Amp Lab – Quilter Labs Bass Block 802

Thermal Management

As far as I can tell from testing this amp, the thermal management is the same as the older version. I really like the cross ventilation and the fact that the fan comes on quickly when the amp is pushed. Even during power testing, this is one of the good ones, as far as keeping the amp running for longer periods at max power.

Gain & THD+N Standout

As with the 800, the 802 is a nice clean amp that is well-behaved. The gain maps show a knee where the max power is reached. The THD+N is low until the max is reached. As with the 800, the knobs at 0 setting is not that interesting for testing, because the gain goes negative and weird results show up. Therefore, I limit the lower limit of the knobs to 2 ticks above the minimum.

Frequency Response Standout

This is the first part of the testing that really sets the two amps apart. The 802 has a fairly standard 4-band EQ. It is generally very well-behaved and accurate. It may be more bland than the 800, so even though this is the major diversion from the 800, there just really is not much to stand out and talk about here.

The “optimally flat” test for the amp (which looks at the limited frequency range of 80Hz to 8kHz) is really just an exercise in splitting hairs. With all knobs at the “noon” position, it is already at +-0.666 dB (my college dorm room number!). But bumping the Depth knob up one tick, I was able to get down to +-0.354 dB.

The strangest thing from the 800 is the weird frequency response that I was getting from the amp when the Contour knob was turned all the way down. The 802 does not show this strangeness. On the 802, the Snap knob controls the EQ at the same frequency, and it does not allow the output to go so low that you can see the problem. Looking at the waveforms on the scope, the 38.25 kHz sine wave overlay still exists on the output of this amp, but the EQ is a bit better behaved now and does not let the output go low enough for the overlay signal to take over the test.

[Pat Quilter note: “Like virtually all class-D amps, there is a visible carrier frequency on the speaker waveform; in our case, at 250kHz. l wonder if this is being ‘folded’ down to 38kHz on the scope trace, which might be possible when using a digital audio test rig.”]

[Editorial response: “We suspected that this might be a possibility, but we have had many similar class-D amps on our Audio Precision test bench, and it appears there is something unique happening with this particular amp.”]

Input Impedance

The passive input measured at 407 kOhm impedance with a 1kHz sine wave. The active input measured at 477 kOhm impedance with the same 1kHz sine wave. There is not much difference between the two inputs as far as impedance, but there is a definite dB drop with the Active input.

[Pat Quilter note: “I am not sure about the method used to measure the impedance externally, but the net impedance across the audio band should be 1meg (two 2meg resistors in parallel) on both active and passive. If impedance is measured by inserting a known resistance in series with the signal and looking at the voltage drop, it is important to do this well below the limiter threshold, but well above the class-D switching residue at 250kHz (or use the output signal from the preamp).”]

[Editorial response: Our standard methodology for measuring input impedance utilizes a 1kHz sine wave input. After double-checking our input impedance measurements, we observed results consistent with our initial measurements. However, we did note that if we ran the impedance test using a 100Hz input, measured impedance was approximately 980Hz.”]

Waveforms

As noted above, the high frequency overlay signal still exists on this amp. It is so small that it does not affect the THD+N values very much, and with the updated EQ, the amp never goes low enough for it to mess up the test bench. I only include it here for completeness; I would not consider this to be a big negative for the amp. It is not audible.

Power – 1kHz Burst and Continuous, 50Hz Burst and Continuous

The 800 and 802 are close enough in design that I was able to get an apples-to-apples comparison of the power numbers. The Gain is at 5.5 and the Master is at max. For the 1kHz testing, I had the THD+N set to 4%. The same higher THD+N occurs on this amp with lower frequencies, so I used the same 11.7% THD+N for the 50Hz power test. I was also able to get through the continuous power test at 50Hz with this amp. The 800 kept going into thermal protection for the 50Hz continuous test, so it appears that there are some upgrades to the power amp and it is indeed better.

All the power numbers are higher on the 802 vs the 800. There is really no doubt at all that this is an 800w amp. I really like the conservative rating on this amp. A lot of manufacturers would list this a 1,000w amp, so it is nice to see that a manufacturer is honest and can more that deliver what is promised.

Conclusion

This is a good amp. I am not sure I am able to give a higher vote than that on any amp. There are always trade-offs and style decisions to be made. Overall, this amp delivers more than what it says it will, and does it cleanly and even has a sturdy, functional case with some style. I can not comment on the playability of the amp, but from an electrical point of view, there is not much I can come up with that would be a downside.

Fig. 2 Gain – Graph of gain with gain knob set at minimum

Master = minimum
Master = midpoint
Master = maximum

Fig. 2 RMS – Graph of output RMS voltage level with gain set at minimum

Master = minimum
Master = midpoint
Master = maximum

Fig. 2 THD+N – Graph of output THD+N ratio for with gain set at minimum

Master = minimum
Master = midpoint
Master = maximum

Fig. 3 Gain – Graph of gain with gain knob set at midpoint

Master = minimum
Master = midpoint
Master = maximum

Fig. 3 RMS – Graph of output RMS voltage level with gain set at midpoint

Master = minimum
Master = midpoint
Master = maximum

Fig. 3 THD+N – Graph of output THD+N ratio for with gain set at midpoint

Master = minimum
Master = midpoint
Master = maximum

Fig. 4 Gain – Graph of gain with gain knob set at maximum

Master = minimum
Master = midpoint
Master = maximum

Fig. 4 RMS – Graph of output RMS voltage level with gain set at maximum

Master = minimum
Master = midpoint
Master = maximum

Fig. 4 THD+N – Graph of output THD+N ratio for with gain set at maximum

Master = minimum
Master = midpoint
Master = maximum

Fig. 5 Gain – Graph of gain with all knobs at midpoint

Active Input
Passive Input

Fig. 5 RMS – Graph of output RMS voltage level with all knobs at midpoint

Active Input
Passive Input

Fig. 5 THD+N – Graph of output THD+N ratio for with all knobs at midpoint

Active Input
Passive Input

Fig. 6 Gain – Graph of gain

depth = minimum
depth = midpoint
depth = maximum

Fig. 6 RMS – Graph of RMS output voltage level 

depth = minimum
depth = midpoint
depth = maximum

Fig. 7 Gain – Graph of gain

woof = minimum
woof = midpoint
woof = maximum

Fig. 7 RMS – Graph of RMS output voltage level 

woof = minimum
woof = midpoint
woof = maximum

Fig. 8 Gain – Graph of gain

bark = minimum
bark = midpoint
bark = maximum

Fig. 8 RMS – Graph of RMS output voltage level

bark = minimum
bark = midpoint
bark = maximum

Fig. 9 Gain – Graph of gain

snap = minimum
snap = midpoint
snap = maximum

Fig. 9 RMS – Graph of RMS output voltage level

snap = minimum
snap = midpoint
snap = maximum

Fig. 10 Gain – Graph of gain

Optimally flat = minimum
All knobs at noon = midpoint

Fig. 10 RMS – Graph of RMS output voltage level 

Optimally flat = minimum
All knobs at noon = midpoint

Fig. 14 Scope – Scope trace of output with 1kHz sine wave input

Fig. 15 Scope – Scope trace of output with 50Hz sine wave input

Fig. 16 Scope – Scope trace of output with 1kHz sine wave input at only 2 mVrms.  It shows the relevance of the high frequency overlay signal at extremely low input voltage levels

1-5 (unacceptable to impeccable)

On-bench