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Old 16th November 2010, 08:15 PM   #1
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Default RIAA By Edgar

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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Old 16th November 2010, 08:23 PM   #2
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That 680k resistor will play holy hell with the noise figure.
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Old 16th November 2010, 08:44 PM   #3
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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.
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Old 16th November 2010, 08:51 PM   #4
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Quote:
Originally Posted by pedroskova View Post
+/-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.
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Old 16th November 2010, 10:34 PM   #5
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Default My Issues

Quote:
Originally Posted by SY View Post
That 680k resistor will play holy hell with the noise figure.
Quote:
Originally Posted by pedroskova View Post
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.
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Originally Posted by analog_sa View Post
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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Last edited by imfree707; 16th November 2010 at 10:42 PM. Reason: To add link.
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Old 16th November 2010, 11:22 PM   #6
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A TL072 op-amp seems like an unusual choice for the first stage of a phono pre-amp.
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Old 17th November 2010, 01:57 AM   #7
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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...
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Old 17th November 2010, 03:12 AM   #8
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Default What Happens...

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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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Old 17th November 2010, 03:29 AM   #9
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Look at my MPP tread how i "misused" your idea.
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Old 17th November 2010, 03:54 AM   #10
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Quote:
Originally Posted by imfree707 View Post
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.
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