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Old 25th February 2014, 06:32 PM   #1
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Default RIAA stage based on LT1028...

I'm thinking to prepare another RIAA pre-amp for MM cart with option for flat operation with ~+30dB gain. This pre-amp is based on example found on LT1115 data sheet + Andy C's RIAA stage using LT1028. I made some changes and simulations with the combined phono -schematic to find suitable changes needed to get original schematic work for flat operation as well. Ended to these changes:

RIAA EQ
Click the image to open in full size.

Click the image to open in full size.

RIAA stage itself is based on the one found from here - http://andyc.diy-audio-engineering.o...o_preamp.xhtml (C/R values can be found from this site). I did change the C4 to 470uF for this stage I'm working with.

Flat pre-amp (By simulation, Gain = ~+30.0dB ((1Hz) 20Hz - 20kHz (>100kHz)) , Phase < +-0.5deg ((1Hz) 20Hz - 20kHz)):
R1 = 54.5kohm
Disable R4, R7, R8 and C1, C2


Am I going in the woods with these "thoughts"? Is the LT1028 good selection (least it's not the cheapest one).

Last edited by jiiteepee; 25th March 2014 at 03:24 PM. Reason: updated schematics, added THD results
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Old 25th February 2014, 11:51 PM   #2
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Quote:
Originally Posted by jiiteepee View Post
I'm thinking to prepare another RIAA pre-amp for MM cart with option for flat operation with ~+30dB gain. LTSpice IV has an educational schematic for phono preamp based on LT1028 (same schematic on page #18). I made some changes and simulations with the educational phono -schematic to find suitable changes needed to get original schematic work for flat operation as well. Ended to these changes:

Flat pre-amp (By simulation, Gain = ~+30.0dB ((1Hz) 20Hz - 20kHz (>100kHz)) , Phase < +-0.5deg ((1Hz) 20Hz - 20kHz)):
R1 = 1.02kohm
R2 = 33.2ohm
Disable R4/C1 + C2

Am I going in the woods with these "thoughts"? Is the LT1028 good selection (least it's not the cheapest because of one LT1028ACN8#PBF costs around 20 + 24% tax + s&h).
jiitteepee fancy meeting you here, PM me and I will send you some AD797's for your project.
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Old 26th February 2014, 12:16 AM   #3
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My suggestion would be to augment the LT1028 with a unity gain buffer (LT1010 might be a suitable choice). For a couple of reasons - first to remove any thermal interactions between the output drive requirements and the input stage. Second to preserve dynamics - the load seen by the LT1028's OPS isn't particularly benign - 0.1uF and 10R in series. Having a separate buffer allows separate decoupling of the supplies and means the LT1028 can work with much cleaner rails.

<edit> On re-reading your post I see you're intending 'flat' operation, meaning no EQ at all? In which case the OPS is going to see a much easier load with those largish fb caps deleted. But then again the output when working 'flat' will be tilted up to HF so dynamics are still a major issue vis-a-vis the supply noise.
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Last edited by abraxalito; 26th February 2014 at 12:21 AM.
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Old 27th February 2014, 06:03 AM   #4
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Quote:
Originally Posted by scott wurcer View Post
jiitteepee fancy meeting you here, PM me and I will send you some AD797's for your project.
Hi Scott!
Thanks for the kind offering but, I'm sticking with this 1028 because of I already have handful of them to use, it's model is bundled with the LTSpice IV so I can make simulations before adding features (this project is more like an experiment / learning happening for me ... ) and it's not very commonly used op-amp in pre-amp design (least I have not seen many).
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Old 27th February 2014, 06:22 AM   #5
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Originally Posted by abraxalito View Post
My suggestion would be to augment the LT1028 with a unity gain buffer (LT1010 might be a suitable choice). For a couple of reasons - first to remove any thermal interactions between the output drive requirements and the input stage. Second to preserve dynamics - the load seen by the LT1028's OPS isn't particularly benign - 0.1uF and 10R in series. Having a separate buffer allows separate decoupling of the supplies and means the LT1028 can work with much cleaner rails.

<edit> On re-reading your post I see you're intending 'flat' operation, meaning no EQ at all? In which case the OPS is going to see a much easier load with those largish fb caps deleted. But then again the output when working 'flat' will be tilted up to HF so dynamics are still a major issue vis-a-vis the supply noise.
Thanks for the info. I added the schematics into my 1st post. Is the LT1010 placed OK there?
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Old 27th February 2014, 06:28 AM   #6
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Its OK as regards the input, but you need to take the feedback from the LT1010's output, not from the LT1028. It should be fast enough to be included in the loop - but if not then hopefully LTspice will reveal any issues.
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Old 27th February 2014, 06:34 AM   #7
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About the power.
I was going to use 12V battery as power source. I need to use a dual output converter to get +-12V. Is the Meanwell DCW03-12 good enough as used on phono preamp project here?
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Old 27th February 2014, 09:00 AM   #8
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Originally Posted by abraxalito View Post
Its OK as regards the input, but you need to take the feedback from the LT1010's output, not from the LT1028. It should be fast enough to be included in the loop - but if not then hopefully LTspice will reveal any issues.
Do you mean I should just send copy of the LT1010 output back to its input or to LT1028 input through some resistor (10k?) or something more complex (by documentation, LT1010 is an unity gain buffer and has less pins to use compared to say LT1028).
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Old 27th February 2014, 09:15 AM   #9
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I mean the output of the LT1028 goes only to the input of the LT1010 (and the bias resistor, if needed). All other components that were connected to the LT1028's output now go to the LT1010's output. See LT's AN4 for several examples of how its used.
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Last edited by abraxalito; 27th February 2014 at 09:17 AM.
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Old 27th February 2014, 09:22 AM   #10
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Originally Posted by abraxalito View Post
I mean the output of the LT1028 goes only to the input of the LT1010 (and the bias resistor, if needed). All other components that were connected to the LT1028's output now go to the LT1010's output. See LT's AN4 for several examples of how its used.
Thanks for the clearing!
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