Who's measured the noise on their RIAA preamp? - diyAudio
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Old 16th February 2004, 09:58 AM   #1
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Default Who's measured the noise on their RIAA preamp?

I made a quick-n-dirty RIAA phono pre-amp with 3 op-amps per channel. I measured the input signal as about +-15mV, and I wanted the output at about +-1.5V (to be followed by a passive attenuator; update on actual output levels coming soon).

When the input is disconnected, the output noise voltage is about* +-10mV, and jumps up to +-30mV when the recordplayer is connected. This appears to be mostly random ultrasonic noise, and includes a 50Hz ground ripple from the 12V adapter in the wall plug. (*About 95% of the noise that's visible on a scope easily fits between +10mV and -10mV).

Improvements I can make to my design (schematic coming soon!) :
-Better schematic with fewer design errors.
-Next time not put the whole thing inside a metal box with exterior dimensions of 58mm x 64mm x 35mm. It was just asking for parasitic component values and parasitic feedback etc.
-Better power supply, but on the other hand I think those single-ended "battery eliminators" are quite convenient.

So, how does your phono preamp perform?

CM
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Old 16th February 2004, 10:05 AM   #2
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In fact I'm hoping to reduce the noise by 10dB or even 20dB or so...
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Old 16th February 2004, 10:51 AM   #3
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Default Noise - wideband.

For a 1.5 Volt output your (output ) noise must be 0.5mV to get -70dB S/N ratio.
You will have to short the input of the preamp with say a 100 ohm resistor to get some meaningful noise figure.
As for hum:
1. In an opamp circuit you should not even see it !
2. Your shielding may be poor.
3. Your power supply must be trash - the opamp can take a bit of crap in any case.
4.Your Phono cartridge to pre-amp wiring may have to be sorted out.
5. You will need a mains earth .
6. Check for ground loops.
7. There should not be any LF noise with half decent opamps at these levels.
Put up a circuit and pictures of the unit. Then more people can give you practical suggestions.

Better not progress to tubes till you have all this sorted out.
Cheers.
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Old 16th February 2004, 06:07 PM   #4
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Wall warts are crap when it comes to 50Hz (or 60Hz if that comes out of the plugs). A small lab supply should not be expensive and far better. Or build one yourself. As the riaa curve amplifies at 50Hz quite a lot of dB's more then at 1kHz, whatever your cartridge and wiring picks up gets amplified a lot. So re-arranging your cables can help.

I don't think what you get is from parasitics of your box. But it could be bad grounding inside your box.
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Old 16th February 2004, 07:08 PM   #5
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As I said, the circuit was "quick-n-dirty", and I know that there are several things wrong it, so I'm not worried at all that the noise is +-10mV. It is +-10mV whether the input is open or shorted; most of the noise is picked up by the turntable, which may be crap but it's a great antenna .

Unfortunately the schematic for, er, version 0.9 is still just a hand-drawn scribble, but when I post it I will also post a much improved version so people can actually see what I'm rambling on about. A lot of the noise is internally generated because of too much gain and some missing components. The mains hum within the +-10mV is due to the single-ended cheap adaptor, and I simply used a few resistors and capacitors to create a bias voltage at half-way rather than use ground as the reference. The bias voltage is a different reference to the one I used for my active crossover - which also happens to use a single-ended adaptor.

Maybe I got off to a bad start, I'm just interested in how much noise people get from their phono preamp, so that helpful comparisons can be made between different designs. It's all very well just admiring the idea of discrete JFETs on the input and using ultra-linear MOSFETs to avoid using negative feedback, but how well do all these different designs actually work?

CM
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Old 16th February 2004, 07:23 PM   #6
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Small update: you made me curious, so I did a "measurement" of my riaa preamp. This is a Barry Porter balanced input design with a couple of LM394H as input transistors, rest are NE5532A/NE5534A with an LF411 as DC servo.

How did I did the measurement: made a plug with a 100 Ohm resistor and connected that to the input of the LEFT channel, the right channel was left open. The output of the preamp was connected to a RME ADI-8 AE AD convertor, and so it was piped into the pc. Recorded 1 minute of the output into CEP at 32bit float/44.1kHz. Then took 30 seconds out of the middle and had a statistic made of the taken samples. After that did a frequency scan of the same selection with 16k point fft and blackman window. This gives a very low noise floor (to get at the statistics reading, you would have to integrate over the frequency scan), so I used it here to show the form of the noise floor.

Results:

...................................Left........... .......Right
Min Sample Value:..........-1.15...........-12.3
Max Sample Value:..........1.16.............12.49
Peak Amplitude:.............-89.05 dB .....-68.38 dB
Possibly Clipped:.............0...................0
DC Offset:.....................0...................0
Minimum RMS Power:......-104.38 dB...-87.65 dB
Maximum RMS Power:.....-91.84 dB.....-69.31 dB
Average RMS Power:......-99.8 dB.......-79.38 dB
Total RMS Power:..........-99.59 dB......-78.88 dB

Using RMS Window of 10 ms
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Old 16th February 2004, 07:29 PM   #7
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The same, but now with the cartridge connected:

The average RMS now sits at -90dB, just because of the 50Hz picked up.
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Old 16th February 2004, 07:40 PM   #8
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And now with the cartridge and the ground wire to the TT removed. Average RMS is now only -65dB anymore and that are all 50Hz harmonics! The small peak at 15kHz and a bit is probably the pc monitor.

For the record, the cartridge is a Stanton 680 HiFi, TT a JBSystems High Q 30D (a DJ type that I use for recording at 78rpm). I could use about 6dB more gain in this setup as it was made for another cartridge with a bit more output.
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Old 17th February 2004, 06:38 AM   #9
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Default Tadaa!

Well, I haven't yet worked out a new schematic that has been improved enough... So for now I'll just attach the schematic of the existing circuit and list some of the things that I know could be improved with it.

It runs on +15V that's supplied to it through a Schottky on another board. Only 1 channel is shown, and the bias is common to both channels.....
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Old 17th February 2004, 07:19 AM   #10
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The opamps are actually NE5532AN btw.

At low frequencies the signal is systematically amplified by a total of 60dB (who cares that it's inverted?). Nothing too major there, except that the DC bias needs to be regulated somehow, so that the subwoofer doesn't generate 3 - 5Hz seismic disturbances from miniscule voltage changes.

At high frequencies the signal is inverted and attenuated slightly, then attenuated some more, and then amplified about 100 times. How silly is that? Preferably it should not be attenuated at all, but simply amplified to a lesser extent, by incorporating the 2 RIAA filter stages with the amplifying stages.

I still need to consider whether an inverting topology will have a lower inherent noise level or not. The 2 stages of the RIAA filter should be combined with the amplifying stages so that noise isn't generated from redundant amplification, and filtering caps should be used so that ultrasonic frequencies are not amplified, or at least not excessively. I'd be reluctant to use just 2 amplifying stages instead of 3, because the much higher gain per stage could produce higher distortion, and more readily pick up noise.

CM
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