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Old 2nd April 2013, 03:25 PM   #2361
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Richard Murdey
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That? Well, that's the complicated bit. It works out to,

1/[1/R7+1/(Z(C2)+R8)+1/Z(C3)] ,

where Z(C2)=j/(2 pi f C2) and Z(C3) = j/(2 pi f C3).

then you divide that by R4 and take the magnitude of the whole thing to calculate the gain.

Works perfectly too, I just checked using MathCAD. The free version is sufficient to do these calculations:

Gain2(20Hz) = 48.393 dB
Gain2(1kHz) = 27.348 dB
Gain2(20kHz) = 7.551 dB

That's just the second stage, mind you, but working through the rest will add nothing to change the story. As I said, your RIAA network uses the wrong values, and the bass response is too high as a result. The only reason it worked in your other circuit was because R7 needed to compensate for a parallel impedance presented by the amplifier circuit itself.

Can we please (pretty please) put the conspiracy theories to bed now?
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Last edited by rjm; 2nd April 2013 at 03:34 PM.
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Old 2nd April 2013, 03:46 PM   #2362
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It's really not that complicated when you let the computer deal with the complex numbers...

(shown for the Phonoclone 3 BOM)
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Last edited by rjm; 2nd April 2013 at 04:03 PM.
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Old 2nd April 2013, 07:51 PM   #2363
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Hi all - I made some modifications to my VSPS. Of course I simulated () the result first.

Here's what I've done:
Click the image to open in full size.

The end-result is a near-flat response
Click the image to open in full size.

The resistor and capacitor attenuate the ever-rising response of the VSPS. I figured it couldn't hurt. Somehow on mine after actual measurement (as in -no sim-), crosstalk improved as well. Oh, you do lose 1dB of gain.
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Old 3rd April 2013, 01:40 AM   #2364
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To "remove" the forth time constant just set R3 to zero ohms (short). There are some arguments in favor of a low pass filter on the output in general, except that your output impedance is now 3.3k, which is little high frankly.

Remember that the response of the VSPS is not "ever rising". In absolute terms it just stops falling. Only when comparing with the RIAA response would it appear to rise.
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Old 3rd April 2013, 05:20 AM   #2365
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Ah, yes, but I followed your remark there: "without R3 the impedance of the feedback loop approaches zero at high frequencies" - causing distortion. So I figured it would have been better to do this with a simple RC-filter after the feedback loop. It's actually just one component more.

Yes, when comparing with the RIAA response it appears to be rising, good correction there. I figured this could be an issue with high-output MC's like the Denon DL110, or an MM like the Nagaoka JT-555,.. The frequency response of those runs way higher than 20kHz.

The change is hardly noticeable at all. Though you can pick up changes if you know where to listen. Certain "hot" records sound a bit less "distorted" when there's high frequency noise. Kind of like less sibilance.

Last edited by Wirehead.be; 3rd April 2013 at 05:29 AM.
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Old 3rd April 2013, 11:42 AM   #2366
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As an insurance measure, an RC filter on the output like you implemented seems like a good idea to me. My only concern is the output impedance is now 3300 ohms.
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Old 3rd April 2013, 12:06 PM   #2367
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Quote:
Originally Posted by rjm View Post
As an insurance measure, an RC filter on the output like you implemented seems like a good idea to me. My only concern is the output impedance is now 3300 ohms.
Ah, yes - that's what I'm still figuring out how to solve that one with minimal components involved. Problem is that it would be a Not So Very Simple Phono Stage

edit: 300Ohm/10nF seems a good match as well. 300Ohm would still meet a decent impedance.

Last edited by Wirehead.be; 3rd April 2013 at 12:28 PM.
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Old 3rd April 2013, 12:44 PM   #2368
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Another addition: 187Ohm (E48) and 15nF (e.g. Wima FKP2 2.5%) would make +- the same sim. And the output impedance is acceptable.

Click the image to open in full size.

Last edited by Wirehead.be; 3rd April 2013 at 12:48 PM.
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Old 5th April 2013, 02:20 AM   #2369
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Well, yes. It's just a simple RC low pass ... double the capacitance, halve the resistance and you get the same response... while the impedance halves.

You might find it instructive to plot the current through R9 as a function of frequency. At high frequencies it is essentially shorted to ground. The condition you wished to avoid (a very low impedance at high frequency in the feedback loop) is resurfacing in the output load.
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Old 5th April 2013, 06:36 AM   #2370
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Hmm, correct.. I'll have to do some further reading - see if I can come up with something simple..
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