Pyle PTA4 Mini Power Amplifier Review and Measurements

Discussion in 'Equipment Reviews & Opinions' started by Brickbat, Sep 18, 2017.

  1. Brickbat

    Brickbat Active Member

    Messages:
    183
    Location:
    Indianapolis
    OK, first of all, I know darn well this isn’t “High End Performance” as Pyle claims. The reason I bought it is because it’s $59. It’s small, and had key features I wanted. I wasn’t expecting much, but had some hope it’d be good enough for my home office room where I have HD tuner, and a couple of PC’s streaming various sources. I design electronics for a living, so I knew the ‘2 x 120W’ claim was a load of crap, but all I wanted for this application was an honest 10W or so, so I sallied forth with the purchase. Pyle makes a half-dozen variants with different features such as MP3 players, Blue teeth, tone controls, more/less power, more/less blinking lights, etc. The PTA4 seemed like the best choice for me.
    2017-09-18 21.47.07.jpg

    Unboxing it revealed a decently-well put together $59 amplifier. There’s no thick (or thin for that matter) aluminum panels, but I wouldn’t describe it as cheap or flimsy either. It’s got a painted steel sheet metal housing. I like that the front panel lettering is white on black. It won’t win any industrial design awards, but you can’t argue the practicality of that choice – it’s easy to read. Popping out a handful of screws allows the top cover to be lifted off, revealing the innards.
    2017-09-18 21.49.41.jpg
    Most of the circuitry is SMT and contained on the main PCB, including all the control pots, front panel jacks, and the power amp chip. It uses a TDA7377 power amp chip. That chip is designed for stereo BTL automotive applications, and is rated at 20W, 4 ohms, 10% THD with a 14.4 VDC supply. Specified THD drops to .03% at 10W, so that meets my needs.

    Interestingly, the ST data sheet for the TDA7377 shows a max operating power supply voltage of 18V, but the supply voltage in the Pyle measure 22 VDC. It’s not as bad as it sounds though, because well-designed automotive chips are pretty tolerant of overvoltage. Still, this is NOT a conservative design choice, and I’d expect poor reliability because of it. I know for a fact that while the TDA7377 data sheet says it’s short circuit tolerant, including shorts to ground and VCC, my first test subject smoked the TDA7377 after I inadvertently grounded the speaker terminal for about 1.3 nanoseconds. So beware of any shorts involving the speaker wiring. And keep in mind that, this being a bridged output, neither the black or red speaker terminals are at ground potential – they sit at around 12V DC, relative to ground.

    The Pyle has a direct 120V line cord – no need for a wall wart to power it up. It uses a run-of-the mill AC transformer instead of switching power supply. No issue there to me, anyway. I doubt that the transformer or heatsink on the power amp will support long-term use at full power, but I don’t intend to find out by testing it to death...again...

    I’d REALLY like to see a schematic for this thing. The only other IC I see in the audio path is a quad op-amp. The volume, bass, and treble controls are single-gang, so at the moment I’m not sure how they’ve implemented that. I was expecting something like a LM1036, but it’s not there. In a way I’m glad it doesn’t use an LM1036, because it looks like that might be too noisy for my application, but we’ll see if the Pyle approach is actually better.

    Other reviewers have whined about the LEDs. Yes, they’re bright. But I like blinking lights, dammit. And this has 10 blinking LEDs on the front panel running from +3 dB down to -15 dB. So, maybe I’ll tone them down by lowering the drive current, or swapping a more easy-on-the-eyes color for the bright blue. Or maybe I’ve just found a need for my eclipse viewing glasses. The controls have decent ‘feel’. They have center detents – fine, whatever. The knobs are cheap black plastic, but not all that bad actually. The key features I wanted are here: tone controls, switching between 3 inputs, about 8” wide, and a ‘tape record’ output. The front panel headphone jack might be handy, if it provides acceptable quality. It has a (useless to me) front panel mic input, and a somewhat mysterious 3.5 mm front panel ‘AUX-IN’. The back is your basic cluster of RCA jacks, a fuse, AC mains voltage switch, linecord, and push-in speaker connectors. They don’t handle wire much bigger than 16 AWG – so don’t plan on using your monster-sized speaker cable with this guy.

    So much for the ‘unboxing’. Basically, it’s what I expected, and didn’t disappoint.

    Technically, my main concern was the background noise level (hum and hiss). IME, THAT matters in an office system because of the close spacing between your ears and the speakers. My ADS L810 speakers are something like 90 dBSPL/W at 1m. My ears happen to be about a meter away. My office is not ‘recording studio quiet’ and I have a touch of tinnitus. So I thought it’d be good if the speaker noise ended up near 30 dB SPL, meaning I need to see 1 uW or less noise power. (This is a very rough analysis, but it gets me in the ballpark.) At 6 ohms, this works out to around 2.5 mV rms of noise, appropriately weighted – and that’s with the volume control at its minimum setting. Measurements to follow.

    Other than that, I wanted an honest 10W of low distortion power, decent flatness over 20-20kHz, tone controls that work, and multiple inputs. So, how’s this little puppy check out on the bench?

    First thing: speaker protection relay. Really? Not expected! And there’s no turn-on or turn-off thump because of it. This is actually somewhat significant and unexpected, because it means that some human, somewhere, has actually decided to spend more money in production of this $59 product to increase the performance and user experience. This is good sign.

    DC offset. So much is posted here about DC offset. This thing ought to deliver 40 Vpp to your speakers. The DC offset measures 28 mV on R and 68 mV on L. Does that matter? Beats me, but I’m not losing any sleep over it. It's within the 150 mV spec of the TDA7377.

    Power Output. I don’t have an analyzer with a differential input right now, so I’m first making some rough measurements with a scope in differential mode. Let’s see here – hook up 8 ohm loads, apply 1V RMS to the CD input, crank up the volume control until the ‘0 dB’ blue light just starts to sear my retina. And the output measures…drumroll…2.89 V rms. Across 8 ohms, that’s just a hint over 1 frickin Watt. Yup. Really. ‘0dB’ on this “120W” amplifier equates to 1 Watt.

    Ok, so sure they can ‘calibrate’ their output LEDs any way they want. But where’s it start to clip? Looks like about 9.5 Vrms. Across 8 ohms, that’s 11.3 W. About what I needed, but Damn. I would have guessed it’d be good for a touch more, given the stupidly high power supply voltage they used, which seems to hold up to around 20V DC, even under load. I’m pretty sure things would look better with 4 ohm loads, but I don’t want to risk smoking a second unit...

    But still, looks like I’ll get the 10W I wanted.

    Tone/Balance controls. At 1 kHz they don’t do much, as expected. Treble provides about +/- 1 dB and Bass does about +/- 0.5 dB. OK by me for a $59 amp. The Treble turnover freq is around 2 kHz. Bass is 400 Hz – too high for me. (Did I mention I’d REALLY like to see a schematic?) Balance provides cut on the opposite channel, but only about 1 dB of boost on the desired. I can live with that.

    The noise floor on my scope is only around 1 mV, so I really can’t make a meaningful hum/hiss measurement. The Pyle measures about 2 mV rms, unweighted. But again, I don’t really have much faith in that measurement. Hooking up a pair of little speakers, there is audible 120Hz hum and a little background hiss. It’s audible to me at about a foot. I’d say this doesn’t bode well…

    More later…
     
    Last edited: Sep 18, 2017
    eizner23 likes this.
  2. Brickbat

    Brickbat Active Member

    Messages:
    183
    Location:
    Indianapolis
    Couple more things. I now have a differential-input audio analyzer. The output noise measures about 1.3 mV rms. It's dominated by a 120 Hz signal.

    THD bottoms out at about 0.2% at 1W, 1 kHz, and creeps up to about 0.5% at 8W.

    Freq response is down 1 dB at 65 Hz and 13 kHz. Its -3dB at 30 Hz and 25 kHz.

    The 'record' outputs work as expected.
     
  3. Brickbat

    Brickbat Active Member

    Messages:
    183
    Location:
    Indianapolis
    I've put this amp into use with the big ADS 810 speakers for a few days. Grr. It's really not gonna cut it. The main problem is the thing I was concerned about from the outset - the background noise level is annoying. 120Hz hum dominates, but there's some 'hiss' as well.

    Too bad, because other than that, I'd recommend it if you're looking for a 10W compact amp.

    Suggestions on a replacement welcomed...
     
  4. AUDIO1MAN

    AUDIO1MAN New Member

    Messages:
    10
    Location:
    Thousand Oaks, CA
    The HUM is part of the PCB layout, Look @ Pin 6, 8 & 9 on the TDA7377. Pin 6 is the ref decoupled audio in and 8 & 9 are the PS & Audio grounds. Their may also be part of the PS filter charge/discharge paths causing the bad HUM.
    Duke
     
  5. Brickbat

    Brickbat Active Member

    Messages:
    183
    Location:
    Indianapolis
    OK - agreed. Bad PCB layout wrt grounds is probably the cause of the 120 Hz hum. What experience do you have with this particular beast?

    Good point about the PS filter paths possibly inducing some voltage across the PA input. Maybe I'll tear into it again and see where they grounded the pin 6 cap...
     
    Last edited: Oct 14, 2017 at 10:41 PM
  6. botrytis

    botrytis Trying not to be a Small Speaker Hoarder Subscriber

    Sounds like a Pyle of S&%T.

    Remember, you get what you pay for....
     
  7. Brickbat

    Brickbat Active Member

    Messages:
    183
    Location:
    Indianapolis
    How interesting. Please go on...
     

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