Isotope: Opamp based MM phono - diyAudio
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Old 23rd December 2016, 04:02 AM   #1
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Default Isotope: Opamp based MM phono

I am working on a MM phono preamp. Its based on the LME49720 datasheet circuit. There are quite a few existing designs based on this circuit like CNC, muffsy etc but I wanted to make my own board.


I have used jumpers to switch between the two sets of input loading. A dpdt switch can also be used for switching this. This way two different TTs can be supported.


I have added a jfet ccs (Idss – 6-8ma) to both gain stages to run them in class A mode. Its a single shared ccs for both gain stages of a channel.


I have quite a few opamps, so will experiment with those, like ne5532, opa2134, 49725, 4562 etc.


I plan to use a dual shunt regulated supply at about a volt less than the max allowed by the opamp.


Any suggestions to make it better?

Updates:
CCS - Removed
Used a different RIAA network. It sims better and sounded better to me.

Final Schematic -
Click the image to open in full size.

Simulation results -

Click the image to open in full size.

PCB Layout -

Click the image to open in full size.


PCBs and Kits are available Now.

Pricing:
Bare PCB – 7$ each.
PCB and 2ch Components kit (Tightly Measured RIAA and Gain Components) – 30$.

Shipping to anywhere:
For PCB only – 2$ for first 2 PCBs, 1$/PCB after that.
For PCB and Kits – 3$ for first two kits. 1.50$/kit after that.
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Last edited by doors666; 2nd March 2017 at 10:04 AM.
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Old 23rd December 2016, 04:01 PM   #2
rsavas is offline rsavas  Canada
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target 100x100mm max to get pcb fab deals
C107/207, move down like C105/205
Thick returns on C104/204
R115/215 does not need to be so high, 100 is fine
C112/212 should be bigger, 10uF min, what a is the worst case load?
copper pour ground to the connector, thermal pad, no need for narrow traces.
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Old 23rd December 2016, 05:27 PM   #3
madis64 is offline madis64  Estonia
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R110 and R111 - shouldn't they be connected "in series"?
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Old 23rd December 2016, 05:43 PM   #4
rsavas is offline rsavas  Canada
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I missed that one
simulate the design in ltspice first, just to be certain you got it right.
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Old 23rd December 2016, 05:55 PM   #5
madis64 is offline madis64  Estonia
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Quote:
Originally Posted by rsavas View Post
simulate the design in ltspice first, just to be certain you got it right.
Quick sim shows it to be "pretty awful" ...

Last edited by madis64; 23rd December 2016 at 06:01 PM.
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Old 27th December 2016, 03:35 PM   #6
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Quote:
Originally Posted by rsavas View Post
target 100x100mm max to get pcb fab deals
C107/207, move down like C105/205
Thick returns on C104/204
R115/215 does not need to be so high, 100 is fine
C112/212 should be bigger, 10uF min, what a is the worst case load?
copper pour ground to the connector, thermal pad, no need for narrow traces.

I plan to use a 2.2uf cap at the output. Cap is 27.5mm pitch, so many 10uf may not fit. It will be driving a preamp, so I guess the worst case load will be 20k. Its probably not usual having a preamp with 10k input impedance.


Quote:
Originally Posted by madis64 View Post
R110 and R111 - shouldn't they be connected "in series"?
Good catch, thanx a lot...

Quote:
Originally Posted by madis64 View Post
Quick sim shows it to be "pretty awful" ...

Actually its not all that bad, but looks like the ccs in the second stage is messing up the hf response. I took out the ccs. The 10R didnt work with the ccs, 100R works ok, but hf issue remains.

I changed the RIAA values for a better response, looks quite good to me. I added the simulation and other images to the first post. The variation is about 0.1db in the response.
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Last edited by doors666; 27th December 2016 at 03:47 PM.
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Old 27th December 2016, 05:15 PM   #7
madis64 is offline madis64  Estonia
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Quote:
Originally Posted by doors666 View Post
I changed the RIAA values for a better response, looks quite good to me.
Can you put up a simulation with CNC phono original filter values?

So that we could see if your version is actually better.
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Old 28th December 2016, 04:15 AM   #8
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Quote:
Can you put up a simulation with CNC phono original filter values?

So that we could see if your version is actually better.
Sure, here's the cnc phono sim. I kept the gain and load same. Its quite good, the plot is within 0.25db. Mine is in 0.1db with a flatter plot.

Click the image to open in full size.
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Old 28th December 2016, 12:28 PM   #9
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I was able to combine both into a single screen shot. Green is cnc and blue is isotope.


Click the image to open in full size.
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Old 28th December 2016, 12:36 PM   #10
madis64 is offline madis64  Estonia
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Where have you taken the signal source schematics from?

I am using one (from :: View topic - RIAA and iRIAA accuraccy in the real and simmed world. ) with a bit other values for the invRIAA-filter and getting a better result with the CNC phono standard values.
Also Rser has been set to 0 .

IMO C1 should be set to something like 100...200 pF to resemble "real life"...

And how do you get the gridlines on the graph?
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