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3e Audio / Alsozone Multi-Channel Amp - Owner's Thread
My PAM 258 lives in an equipment closet, so I haven't been watching it very closely. I hooked up my Atmos and surround rears, leaving one set of channels unused, and put it on a 12v trigger. I noticed that the trigger lit up all the channel lights on the front panel, including the unused ones. Recently, I switched the amp from Trigger to Auto mode. And interestingly, the unused channels did not light up. So it appears the amp is actually quite smart about this. I had asked Google Gemini about this, it got it wrong. So I give AI an F on this one, but top marks for the amp. It offers a really good feature set, regardless of price.
Roland68
Master Contributor
It is very simple: every 4 modules share a single power supply.
This means that units with 2, 3, or 4 modules have one power supply, while units with 5 to 8 modules have two identical power supplies.
There is a single 48-volt version for the TPA3255 units (PAM25X) and a single 38-volt version for the TPA3251 units (PAM15X).
This makes it possible to upgrade any unit to the maximum number of amplifier modules without needing to modify the power supplies.
Naturally, PAM25X units can only be upgraded with TPA3255 modules, and PAM15X units can only be upgraded with TPA3251 modules.
Theoretically, it is also possible—upon request—to configure the large 8-slot chassis with a mix of up to four TPA3255 modules (using a 48V power supply) and up to four TPA3251 modules (using a 38V power supply), though @3eaudio would need to confirm this.
I've done some more investigating into the hiss that I was hearing from my tweeters when using the PAM158.
By putting my ear up against the tweeter, I was able to hear hiss from my existing amplifier (Aiyima A07) even when no inputs were connected to the amp. The hiss was quieter than that produced via the PAM158, but it was there. As with the PAM158, the hiss volume stayed the same whether the input cables were plugged in or unplugged. The volume of the hiss didn't change no matter how I positioned the volume (gain) knob on the A07, just as it stayed the same on either of the PAM158's gain settings. Although I didn't short the RCA inputs on my A07, I had found that shorting the inputs on the PAM158 made no difference. That gave me confidence that the RCA inputs weren't acting as antennae and that the hiss wasn't an induced signal.
Because the gain setting (or volume knob) has no effect on the hiss level, I figured I must be hearing a constant internal noise floor from the amplifier bleeding through to the tweeters. As such, it must be a fixed characteristic of the amplifier's internal electronics.
The PAM158 is built using the TPA3251 Class D chip. According to data I found at QuantAsylum, the TPA3251 has a minimum residual noise floor of 60 μVrms (A-weighted) and that the actual circuit layout, op-amp input buffers, and power routing can push that residual noise up to 80 μV to 150 μV.
@3eaudio can you share what the noise floor is for the PAM15X and PAM25X amplifiers?
I would have thought that the noise floor would be a bit lower than that (i.e., in the tens of microvolts) and that my tweeters wouldn't be sensitive enough to reproduce that signal. However I am using very sensitive (106 dB SPL @1W/1m B&C DE120) compression drivers on horns as my tweeters. Those tweeters are far more efficient than "regular" tweeters (domes are typically around 86 dB). Consequently, they could reveal electrical noise floors that would be completely silent on conventional speakers.
I have concluded that my very high efficiency tweeters make me a "corner case" and that my speakers are not well suited for the PAM158 or other TPA325X-based amps. Although the inherent 60 μV to 100 μV residual noise floor of a standard TPA3251 configuration is harmless on ordinary speakers, when pushed into a 106 dB sensitive compression driver, that tiny voltage floor translates directly into an easily audible, constant "shhh" acoustic hiss, even from across the room.
@3eaudio would you agree with my conclusion?
Unfortunately, I cannot confirm my suspicions because neither my oscilloscope nor my differential probes are sensitive enough to measure in the sub-millivolt range. My differential probes have a minimum attenuation of 20X (and a noise floor specification of about 22 mVrms on that setting) so that the probe's internal electronic noise would completely mask the amplifier's natural noise.
What to do?
For my HF drivers (such as the B&C DE-120 tweeters) I could switch to low-gain, low-noise amplifiers, ideally something with low residual noise floor (i.e., under 10 μV to 15 μV). A smaller Class A, Class A/B, or high-end composite op-amp based amplifier (like a Neurochrome design) would be more appropriate. I note that the Topping Mini300 claims to have a noise floor of <15 μV.
Alternatively, I could add a passive fixed L-Pad between the amp's outputs and the tweeter. Adding a -12dB to -18dB L-pad should drop all elements of the signal going to the tweeter --- including the noise --- by that amount, rendering it inaudible.
To compensate for the attenuation due to the L-pad, I could turn up the gain on my DSP for the signal to the tweeters.
*** UPDATE ***
I ordered a Topping Mini300 amp (noise floor of <15 μV) and tried it with my speakers. It was dead quiet, even with my ear pressed right against the tweeter horn.
I am convinced that the hiss I was hearing with the PAM158 was due to the inherent noise floor of that amplifier. The noise floor of the PAM158 was high enough that it turned out to be a poor match for my speakers.
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Well, my PAM256 came today, and it has one dead channel (I tested it). I went on the ALSOZONE website, and there is zero information on how to get support (including in the manual). I DM'd @3eaudio , but it's not a good start to the experiment.
As an aside, it will rack mount on a shelf, but you kinda have to angle it in. Hopefully they release a proper rackmount kit eventually.
As an aside, it will rack mount on a shelf, but you kinda have to angle it in. Hopefully they release a proper rackmount kit eventually.
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Yes. They are standard size and will fit regular banana plugs.
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