Higher amp input sensitivity = greater volume?

[Mazzeroph]

I hope this is the right forum to ask this.

I'm getting too much distortion from my miniDSP flex into my Rotel RB-1552 power amp (130 watts, 1.5volt input sensitivity) in a medium sized room.

Apparently when using Dirac Live you're not supposed to turn the Flex up past -10 so that Dirac can have the headroom to work it's magic, which, sure enough, is about when I start hearing distortion. Problem is, some streaming apps do great with that. You tube tends to be loud and clear at -10 while Spotify and KODI tend to be too quiet.

So I'm trying to understand how to get a cleaner signal here. Does the problem lie with the Flex? Amir's tests did find rising distortion above -20 with the Flex but since it peaks at 4.4 volts output he wasn't concerned. Or do I need to replace the Flex with RCA outputs for a digital out and add a DAC/Preamp with better gain?

Is the problem the amp's input sensitivity? Do I need to go with something like the Topping Pa3 which gets to max power at 2volts?

Trying to understand how gain and input sensitivity works so I can plan how to move forward.

Appreciate any help!!

 

[AnalogSteph]

You tube tends to be loud and clear at -10 while Spotify and KODI tend to be too quiet.

YT normalizes to a relatively high level of -14 LUFS. That's not even Foobar2000 RG target level (-18 LUFS), let alone the EBU R.128 recommendation of -23 LUFS. I used to set my YT player volume to 50%, nowadays it's 35% (with a RG target level of -18 LUFS - 5.9 dB ~= -24 LUFS).

Your best bet may be adding some analog gain between the Flex and Rotel, e.g. via a $100-140 class headphone amplifier sporting an RCA preamp output. A JDS Labs Atom Amp 2 would get you 4.7 = +13.4 dB, a Schiit Magni is similar with 5 = +15 dB, and a Topping L30 II has +16.5 dB to offer at high gain, giving you anywhere from 40 to 43 dB of total gain or close to as much as you would have had in an integrated amp. This should be quite enough to reveal the Flex' DAC noise floor; adjust the volume knob to strike a good balance.

 

[JIW]

I hope this is the right forum to ask this.

I'm getting too much distortion from my miniDSP flex into my Rotel RB-1552 power amp (130 watts, 1.5volt input sensitivity) in a medium sized room.

Apparently when using Dirac Live you're not supposed to turn the Flex up past -10 so that Dirac can have the headroom to work it's magic, which, sure enough, is about when I start hearing distortion. Problem is, some streaming apps do great with that. You tube tends to be loud and clear at -10 while Spotify and KODI tend to be too quiet.

So I'm trying to understand how to get a cleaner signal here. Does the problem lie with the Flex? Amir's tests did find rising distortion above -20 with the Flex but since it peaks at 4.4 volts output he wasn't concerned. Or do I need to replace the Flex with RCA outputs for a digital out and add a DAC/Preamp with better gain?

Is the problem the amp's input sensitivity? Do I need to go with something like the Topping Pa3 which gets to max power at 2volts?

Trying to understand how gain and input sensitivity works so I can plan how to move forward.

Appreciate any help!!

If volume is too low with your amp, you want a higher input sensitivity which means a lower input voltage being required for producing the full rated power. The Rotel's maximum output voltage is around 30.5 dBV since for XLR input, its gain is 22.5 dB and its input sensitivity is 2.5 V RMS which is 7.9588 dBV.

The miniDSP flex has a maximum output of 4 V RMS or 12 dBV using the balanced output. At -10 dB, without EQ, a 0 dBFS sine would be at 2 dBV producing 24.5 dBV out of the amp. Thus, adding 6 dB of gain would be enough for the amp to reach full power. With an EQed signal, that might be less. Using the unbalanced output instead, the maximum output voltage is 2 V RMS or 6 dBV. At -10 dB, this is -4 dBV. The Rotel's gain for the unbalanced input is 26.5 dB and thus produces 22.5 dB - 8 dB short of its maximum.

You can add a preamp or a headphone amp in between the miniDSP and the Rotel as @AnalogSteph suggested or look for an amplifier with low input sensitivity voltage.

Sources: Rotel, miniDSP.

 

[mdsimon2]

Dirac can boost up to 10 dB, that is where the -10 dB volume recommendation comes from. However, that depends on input level.

If the input level is high (approaching 0 dBFS), and you do not attenuate -10 dB (either via volume control or other gain adjustments in the DSP), the output of the Flex will clip, and result will be nasty sounding distortion. This would also typically be accompanied by very high output levels from your system, opening the door to all other sorts of distortion from the downstream amplifier and speaker as well.

If the input level is well below 0 dBFS you can safely increase the volume to above -10 dB. All that matter is that the output meters show less than 0 dB.

Do you experience distortion even with the lower-level signals from Spotify and KODI? I would be surprised if you do.

You can measure exactly what Dirac is doing but using the bi-directional USB audio of the Flex, it is easy to do, let me know if you want to try this.

Michael

 

[Mazzeroph]

Thanks everyone for your generous help.

If volume is too low with your amp, you want a higher input sensitivity which means a lower input voltage being required for producing the full rated power.

To be clear I have the mk1 version of the Rotel 1552 so it's input (RCA only) sensitivity is 1.5v. I can't find a gain spec for this model. It seems, from what you're saying, that because the input voltage of 1.5 on the Rotel is so much lower than the Flex's 4v RMS I should be getting very clean non-clipping power. Am I understanding correctly?

If I want to replace the amp what kind of input sensitivity or gain would I need to look for?

Do you experience distortion even with the lower-level signals from Spotify and KODI? I would be surprised if you do.

I get distortion on all three, just different volumes.

 

[AnalogSteph]

To be clear I have the mk1 version of the Rotel 1552 so it's input (RCA only) sensitivity is 1.5v. I can't find a gain spec for this model.

You can calculate it based on rated power and input sensitivity.
Gv = √(130 W * 8Ω) / 1.5 V
Gv[dB] = 20 log10(Gv) ≈ 26,6 dB

It seems, from what you're saying, that because the input voltage of 1.5 on the Rotel is so much lower than the Flex's 4v RMS I should be getting very clean non-clipping power.

Or at least, the power amp should be maxed out before you get clipping in the digital domain. (BTW, the RCA version of the Flex expectedly only sports a nominal 2 Vrms max.) Which in turn shouldn't happen easily unless you listen to insensitive speakers in a big well-damped space. (Let's say 82 dB/2.83V/m @ 4 m.) So I am not exactly sure where in the signal chain your problem is.

I'd be tempted to poke around in the settings via miniDSP Device Console. Maybe take some screenshots of the app and post them here. (I see there's settings for input gain and output gain. Does dialing in some negative input gain with the same amount of positive output gain improve anything? You can also watch your signal levels there.) The Dirac EQ settings would also be of interest, maybe there's some absurd bass boost going on or something.

 

[mdsimon2]

I get distortion on all three, just different volumes.

When you say different volumes, different physical volumes (SPL) or different volume control settings on the Flex? If volume control settings on the Flex, what are those settings?

Would you mind sharing your configuration file?

Michael

 

[JIW]

Thanks everyone for your generous help.


To be clear I have the mk1 version of the Rotel 1552 so it's input (RCA only) sensitivity is 1.5v. I can't find a gain spec for this model. It seems, from what you're saying, that because the input voltage of 1.5 on the Rotel is so much lower than the Flex's 4v RMS I should be getting very clean non-clipping power. Am I understanding correctly?

If I want to replace the amp what kind of input sensitivity or gain would I need to look for?

I get distortion on all three, just different volumes.

As @AnalogSteph said, gain on the mk. I seems to be 26.6 dB. Since the mk. II has a gain of 26.5 dB for RCA input, I assume it is the same as that. Thus, maximum output is 20*log10(1.5) + 26.5 = 30.0218 dBV or 10*log10(130*8) = 30.1703 dBV. The miniDSP at -10 dB volume produces -4 dBV for a 0 dBFS sine without EQ. This leads the amp to output 22.5 dBV about 7.5 dB below its maximum.

Since you are using RCA connection, inserting a headphone amp with a gain of at least 7.5 dB between the miniDSP and the Rotel can make up the difference. All you need is a TRS to 2xRCA cable if the headphone amp has no preamp function. @AnalogSteph has pointed some out to you earlier.

For an amp with rated power P at resistance R, the maximum voltage output is V = sqrt(P*R) and thus 10*log10(P*R) dBV. For a given gain G in absolute value, the input sensitivity is V/G or in 10*log10(P*R) dBV - gain in dB (20*log10(G)) dBV. Look for an input sensitivity of -4 dBV or below, i.e. 0.631 V or below.

 

[Mazzeroph]

Quick update and a thank you to the helpful responses here.

Turns out my speakers are the issue. They distort in the upper mid bass and it wasn't an issue until Dirac pushed those up a few db.

Also the volume issue was due to an overall low output voltage. Once I turned up the gain in the Flex the volume normalized.

Didn't want to leave a bad taste in anybody's mouth for the minidsp or Dirac.

 

[Chrispy]

Was just about to ask some details, glad you sorted it out. My main question was how were you determining the level of distortion from the electronics but if it was the speakers, well then nm