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Any value to upgrades for low SPL listening?

seashell

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I recently decided to put together a stereo system in my bedroom starting with a raspberry pi 4 as a player feeding a system of long unused parts I had lying around. I ended up with the following initial setup.

Rpi4 -> HDMI OUT -> HDMI Audio Extractor to Toslink -> Marantz SR8000 -> Phasetec Teatro 10 Speakers.

Sadly over the years of sitting idle the Teatro's plate amplifiers for the woofer have moved from emitting a faint transformer buzz to a room filling transformer buzz. (Both speakers do it and a DC offset rejecting AC outlet did nothing to reduce it.)

This led me to consider upgrading the system and I found this site with all it's helpful technical data which is greatly appreciated. I happen to live in a large town that is close enough to a much larger town there is basically nothing in the way of AV dealers other than Best Buy and custom integrators with no actual retail locations which makes experiencing anything in person basically impossible. And yes I know from past experience even if I could there is limited value to in store testing. So it's great to see empirical measurements to help inform buying decisions.

The first thing I did was get new speakers. I wanted smaller this time as I'm disabled and have difficulty moving anything large or heavy these days so I decided to go the bookshelf route. I ended up with a pair of Revel M106s. Doing an A/B against the Teatros they're a clear step up in terms of clarity in the mids and highs. (Unfair to judge the low end as the Teatros are missing a large part of their low end w/ their woofer's disabled.)

Background complete, I move on to my question. I see there are DACs and amps that measure wonderfully these days and conveniently they come in small and light packages. My question is are these likely to provide any audible benefit over the SR8000 (which is serving as both DAC and amp) at low SPL?

To be clear when I say low SPL I'll give numbers from my old Radioshack SPL meter. As measured at my listening position I find 75 dBC SPL to be very loud. I more often listen in the low 50s dBC and frequently lower than that although 50 is as low as the meter goes.

At these SPLs is the improvement in noise floor and distortion likely audible?

Here's the listed audio specs from the SR8000 manual. How close it is to these specs in reality (either new or now) I have no idea.

AUDIO SECTION
Continuous Power Output
FRONT (20 Hz - 20 kHz) .............................................................................................................................. 8 ohms 100W / Ch
THD Front (20 Hz - 20 kHz) ..................................................................................................................................... 8 ohms 0.05 %
Input Sensitivity/Impedance
Linear ........................................................................................................................................................... 316 mV/ 47 Kohms
Signal to Noise Ratio
Linear .............................................................................................................................................................................. 105 dB
 

AnalogSteph

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Sadly over the years of sitting idle the Teatro's plate amplifiers for the woofer have moved from emitting a faint transformer buzz to a room filling transformer buzz. (Both speakers do it and a DC offset rejecting AC outlet did nothing to reduce it.)
Someone used cheap Chinese electrolytic capacitors by the looks of it. These are a case for someone who knows how to wield a soldering iron.
Background complete, I move on to my question. I see there are DACs and amps that measure wonderfully these days and conveniently they come in small and light packages. My question is are these likely to provide any audible benefit over the SR8000 (which is serving as both DAC and amp) at low SPL?
Maybe.

Do you get audible hiss with the Marantz? If it sticks to its specs, noise level with the M106s should be about 0 dB SPL(A) @ 1 m, a complete non-issue. (Very good performance for a near 20-year-old AV receiver btw, showing why these were their TOTL.)

The biggest potential problem is that the unit is now old enough to be seeing issues with its output / protection relays. This is most evident once it's become so bad that the volume of individual channels clearly varies between each turn-off/turn-on cycle, but a smaller change in frequency response caused by high but not yet hopeless output impedance may not be as obvious. The exact magnitude of this problem will depend on a multitude of factors, including relay quality, operating history and air quality history (yes, air quality - these relay contacts are generally Ag-Ni coated, and silver tarnishes when it encounters sulphur compounds).
Relays can be removed and either opened and contacts polished up (if they're just tarnished and not burned) or replaced altogether, it's about the same amount of time/effort either way. (As they seem to have lasted this long as-is, I may be leaning towards the former.) Not a super big deal for an experienced tech, or even a not-so-experienced one for that matter.
(I still have an old Onkyo ProLogic receiver from '97 gathering dust that has been waiting for some new relays since 2006. Well, my placement options for Proper Speakers have always been subpar, so I ended up just listening on my computer instead...)

Reviewing the service manual, the digital section uses arguably the fanciest stereo DAC chip you could get at the time for the front channels, the AD1855 (113 dB DR, -97 dB THD+N - probably not reached here). It is paired with a CS8414 S/P-DIF receiver (not AK4114 level but half-decent - see Cirrus Logic appnote AN339) and a YSS912 DSP. I wouldn't expect DAC performance to hit datasheet specs, not necessarily audible issues though. If you don't trust it or the DSP action, you can always attach a competent modern DAC to the 6CH DIRECT IN, shouldn't cost you a great deal more than $100 or so. I wouldn't be expecting anything major.

Not sure what player you are using on the Pi, but just in case you don't have volume normalization like ReplayGain active, a small amount of digital attenuation to keep peak levels away from 0 dBFS may be advisable... maybe 2-3 dB. Output data format should also be 24 bit.
 
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seashell

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Thanks for your reply. My first thought on the Teatros was the caps, but DC offset was easier to test - just plug it in. I'll pass them along to someone else who can replace the caps and have a cheap set of decent speakers.

On the hiss, apparently the Marantz has a mute circuit when nothing is coming in from a digital source as the speakers are dead quiet at idle. However, playing zeros I can hear a hiss at 1m from the speakers when the Marantz is at about +5dB on its volume, but at my normal listening position I can't really hear the hiss even at the max volume of +15 dB. Switching to the analog inputs (unconnected) it's louder unless I turn on "Source Direct" and then it's quieter. I'm guessing without source direct it's analog in => ADC => DSP (bass management and tone controls) => DAC => amp, and with source direct it skips ADC, DSP, DAC. So it seems the DAC does add some noise, and the ADC/DSP a bit more. But not audible at the volumes/distance I listen.

Corroded relays never even occurred me to. I just this morning cleaned up some horrible corrosion on a maglite flashlight that had batteries leak. It wouldn't even light up as the contacts had a few Mohm resistance. Fortunately I'm certain the Marantz's relays aren't in anywhere near that bad of shape. I hear no cycle to cycle difference in sound, but I understand your concern. I plan on doing some room measurements at some point. I suppose I could look at the impedance measurements of the M106 and a spatially averaged room response and see if there's dips tracking the M106 impedance curve? Nice to hear the Marantz was actually made with decent parts. A relic from before they were sucked into the conglomerates.

I'm using Moode as the player on the RPI, which is one of the many MPD based audio players. Out of the three I tried it had the least buggy and annoying UI, although as I use it more I find more issues but really when isn't that the case for a UI? I do have ReplayGain enabled in track mode and all my music is tagged for both album and track ReplayGain. Just checking /proc/... the output format for 16 bit files (all that I have) reports 16 bits. I've failed to spot any place to change this for the HDMI output which seems to be an afterthought for all the RPI/MPD based players. The plan is to put something like brutefir in the path to add some room correction eventually and hopefully that path will allow for 24 bit output.

So you've given me a simple test that tells me noise isn't an issue at the SPL/distance I listen. Thank you. But you've given me the scary relay problem to think about now. So cruel. :p Seriously though thanks again for the reply and the information. It's all appreciated.

The relay problem does cast a new light on the talk I hear about people buying "forever amps". I think some of the newer ones use electronic protection circuits rather than relays though, but the caps will always be there. As they say, father time is undefeated.
 
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