Discrete Op Amps used in a Phonostage

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JohnG
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Discrete Op Amps used in a Phonostage

Post by JohnG » Sun Jan 21, 2018 8:57 pm

I have a longterm owned MM Phonostage that I now have the knowledge of how to convert it to a MC.
It is a case of adjusting the Load Resistor, and exchanging two of the AD743 Op Amps for two AD797 types.
I am very familiar with the performance of this device, so am curious to investigate how to get a improved performance from it.
Would using Discrete Op Amps for the AD743 ( part not found yet ) and AD797 ( various options found ) be a better choice of component ?

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jack
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Re: Discrete Op Amps used in a Phonostage

Post by jack » Sun Feb 04, 2018 8:26 am

It'd be slightly interesting - the 797 is a very well known low noise opamp - you are looking at a MASSIVE drop in THD from .0003% to 0.0001% which is basically as a result of the 797 having a noise figure of 0.9 nV/rootHz vs. the 743s 2.9 nV/rootHz. The figure that you might be able to hear is the genuinely much improved slew rate of 20 V/uS vs. the 743's 2.8 V/uS. The 797 has a much better GBW (gain bandwidth product) - 110 MHz vs. 4.5 MHz - this is very important in this application - see below.

Opamp rolling is a pretty standard thing to do, but at those levels it'll likely be the slew rate not the THD that you'll notice...

Main thing is the gain difference - MM gives about 5mV, MC maybe 0,2mV, so a 25x (or thereabouts) greater gain is needed from the opamp, i.e. the resistor you're going to change will up the block gain by a factor of 25 or so. As you up the gain, the bandwidth drops proportionally.

It just so happens that the 797 has about a 25x better GBW than the 743, so the overall bandwidth with the new higher gain is maintained.

i.e. probably a decent way to go.
Would using Discrete Op Amps for the AD743 ( part not found yet ) and AD797 ( various options found ) be a better choice of component ?
Not sure what you mean by this - if you genuinely mean trying to make a better opamp yourself out of discrete components, then good luck with that. AD and others spend 10s of millions of USD getting this stuff right. Unless you are a really determined specialist EE designer, it's highly unlikely that you'll achieve a better spec. And, don't forget, the noise & THD levels from the 797 are so low anyway that you're highly unlikely to be able to hear the difference. Other noise & distortion factors will likely make a larger difference anyway - choice of capacitors, PSU design, PSRR, board layout, screening, Johnson-Nyquist, Shot & Flicker noise etc...

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Nick
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Re: Discrete Op Amps used in a Phonostage

Post by Nick » Sun Feb 04, 2018 10:45 am

Also with the 797, make sure you have decoupling that matches the spec sheet, they are lively little buggers. And another also is to pay attention to the resistor values around the thing as you are in the place where the noise of 47R resistor matters.
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Re: Discrete Op Amps used in a Phonostage

Post by jack » Sun Feb 04, 2018 11:15 am

Nick wrote:
Sun Feb 04, 2018 10:45 am
Also with the 797, make sure you have decoupling that matches the spec sheet, they are lively little buggers. And another also is to pay attention to the resistor values around the thing as you are in the place where the noise of 47R resistor matters.
Very true - decoupling as per the datasheet - right up close to the pins. High GBW opamps are great, but they do have a tendency to HF oscillate. In this case, the bandwidth has been bought right down by the high gain, but do double check - page 13, figure 36 of the datasheet is very explicit on this.

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Re: Discrete Op Amps used in a Phonostage

Post by JohnG » Sun Feb 04, 2018 6:14 pm

Thank you for your guidance, it is very technical, so is a little bit beyond my capacity to grasp, if I made the schematic available this may help with your observations, I have not as yet posted a image on this forum, I will familiarise myself with the process.
As for building a discrete device from scratch, that is not a possibility for me, it is way beyond my wire a plug capability.
I am thinking of a Burson Discrete or competitor brand. Burson do a 30 Day trial, so that seems the best place to start, as the experience of the device should only cost the postage if not felt a valuable contribution.
I also discovered a interesting device this weekend, a single AD747 8 pin dip conversion to Dual type.

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Re: Discrete Op Amps used in a Phonostage

Post by jack » Tue Feb 06, 2018 12:13 am

There are many breakout/carrier boards available to convert package formats.

One caveat on these - they increase the inter-pin capacitances and can adversely affect the top end frequency response of the stage. Also, as mentioned, decoupling should be right up against the chip. With carrier boards, this can be difficult/fiddly.

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