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Thread: INA217 Build... a problem

  1. #1
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    INA217 Build... a problem

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    Hi all!

    For my final class of college, I'm building an INA217 preamp based just on the schematic in the data sheet and playing around with it. I'm also comparing it to some other pres and writing about a 26p report on it, but thats rather boring.

    I got a power supply from FiveFishStudios (Great supply, BTW) and just put the circuit together on some small perf boards from ratshack.

    All of my pics can be found here, but here are a few:









    Sounds AWESOME! Very, very clean and UNBELIEVABLY quiet! I love it! Its at least a step above the pres in my M-Audio unit. Definitely usable.

    Now for the problem... I can't get the gain low enough!

    I'm not completely sure I understand how to setup my pot for gain. mshilarious, where are you?

    Any help would be much appreciated.

    So far I've connected a 1k pot and a 500ohm resistor (barely moved when I used the pot), a 25k pot (just for the heck of it ), a 1k pot with 10r resistor, 1k pot with 100r resistor, etc. Nothing really gets quiet at all... and this goes for both channels I've got and I was testing with a dynamic mic.

    Any suggestions? The schematic calls for (if I'm reading it correctly) a 1.6k pot and a 8ohm resistor in series. Maybe I've got my pot hooked up wrong. Hmmm... need to go back and look... The way I have it wired is this: The 10ohm is connected to pin 1 on the INA and the wiper on the pot. One side of the pot is wired to pin 8 to finish the loop.

    Hmmm... do I need the wiper and side connection connected together and then those two in series with the 10r? Maybe... I think that may be it...

    Anyone have any ideas?

    Thanks! And BTW, these things are definitely worth 4 channels at $130 or so.

    Brandon

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    Ah man, you're making me read a datasheet on a Saturday night? OK, I'm not an instrumentation amp guy, usually I just use opamps. But Mr. Datasheet says:

    Gain = 1 + 10,000/Rg

    OK, the datasheet schemo does say the 1k6 pot (what the hell kinda pot value is that? And they want reverse log taper?) in series with an 8R. That gives variable gain of 7 to over 1,000, or in dB 20*log(gain), or 17 to 60dB. Actually max gain indicated would be higher than 60dB, but apparently 60dB is the limit of the chip. It shouldn't matter if you let the extra leg of the pot float or connect to the resistor.

    So if 17dB is too high a minimum gain, use a larger value pot. Or you could add an input or output pad.

    The question is, what you are connecting this to that 17dB of minimum gain with a dynamic is too much? If you are in doubt about the gain, get some sort of signal generator (I use a tuner), and measure VAC off the input and output. That should confirm the gain from the chip.

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    Bless you for reading the data sheet on a Saturday.

    I agree... what kind of flipping value is 1.6k? Mouser didn't have one, at least that I could find.

    The only thing I can figure is that my pot value of 1k is whats messing me up.

    I'll find a source and see if I can check the gain amounts.

    How are you using a tuner as a source?

    Brandon

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    Quote Originally Posted by NeveSSL View Post
    Bless you for reading the data sheet on a Saturday.

    I agree... what kind of flipping value is 1.6k? Mouser didn't have one, at least that I could find.
    They're not that hard to find in a Google product search, but worst comes to worst, a 1k and a ~600 ohm in series will do the job.... *shrugs*

    Oh, wait... did you say 1.6k POT!?!?!?! That has to be a mistake. Either that or the schematic was drawn up by a theoretician....
    Quote Originally Posted by Obi-Wan
    If you mod me down, I will only grow stronger.

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    Yep, a pot.

    Maybe they did that so you would be forced to come up with your own value?

    Brandon

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    10K pot

    So far I've connected a 1k pot and a 500ohm resistor (barely moved when I used the pot), a 25k pot (just for the heck of it ), a 1k pot with 10r resistor, 1k pot with 100r resistor, etc. Nothing really gets quiet at all... and this goes for both channels I've got and I was testing with a dynamic mic.
    Use a 10K pot (reverse log audio taper) and that should bring your minimum gain to 6dB at the max. wiper setting of 10K.

    Now, if you open the connection between Rga and Rgb (or use a very large resistor), you can have 0dB gain (i.e. no gain).

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    Thanks for the reply, fivefishdiy.

    Whats weird is when I open the connection (playing around with resistor and pot values hooked up to the breadboard), it gives me a lower gain, but there is still quite a bit there. And thats frustrating. In ProTools, it still hits around -30db or so just speaking loudly into it. Kinda weird. Thats what I'd love to have as my lowest, and even that seems a little hot for all the way down (keep thinking about recording guitars with a 414 or something).

    Could it be a biasing thing? I didn't think I would need the offset circuit, but maybe I do.

    Also, I ran my rails up to 16v instead of 15v. Not a ton of difference, but it may not hurt to pull them back down to 15 or maybe even 14v. I may try that tomorrow.

    I'll dig around and see what other pot values I can find laying around. If nothing else, I need to order some phantom power mini toggles... I may do another order and grab some 10k reverse audio pots. I just hope they work! Something seems a bit off... We'll see!

    Thanks guys!

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    Quote Originally Posted by NeveSSL View Post
    Thanks for the reply, fivefishdiy.

    Whats weird is when I open the connection (playing around with resistor and pot values hooked up to the breadboard), it gives me a lower gain, but there is still quite a bit there. And thats frustrating. In ProTools, it still hits around -30db or so just speaking loudly into it. Kinda weird.
    Again, what are you connecting the pre to? Another gain stage? And speaking loudly into a dynamic at close range could result in -30dBV, which shouldn't translate into -30dBFS, but maybe 18dB of gain and you're there.

    Could it be a biasing thing? I didn't think I would need the offset circuit, but maybe I do.
    No, a DC offset just chews up your headroom. It should be minimal or zero at the input, a lot of gain can cause a small DC offset to be a larger one at the output, but that's usually only a big deal if you have a direct coupled gain stage that follows. Also, it's sloppy to leak DC out of a device. You can use the servo in the schemo, or just a humble cap. You can also measure DC offset at the output with your multimeter.

    Also, I ran my rails up to 16v instead of 15v. Not a ton of difference, but it may not hurt to pull them back down to 15 or maybe even 14v. I may try that tomorrow.
    Lowering the power rail doesn't reduce gain, just headroom. I guess if you lower headroom enough, you'll have less gain via clipping! But that isn't what you want

    But the change in headroom is rather small: 20*log(14/15) = -0.6dB

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    Quote Originally Posted by fivefishdiy View Post
    Use a 10K pot (reverse log audio taper) and that should bring your minimum gain to 6dB at the max. wiper setting of 10K.

    Now, if you open the connection between Rga and Rgb (or use a very large resistor), you can have 0dB gain (i.e. no gain).
    Hey, fivefish, I saw your new battery powered pre, you should plug that on taperssection.com in the Retail section, they love stuff like that. Just be sure to spec the phantom power current, most portable recorders with phantom can't manage more than 3mA or so, and some of them aren't 48V, so if you've got that it's a big selling point.

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    48v

    Hey, fivefish, I saw your new battery powered pre, you should plug that on taperssection.com in the Retail section, they love stuff like that.
    Thanks. I should get the final, final, final Prototype PCB from the factory this week. Then I'll be able to do one final testing.


    Just be sure to spec the phantom power current, most portable recorders with phantom can't manage more than 3mA or so, and some of them aren't 48V, so if you've got that it's a big selling point.
    Well, the chip I used for the 48V section is rated 1.5 Amp (1500mA), the inductor 0.8 Amp (800mA), so I think I can safely say yeah, it will be capable of more than 3mA I haven't done actual measurement yet, but from my testing, it can sufficiently power one of my power-hungry FET condenser mic. (Other condenser mics will work even at 18V phantom power, but not this FET mic... it wants 48V and sucks up power more than my other mics.)
    Last edited by fivefishdiy; 06-01-2008 at 10:58. Reason: Speeling error.

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