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Why do I need a Phono Stage?

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BAK View Drop Down
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    Posted: 22 Dec 2021 at 11:02pm

The audio signal generated by a phonograph cartridge (or a dynamic microphone) can be compared to the small heartbeat pulses picked up by a EKG machine, all on an order of magnitude close to 1/1,000th of a volt. The lower the level of signal voltage, the more noise and interference can affect it. 1/1,000th volt is closer to 1/1,000,000th of a volt where noise "lives"... closer to the noise than 1/10th volt to 1/2 volt, normal auxiliary audio input levels. 


A phono stage is a special pre-amplifier needed to bring the small and delicate cartridge signal up "out of the mud" called noise.


Medical electronics engineers design heart monitors to detect and amplify the small 1mV heart pulses. 

(Brainwaves picked up on an EEG are smaller, under 0.1mV.)

They found the pulses can be more accurate with less noise by pre-amplifying the tiny signal closest to the source. The pre-amplifier is built into the pickup wires attached to the patient. The pickup pre-amp is then connected to the ECG or EKG or heart monitor by a cable up to 10 foot long (about 3 meters).

Heart beat signals are EQ'd to a tight frequency bandwidth, only 1Hz to 100Hz or 10Hz to 10kHz for diagnostics... always less than 20 to 20k.

Only after pre-amplifying and EQ-ing at the source are the medical signals then digitized and processed further and/or recorded. 

Why do they do that?

Because the wanted signal is purest with less noise at the source where it is generated.

This is the same for all signals. 

(This was discovered for microphones to have a pre-amp built-in... about the same time medical did.)

It makes sense to pre-amplify all small voltage signals at the source... likewise it is best to EQ those signals in the first few amplifying stages to reduce noise and phase anomalies. 

Medical Grade electronics are some of the most critical designs, mostly with narrower frequency bandwidths than any high quality audio system.

Having myself designed and maintained most medical electronics, this process is the best to reproduce any small signals less than 0.5volts.



Edited by BAK - 23 Dec 2021 at 4:59am
Bruce
AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Reflex M or C, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Using Proprius
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Ash View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Ash Quote  Post ReplyReply Direct Link To This Post Posted: 22 Dec 2021 at 11:29pm
The smaller the order of magnitude of a signal, the more vulnerable it is to EM interference, as the interfering amplitude is a greater percentage of the signal amplitude. Then the result gets amplified by the downsource stages.

Going to prove this to myself when I finally pair the DAK 5V 3A supply with my soundcard and put it into the Enigma-powered Majestic.


Edited by Ash - 22 Dec 2021 at 11:30pm
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BAK View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 23 Dec 2021 at 5:00am
I added a paragraph to my post above.
Bruce
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Post Options Post Options   Thanks (0) Thanks(0)   Quote discrete badger Quote  Post ReplyReply Direct Link To This Post Posted: 23 Dec 2021 at 8:45am
I understand the point being made.

If misunderstood, however, it can be read as a case for putting a phono pre-amp in the cartridge!
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