Which would be my weakest link?
Reinventing the wheel...? You will find out what you didn't do correctly after you finish. I know I always do. :lol:

I still prefer using split supply to not create any unanticipated issues using chip sets that appear engineered for use with split supplies. If the manufacturer provides app notes for single supply use disregard me,, I have never used a canned preamp chip so I am not the expert on applying them.

But I am generally not inclined to design another mixer either..

JR
Cancel the "cancel culture", do not support mob hatred.

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