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Old 20th May 2016, 11:53 PM   #1
Nanook is offline Nanook  Canada
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Default phono stage building blocks...

Hi all,

So although no electronics idiot (okay, I probably am), I've never had anything to do with phono stages, other than owning them. I would like to understand fully what actually goes on in an RIAA phono stage (other than the RIAA equalizer). I understand the concepts of time constants, etc, and understand that in various regions of the RIAA curve different time constants are required. There are many excellent kits and DIY phono stages out there, but I just need some help coming up with a block diagram of what is needed (I do have an intended use for the outcome of this thought experiment).

So as I understand it, an RIAA can be completely passive (no active components, except an output stage), Transformers can be used on the inputs to step up voltages (or a small amplifier to provided 20dB-40dB input gain prior to the actual phono equalizer if the cartridge is a LOMC.) and another small amplifier can be used as a gain stage on the output of the phono equalizer to allow lower impedances to be driven (so essentially a buffer stage). Is this correct?
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Old 21st May 2016, 12:12 AM   #2
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From what you have written you already know pretty much everything there is to know.
Your diagram looks ok to me.
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Old 21st May 2016, 03:01 AM   #3
Nanook is offline Nanook  Canada
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Default thanks Nigel...

... I guess I just needed someone to verify that for me.
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Old 21st May 2016, 11:03 AM   #4
DF96 is offline DF96  England
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Quote:
Originally Posted by Nanook
So as I understand it, an RIAA can be completely passive (no active components, except an output stage), Transformers can be used on the inputs to step up voltages (or a small amplifier to provided 20dB-40dB input gain prior to the actual phono equalizer if the cartridge is a LOMC.)
No. The output from MM is far too small for just a transformer. You need some power gain; all a transformer does is change impedance. MC may use a transformer plus a gain stage.
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Old 21st May 2016, 02:12 PM   #5
piano3 is offline piano3  United Kingdom
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Even for a MM cartridge, the active devices need to provide, say, 78 dB of gain which the RIAA network will reduce by 20 dB at mid-frequencies relative to the lowest.

Occasionally, those involved in transferring old recordings (78's) use passive equalisers or even do the equalisation in the digital domain but this is after a flat preamplifier with considerable power gain and commensurately low output impedance.
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Old 21st May 2016, 02:15 PM   #6
adason is offline adason  United States
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Nanook, you are missing the understanding of impedances. Each block you have can not just simply drive next block, it does need current drive capability, that means low output impedance.
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Old 21st May 2016, 06:35 PM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Nanook View Post

So as I understand it, an RIAA can be completely passive (no active components, except an output stage), Transformers can be used on the inputs to step up voltages (or a small amplifier to provided 20dB-40dB input gain prior to the actual phono equalizer if the cartridge is a LOMC.) and another small amplifier can be used as a gain stage on the output of the phono equalizer to allow lower impedances to be driven (so essentially a buffer stage). Is this correct?

Hi,

Whilst fully passive RIAA is allegedly the only way to do it
according to hifi snobs, the technical reality is very different.

RIAA stages slug the response with a baseball bat 1st stage and then
repeat the same treatment 2nd stage. Much nonsense is written about
the subtleties of what is effectively pretty much a glorified integrator.

Passive is a bad way to do it, as any technical analysis will indicate.
Active is far better in all technical respects. Nobody builds passive
integrators because the active version is just so simply superior.

However if you want to go pointlessly complicated, go passive.

rgds, sreten.

FWIW you can make a killer specification MM RIAA
stage with a 5532 dual op-amp wired in parallel
and all the RIAA EQ built into its feedback loop.

Last edited by sreten; 21st May 2016 at 06:42 PM.
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Old 21st May 2016, 07:05 PM   #8
piano3 is offline piano3  United Kingdom
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There is a very interesting chapter on equalisation in "Understanding hi-fi circuits" by Crowhurst (1957). His conclusion is that RIAA is not most accurately achieved in a feedback loop.
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Old 21st May 2016, 07:31 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by piano3 View Post
There is a very interesting chapter on equalisation in "Understanding hi-fi circuits" by Crowhurst
(1957). His conclusion is that RIAA is not most accurately achieved in a feedback loop.
Hi,

The calculations to do RIAA all at once in a feedback loop accurately are
horrendous (due to interaction) compared to a two stage implementation.
(Two stage can be passive or active, neither is complicated for accuracy.)
However they were done by Lipshitz in 1979, 22 years later.

rgds, sreten.

See D.Self's "Precision Preamplifier" for values.

Last edited by sreten; 21st May 2016 at 07:38 PM.
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Old 21st May 2016, 08:07 PM   #10
SY is offline SY  United States
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Piano, that was true in the days when open loop gain was limited. These days, one can achieve excellent results with passive, active, or a combination.
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