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Building phono preamp based on platINA
Building phono preamp based on platINA
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Old 24th October 2017, 07:07 PM   #1
ghzgary is offline ghzgary  United States
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Default Building phono preamp based on platINA

Hi all, I plan on building an RIAA phono preamp board from scratch for personal use. I really like the platINA design for a number of reasons:

1. It's balanced input. The cartridge is a balanced device anyway so why not go that way and get rid of all the common mode noise.
2. The configurabilty with regards to gain and cartridge loading.
3. Relatively simple design (I will be doing my own PCB)
4. The use of an INA, that seems purpose made for a true balanced design.

My two sources of information are:
1. platINA | Crazy Audio
2. Calvin-phono/PlatINA/RP1 - calvins-audio-pages

If there are any other sources I should be looking at please let me know.

I plan to design and layout my own PCB (using Altium). It will either be a 2 layer or 4 layer design. I don't have any qualms about spending more if there is a technical reason to use 4 layers. I also don't have any qualms about using SMT, actually prefer it over through-hole. I have good soldering skills and equipment including a microscope.

This PCB I plan to mount directly inside my turntable (in an underneath cavity) going on the theory to place small signal amplification as close to the source as possible. I plan to power the amp with a couple lead-acid batteries, at least initially. Maybe later I will add a line supply.

I have gone through the BOM listed on source 1 and many of the parts are no longer available. I have found alternate parts for most all of them, trying to use the same type of part where possible. I plan to use the INA163 rather than the INA103P, no big reason, just that the INA163 looks to be slightly better on paper.

I am attaching the schematic so far. On the adjustable R's and C's at the input, I am using 8 position dip switches (rather than 6 position)so I have a few more options. Not a lot of science has gone into picking the extra values.

At this point, I have started laying out my PCB, but I wanted to get some feedback from the forum first to make sure I wasn't doing anything stupid before I got too far. I also want to publicly thank the designer(s) of the platINA, it looks to be a well thought out design!

I look forward to constructive criticism!
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Old 25th October 2017, 01:01 AM   #2
PRR is offline PRR  United States
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It is a well-considered circuit, but nits can be picked.

The INA103** has low hiss voltage but high hiss current. It is optimized for around 500 Ohms (for audio). The canonical phono cartridge may be <1K below 500Hz, but rises to 47K around 15KHz. Then we apply a low-pass. The result can not be expressed simply, but for "audible hiss" the relevant source impedance is several K Ohms, not 500 Ohms. I would expect hiss to be higher than some other phono designs.

The phono cartridge is a floating source and inevitably used with short wires. Balanced is not a real blessing except in adverse conditions (long lines or nearby interference). I've always had good results with grounded inputs or semi-floating inputs. *Especially* for preamp inside table with no preamp power transformer in sight (but AC motor?). IMHO balanced input just makes the designer's job harder. (The existence of pre-designed parts like INA103 muddies the water-- but INA103 is optimized for lower-Z sources than a phono....)

Others will have different opinions on these points. Balanced is not wrong. Preamp hiss should never be the limit when the needle is in the groove- groove quality dominates.

Number of layers.... _I_ would lay it out Single Sided. This is not a "messy" topology with hundreds of crossovers. Certainly more complex things have been built single-sided. Because double-sided is now the default at most send-away fab houses, I'd get 99% done single sided and use the other side to avoid hand-jumpers and be a ground plane. I sure can not see 4-layer etc. Having been a 3-D tic-tac-toe player in my youth, I know how mind-bending the middle layers can be, and I see no reason you should need them.

** I did not see INA163. It seems to be optimum for 1K sources (at audio). This lessens my concern for current hiss. I still think there are other paths.
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Old 25th October 2017, 03:52 AM   #3
Calvin is offline Calvin  Germany
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Hi,

the afore mentioned INAs have a optimum noise impedance around 200-500Ohms ... which makes high output MCs the best choice noisewise.
Still though the voltage and current noise figures are so well balanced that it suits MCs from ~150uV to MMs with up to 10mV.
The very wide possible gain range of the INAs from a couple of dBs up to 60dB allows to accomodate for almost any cartridge on the market.
Together with the input loading this allows for a very high flexibility of the phono stage.
Built with OPAmps instead one almost always needs to design an extra MC-prepre, ahead of a dedicated MM input stage --> added complexity.
The possibility of a balanced input is then more of a cream topping than a necessity, especially if the cabling can be kept very short.
But then ... the transformers typically used in turntables are cheap types with considerably spread stray fields.
In praxis the PlatINA has proven to be a very lownoise design, regardless of the connected cartridge.
When built with sufficiently tight tolerated parts the linearity of the amplitude response is very high and one doesn't need to match or screen parts.
The chance of building success is very high ..... a multimeter is recommended but not necessary.
Fancy measuring equipment is not required ... there's nothing that requires tuning or trimming.

jauu
Calvin
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Old 27th October 2017, 12:29 PM   #4
ghzgary is offline ghzgary  United States
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Thank you PRR and Calvin for your thoughts. I will be posting more on this thread as I progress on the project.
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