Question about preamp+amp performance

[REK2575]

Recently picked up a couple of Rotel components, RC-1572 MKII preamp & RB-1552 MKII stereo amp. The amp is rated at 130W per channel @8 ohms. It's replacing a 20+ year old Denon AVR that was rated for 60W @8 ohms.

Most of my listening is via streaming Qobuz:

Bluesound Node 2i --coaxial--> RC-1572 preamp --XLR--> RB-1552 amp --> ELAC DBR62 speakers (6 ohms)

Overall I'm very pleased with this setup, but I'm a little surprised at the performance of the preamp+amp: namely, not as powerful as I would have expected.

The RC-1572 volume control is from 0 to 100. It defaults to 45 when powered on. At 45, the volume from the speakers is surprisingly low, much quieter than is viable for even casual listening in my living room.

Am I interpreting this correctly? At 45% power, my 130W amp doesn't put out enough volume for even casual listening? It has more than twice the power of my old 60W Denon AVR. At ~50% volume the Denon was definitely louder than 'very quiet'!

I'm finding that the practical volume range is all between 50 and 80 -- i.e. from moderate-soft to Loud. Lower than 50 volume is essentially useless. Higher than 80 and things are getting QUITE loud.

Does this sound normal? I guess I don't understand why 45-50% of the volume range on the preamp is useless i.e. too quiet. I figured with 130W things would already be very loud at 50%?

 

[MaxwellsEq]

There isn't that much difference between 60W and 130W in terms of how loud an amplifier will drive speakers. You would assume it will be twice as loud, but it won't be.

This is really about amplifier gain and how the volume control works. It doesn't really mean very much where the volume control is for a certain sound level.

 

[DVDdoug]

...not as powerful as I would have expected.

...Higher than 80 and things are getting QUITE loud.

So it's not a question of power, it's just the volume control "profile". I assume the volume control is an rotary encoder so that depends on the software (firmware) and it was a design choice.

It tends to be less of a problem with a rotary encoder because it can be programmed so that it takes more than one turn for 0-100% and you can get finer control. Or you can program-in acceleration for a bigger change when you move the knob faster, etc.

The thing the preamp designer doesn't know is the exact-level your feeding-in and different hardware will be different. And with a preamp you might not be using the "matching" power amp so they don't know how much gain the power amp has. And they don't know the sensitivity of your speakers or how close you are to the speakers.... i.e. They have no idea how "loud" "45" is.

A regular analog volume control is a potentiometer with an "audio taper" (approximately logarithmic, or maybe it's anti-logarithmic). It's similar to your setup with most of the voltage/signal change happening at the loud-end, but it's more natural and yours is probably more extreme.

A regular-linear pot would be at 50% (-6db from the maximum) at the mid-point and that would be way too loud and you'd have the opposite problem.

 

[REK2575]

I appreciate the explanations. Many thanks.

@DVDdoug -- I see what you mean by preamp not being able to 'predict' (so to speak) pairings with a vast array of other hardware, and so the volume # can only be a very relative measure of actual volume. However, I thought because the Rotel 'MkII' preamp was clearly designed to complement the 'MkII' power amp, there would be more refinement / nuance in how the preamp's volume control affects the power amp. In other words, I would have expected more of the 0-100 volume range to register meaningful differences in volume, whereas I'm finding I'll never even use 0-40 range because it's way too soft.

Anyway, the most important point for me is that there's nothing actually 'wrong' with my setup! I was worried that I was somehow not getting the full potential of the power amp.

 

[djtetei]

The RC-1572 volume control is from 0 to 100. It defaults to 45 when powered on. At 45, the volume from the speakers is surprisingly low, much quieter than is viable for even casual listening in my living room.

Your preamplifier maximum output voltage on balanced XLR outputs is 2 Vrms, while your amplifier input sensitivity (max level) is 2.5 Vrms. As you probably know, input sensitivity is the maximum input signal level required to drive the amplifier to its maximum rated power.
Preamplifier maximum output level = 2Vrms = 8.23 dBU
Amplifier maximum input level = 2.5 Vrms = 10.17 dBU
Your preamplifier maximum output level is 1.94 dBU (almost 2 dBU) lower than the maximum input level required to drive the amplifier to its maximum rated power.
Setting your preamplifier volume knob at 75-80% feeds the amplifier an even weaker signal, preventing it to reach it maximum rated power.
Amplifier amplification factor (X): SQRT(130x8)/2.5=12.899
Amplifier gain: 20log(12.899)=22.211 dB
2.5 Vrms (required input level) x 12.899 (amplification factor) = 32.2475 Vrms
SQR(32.2475)/8=129.98 Watt (rated power)
Suppose input signal level is 2Vrms (maximum delivered by your preamplifier), then:
2 Vrms (actual input signal level) x 12 899 (amplification factor) = 25.798 Vrms
SQR(25.798)/8=83.192 Watt (actual power - 64%)
So, your system would run 1.9379 dB (almost 2db) lower than possible, actually much lower with 80% volume setting, and this is noticeable to the ear, especially on your 6 ohm / 86 dB sensitive speakers.

 

[zermak]

To make what @djtetei said simple, and if my math is correct, it means that when your pre is at full then your amplifier (with its 22.5dB gain) is putting out only 88,8W-ish watts into 8ohm.

 

[djtetei]

Given the fact that you already have the preamplifier, what you can do to bring your system to optimum level? There's no need to change your existing preamplifier and spend a tidy sum for another. Just insert a well built, high quality mixer, with independent channel input gain control, balanced master outputs and accurate level meters, between the preamplifier and amplifier. You will have total control over the input signal level and output signal level, ensuring optimum gain stage and preventing your amplifier to ever going into clip from overdriving its inputs.

 

[REK2575]

@djtetei @zermak Thank you both. I appreciate the calculations. I am certainly surprised by them!

I assumed, naively, it turns out, that Rotel's preamps would be designed to get the most (or nearly the most!) out of Rotel's power amps. I know that Rotel makes several models of amp, but the one I own (RB 1552 MkII) is not the most powerful. The performance of the 1572mkii preamp is disappointing in this regard.

Rotel's next-tier preamp, the RC 1590mkII, also does not look like it can fully exploit the 130Wpc of the 1552mkii. From the 1590mkii manual,

Output Level
Line Level (RCA) 1 V
Balance (XLR) 2 V

It also gives "Preamplifier Output level" as "1.65V (at -- 20dB)". For my 1572mkii, it's "1.44V (at -- 20dB)". I don't understand the difference between "Output Level" and "Preamplifier Output Level," but the Output Level on both the 1572 and the 1590 preamps is 2V (XLR, which is what I'm using).

The essential saving grace for me, though, is that there's no way I would ever need to have my Rotel amp at full 130W output. Even on volume setting above 80 on my Rotel preamp, it's too loud for me to listen to. I'm generally in the 65-75 region. Regardless, though, I do feel slightly burned by this purchase! But it was my own ignorance that's to blame.

 

[REK2575]

Just insert a well built, high quality mixer, with independent channel input gain control, balanced master outputs and accurate level meters, between the preamplifier and amplifier.

I'm likely to stay put with my current setup, but out of curiosity, could you give me an example of a high quality mixer?

 

[djtetei]

You don't have to fell bad by the preamplifier performance. Think about it: it is able to output 2 Vrms (+8dBU), which is alot.
It's just that your amplifier requires 2 dB more input signal to achieve its rated power output and your speakers sensitivity need that power.
As for mixer options, you can use a zone mixer, a pa mixer, a studio mixer, or a DJ mixer. The important thing is to be high quality, high headroom, low THD. As for high quality mixer manufacturers, you can look at Allen Heath, Denon, Pioneer, Ecler Soundcraft, RCF, Yamaha, and you will not be disappointed. If you need more specific information, just tell me.

 

[pjug]

It was pointed out in another thread that the preamplifier output specs are sometimes nominal values, not the max output. I bet your preamp will be able to drive the power amp to clipping with your setup.

 

[LTig]

Possibly true.
@REK2575 If you set volume to 100 is the sound clear or distorted?

 

[djtetei]

preamplifier output specs are sometimes nominal values

If a specification is given as nominal or maximum, the manufacturer just have to state this specifically. A line level output, especially for consumer or home audio grade, is usually specified as maximum output level, with usual values between 1Vrms and 2Vrns.
If one really wants to know for sure, a true RMS voltmeter or an oscilloscope can be very handy and clarify the matter.
When in doubt, one can always ask the manufacturer directly.

 

[REK2575]

Possibly true.
@REK2575 If you set volume to 100 is the sound clear or distorted?

The best I can say at the moment is that it sounds uncomfortably loud! I haven't tried a using my phone's dB meter app at max volume, but I probably should (...when wife and kids are not in house!). My totally subjective sense is that even at Max volume, my preamp isn't pushing my amp to full power. It's uncomfortably loud, but not unbearably loud at max. But again, a totally subjective impression.

 

[sergeauckland]

It was pointed out in another thread that the preamplifier output specs are sometimes nominal values, not the max output. I bet your preamp will be able to drive the power amp to clipping with your setup.

Indeed this. With a digital input, the pre-amp is rated at 1.4V output at -20dBFS. Consequently, the pre-amp can output 14.4 V at 0dBFS. That's way over what you need to drive the power amp fully. Analogue inputs are good for 4&5 V input before overload.

Volume control position has no correlation whatsoever with loudness, it's entirely dependant on the volume control's law, so there's no problem whatsoever using that pre-amp with that power amp. You just have to get used to different numbers on the volume control.

S.

 

[djtetei]

totally subjective impression

Don't guess, measure, just to be sure and for piece of mind.
Put the preamplifier volume control knob to full tilt, feed pink noise to its inputs and measure the XLR output voltage RMS.

 

[LTig]

The best I can say at the moment is that it sounds uncomfortably loud! I haven't tried a using my phone's dB meter app at max volume, but I probably should (...when wife and kids are not in house!). My totally subjective sense is that even at Max volume, my preamp isn't pushing my amp to full power. It's uncomfortably loud, but not unbearably loud at max. But again, a totally subjective impression.

Then it is more than sufficiently loud. No need to worry or change anything.

 

[OJAUSSIE]

Recently picked up a couple of Rotel components, RC-1572 MKII preamp & RB-1552 MKII stereo amp. The amp is rated at 130W per channel @8 ohms. It's replacing a 20+ year old Denon AVR that was rated for 60W @8 ohms.

Most of my listening is via streaming Qobuz:

Bluesound Node 2i --coaxial--> RC-1572 preamp --XLR--> RB-1552 amp --> ELAC DBR62 speakers (6 ohms)

Overall I'm very pleased with this setup, but I'm a little surprised at the performance of the preamp+amp: namely, not as powerful as I would have expected.

The RC-1572 volume control is from 0 to 100. It defaults to 45 when powered on. At 45, the volume from the speakers is surprisingly low, much quieter than is viable for even casual listening in my living room.

Am I interpreting this correctly? At 45% power, my 130W amp doesn't put out enough volume for even casual listening? It has more than twice the power of my old 60W Denon AVR. At ~50% volume the Denon was definitely louder than 'very quiet'!

I'm finding that the practical volume range is all between 50 and 80 -- i.e. from moderate-soft to Loud. Lower than 50 volume is essentially useless. Higher than 80 and things are getting QUITE loud.

Does this sound normal? I guess I don't understand why 45-50% of the volume range on the preamp is useless i.e. too quiet. I figured with 130W things would already be very loud at 50%?

The RB 1552MKII is not specified to drive 4 ohm speakers at all. If you have 6 ohm nominal speakers there is a fair chance they will dip down to 3 or 4 ohm on a lot of material. I bought the RB 1582MKII which is rated at 200 watts per channel into 8 OR 4 ohms. It is a dual monoblock design where the 1552MKII is not. It is NOT the RC1572MKII's fault as I also have the same pre amp. 45dB on mine default is

The best I can say at the moment is that it sounds uncomfortably loud! I haven't tried a using my phone's dB meter app at max volume, but I probably should (...when wife and kids are not in house!). My totally subjective sense is that even at Max volume, my preamp isn't pushing my amp to full power. It's uncomfortably loud, but not unbearably loud at max. But again, a totally subjective impression.

This is because the RB1552MKII is not designed to run 6 ohm nominal loads. IT would sound VERY strained indeed. Silly Rotel!

 

[OJAUSSIE]

To make what @djtetei said simple, and if my math is correct, it means that when your pre is at full then your amplifier (with its 22.5dB gain) is putting out only 88,8W-ish watts into 8ohm.

The RB 1552MKII will put out a genuine 130 watts RMS per channel into both channels with 8 ohm loads. What it CANNOT do is put out 130 watts RMS per channel into both channels with 6 ohm loads and his speakers are apparently stated as 6 ohm nominal so would certainly spend plenty of time at 4 ohm or even below. Then your math is spot on at 88 watts RMS per channel. An RB1582MKII puts out 200 watts RMS per channel into both channels with 8 ohm loads however it can deliver 300 watts per channel into both channels with 4 ohm loads and near on 400 watts into both channels with 6 ohm loads. They are totally different amps!

 

[Chrispy]

The RB 1552MKII is not specified to drive 4 ohm speakers at all. If you have 6 ohm nominal speakers there is a fair chance they will dip down to 3 or 4 ohm on a lot of material. I bought the RB 1582MKII which is rated at 200 watts per channel into 8 OR 4 ohms. It is a dual monoblock design where the 1552MKII is not. It is NOT the RC1572MKII's fault as I also have the same pre amp. 45dB on mine default is

This is because the RB1552MKII is not designed to run 6 ohm nominal loads. IT would sound VERY strained indeed. Silly Rotel!

The RB 1552MKII will put out a genuine 130 watts RMS per channel into both channels with 8 ohm loads. What it CANNOT do is put out 130 watts RMS per channel into both channels with 6 ohm loads and his speakers are apparently stated as 6 ohm nominal so would certainly spend plenty of time at 4 ohm or even below. Then your math is spot on at 88 watts RMS per channel. An RB1582MKII puts out 200 watts RMS per channel into both channels with 8 ohm loads however it can deliver 300 watts per channel into both channels with 4 ohm loads and near on 400 watts into both channels with 6 ohm loads. They are totally different amps!

Looking at Rotel's site specs, the 1852mk2 is just an 8 ohm (low thd%) for 200wpc. The spec on the 1552mk2 is 130wpc at 8 ohm, 185wpc at 4 ohm. Why on earth would you think such an amps couldn't handle 6 ohm or 4 ohm?