Author Topic: Input Impedance Tollerances for the NE5532??  (Read 3383 times)

Minion

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Input Impedance Tollerances for the NE5532??
« on: August 31, 2006, 03:18:31 PM »
Hi Folks, I am designing a Transformerless Mic preamp that has an Optional Tube Stage useing a couple Power amp pentodes to add a Bit of Color to the sound but I am haveing a Problem if I will need some sort of Impedance buffer after the Tubes before going into the NE5532 for Ballanceing??

Here is the basic Topology of my design...It has a Transistor front end which then goes into a INA103 Instrumentation Opamp which then goes into the Tube stage (6418 Submini Power amp pentode tubes) and after the Tube stage it goes into a Ne5532 for Creating a Ballanced output and then to the Output....

What I am wondering is I should use a Fet Impedance Buffer after the Tubes to lower the Impedance before the Signal goes into the NE5532??

The output impedance after the Tube stage is Close to 1M ohm which I think might cause problems for the NE5532 so I was thinking of useing a Fet Impedance Buffer to lower the impedance for the NE5532.....

And if I do need some sort of Buffer can someone Point me to a simple Buffer design??


Thanx a Lot!!!


PS: I know poeple from the Golden ears Brigade will say "You should use an Ouput Transfermer after the Tubes" But this is absolutly NOT an option for me because of the Cost of Output transformers and the Space requirements I have.....

 :thumb:


SSLtech

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Input Impedance Tollerances for the NE5532??
« Reply #1 on: August 31, 2006, 03:51:55 PM »
I use an NE5532 as a buffer.

No, I'm not joking.

-You didn't mention the topology: inverting or not? (HINT! -V-E-R-Y important!)

Keith
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Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

Minion

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Input Impedance Tollerances for the NE5532??
« Reply #2 on: August 31, 2006, 04:15:37 PM »
Hi, Thanx, I belive it is a Non Inverting config...(I am such a newbie I had to look up the Differance between the two and still don"t really understand)

Thanx

SSLtech

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Input Impedance Tollerances for the NE5532??
« Reply #3 on: August 31, 2006, 04:31:32 PM »
If it's non-inverting, the input impedance can be megohms. It just needs to see path to a DC reference, (which usually defines the input impedance when non-inverting) but that's about it.

Keith
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Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

Svart

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Input Impedance Tollerances for the NE5532??
« Reply #4 on: August 31, 2006, 06:34:27 PM »
why have a buffer before the INA103 at all?
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Svart

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Input Impedance Tollerances for the NE5532??
« Reply #5 on: August 31, 2006, 06:35:24 PM »
and Edcor for output trafos, ~10$ us for a regular output trafo..
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NewYorkDave

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Input Impedance Tollerances for the NE5532??
« Reply #6 on: August 31, 2006, 07:44:41 PM »
The 5532 is a bipolar-input op-amp. The specified input resistance is around 300K, if I remember correctly.

Minion

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Input Impedance Tollerances for the NE5532??
« Reply #7 on: September 01, 2006, 02:10:46 PM »
Quote
why have a buffer before the INA103 at all?



I was going to use it as an extra Gain stage because I don"t get as much gain out of the INA103 as I would Like......


So if the Input Impedance Max for the NE5532 is 300K then I will have to put a Buffer after the Tube stage as it puts out a Very High impedance signal of about 1m.....


Thanx guys, You have been a great help!!!

Samuel Groner

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Input Impedance Tollerances for the NE5532??
« Reply #8 on: September 01, 2006, 02:42:57 PM »
Quote
It has a transistor front end which then goes into an INA103 instrumentation amp which then goes into the tube stage (6418 submini power amp pentode tubes) and after the tube stage it goes into a NE5532 for creating a ballanced output...

Your plan sounds rather adventurously. What's the point of the tube stage? [Edit: Sorry, you actually said that in your first post!]

Quote
I was going to use it as an extra gain stage because I don't get as much gain out of the INA103 as I would like...

So why do you need more than 60 dB gain? And if there's really a reason for it, you'd better add additional gain after the INA103 as you'll likely spoil the excellent noise figure of the later otherwise. Doesn't your tube stage has some gain as well?

I don't know for what you are building this stuff but if I were building a simple INA103-based preamp with more than 60 dB gain I'd make the INA103 switchable from 0 dB to 55 dB gain (in 5 dB steps) and add a NE5532 as a balanced output stage with continuously variable gain (say from 0 dB to 20 dB).

Quote
I am such a newbie I had to look up the difference between the two and still don't really understand.

Are you sure that you will tackle this project then? Just wanting to protect you from a minor disaster... :thumb:

Quote
The specified input resistance is around 300K, if I remember correctly.

That's open-loop, isn't it? Closed-loop it will be very high.

Samuel

Minion

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Input Impedance Tollerances for the NE5532??
« Reply #9 on: September 01, 2006, 03:38:13 PM »
Well I have allready built one useing the INA103 with the NE5532 and it didn"t provide very much gain (Maybe 50db) but was very clean and clear but lacked the Dynamic range of other pre"s...

The transistor front end actually has as good or better noise Figures than the INA103 so I was just going to use it to add Maybe another 10db of gain useing a resistor as opposed to haveing a Trim knob....

The tube stage adds very little gain, only about 12db as these Tubes are Power amp pentodes and are only about the Size of a Mid sized Capacitor and run off of low Volatge and current....

I actually have the whole schematic drawn out accept for the Last stage (actually the PCB design, the schematic is in my head) ....

Yes this Project is probably way over my head but I figured that this is the best way for me to learn in a Hands on way....Poeple here keep telling me "This is DIY so don"t ask us Just do it your self and find out" so I am trying not to ask Newbie questions and rather just jump right in and try to learn for my Mistakes... I actually have a Pretty good Mind for figureing out this stuff but it is the Math that gets me every time......


Thanx a Lot Guys.....


Samuel Groner

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Input Impedance Tollerances for the NE5532??
« Reply #10 on: September 01, 2006, 04:01:19 PM »
Quote
Well I have allready built one using the INA103 with the NE5532 and it didn't provide very much gain (maybe 50 dB).

You can easily configure an INA103 to 60 dB and more by the means of a single resistor.

Quote
The transistor front end actually has as good or better noise figure than the INA103.

I'd be interested in that frontend design--do you mind posting the schematic? Thanks!

Samuel

jensenmann

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Input Impedance Tollerances for the NE5532??
« Reply #11 on: September 01, 2006, 04:48:25 PM »
Quote from: "Minion"

Yes this Project is probably way over my head but I figured that this is the best way for me to learn in a Hands on way....Poeple here keep telling me "This is DIY so don"t ask us Just do it your self and find out" so I am trying not to ask Newbie questions and rather just jump right in and try to learn for my Mistakes


that´s great attitude :thumb:
respect
Jens
Quote from: PRR
>The tubes of course don't care what frequency they distort

Minion

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Input Impedance Tollerances for the NE5532??
« Reply #12 on: September 01, 2006, 07:16:43 PM »
Quote
The transistor front end actually has as good or better noise figure than the INA103.

I'd be interested in that frontend design--do you mind posting the schematic? Thanks!

Samuel[/quote]


Well the design is mostly Part of another Mic pre design I saw on the Net and I just took part of it out to use with what I"m working on....

this is it:



These are the Noise Figures that were Posted for the whole Preamp (I figured that the Front end had to have as good of noise figures as the whole amp as i don"t think you can get rid of noise once it has been introduced into the signal??)

EIN = 0.27 uV rms in 20 kHz bandwidth with 200 ohm source
= 1.9 nV per root Hz
Noise Figure = 0.9 dB rel 200 ohms.


Cheers

Svart

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Input Impedance Tollerances for the NE5532??
« Reply #13 on: September 01, 2006, 10:19:28 PM »
the INA103 has astounding noise figures, even discretes selected for noise will have a hard time matching it.  If it's noisy then there is something else wrong.

The INA103 also has the ability to give gain of over 100 easy.  The discrete front end is likely identical to the frontend of the INA parts, no sense in doing it twice..  If the INA doesn't give you enough gain, I would set it for unity and use a second gain stage behind it.  Let the INA103 do your debalancing, that is what it is good at, then use something like an opa134 for gain and drive.

just my thoughts, I could be wrong.

Also, you give a voltage noise figure, but do you know what your current noise is?  How about noise at specified frequencies?

@1khz the INA has 1nVrtHZ and 2pArtHZ noise, they also give noise at other freqs.
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
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Minion

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Input Impedance Tollerances for the NE5532??
« Reply #14 on: September 01, 2006, 11:31:33 PM »
Hi, With the Pre I made useing the INA103 I basicly followed the Data sheet which had a Mic pre design in there and for setting the Gain in that design is said to use a 1K Pot in series with a 10R resistor for gain (Well that is how I interpreted the schematic anyways...

this is it:



Were it has the set gain I read it as joining Pins 15 & 13 and Putting a 10R resistor on that Pin and put the 1K Pot with Pins 15 and 6 on each end of the Pot and Pin 2 on the Wiper....

Is that correct?? The only changes in that curcuit was I used an OPA134 for DC Offset instead of the OPA627 which is a $10+ part and added the CMR adjust feature and I put a Ne5532 for Ballanceing.... The problem I had was I didn"t get quite enough gain and with the Gain Pot turned all the way down I was still getting a Bit of a signal on the output....

I Think there is also a Gain adjustment for setting a "Post Gain" with this IC but I didn"t know how it worked (If it needed adjustment or if I had to use either the Post or Pre stage or what??) Do you think this is a way to get more gain out of the Opamp??



I think I am going to try your suggestion and Use the INA103 in the First stage and instead use half of the Transistor stage as the Impedance Buffer for the Tube stage and then into the NE5532 for ballanceing as there is no need to the Transistor stage there if I don"t need it....

If you can tell me how that Post gain stage works (is it a Fixed gain set by resistors and does it work in tandem with the Pre gain stage ect?) in the above diagram or if I somehow swrewed up my interpretation on the INA103 pre schematic??

Thanx a Lot!!

Svart

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Input Impedance Tollerances for the NE5532??
« Reply #15 on: September 01, 2006, 11:55:01 PM »
I would try setting the gain on the INA103 for 1.  I think the datasheet says to leave the gain pins unhooked for that but you'll need to check it out.

as for the gain of the second stage, that is set with the feedback of the opamp.

I would feed the output of the INA103 to one side of a 10k pot and take the other side of the pot to ground.  the wiper is now your output to the Non inverting input of a 5532.  use a feeback resistor of 4k3 and a gain resistor of 2k and try that for a start.

again, this is just off the top of my head, someone will likely be along shortly to tell me why this won't work.. :wink:

As for the INA103 preamp, that doesn't use all of it's gain by far.

I believe that the output gain of the INA103 is separate than the input gain.
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Samuel Groner

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Input Impedance Tollerances for the NE5532??
« Reply #16 on: September 02, 2006, 09:51:11 AM »
Quote
I would try setting the gain on the INA103 for 1.

 :?:

I quickly drew an output stage with variable gain (0 dB to 14.3 dB): [removed]

Rate the electrolytic caps for 25 V and use NP0/C0G ceramic or film for the other two caps.

I really advise you to skip that additional discrete stage, it will--as already noted by another poster--greatly reduce the noise performance of the INA103 as does setting its gain to 1 and do all the amplification afterwards (that's why the above quote got a big question mark).

Samuel

sbeach

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Input Impedance Tollerances for the NE5532??
« Reply #17 on: September 02, 2006, 10:58:36 AM »
Hi,

If you need more gain from your INA103, remove the 10 ohm resistor in the gain loop & try replacing it with a 4 or 5 ohm resistor.  Why?  I think your 1k pot is not actually measuring "0" ohms at it's minimum - you probably have 2 to 4 ohms at the minimum position.  That's fairly common.

Using the gain formula included in the INA103's data sheet, a true 10 ohms in the gain loop will only give you 56 db of gain.  I would guess that 10 ohms, plus whatever residual resistance your particular 1K pot has, is just not providing enough gain for your needs.  

A 6 ohm total resistance in that gain loop gives you 60 db of gain - that's probably enough for most applications.  Also, by using the same gain formula, 1010 ohms (your 1K pot plus resistor) in the gain loop still gives about 16.5 db of gain.  So "off" on your pot is not "off" on the gain setting of the device.  That's why you hear a faint signal in the off position.

If you really want to be precise with the gain settings of this chip, drop the pot & use a shorting rotary switch with 1% resistors for gain setting. The 1k gain pot approach is "ballpark" at best.

If you try to increase the gain on the output of the chip - as shown in the data sheet - you will uncover one of the INA103's major shortcomings: a wimpy output stage in it's instrumentation amp architecture.

So, I hope that your next amplification stage has a very high input impedance. If not, the INA103 will have difficulty putting out enough current without some distortion.  To solve this inherent problem, maybe a current booster circuit in the INA103 feedback loop would be in order.  That would entail adding a suitable current booster or buffer circuit on the INA103's output,  taking a parallel feed from the output of the current booster & going back into the INA103 at pin 11.  

If you try to add more gain by re-configuring the 103's output stage with additional resistance in the feedback loop (as shown in the data sheet) you may get into aural trouble - especially if the next stage's input impedance is below 100K ohms.

The INA103 is a pretty cool instrumentation amp. It allows access to the feedback loop on the output stage which is something the SSM2017, SSM2019 or the INA217 do not offer.  This chip just needs to be goosed on the output current & then have the gain set accordingly.  Good luck!!

Skip

Minion

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Input Impedance Tollerances for the NE5532??
« Reply #18 on: September 02, 2006, 02:48:35 PM »
Samuel: Thanx for writeing out that Schematic, What type of Opamp do you suggest for this Curcuit?? I have some NE5532"s, TL074,OPA2107, OPA134,OPA137,OPA627,LM2900....Can I use a Dual instead of 2 Single opamps??

Also I am looking for a Impedance Buffer design (Probably useing Fets??) to lower the output impedance of the Tube stage which is close to 1m ohm, I was going to use Part of the Transistor front end but realized that it doesn"t perform well with High impedance sources....

I started writeing out the schematioc last night for the new design and I Finnished the INA103 Stage with the DC Offset option and the CMR Trim feature and added a couple freeite beads on the input for RF supression and finnished the Tube stage but without the Impedance buffer, I still have to do the Impedance Buffer and the output stage and the Ballanceing stage.....

The tubes I am useing run off of a 22v to 30v Plate voltage which I am powering off of the +48v Phantom rail (before the 6.8k resistors) and am useing a 10K resistor and a 30v Zener as a Regulator, the Filament runs off of 1.25v which I am getting off of the +15v Rail through a 1k resistor and useing a 1.5v LED as a Regulator..... I have never used Diodes as regulators before and didn"t really know how to implement them but I tried a few things and noticed that If it Tied the Negitive end of the Zenner Diode to Ground the Voltage gets clamped as the Diodes forward voltage which acts as a regulator....


SBeach: Thanx for the Tips, I will take some of your suggestions and use them, I think I will replace the 10R resistor in series with the 1K Pot and replace it with a Multi Turn trimmer pot (In have some 20R ones that should work) so I can dial the Gain down so It is quiet with the Gain knob all the way Down....

I was thinking about useing a rotary switch but they are pretty expensive were I live and I don"t know how to calculate the Resistors needed for to get gain I want for each position on the switch and I had a Hell of a Time with my Rotary switches when makeing my Green Pre"s.....


Thanx a Lot guys.....

Samuel Groner

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Input Impedance Tollerances for the NE5532??
« Reply #19 on: September 02, 2006, 03:41:10 PM »
I designed the circuit bearing the NE5532 in my mind, but OPA627, OPA134 and OPA2107 should work well too. The other amplifiers are not suitable for various reasons.

A simple solution for the buffer would be to use a OPA627 connected as voltage follower. However I'm somewhat in doubt about the 1 MOhm figure you are quoting--can you post a schematic of the tube amplifier?

Quote
If you try to increase the gain on the output of the chip--as shown in the data sheet--you will uncover one of the INA103's major shortcomings: a wimpy output stage in its instrumentation amp architecture.

Do you have some data to offer on how the distortion increases with load?

Samuel


 

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