Converting the AD797 MC Phono Pre to a MM Phono Pre - A Mixed Design.

[USMC Spike]

This thread will discuss the changes necessary to convert the AD797 Moving Coil (MC) phono preamp to
use a Moving Magnet (MM) cartridge.

Hypno Toad worked to create the design of both the AD797MCPRE Printed Circuit Board (PCB) and a MMPRE PCB. These are through hole designed boards.
AD797 thread located here:
http://audiokarma.org/forums/index.php?threads/ad797-phono-stage-build-and-help-desk-thread.501186/

MM thread located here:
http://www.audiokarma.org/forums/in...igh-end-phono-stage-no-expense-spared.371889/

sacgu888 worked to have these boards produced along with a power supply board
for AK members, here is the LINK

My inquiry started with this post:
http://audiokarma.org/forums/index....help-desk-thread.501186/page-71#post-12273730
followed by some additional questions, answers, suggestions, etc.

Wyn Palmer, a retired Analog Devices, Inc. engineer, graciously suggested we create a new thread.

Well, if you want I can explain the differences between the various approaches in terms of overload etc. and give detailed instructions on how to convert the AD797 board to the mixed design. I've done this previously for a friend who wanted a 2.5mv input configuration and I could easily do it for 5.5mv.
If you want you could start a new thread. I redesigned/explained in detail the Leach MC preamp for someone before and it was done in a new thread.
http://audiokarma.org/forums/index.php?threads/the-leach-moving-coil-pre-preamp.838004/

Here is a summary of previous information:
Is there a way to adjust the gain of this Phono amp so that It can be used with
a MM Ortefon red cartridge? Specs as follows:
Output: 5.5 mV
Frequ at -3dB: 20-22000 Hz.
The following are recommended:
Load R: 47kOhms
Load C:150-300 pf

Devices in my inventory:
AD797
LME49990
LME49870
LME49710HA
LME49720HA
LT1115
LT1028
LT1128
LT1468
OPA 134
OPA2134
OPA1641

I've read the RIAA Phono Preamplifier section from Jung, W. (2002). Op Amp Applications,
Analog Devices, Inc. (ch 6.11 - 6.27). While it's good background material, it's not applicable to this discussion.

My turntable is nothing fancy, a working Thorens TD-147, not currently in my system
as it's slated for electrolytic cap replacement and cartridge fitting.

The cartridge I've chosen is suitable for a reentry into vinyl of which I have one milk
crate left of various LPs. The cartridge specs follow:
Ortefon Red MM Cartridge
Output: 5.5 mV
Freq at -3dB: 20-22000 Hz.
The following are recommended:
Load R: 47kOhms
Load C:150-300 pf

 

[USMC Spike]

Here is a brief summary of our discussion. I've tried to edit out some of the non essential information.
Can the AD797MCPRE be changed so that I can use a MM cartridge with it?

Well, the answer to the question is yes- after all the AD797MCPRE design is essentially a fixed gain stage (approximately 80 for a 250uv @1kHz cartridge- the AD797) plus an RIAA de-emphasis stage (The LME49710), but there are trade-offs- such as the cartridge termination RC will be different (i.e. the 47k vs, say, 200 ohms). The MM cartridge also has a large DC resistance (unspecified but useful to know) which makes the AD797 not the best choice for noise/offset.
To accommodate a MM cartridge like the one you are talking about you could just reduce the input stage gain to about 3.6 or bypass the AD797 completely and just use the 2nd opamp with a somewhat increased gain. In fact having the RIAA stage at the input gives the best overload characteristic of all.

Ok, I'll have to figure out how to measure it.

So, are you actually asking for mods to the AD797MCPRE board that will make it compatible with a MM cartridge and not a dual purpose design? It's not that difficult to mod the board- it's just a couple of opamps per channel after all, and the output stage contains all the RIAA "stuff".

Yes

See post #1185 in this thread for the provenance of the original design. (the AD797MCPRE thread, link).

Yes, Walt always did do good work. Well worth reading.

The biggest problem with the AD797 as a MM input amp is that the input bias current is very high. MM cartridges generally have coil DC resistances in the 1k+ region- for example the Shure M97 is 1.55k- and it becomes hard to configure the AD797 so that the output offset is not very high- possibly several mv- and hence the output voltage from the RIAA amp could be c.4v. This is still within the acceptable operating range of the output opamp with 1v RMS out, but it's hardly ideal. In my opinion, you'd be better off trying an amp like the LME49710 which will actually give you lower noise and much lower offset and similar distortion.

Also, with the AD797 input amp if you were to disconnect the phono input at any time while the amp was active the output would likely go to one of the rails which I can't believe would be a good thing for your speakers...
So, this is what to do. Replace the ImpR/L pot with a fixed 47k R. Leave off the input cap (C25/C26).

Ok for the moment.

Set the first opamp GainR/L pot to the max (2k), replace the 47ohm R (R1/2) that goes from the input inverting terminal to ground with 820ohms. Don't make any of my previously recommended changes- which will leave a very large ultrasonic peak in the gain at about 3MHz- or just add a 2200-3300pF cap from the non-inverting input of the 2nd opamp to ground (output opamp side of 390 ohm R (R9/R10)).
This improves the overload characteristics of the amp significantly and also helps to eliminate RFI interference issues.
This will give you a 1v RMS output at 1kHz for a 5.5mv input.

 

[wyn palmer]

Here is a brief summary of our discussion. I've tried to edit out some of the non essential information.
Can the AD797MCPRE be changed so that I can use a MM cartridge with it?

Ok, I'll have to figure out how to measure it.


Yes



Ok for the moment.

So, how do you want to proceed with this? Do you want to ask questions and I will answer- sort of how the Leach redesign thread went?
I'm willing to add fairly detailed explanations with schematics and simulations- then instructions on how to actually implement the changes on the board.
It's a bit of a trade off between too high levels of technical detail that many people will not follow and providing sufficient information so that the changes are not opaque.
It would be really good if you could "follow along" by running LTspice simulations in parallel, just as happened in the Leach thread. It does require a bit of learned expertise to do so, but you might enjoy the experience.

 

[USMC Spike]

Hi Wyn, Yes, and no. I'm more proficient at using NI Multi sim as that is what we use at our community college.
So I can simulate it with that. Also, I have a various general types test equipment.
I think there is a way for me to import spice models into Multisim.

The problem is I don't have a schematic to start with.
I just ordered a couple of boards, so it will be a little
bit before they arrive. In the mean time Lets go.

Thanks,

I sent you a PM (conversation)

 

[wyn palmer]

Hi Wyn, Yes, and no. I'm more proficient at using NI Multi sim as that is what we use at our community college.
So I can simulate it with that. Also, I have a various general types test equipment.
I think there is a way for me to import spice models into Multisim.

The problem is I don't have a schematic to start with.
I just ordered a couple of boards, so it will be a little
bit before they arrive. In the mean time Lets go.

Thanks,

I send you a PM

I don't have access to NI's Multisim. LTspice is free, straightforward/intuitive to use, and has embedded models for several of the opamps you have and adding the LME49710 is simple and I will provide instructions. Heck, although not exactly the same (amongst other things the model has some odd HF behavior), the Lt1115 model that is included in the LTspice libraries is a near drop in replacement for the 49710.
I also can send/post circuit files that you can use.

 

[USMC Spike]

Looks like LT Spice it is.

Hypno Toad send me the link to the schematic, which I'll include where
as the starting point.

Well the image that was below has been removed.

 

[USMC Spike]

Oh the joy, trying to add j2 into a new schematic. one hour and counting.
LTSpice XVII doesn't like me.
Help doesn't recognize "input" RCA Jack, Jack etc.
Can't even place and AC Signal input on the screen.
I have a blue screen with nothing on it in my sim.
Now it wants a search string and won't do anything else.
ESC key does nothing.
Enter closes that dialog box.
I click on the thing that looks like an electrical plug.
select voltage, click ok, then my mouse pointer disappears.
I thought I'd left click and it would place it, NOT.
I right click and zoom out the screen turns gray.
Okay I try again, to place a voltage, no dice.

Now I'll try to place something else.
I picked the resistor ICON and then clicked
into the work gray work space twice.
There were two dots placed on the gray matter.
So small I can't see them, never fear, the + magnifier is there.
I click it, and make is square over the two dots.
the dots disapear. -- More Joy.

I clock the capacitor icon then I
click into the gray space. 10 somethings
that look like fleas on a dog. I can't wait
to see how these will disappear also.
They don't this time, as I went View:ZoomArea.
The small flees moved to the middle of the screen.
Now when I left click, I get more fleas. -- Even more joy.

Back to view: Zoom to fit
whamooo, they move to just below my screen, with
just a tiny piece of the flea on the boarder. I check and
see that both the application and the sim screen are maximized.
somehow I fake it out with the zoom icon and magnify it.
It moves to center screen, by this time, I click the zoom icon
again and drag over the fleas. About 10 times.
I have something, but now it's too big.
I press Ctrl + Z keys to undo what ever I did.
That doesn't work. Wow, user friendly.
There is no more joy to me. I'll post screen shot for
Everyman to view.
Sadly I'll now quit--and I'm not even ahead.



Oh the More, More, More, More, did I say more joy?

What a way to start the new year.

 

[wyn palmer]

I'll send schematics tomorrow. It's pretty simple so I've no idea what the issue is.
Hopefully that will help. Are you using a PC?

 

[sregor]

Looks like LT Spice it is.

Hypno Toad send me the link to the schematic, which I'll include where
as the starting point.

There appears to be an error in the feedback loop of the second IC I believe it should be:

 

[wyn palmer]

There appears to be an error in the feedback loop of the second IC I believe it should be:
View attachment 1373467

Yes, that's correct. There's also the supply decoupling network. I have updated the schematic to include it. I'll send it and my modified MM schematic with the updated RIAA values tomorrow am. Then I can send the split design and simulations.

 

[USMC Spike]

Yes, I'm on a PC. I did finally manage to place a ground arrow.

Then I got about 1/2 devices in the sim, when I tried to rotate
the resistor between ICs I fat fingered the mouse on something
everything disappeared, nothing remains.

F9 and shitF9 says there is nothing to redo or undo.

I can tell this feels like free ware.
So where can I find pots, and RCA, BNC, Jacks and connectors.
LT doesn't seem to have any. Couldn't find the rotate button.
LT can't find "rotate" device, object, jack, etc.

But I did manage to come up with this
in about an hour:



Then I tried to add power (more joy):



Now I can't tell you how much longer that took, because
the first time the AK system said it was 58 minutes.
When I looked at the AK system it now says 8:50 PM.
Where is that happy banana, I want my little girl to start
screaming with joy!

 

[wyn palmer]

Below is the left channel full schematic- original design. The device numbers do not necessarily conform to the identifiers on the board.

Below is the modified (new) design, with a tighter RIAA characteristic and RF rejection added.
The sensitivity for 1v rms out at 1kHz is 432uv.

Below is the MM mod of the above (new) design with the AD797 at the input replaced with an LME49710 and the sensitivity reduced to 5.5mv. RIAA compliance is about +/-30mdB 20Hz-20kHz.


Below is the MM mod- but with a distributed pole-zero implementation of the RIAA (mixed).

Either an LT1115 or a LME49710 can be used interchangeably in this last one with essentially identical results.
The 10pF (C27,C28) is necessary for the LT but not the LME. Without it the LT is marginally stable at HF.
The RIAA compliance is c. +/- 25mdB nominally.
If you would like .asc files of any of the above please ask.
If you have questions about the difference, performance levels etc. please ask and I will add simulation results with commentary.
All of the mods can be relatively easily made on the AD797MCPRE board as they consist mostly of the replacement of components or the placing of two Rs in series or the addition of a shunt cap- either across a resistor or to ground from a resistor. I've tried it. It's not terrifically pretty- but it works.

 

[sregor]

Yes, I'm on a PC. I did finally manage to place a ground arrow.

Then I got about 1/2 devices in the sim, when I tried to rotate
the resistor between ICs I fat fingered the mouse on something
everything disappeared, nothing remains.

F9 and shitF9 says there is nothing to redo or undo.

I can tell this feels like free ware.
So where can I find pots, and RCA, BNC, Jacks and connectors.
LT doesn't seem to have any. Couldn't find the rotate button.
LT can't find "rotate" device, object, jack, etc.

But I did manage to come up with this
in about an hour:

View attachment 1373589

Then I tried to add power (more joy):

View attachment 1373606

Now I can't tell you how much longer that took, because
the first time the AK system said it was 58 minutes.
When I looked at the AK system it now says 8:50 PM.
Where is that happy banana, I want my little girl to start
screaming with joy!

Good work - almost correct. You need to remove the connection between the output of U1 to R4, and to move the connection between R5 and C4 to the other end of C4. I don't see any other errors, but....

 

[wyn palmer]

Yes, I'm on a PC. I did finally manage to place a ground arrow.

Then I got about 1/2 devices in the sim, when I tried to rotate
the resistor between ICs I fat fingered the mouse on something
everything disappeared, nothing remains.

F9 and shitF9 says there is nothing to redo or undo.

I can tell this feels like free ware.
So where can I find pots, and RCA, BNC, Jacks and connectors.
LT doesn't seem to have any. Couldn't find the rotate button.
LT can't find "rotate" device, object, jack, etc.

But I did manage to come up with this
in about an hour:

View attachment 1373589

Then I tried to add power (more joy):

View attachment 1373606

Now I can't tell you how much longer that took, because
the first time the AK system said it was 58 minutes.
When I looked at the AK system it now says 8:50 PM.
Where is that happy banana, I want my little girl to start
screaming with joy!

There are a few errors.
I've attached a schematic that might be of some help.
It includes a couple of the prior mentioned schematics with the LME49710 replaced with the LT1115.
It does include an AD797A which you will not be able to simulate without adding the device to your libraries, but it should get you off to a good start.
You need to change the .txt extension to .asc then open it in LTspice. I think that works even if you don't have the model for the AD797A.
By the way, I should make it clear- I prefer the LME to the LT. The LT model has some pathological issues in the 2-10MHz region that I don't like. The LME has none of these issues and in my opinion is a more solid design- assuming that the macro models are accurate.

 

[USMC Spike]

Ok, I see them. I am unsure of what they mean, so please forgive my ignorance.
One thing that would help, is how to rotate components in LT spice.

AND

Generally, how do the sims correlate to real world listening of the devices that
we are discussing?
AD797
LME49990
LME49710
LT1115

Can you also send the LTSpice info for the devices to include into my library?

I'm not sure where to go from there at the moment. So, if there is something I need
to know or that would be helpful background wise for those following along please
continue.

That being said I thought of some generalities perhaps you could explain.
I read alot and see in the data sheets specifications for the following:
Power Supply Rejection Ratio (PSRR)
Common Mode Rejection Ration (CMRR)

For example, the LME4990 has the following:
PSRR: 144dB
CMRR: 137dB

If I understand it that would be the ICs ability to separate the music signal from the noise
that is in the background resulting from the powering the IC and the electro-mechanical noise
of the devices on the board etc.

Somewhere among the PCB layout there is susceptibility of RFI that can enter the signal
chain and corrupt the music signal and cause problems with the various SS devices, yes?
I'm not sure I have a good understanding of the causes. For that matter, I'm also not sure
that I understand the solutions...other then button things up and keep parts isolated from
other parts in the circuit.

I've read about this on the boards and forums and also we discussed it briefly in my some
of my EE tech classes. When I asked for specifics they were short on answers.
When I inquired as to why Florescent light figures were noise or mentioned that
cheap light dimmers can be noisy I was flat out told they were not. For the latter,
I am not talking about the large rheostat type dimmer on isolated plates that are
used for studio control room dimming.

In the mean time I'll try and get the attached sim into my LTSpice and get it running.
If you don't mind you can send the the other 2 or three sims that you provided images
to, so that I have them in my sim library.

Thanks,

Yes, there is a void where the IC would be....right dead smack in the middle.

 

[wyn palmer]

Ok, I see them. I am unsure of what they mean, so please forgive my ignorance.
One thing that would help, is how to rotate components in LT spice.

AND

Generally, how do the sims correlate to real world listening of the devices that
we are discussing?
AD797
LME49990
LME49710
LT1115

Can you also send the LTSpice info for the devices to include into my library?

I'm not sure where to go from there at the moment. So, if there is something I need
to know or that would be helpful background wise for those following along please
continue.

That being said I thought of some generalities perhaps you could explain.
I read alot and see in the data sheets specifications for the following:
Power Supply Rejection Ratio (PSRR)
Common Mode Rejection Ration (CMRR)

For example, the LME4990 has the following:
PSRR: 144dB
CMRR: 137dB

If I understand it that would be the ICs ability to separate the music signal from the noise
that is in the background resulting from the powering the IC and the electro-mechanical noise
of the devices on the board etc.
Somewhere among the PCB layout there is susceptibility of RFI that can enter the signal
chain and corrupt the music signal and cause problems with the various SS devices, yes?
I'm not sure I have a good understanding of the causes. For that matter, I'm also not sure
that I understand the solutions...other then button things up and keep parts isolated from
other parts in the circuit.

I've read about this on the boards and forums and also we discussed it briefly in my some
of my EE tech classes. When I asked for specifics they were short on answers.
When I inquired as to why Florescent light figures were noise or mentioned that
cheap light dimmers can be noisy I was flat out told they were not. For the latter,
I am not talking about the large rheostat type dimmer on isolated plates that are
used for studio control room dimming.

In the mean time I'll try and get the attached sim into my LTSpice and get it running.
If you don't mind you can send the the other 2 or three sims that you provided images
to, so that I have them in my sim library.

Thanks,

Yes, there is a void where the IC would be....right dead smack in the middle.
View attachment 1373901

The rotation commands are the clockwise and counterclockwise circular arrows about four icons from the right on the control bar at the top.

http://audiokarma.org/forums/index....age-build-and-help-desk-thread.501186/page-61
Post 1210 explains how to add the AD797A model. The LME49710 is a bit different and I'll have to dig through my memory to remember how I did it.
I'll post the info later.

How do the sims relate to real world listening?
Who the heck knows. People claim all sorts of preferences based on, well, not very much that makes any kind of sense from an engineering perspective, and there's an awful lot of confirmation bias out there. So, ultimately caveat emptor...
There really is not much that I can say in a few paragraphs that will not create controversy- which is not my goal- so I'll try to be as objective and as terse as possible while not pretending that the lists of the relative merits are all inclusive.
In my opinion, the one that generally makes the least sense from an engineering perspective is the passive design (the CNC preamp)- it tends to have the worst noise performance and the greatest susceptibility to overload- but it does have the magic of no capacitors in any feedback loop and the interstage filter that defines the equalization has excellent RF rejection. And yes, I know that capacitors have non-linearities, but people also prefer caps in some cases that are measurably worse than others they dislike- so what can I say?
It's also possible to design a passive equalization stage that doesn't have the built in voltage attenuation, but that's beyond the scope of this exercise.
The "all the equalization at the input stage" approach has the best noise performance and the best overload characteristic, but has poor intrinsic RF noise rejection as the gain remains unity up until very high frequencies, and having all the gain in one stage leads to less feedback at HF which can increase the deviations from Ideal for the RIAA stage and also the distortion.
The mixed design has excellent overload characteristics as the HF pole is placed directly at the input. The input opamp becomes unity gain at frequencies of a few kHz and above which counteracts the RIAA preemphasis boost prior to the low pass needed for the LP cutter at or above 40kHz . It is then followed by a second HF pole which provides RF rejection and even better overload protection for the output stage. The final opamp provides the low frequency boost needed for the equalization- at frequencies where there is lots of gain for distortion reduction- then it becomes constant gain at frequencies above 500Hz or so. It also has good noise performance so it's a decent solution on all fronts.
Yes, opamps can have very large power supply rejection and common mode rejection. Both of these decline with increasing frequency. for a variety of reasons.
The mechanisms for noise pickup are complex and beyond the scope of this write up. However, it's beneficial to have good HF power supply rejection and good RF rejection in the amp signal path. RF components can cause intermodulation products with desired signals that can fold down into the audio band and cause audible issues.

 

[EngineerNate]

This thread is fun. Subbed.

 

[USMC Spike]

Getting closer, it still doesn't work.
I was able to clean up the schematic a bit.



Maybe someone can assist who's following along.
Wyn, do we have the LME49710HA Data or do we need
to find it?

I've spent over and hour on the ADI website reviewing a video
it was overly complex so I stopped. I placed something in the
sim relating to the AD797 but got stuck over in the
I for get the name, I used opamp2, then right clicked
it and remained it. I'll be damned if I could figure out how
to place the device in LTSpice.

So then LTspice is ok for getting the electronics part working
to see if we are in the ball park and hopefully won't blow everything
up; (i.e., letting the sacred smoke out of the devices.)

From there we have to assemble the PCB with components, test it
then start listening and measuring.

Engineering versus Listening, I recall something from that guy from
the U.K., who spent time in India's sunny climb, a serving of Her Magistracy
the Queen....

Now east is east and west is west and ne'er the twain shall meet. -- Rudyard Kipling.

At least we know that going in.

NOTE: Did the additions according to the post#1210.

How do I create a, "A .lib spice directive needs to be added for AD797.sub"?
I know these are kind of basic questions... bare with me.

Post Script - Yes, I found the rotation icons up there finally.
Before, I was using the undo circular back arrow which was
causing me fits.

Post Post Script, I'll try adding the AD797 to it and see if it works.
Realization, that now I have included the device to /subs and /lbs
Your schematic might work on my 'puter.

 

[EngineerNate]

I see your problem, at least one of them. You've set V3 and V4 to -15V and then hooked the opamps up to the '-' side of the voltage source. If you input a negative voltage they'll output a negative voltage on the "+" side of the voltage source. As a rule I hook the minus side of DC voltage sources in LTSpice to ground.

My preference, if the schematic can accomodate it, is to setup Vplus and Vminus "rails" in LTSpice with a single voltage source for each voltage. I'll only add a second if it's really really cumbersome to route that voltage where it needs to go with wires. In your schematic, I'd have a +15 and a -15 off either to the left or right above and below the existing schematic with horizontal "rails" drawn in and run wires down to each opamp for power.

 

[sregor]

Looks like you have a ground on the input signal line.