Low frequency response -- Gain loop questions!

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riggler

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So, I snagged 4 MS 1b mic pres. Investigating the schematic:

http://www.rane.com/pdf/ms1bsch.pdf

We can see C36 (470uF, 16V) in the gain loop with the gain pot. The preamp has a bit off low end rolloff and I'm thinking that more gain = more rolloff. (It sounds that way).

So can someone set me straight on the theory behind this? Do R45 and R2 set maximum gain? I do not think that changing these would do anything to help bass response. I think that C36 is the culprit though.

So, what would be a good value to replace C36 with? I do want to increase it's size I believe, right?

Am I right in thinking that the cap is there for killing any DC?

Been reading a lot but I'm not sure I fully understand what's going on in this gain loop. Thanks for any info!
 
There is circuitry inside the instrumentation amplifier gain block with resistor values you need to know for the full gain equation but in general the visible parts are in the denominator of that gain equation, so indeed LF pole frequency rises at higher commanded gain as series R drops.

This is sometimes "gamed" by product designers as frequency response is specified at lower nominal gain, and this LF roll off at very high gain can give the appearance of a quieter noise floor.

In general the quality of this capacitor in series with the gain leg can be important since at high gain it is operating into a pretty low resistance, so working harder than common blocking applications.

JR
 
You can use a 6.3 V rated capacitor here. It looks like there's plenty of space on the PCB around that part so you might be able to make a 4700 uF or even 6800 uF fit.

Samuel
 
Thanks guys for the information. But, can anyone explain what is happening with the capacitor in the circuit?

I *think* I understand that:

1. If the gain loop were jumped with a conductor, we'd get maximum theoretical gain from INA163.
2. The amount of resistance on this loop determines gain. The
3. The hard-wired resistors set a limit of maximum gain (2 in series being 5 ohms of resistance).

In a crossover, I understand how a capacitor will effectively "dissipate" high-energy bass.

But, am I wrong in thinking of transistor-like flow of signal where our gain loop is merely a control valve for the signal? Inside the IC it must be part of the audio path then if I am wrong.

If that is true than it makes sense that not only should that cap be of good quality (thinking poly here), but the pot maybe should be changed for a decent rotary switch with fixed resistors.

So, I'm thinking a nice Panasonic poly will be nice for this?

Still learning!
 
but the pot maybe should be changed for a decent rotary switch with fixed resistors.

Yes, you can.

But if you use a rotary switch, I'd leave that capacitor in place and use a bigger value cap here. Like 3300 at least or 6800uf as Samuel said.

But, can anyone explain what is happening with the capacitor in the circuit?

The cap here will prevent/reduce the "thumping" sound when you change gain using the rotary switch (i.e. as the resistance changes). Otherwise, it sounds real nasty with the loud popping.

If you're just using an ordinary pot, you can leave the capacitor out... but as the pot gets worn out and becomes scratchy, it's going to have this popping/thumping sound as you rotate the pot. The capacitor here prevents that from happening.
 
> it makes sense that not only should that cap be of good quality (thinking poly here), but the pot maybe should be changed for a decent rotary switch with fixed resistors.

Well, yeah.... but then you would not be thrilled to snag preamps which are "great value for a dollar(*)". Instead you would be looking at $5-$50 cap, $25 switch (not a good place for a $2 switch), or some totally different topology.

And really: how often do you really need a gain of 1,000 in one box? How often do you have a room where gain of 60dB doesn't just bring low-frequency rumble well up the VU meter? I run 54dB gain on soft choral pieces direct to disk, in an old church which has no blowers; that's unusual. I'd cut-out R45: who needs that much gain? In my regular room, 40dB gain flutters the VU with blower rumble. Some bass-cut is almost essential at very high gain in most modern rooms. If your low levels are due to lo-output mikes, then the ~0.5uV hiss of naked-BJT input is masking your mike self-noise, and you oughta consider a transformer input.

(*) Yeah, the raw parts are worth more than several dollars. The box costs more than the contents. I just noticed the whole box sells for $150 for one channel. That seems fair to me. And fair to you. Frankly there are good "one buck" preamps, and bad "half-buck" preamps, sold from $900/ch to $100/8ch. This looks like what we expect from RANE: very robust, very transparent, no nonsense BS.
 
I run 54dB gain on soft choral pieces direct to disk, in an old church which has no blowers; that's unusual.

This is exactly my application. --Choir and orchestral recording. And I'm in older stone churches. High gain is a necessity which is why the MS1b was a good candidate, and the INA163 sounds pretty good.

Just like you said a no BS design from Rane.

Some bass-cut is almost essential at very high gain in most modern rooms.

I've run into this (mainly air handlers and sometimes just air sound even), but I do think the rolloff in this box is a little bit much (at high gain)-- not a worry for choral applications, but for orchestral work in some cases we do have low frequency content around the subsonic range.

But maybe I'm getting a little crazy for a $150 pre. I'm going to run stock on an ensemble, change the caps, see what happens. After all it's two solder joints!


Sure--if you can find a 6800 uF film fitting in your control room...

I thought that may be an issue as well. A 4700 may even be too big.

-D
 
PRR's point about gain is well taken though. Do you really need that much?

To a degree, yes. Classical work is the main use here. However, do I need 60dB? Maybe 55 is fine.

How much 'color' is that additional resistor adding? It should be not much in my opinion, but I am by no means an expert here!

Just make the 470 uF larger and bypass it with a film of modest value.
I'm think modest being ~2000uF. Not sure about what else will fit.
 
in parallel with a film cap of 0.1 uF or so.

Why would we do this? Why not just replace the stock cap with a larger one?
(Remember I'm learning here!)

Is it for coloration purposes? (Can't get film capacitors that are very large).
:?:
 
I do not belive that adding a 100 nF does any good for audio, but neither do I believe that people will start not doing it any more. Do the math: at which frequency does a 100 nF reduce impedance for a (much larger) capacitor with 10 Ohm ESR? Any recent good quality capacitor will be an order of magnitude better than this, so we end up in the MHz region.

Samuel
 
In case this hasn't been clearly stated the mechanism for LF rolloff is precisely the RC formed by the gain R and series C.

As you should be grasping there is no simple answer for a good performance capacitor at this point. I am philosophically with Wayne here. The best cap is no cap. The purpose of the capacitor at that circuit node is to isolate small DC errors from the gain pot which will cause a scratchy sound as that DC error gets modulated by the changing closed loop gain.

If you can tolerate the scratchy noise (only while changing gain) and the nominal DC operating point of the circuitry isn't compromised, you could DC couple at that point and incorporate a HPF in some later stage with a good quality film capacitor. Another benefit of this later fixed HPF is that it wouldn't change with gain. This (big electro in low impedance audio path) is a common trade off made in product design since the vast majority of customers would be too turned off by the scratchy gain pot to notice that the preamp actually sounds better performing it's primary function.

My personal experiments with paralleld capacitors required a ratio of 1/10th for near ideal performance. More recent reports from others suggest a ratio of 1/100th is adequate. I expect modern caps in general are better, and some series better than others.

Good luck.

JR
 
[quote author="lydmann"]Would it be possible to use a Vactrol instead of R1 and C36?[/quote]
Typical on resistance (about 200 to 600 ohms) will exceed the desired near zero value for max gain. Never seen a vactrol below 60R. No idea about tolerances, especially for stereo tracking. May work better with higher Rfb in non inverting opamp circuits, unfortunately leading to more resistive noise. I think, they're more useful in attenuator/compressor circuits. YMMV.
vtl5c1, vtl5c2, ..
link edited
 
> Choir and orchestral recording. And I'm in older stone churches. High gain is a necessity

Choir has nothing below 80Hz.

Small orchestra won't have enough 42Hz to rise above the rumble of the diesel on the other side of town (even in stone church).

Big orchystra with a row of bass fiddles can have big 42-60Hz... but will also have rows of small and medium fiddles and assorted tooters filling up the 100Hz-800Hz zone with high power. You could not use 60dB gain in the loud passages, and on modern media you probably should not be raising gain in the soft passage (goodness knows I did a lot of that to slow tape).

Hmmmm.... 470uFd and 5 ohms gives -3dB at 68Hz, -1dB at 135Hz. That does fade the ballz on small/medium male/mixed chorus. But trim off 6dB, 54dB gain, it is -3dB at 34Hz, -1dB at 68Hz. I'd expect any orchestra with significant bottom (not one guy sawing a note his neighbor can't hear) to use 10dB less gain. Now the -1dB point is 23Hz. -0.5dB on the lowest note of standard bass. Piano has a note called 28Hz but the fundamental is hardly audible on big Steinways (we got this Bosendorfer with 28Hz thunder and a couple extra keys to the left of that....).

And if you do find the low singers' lowest note 2dB down, can't you fix it in the mix? When I did an organ with honest 32Hz, with mikes that gently sloped below 40Hz, I brought the track up in the PC, applied a few-dB rise and then a steep slope to cut a diesel out on the highway and the universal 1/f noise. Waste of time because no available (in my life) speaker bothers to try below 50Hz. If I ever find the speaker, I have the track.

Rane can't make everybody happy. Another 90 cents of cap may not even be the issue. If you put that 4,700 on there, and the rockers drop it out of the van, the cap will break loose. Unhappy customer. If you are tracking a young untrained female singer on ribbon mike in a modern bedroom, you may well need 60dB gain to peak near the top of the meters, but you also bring up an astonishing amount of sub-100Hz rumble. Sounds like garbage. Unhappy customer. And a LOT of people use hot condensers on hot music: big bass gain is wasted on them. We choral capturers are a small minority. And I could accept the 1B's choice of gain/bass-response.
 
470uFd and 5 ohms gives -3dB at 68Hz, -1dB at 135Hz. That does fade the ballz on small/medium male/mixed chorus. But trim off 6dB, 54dB gain, it is -3dB at 34Hz, -1dB at 68Hz.

You make a good argument for reducing gain. But then I could just as well not turn up the knob (stock) so high and be in virtually the same context, no?

Looking at the Hardy design, we could do away with the cap altogether, but understanding that design is truthfully a little over my head.

And I do concur: Rane did an awesome job with this box.
 

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