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Matching Gain to Output Voltage

Epos7

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I have recently ordered a Parasound P6 preamp as well as a March Audio 452 stereo power amp. There were two gain options available for the power amp - one optimized for 4V XLR and one for 2V RCA. I selected the gain corresponding to 4V XLR as that is the connection I am planning on using from the preamp. I just want to make sure I am understanding things correctly.

For home theater, the P6 will be used in bypass mode with a Denon X3700H. Will the P6 XLR outputs produce 4V in this situation? For the Denon, my understanding is it's best not to exceed 1.4V of output voltage. Will the P6 still produce 4V output in this configuration?

For music, the P6 will be connected to a couple yet undetermined sources. As bypass mode won't be engaged, I assume it will output 4V over XLR.
 

NTK

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Apparently theater bypass is a direct connection between input and output (from manual page 8).

P6 Bypass.PNG
 
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Epos7

Epos7

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Good catch, thanks. So it seems I would need to add some additional gain to the Denon outputs. Hmmmm.
 
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Epos7

Epos7

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It's not adjustable, there are two choices when you order. I don't believe mine has shipped yet so I could possibly change to the 2V optimized (26.5dB) option. However, if the P6 is a direct connection, 1.4V out of the Denon might still be a little low.
 
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Epos7

Epos7

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Hmmm, the rear of the P6 indicates the bypass inputs are routed to the outputs "at full volume" - I suppose this means at the same volume as the input, not at the P6's "full" 10dB gain applied...

1632946810789.png
 
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Epos7

Epos7

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Confirmed with Parasound there is no gain applied, so full volume is not in reference to the P6 gain.

I'm not quite sure how to tackle this as other sources I connect to the P6 will be output at 4V XLR.
 

NTK

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One option is to use a headphone amp to amplify the L&R signal from the AVR to the P6. The AVR should be able to take care of the differences as they can compensate as if you have speakers with different sensitivities.
 
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Epos7

Epos7

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That's a rather ingenious solution, although a second headphone amp would likely be needed for the subwoofer outs?
 

NTK

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That's a rather ingenious solution, although a second headphone amp would likely be needed for the subwoofer outs?
Your P6 has a knob for sub volume control in the front. You should be able to dial down the P6 sub output level to match the AVR sub output level, and adjust the volume control in the sub to set the overall level.
 
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Epos7

Epos7

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Your P6 has a knob for sub volume control in the front. You should be able to dial down the P6 sub output level to match the AVR sub output level, and adjust the volume control in the sub to set the overall level.

Not sure if I completely understand - I don't think the sub output level is enforced when in bypass mode? I need to turn this over in my head a few times to make sure I'm understanding correctly.
 

NTK

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Not sure if I completely understand - I don't think the sub output level is enforced when in bypass mode? I need to turn this over in my head a few times to make sure I'm understanding correctly.
Let's see if this attempt to explain is successful. I can try again if not :)

Your original problem is that you want to drive your March Audio P452 amp with a 4 V input, which is the optimal setting to pair with your Parasound. The issue is that the "usable" pre-out output from your AVR is only ~1.4 V. For your AVR to be able to drive the P452 to its full output, you need to find a way to boost the AVR pre-out output, and you can do it with help from an inexpensive headphone amplifier.

For the subwoofer, the situation is a little different. First, the sound quality requirements are less strict, so your don't need to strive for perfect gain matching. Second, the subwoofer mostly likely has a gain knob for flexible gain adjustments (unlike the P452). Therefore, you can raise the gain at the subwoofer so it matches better to your AVR's lower output level. For the Parasound, you can compensate for the "increased sensitivity" of the subwoofer by dialing down the subwoofer volume knob*.

* I think this problem may not even exist if you are driving your subwoofer with the RCA output from the Parasound. Since the standard for RCA is 2 V, the sub outputs from the Parasound and your AVR may not differ by much. You can simply let the automatic room EQ of your AVR takes care of setting the correct subwoofer level based on its measurements. (And the same for your 2-channel system if you use room EQ there too.)
 
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Epos7

Epos7

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Let's see if this attempt to explain is successful. I can try again if not :)
That makes sense, thanks for the detailed explanation!

I'm planning to use XLR cables for the subwoofers, so the output will be 4V. I could be convinced to use the RCA outputs - Rythmik recommends XLR which is why I was leaning that way. These Rythmik subs do have gain adjustments from +3dB to -12dB. So the setup you suggest may work quite nicely if 3dB of gain at the subwoofers will be enough.
 
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Epos7

Epos7

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I checked in with Rythmik and they suggested an RCA cable would work just fine, recommending the LC-1 from Blue Jeans Cable. Blue Jeans has been my preferred cable supplier for over a decade so the LC-1 should do nicely.

I also realized if I'd used XLR, I wouldn't be able to add a miniDSP 2x4 HD to the chain (on the P6 sub outputs). On top of that, the 2x4 HD would (I presume) allow me to add some additional gain if the 3dB provided by the subwoofer isn't enough.
 

NTK

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Looks like you have your problems squared away. The 3 dB gain (provided by the Rythmik) is 1.4X, so you should be able to drive your Rythmik sub(s) to full output with the 1.4 V output from your AVR without any additional assistance.

Your 2 channel system got some first class electronics and I have little doubt they will give you most satisfying results. Enjoy!
 
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Epos7

Epos7

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Looks like you have your problems squared away. The 3 dB gain (provided by the Rythmik) is 1.4X, so you should be able to drive your Rythmik sub(s) to full output with the 1.4 V output from your AVR without any additional assistance.

Your 2 channel system got some first class electronics and I have little doubt they will give you most satisfying results. Enjoy!

Thanks! I'm looking forward to getting everything set up. My only concern is the P6's advertised 88dB SINAD is significantly lower than the other components in the chain. I don't know if it's something that would be audible but I was hoping to find something with comparable features that performed a little better in that department.
 

NTK

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I wouldn't worry about it. Let's do some numbers.

I couldn't find any measurements of the P6, and will settle on the Stereophile P5 measurements. I assume the newer model's performance did not regress.
P5fig4.jpg


The shape of the curve suggested THD+N is noise dominated. Using the 1 V THD+N of 0.01%, we can calculate that the noise as 1*0.01/100 = 0.0001 V. We'll assume the P6 idle noise is equal to this value.

The gain of the P452* for 4 V input is 20.5 dB, which is 10.6X. The amplified output from the P6 idle noise will be 0.00106 V, which is 20*log10(0.00106/2.83) = -68.5 dB below the standard voltage of 2.83 V at which we spec speaker sensitivities.

If your speakers have the typical sensitivity of 89 dB SPL @ 2.83 V, the noise volume at 1 m distance will be 89 - 68.5 = 20.5 dB SPL.

Given that in most domestic rooms the background noise level is >30 dB SPL [reference], it will be inaudible at normal listening distances. You may be able to hear noise if you get very close to the speaker, but that's all about it.

* Per Purifi spec, the A-weighted idle noise of the 1ET400A is 11.5 μV. It is totally insignificant compared to those from the P6 and can be safely ignored in the calculations.
 
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