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Old 4th February 2013, 05:23 PM   #11
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Thanks for all the tips, insights, etc.

Surface-mount fears aside, I like the idea of using something like the INA163 for the MM stage. In general it seems harder to find instrumentation amps that have both low current and voltage noise. I went with the 3-op-amp method largely because I have parts on hand, and I've used the NE5532 enough that I have a good feel for what it can do; it seemed pretty well suited to MM impedance in terms of noise specs. I'm not married to it, however, and it sounds like there are some other contenders.

On that note, I noticed that the LT1028 is gain of -1 stable, while the LT1128 is +1 stable. For the unity differential configuration (op amp #3 in the in-amp), which is preferable?

I'm now working on the pin-1 issue, and I appreciate the suggestion to rethink it. Given the inconsistencies in other gear I have, and cables/connectors, I'm thinking of just adding chassis/signal/float switches on the inputs' and outputs' #1 pins. Switches are cheap and allow for easy experimentation with a given configuration.
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Old 5th February 2013, 05:19 PM   #12
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Hi,

while it is clear to me what the 3k and 6k resistor positions are, Im not sure where You put the 30 and 120Ohms in Your design. The only position I´d expect such low values would be for the Rg. The LT1028 would be seriously loaded down if it had to drive a following load plus a lowimpedance network.
The Rg is also the dominant noise source resistor. For the INA103/163 the useful gain range is between 20 and 1.000, giving Rg values between 316 and 6 Ohms. So the noise contribution from Rg fortunately gets the lower the higher the chosen gain is. The 3k feedback resistors hardly effect noise at all in presence of such a low Rg.
Since the differential stage just buffers and sums its input signals its noisegain is negligible too. Higher resistor values only play a minor role here.

jauu
Calvin
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Old 5th February 2013, 06:05 PM   #13
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Easy enoug to see if one bothers to look at data sheets they are called R in litle dwg I posted

And if you listen to LT1028 it does like to run hot
How impedance network take any part in it as design is for split passive and has 2 more gain stages as in post 1 those wuld be the triangular shaped things in the drawing.

Here is data sheet and hey presto sch is on page 1
http://www.ti.com/lit/ds/symlink/ina163.pdf

Last edited by Bksabath; 5th February 2013 at 06:07 PM.
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Old 6th February 2013, 08:02 AM   #14
Calvin is online now Calvin  Germany
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Hi,

if it were that clear, I sure wouldn't have asked.
First because such low impedance values are far off of the usual value range for the feedback resistors and are rather typical for Rg.
Such low feedback-R values would ask for Rg values of 1ohm or even less.
How do you deal with gain variations due to the increased sensitivity to tolerances, temperature and ageing and gain differences between the two stereo channels? Have you had the opportunity to verify the behaviour of the LTs regarding noise, THD and headroom under those heavy loading conditions?

Second because there are six resistor positions -or better two times three- for two resistor values, allowing for more than just one single combination.
The differential stage doesn't need to be a unity gain buffer as shown in the simplified schematics. It could as well be a differential gain stage. (remark: I refer to 'differential stage' as the third OPamp within the three-OPamp instrumentation amplifier schematic)

Quote:
How impedance network take any part in it as design is for split passive and has 2 more gain stages as in post 1 those wuld be the triangular shaped things in the drawing
??? And the message is??? What?

Jauu
Calvin

ps: OMG, have been building balanced in phono stages for 20years now and never realized that triangles were gain stages or OPamps, nor that there are datasheets for info I must be a truely dumb nut. thanks for clearing that one.
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Old 6th February 2013, 11:33 AM   #15
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Well last post for me here
From the data sheet you could clearly see the resistors values used in the INA163
there are 4 X 6K and 2 X 3 K resistors and this IMO is limiting design choices

It did sound prety good with a prety 3 dimentional stage and good tollerance to scratches and such

Wow 20 years design experience I beter let you carry on as I think wagneric has the answer he required about loading
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Old 6th February 2013, 04:45 PM   #16
Calvin is online now Calvin  Germany
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Hi,

Yes, the DS states the resistor values as 3k and 6k.
The first value allows to calculate the gain for a given Rg or vice versa the required Rg for a given gain. The Q is, which resistor contributes how much to the output noise?
For noise calculations the 3k are in parallel to Rg at the negative input. For a gain of 100, Rg is 61Ohms, for 1000 its 6.006Ohms. So the 3k ´in parallel´ doesn´t affect the ´equivalent source resistor´ value (59.78Ohms and 5.994) in a noticable order. The Pickup impedance in parallel to the input loading resistor defines the equivalent source impedance for the positive input.
A AT33PTG has a coil dc resistance of 17Ohms, giving 14.52ohms with 100Ohm loading. Same result here....Pickup and Rg are the dominating factors of noise calculation, not the 100Ohm Input loading nor the 3k feedback resistor.
As explained in #12 the 6k resistors of the differential buffer also contribute negligible to noise, since thats gain is much lower than the input stage´s.
In fact the noise penalty of typical resistor values compared to the extreme low values in the LT1028 INA is < 2dB(!). The extreme low resistor values increase tolerance sensitivity and heat. THD is increased grossly (btw. there´s a graphic in LT1028s DS of THD vers. Frequency, vers. Load resistance), which may be the source of the sound You like about the hot running LT1028.

jauu
Calvin
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Old 4th April 2013, 07:23 PM   #17
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Just a point regarding the INA217. This was pointed out to me in my phono design which uses this IC. It is something easily overlooked. The INA217 has a high input bias, upto 12uA which will flow back into the cartridge windings. This is not good.

Paul.
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