RIAA By Edgar

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Hi Guys, I posted this in another thread, but wanted my new preamp to be more visible, as I haven't seen any opinions on it yet. Thanks

My design uses a passive, tweakable HF rolloff and an active LF boost, both done in the first stage. The HF adjust trimmers are rather noisy while being adjusted, but don't cause objectionable noise when stationary. To my ears, the circuit sounds fairly quiet and accurate to the RIAA Playback curve. The second stage is a flat-response X100 voltage amp, so mic-level equalized audio can be taken immediately after the first stage.
 

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My Issues

That 680k resistor will play holy hell with the noise figure.

I'm not an EE, but I did rob a Holiday Inn Express last night.

+/-5 volts? I would worry about clicks and pops sending the circuit into clipping.

The 75uS is preceding the first stage, so that's not really a worry. The 680k otoh is a real noise disaster.



Yes, the 680 K did play hell with the noise figure. Noise was rather high with the "low noise" LM4562 I tried first. The FET input TL072 handles the hi Z input well enough, 10-20 db quieter, than the LM4563, from what I could tell by ear.

Using a 68 K and a 1uf mylar could bless my sox off! Need to try that! If that change doesn't improve the TLO072, it may give a low enough Z for an LM4562 to do low-noise performance.


Yes, +/- 5 V is pretty low for a phono preamp, but I had to use a pair of resistors to change a single-ended 24 V supply into +/- 12 V supply, the only pair of regulators I had were a T78L05 and T79L05, and 5 V gave me enough headroom to allow for unbalance under load. The output still clips at a reasonable 8 V P/P. Stage 2 has adjustable gain and fast-acting LED Level-OK and Level-Hi indicators at 1 V P/P, OK and 6 v p/p, Hi. It's linear enough and pops and ticks rarely blink Red. I do confess that it would be nice to push that noise floor down another 10db, but it sounds so much better than mp3's that the barely audible noise does not offend me a bit. Thanks for the great input, Guys.

Bass boost stops somewhere below 70hz and above 20 hz, giving improved immunity to subsonic problems.

Hear it!
 
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What Happens...

Kevin, normally that's true; that opamp is not a champ in the en department. But with an ENR of 680k in series with the source, noise is kind of moot...

At age 55 and ADD, I'm Noise Math illiterate, just reaching the point of learning that resistor noise is a serious consideration hi gain audio amplifiers (learned from some guys in VLF Natural Radio a few months ago) and can actually be of higher magnitude than OP amp noise. I looked in Mouser and saw that 680 K is available in 1/4 W, metal film. Anyone care to give me a informal estimate of about how many db noise improvement I would get by going form cheapest carbon to metal film? Thanks
 
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At age 55 and ADD, I'm Noise Math illiterate, just reaching the point of learning that resistor noise is a serious consideration hi gain audio amplifiers (learned from some guys in VLF Natural Radio a few months ago) and can actually be of higher magnitude than OP amp noise. I looked in Mouser and saw that 680 K is available in 1/4 W, metal film. Anyone care to give me a informal estimate of about how many db noise improvement I would get by going form cheapest carbon to metal film? Thanks

Hi Z input unfortunately creates high self noise, Johnson (thermal) noise its called. If your cartridge is MM with 1kOhm generator spec it creates 4nVrtHz. Your 680k creates 104nVrtHz. That figure expands over a bandwidth. For 20kHz audio bandwidth 1k will be 0.56uV RMS times your circuit's total gain to get at output. Also circuit's other own noise sources. 680k will be 14.83uV RMS times your circuit's total gain. Rather much.
 
Sad to say, I'm also 55 (though not for much longer).

Going from carbon to metal film will reduce so-called excess noise, but the real question is how much Johnson noise (that's irreducible noise, a property of fundamental physics, something you can't fix with better components) you're introducing. We can consider it in dB. The ratio of the Johnson noise of two sources is equal to the square root of the ratio of the resistances. The resistance of a cartridge is (roughly) 1k, so the ratio is 680k/1k. Square root is about 26. So that resistor is introducing 20log 26 = 28dB of noise above and beyond the limits of the cartridge. Metal films will give you something close to that number, carbon might be a couple dB worse.

Moral- keep the source resistance at the input to a minimum.
 
Mitigating Factors?

Hi Z input unfortunately creates high self noise, Johnson (thermal) noise its called. If your cartridge is MM with 1kOhm generator spec it creates 4nVrtHz. Your 680k creates 104nVrtHz. That figure expands over a bandwidth. For 20kHz audio bandwidth 1k will be 0.56uV RMS times your circuit's total gain to get at output. Also circuit's other own noise sources. 680k will be 14.83uV RMS times your circuit's total gain. Rather much.

The amp sounds quieter and looks quieter on my oscilloscope than the math would indicate. I set the gain to give 2 Volts peak-to-peak on an average volume recorded LP record. The turntable has no muting switch. Hum and noise, mostly hum, is a little under 20mv peak-to-peak and would be about 10 mv pesk-to-peak without that hum. That's 40-46db below signal level and that's mostly hum, obviously not impressively quiet. I think the 90 pf trimmer and 100 pf cap could be loading HF noise somewhat and the TL072 is probably contributing minimal percieved noise because it is unity gain at 1 khz and over. It sounds good enough for my use. Thank you for the information, Guys, and I'll experiment some more, soon.
 
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Actually it does as predicted when filtered. You show 10x and 100x gain stages on your diagram. So a total gain of 1000. Since you got a passive HF filter before any gain, that 680k resistor's noise bandwidth is limited and your first stage does not amplify over 1kHz.
680kOhm for 1kHz bandwidth is 3.3uVxG1000=3.3mVRMS+0.3mV from that 5K6. 2VP-P is 710mVRMS/3.6=197 times bigger. 197log20=46dB SNR. Sounds familiar. You should achieve over 60dB SNR to play vinyl relatively OK though.;)
 
Cool!

Actually it does as predicted when filtered. You show 10x and 100x gain stages on your diagram. So a total gain of 1000. Since you got a passive HF filter before any gain, that 680k resistor's noise bandwidth is limited and your first stage does not amplify over 1kHz.
680kOhm for 1kHz bandwidth is 3.3uVxG1000=3.3mVRMS+0.3mV from that 5K6. 2VP-P is 710mVRMS/3.6=197 times bigger. 197log20=46dB SNR. Sounds familiar. You should achieve over 60dB SNR to play vinyl relatively OK though.;)

Thanks, Guy, that sounds awesome, as the numbers pretty well describe what I'm hearing from the preamp.:)
 
The LF active RIAA looks a bit suspect. Time constants should be 318 and 3180us. Yours look more like 500 and 5000us so your LF will be low below 500Hz. Tuning HF RIAA by ear means either that your ears are incredibly good (to achieve what is difficult with test equipment, but easy with a calculator) or rather undiscriminating (so you are happy with fairly serious errors). As others have said, noise will be awful.

Overall, you might get better results from a good ceramic cartridge into a high impedance input. There is less to get wrong.
 
The amp sounds quieter and looks quieter on my oscilloscope than the math would indicate. ...snipped...I'll experiment some more, soon.

Actually it does as predicted when filtered. You show 10x and 100x gain stages on your diagram. So a total gain of 1000. Since you got a passive HF filter before any gain, that 680k resistor's noise bandwidth is limited and your first stage does not amplify over 1kHz.
680kOhm for 1kHz bandwidth is 3.3uVxG1000=3.3mVRMS+0.3mV from that 5K6. 2VP-P is 710mVRMS/3.6=197 times bigger. 197log20=46dB SNR. Sounds familiar. You should achieve over 60dB SNR to play vinyl relatively OK though.;)

The LF active RIAA looks a bit suspect. Time constants should be 318 and 3180us. Yours look more like 500 and 5000us so your LF will be low below 500Hz. Tuning HF RIAA by ear means either that your ears are incredibly good (to achieve what is difficult with test equipment, but easy with a calculator) or rather undiscriminating (so you are happy with fairly serious errors). As others have said, noise will be awful.

Overall, you might get better results from a good ceramic cartridge into a high impedance input. There is less to get wrong.

I use an audio generator and oscilloscope to tune the HF response.

Salas, my ears, and my test equipment agree with each other, vinyl plays OK on that preamp.

Regards, Edgar
 
Tweaked a Little More

In my configuration of the pre-amp, c-1 is only about 1/4 of the way closed, probably set around 25-35pf, or so, with the C-1+C-2 total being around 125-135pf.

I had been aware that U-1 and the circuits around it were picking up a little hum, for a few days, now. I put a shield cover over U-1 and it's supporting components and after that I discovered that wiring on the back of the RCA type input jacks was picking up hum, as well. After shielding those 2 areas I ended up with 5mv P/P noise against a 2.4 V P/P signal with an average level recorded LP record. I think I created a better preamp than the one in my, still in storage, 1977 Pioneer SX-650. Mine's a little noisier, but way less distorted and rings a lot less on "S'es". I also have a lot less subsonic content in my output signal now than I had with the SX-650. The Direct Drive Turntable and my RIAA LF boost ending above 20 hz probably share credit for reduced subsonics. Best Regards To All, Edgar
 
So you have -54dB S/N - quite a poor result. The RIAA LF boost is supposed to rolloff from 50Hz, after boosting from 500Hz down. Yours will boost from around 300Hz down to 30Hz. RIAA also adds an optional rolloff from 20Hz down to reduce subsonics.

I would be surprised if the Pioneer RIAA is as inaccurate as this, but its your system and if your ears are happy with it . . .
 
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