Forum

Magnetic Phono Prea...
 

Magnetic Phono Preamp  

  RSS

Speaker Builder
(@speaker-builder)
Reputable Member Registered
Joined: 3 months ago
Posts: 251
06/06/2019 9:16 pm  

This is the magnetic phono preamp I built into my DIY preamplifier years and years ago.

The original design used th UA741 op-amp. This re-design uses the NE5532 Op-amp and expands the circuit to accomodate both MC and MM cartidges with RIAA equalization. In spice testing the output is -0.5db at 20hz and 20khz, and almost totally flat through the rest.

A unique design feature is the output level control that lets you adjust for different cartridges and match levels with the rest of your gear. The maximum output is about 2 volts peak to peak, so start with the level control at mid point.

You can use any supply voltage from 9 to 24 volts. The current draw is only a few milliamps so you could even use a battery.

Of course you need to build two of these for stereo.

Now, fair warning... this is not a production circuit. The original design worked perfectly. But, you may have to do some tinkering to get this update working properly... but that's what DIY is all about... right?

Magnetic Phono

Mag Phono Frequency

Any and all feedback on this design would be appreciated. Thank you.


Quote
Topic Tags
JonnyAudio
(@jonnyaudio)
Active Member Registered
Joined: 6 months ago
Posts: 19
07/06/2019 1:30 pm  
Posted by: Douglas Blake

This is the magnetic phono preamp I built into my DIY preamplifier years and years ago.

The original design used th UA741 op-amp. This re-design uses the NE5532 Op-amp and expands the circuit to accomodate both MC and MM cartidges with RIAA equalization. In spice testing the output is -0.5db at 20hz and 20khz, and almost totally flat through the rest.

A unique design feature is the output level control that lets you adjust for different cartridges and match levels with the rest of your gear. The maximum output is about 2 volts peak to peak, so start with the level control at mid point.

You can use any supply voltage from 9 to 24 volts. The current draw is only a few milliamps so you could even use a battery.

Of course you need to build two of these for stereo.

Now, fair warning... this is not a production circuit. The original design worked perfectly. But, you may have to do some tinkering to get this update working properly... but that's what DIY is all about... right?

Magnetic Phono

Mag Phono Frequency

Any and all feedback on this design would be appreciated. Thank you.

Would you be able to used the NE5532 in +-15V / +-12v Instead, as the gain needed for single rail would be a lot higher if remember correct from my experience building a car active crossover as the feedback had to be a lot higher to get a decent output ?


ReplyQuote

Speaker Builder
(@speaker-builder)
Reputable Member Registered
Joined: 3 months ago
Posts: 251
07/06/2019 2:19 pm  
Posted by: JonnyAudio
Would you be able to used the NE5532 in +-15V / +-12v Instead, as the gain needed for single rail would be a lot higher if remember correct from my experience building a car active crossover as the feedback had to be a lot higher to get a decent output ?

Yes you can. That would let you eliminate a couple of capacitors (C1 and C7) and of course the V- pins of the op-amps would be moved to the new negative supply rail. The rest of the circuit would remain the same, as would it's gain. Op-Amps are very flexible about supply voltages, so it should work just fine.

However, what you might gain in the pre-amp you risk losing in the power supply. Now instead of an  external SMPS brick, or even a simple 9v battery, you are having to design and include your own power supply which places a pretty big source of hum and interference in close proximity to a circuit with 60 db of gain. 

If you need more gain, for some reason, you would get it by adjusting R7 and R8 in the second stage. The open loop gain of an NE5532 is in the hundreds of thousands so there is some wiggle room if you have a specific need. ( https://www.ti.com/lit/ds/symlink/ne5532a.pdf )

But, the goal here is not infinite gain. What the circuit aims to do is to boost the 4mv output from a MM cartridge (or 0.4mv for MC) to what they call "Consumer Line Level" (CLL) which is about 800mv peak to peak. ( https://en.wikipedia.org/wiki/Line_level )

I've designed for 1 volt from an "average" phono cartridge and included the output level control to fine tune it for various cartridges.  From there the circuit would feed into the AUX input of a regular pre-amp or integrated amp with it's own gain and a volume control.


ReplyQuote
Speaker Builder
(@speaker-builder)
Reputable Member Registered
Joined: 3 months ago
Posts: 251
06/08/2019 10:17 pm  

Oh wow ... PS Audio has just announced a magnetic phono preamp as part of their Stellar line... And it's only $2500 ... That's freaking insane... When I saw the press release I was certain it was a typo ... but it's not.

https://www.analogplanet.com/content/ps-audio-launches-stellar-phono-preamp

 

 


123Toid liked
ReplyQuote

123Toid
(@123toid)
Illustrious Member Admin
Joined: 2 years ago
Posts: 545
06/08/2019 10:20 pm  

@douglas-blake

"I wanted to make it sound the way I wanted it to sound."  Not a bad idea for personal use, but maybe not the best approach for HiFi.  At least IMO.  😀 


ReplyQuote
Speaker Builder
(@speaker-builder)
Reputable Member Registered
Joined: 3 months ago
Posts: 251
06/08/2019 10:24 pm  
Posted by: @123toid

@douglas-blake

"I wanted to make it sound the way I wanted it to sound."  Not a bad idea for personal use, but maybe not the best approach for HiFi.  At least IMO.  😀 

Doublespeak for "I couldn't get the RIAA curve right"...

Granted it's a bit tricky, but not $2500 tricky.  Only a total idiot spends that kind of money for a $39 preamp.

Here ya go ... Behringer, one of the best... https://www.amazon.com/dp/B000H2BC4E


123Toid liked
ReplyQuote

Share:

%d bloggers like this:

Please Login or Register