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Old 1st February 2013, 08:44 PM   #1
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Talking Phono Stage w/split passive RIAA, Instrumentation input

Here is my stab at a phono preamp, with differential MM and MC input stages, passive split RIAA, and twin secondary+output stages, inspired by the Jensen twin servo mic preamp.

I'm building in an industrial Viking RP120, a 60s-era tape preamp, with four XLR inputs and two out, which had four octal-socket input modules for balanced or unbalanced line/mic, that I am now using two of for the MC stage (could be either active, as designed, or transformer), and two for the RIAA modules. So far, these modules are all I have actually built, so scrutiny is only welcome everywhere else.

Input stages use an instrumentation amplifier topology: 3 op-amps (NE5532/4) for MM, and INA217 (or SSM2017) for MC. I'm using a mil-spec sealed relay with gold contacts for switching between the MC and MM stages' outputs, prior to DC filtering since I figure the more electrons flowing through the relay the better, at the expense of a potential thump when switching. RIAA EQ is done in two stages, and the values of the resistors in the DC filters are accounted for in each.

I'm not 100% sure of the input loading. I wanted a load across the input terminals, and I know I needed paths to ground (float->bal), so I did both equally. Is there a method to use here? Thanks in advance for taking a look!
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Old 2nd February 2013, 07:22 AM   #2
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I have built a similar one with LT1028 for low impedance MC (60dB)
This alowd for much smaller resistors on the Imput than SSM2017
It was 3/4 years ago and I will have to dig up the SCH
cartridge loading is just a reistor / capacitor in parallel to carttridge
No need to go to ground as you can keep cartridge floating and the third op amp is grounded
This configuration of the INstumentation Amplifier that let you change gain with just one resistor easy peasy to get total gains from 40 to 60dB and you can use paralel combinations to switch on and make signal path simpler
so MC MM is done at imput and passive RIAA s are not disturbed
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Old 2nd February 2013, 09:35 AM   #3
Calvin is offline Calvin  Germany
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Hi,

signal- and noise-level-wise a symmetric input stage using an instrumentation amp is absolutely sufficient to accommodate for lowoutput MCs up to highoutput MMs. But if You intend to make switchtable inputs, I think itŽs a good idea to have some gain first, since most relays are not even specced below 100”V. Also some relays like the Omron G6K react very sensitive to coil supply and layout and may show high crosstalk. IŽd rather parallel the relay contacts and give each channel its own relay, than sharing the relay between the channels.
Since most MMs are not balanced sources, IŽd choose an unbalanced MM input stage using a lownoise JFET OPamp like the AD745 that offers a unique combination of low voltage and current noise.
For the balanced MC-Input You may also look at the INA103 and INA163 of BB/TI. The ADI SSM2017 requires quite high input bias currents, app. double that of the BB/TI parts, is specced for >2kOhm output loads and requires larger Gain setting resistors. The BB/TI work very especially well at gains around 100x (+40dB). Due to the high gains, one must take into account considerable amounts of output offset voltage which may reduce the headroom. A active DC-servo, feeding into the ref-pin can take care of this and would render the additional LF-Filter between stages 2 and 3 obsolete.
If the -3dB freq is designed to be 20Hz instead of the typical 100mHz You meet the RIAA/IEC1963 requirements at the same.
IŽd also have a lowpass after the input gain stage, but just the passive 75”/2120Hz Filter. IŽd then choose a gain stage with the 3180/318” equalization in the feedback path. This would require 20dB less gain.
Imho any possible advantage of a pure passive equalization will be completely swamped by the disadvantages of +20dB higher required gain and more required parts As Opamp IŽd choose a FET-type also, since it is more immune against the frequency dependant source impedance and impedance differences between the OPamps inputs. A OPA134 would come to mind. If You use a Dual Opamp like the OPA2134 You can use the second OPamp as the DC-servo for the input stage. The second (and 3rd.) stage donŽt require varying gain. The gain setting of the input stage will be all You need. You wouldnŽt need a Buffer stage after the OPamp but You could add a buffer, either on its own, or inclosed to the OPamps feedback loop. If balanced outputs are wished for a dedicated balanced line driver like the SSM2142 or DRV134 may be used.
All this put together resukts in a short, straightforward signal path with a low parts count, high flexibility and excellent results.
Examples for such phono stages can be found here at DIYaudio and on my website

jauu
Calvin

ps. btw. You might rethink Your Pin1 connection.
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Old 2nd February 2013, 04:21 PM   #4
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And a prety good one is FDW balanced
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Old 3rd February 2013, 07:06 AM   #5
Calvin is offline Calvin  Germany
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Hi,

I kindly disagree
The FDW is kind of a 'questionable' overkill in the power supply. Stuffing a supply with tons of caps doesn't guarantee for best results. Imho it is rather wasted money and space.
The design of the Input stages is off of optimum. I don't know why one would want an OPamp for a balanced input stage if there are enough high quality INAs around? Even built from 3 OPamps or as with a discrete frontend it'll be very hard to beat the INAs.
A OPAmp input is also not balanced at all. The inverting and noninverting ins don't behave the same and gain setting will be problematic. INAs are much more conveniant not only in this regard.
Configured as they are in the FDW the high impedances add noise, and probabely are the input resistors the dominant noise source here. A low current noise OPamp would be needed, rather a FET-type than a bipolar.
The layout also leaves room for improvements. The supply current paths layout is highly important, the more so the 'faster' the OPamps are. Surprisingly each OPamp is just HF blocked, but no local Joule reservoir. Instead there are long inductive traces. Its sad to see so many precious devices beeing compromised in their possible performance by inferior implementation.
Besides, its claimed to be a low parts count design, but it certainly isn't.

You may want to read the thread "Schematic for Pro-ject phono box" from #62 for a INA based balanced Input stage.

jauu
Calvin
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Old 3rd February 2013, 02:52 PM   #6
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Quote:
Originally Posted by Calvin View Post
Hi,

I kindly disagree
The FDW is kind of a 'questionable' Calvin
Sorry Calvin you asked and I did reply my big mistake so not to worry and ...
Why FDW is questionable did you ear it? did you build it?

I did ....
Al the best and have nice day
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Old 3rd February 2013, 02:58 PM   #7
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Quote:
Originally Posted by Calvin View Post
Hi,

I kindly disagree .... I don't know why one would want an OPamp for a balanced input stage if there are enough high quality INAs around? Even built from 3 OPamps or as with a discrete frontend it'll be very hard to beat the INAs.
jauu
Calvin
Just have a look at the resistors values and as it seems that you know quite a fair bit about things do your own calculations for Jhonson noise.

I had 3 X LT1028 with resistors as low as 30 Homs and could have gone lower.

But this is just not what you want to ear so please do ignore it
Have a nice day again...
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Old 3rd February 2013, 03:15 PM   #8
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Quote:
Originally Posted by Calvin View Post
Hi,

I kindly disagree
A OPAmp input is also not balanced at all. The inverting and noninverting ins don't behave the same and gain setting will be problematic. Calvin
Sems to me that this go nought to do whit litle SCH I have posted
and the reason I posted that is that it does show the connection to ground on one resistor, so cartridge can be floating...and the loading part goes just in parralel as standard, which I think is just an answer to what was asked but if one realy realy need to argue about....

Why is a MM not balanced?
it is just a bit of vire wound up in to a coil same as MC
one got coil moving against a magnet other is a magnet moving against a coil

Same do have tag to Ground that can be cut by the way but the bit of vire wound up does not change so where did you read about it
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Old 4th February 2013, 06:07 AM   #9
Calvin is offline Calvin  Germany
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Hi,

yes, #5 commented only about the FDW, and on a strictly technical level, as I may add.

If You want to use or build an instrumentation amp, the 3-OPamp structure is the most consequent and the LT1028 could be a good choice for a lownoise design. I'd probabely just choose a different OPamp for the 3.rd, the differential stage. Still though the Q remains, what could be the advantage of a 3-OPamp implementation against the integrated solution in form of the above named devices from ADI or TI? I haven't found any that is of major importance

Quote:
Why is a MM not balanced?
I didn't say that! The signal generating principle is of a balanced kind, same as for MCs. That 'most MMs are no balanced sources' (cited correctly) though is due to the ground tag you mentioned yourself.


jauu
Calvin
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Old 4th February 2013, 08:54 AM   #10
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LT1028 is a low voltage noise and on paper it looks good till one find out about the current noise it has

On a pure technical point using an complete INA chip limits the choice of values for the resistors in the circuit
INA163 use 3 K and 6K resistors for example, with LT1028 I was using 30 and 120 Hom's
this will also warrant the use of different values for the gain resistor
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