Gain resistor values in PPA
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IpsilonSound

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I was reading about some issues regarding PR amp and different set of phones, some phones are too sensitive with low impedance which creates problem with the smooth volume for them. Larry fixed that issue by adjusting the gain resistor value for the PR.
My PPA will likely to be used with different set of phones, and having to adjust a R4 gain resistor every time seems to me like a workaround rather than a solution. I'll see what happens with a generic 10k value for it, if it doesn't work well, try to create some tweak here. For example, NAD has a built-in circuitry to recognize different speaker impedance.
 
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ITZBITZ

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The general formula here is 1+R4/R3. Since R3 is 1K by default, you get a gain of 11 with a 10K resistor. A 4.32K would give you a gain around 5.

The super efficient phone like the in-ear canalphones don't need as much gain so reducing it helps.

You could wire up a switch to the resistor position to make it selectable if necessary.
 
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IpsilonSound

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Thanks, ITZBITZ. That's what I had on my mind, putting a multi-swith or a circuit to detect the phone impedeance.
 
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DCameronMauch

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Also keep in mind that if you use the recommended AD8610 opamps, your rails can only be +-13Vdc max. Given a standard 2Vrms line level input, the very most voltage gain you could get without clipping into the rails would be around 4x. So, if these two typical assumptions match your situation, be very carefull going over 3k for R4. I currently use a pair of HD600s, with a impedance around 300, which is on the higher end of normal for headphones. And 4x voltage gain is more than enough. With some other headphones, like especially Grados, their impedance is so low, and efficienty high enough, you probably don't need any voltage gain at all. Just current gain, which the buffers handle.

Taking a look at the actual specification for some headphones... HD600s are rated 200mW and 300 ohms. That gives a maximum voltage of around 8Vrms. Like I said, 4x voltage gain. The specifications for the Grados must be wrong... It says 32 ohms, but with a 1mVrms sensitivity of 98dB SPL. If that is correct, than 2Vrms would create over 130dB SPL at your ears. That is 10dB past the threshold of physical pain, and well into the land of immediate permanent hearing damage. Someone let me know if my calculations are incorrect.
 
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IpsilonSound

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I read somewhere about the incorrect sensitivity data posted on the Grado website. They are way more sensitive and can be driven by portable unamp easy, compared to Senns which definitely need an amp with them.
The trick is matching the R4 in such a way, so that most of the phones would sound rather well with PPA, high and low-sensitive ones.
 
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DCameronMauch

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Hey. I agree with ITZBITZ. If I were in your position, I would use a switch to control the voltage gain. But only if you are not using the optional bass boost. Changing R4 alters the response. So the boost may sound different depending on the gain setting.
 
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IpsilonSound

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Good point, DCameronMauch. I totally missed the impact of gain change onto bass boost.
 
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jboehle

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I wouldn't worry much about R4 and the change to the bass boost. Quoted from Tangent's PPA Tweaks page:

Quote:

you can vary R4 without worrying about what it will do to the bass boost behavior


Check the graph on Tangent's site, the bass boost changes with R4, but not much.

-Jason
 
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The Most practical way to have user selectible gain in the PPA and PR is have the Bass boost switch funtion as a gain selection switch rather than a Bass Boost. simply do not install C7L and C7R the Bass Boost capaicitors. then select R7L and R7R that when in series with the feedback resistor R4L & R4R have your desired gain points
 
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