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Equipment Measurements

February 2006

Grommes 360 Mono Amplifier: Measurements

All amplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test amplifiers there. All measurement data and graphical information displayed below are the property of SoundStage! and Schneider Publishing Inc. Reproduction in any format is not permitted.

Additional Data
  • Measurements were made with 120V AC line voltage and one channel driven (this is a mono amplifier). Data shown for the unbalanced input unless otherwise noted.
  • Input impedance
    • Unbalanced: 9.8k ohms.
    • Balanced: 14.1k ohms.
  • Gain
    • Pentode: 49x, 33.8dB.
    • Ultralinear: 30.5x, 29.7dB.
  • Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination
    • Pentode: wideband 1.3mV, -66.7dBW; A weighted 0.26mV, -80.7dBW.
    • Ultralinear: wideband 0.81mV, -70.9dBW; A weighted 0.12mV, -87.4dBW.
  • Output noise, 8-ohm load, balanced input, 600-ohm input termination
    • Pentode: wideband 1.0mV, -69.0dBW; A weighted 0.21mV, -82.6dBW.
    • Ultralinear: wideband 0.82mV, -70.7dBW; A weighted 0.17mV, -84.4dBW.
  • AC line current draw at idle: 1.45A.
  • Output impedance at 50Hz
    • Pentode: 17.3 ohms.
    • Ultralinear: 8.5 ohms.
  • This amplifier does not invert polarity.
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD (P): 14W
  • 8-ohm load at 1% THD (UL): 12W
  • 8-ohm load at 10% THD (P): 77W
  • 8-ohm load at 10% THD (UL): 75W

  • 4-ohm load at 1% THD (P): 6.3W
  • 4-ohm load at 1% THD (UL): 7.5W
  • 4-ohm load at 10% THD (P): 88W
  • 4-ohm load at 10% THD (UL): 75W

General

The Grommes 360 is a medium-power, mono, push-pull tube power amplifier utilizing one pair of KT88 output tubes. Not usual in this day and age is the use of two tube high-voltage rectifiers and a 6L6 tube used presumably as a voltage regulator for either output-tube screen grid voltage or front-end-tube supply voltage.

Distortion behavior of the amp was essentially the same for balanced or unbalanced inputs. One thing that is a bit puzzling is the low input impedance. This parameter, for tube circuits, can easily be much higher than this and is typically 50k ohms or higher for most tube-amp designs. The low input impedance of this amp could penalize the performance of some otherwise very good tube preamps used to drive it.

Chart 1 shows the frequency response of the amp with varying loads for pentode and ultralinear modes. The output impedance, as judged by the closeness of spacing between the curves of open-circuit, 8-ohm, and 4-ohm loading in the pentode mode, is unusually high and would cause major aberrations in the frequency response of many loudspeakers. For instance, with the NHT dummy speaker load, the variation is some +/-5dB. In ultralinear mode, things are a bit better, but still the output impedance, in my opinion, is unacceptably high. All of this technical logic aside, it may well be that this amp with some speakers may be complementary to each other and sound very good.

Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for a 1kHz and SMPTE IM test signals and amplifier output load for both pentode and ultralinear modes. This design, with its single output connection for speaker loads, is about equally good for either 4- or 8-ohm loads in either pentode or ultralinear modes although, as usual, distortion is higher for the 4-ohm loading.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3 for both output-stage modes. Amount of rise in distortion at low frequencies is quite pronounced, but it is not atypical for many tube power amps. High-frequency-distortion rise is moderate and reasonably good.

Damping factor vs. frequency is shown in Chart 4. Here, we can see the unusually low damping factor in pentode mode and the approaching acceptable and typical value for some tube amps in ultralinear mode.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Chart 5 for the ultralinear mode. The pentode-mode signal spectrum was very similar, but had more hum components. The principal signal harmonics are second and third with the remaining harmonics about 20dB below the level of the second and third harmonics. However, there are some spurious non-harmonic-related components present in both modes.

Chart 1 - Frequency Response of Output Voltage as a Function of Output Loading

Pentode

Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load
Cyan line = NHT dummy-speaker load

Ultralinear

Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load
Cyan line = NHT dummy-speaker load

Chart 2 - Distortion as a Function of Power Output and Output Loading

Pentode

(line up at 1W to determine lines)
Top line: 4-ohm SMPTE IM
Second line: 8-ohm SMPTE IM
Third line: 4-ohm THD+N
Bottom line: 8-ohm THD+N

Ultralinear

(line up at 20W to determine lines)
Top line: 4-ohm SMPTE IM
Second line: 8-ohm SMPTE IM
Third line: 4-ohm THD+N
Bottom line: 8-ohm THD+N

Chart 3 - Distortion as a Function of Power Output and Frequency

Pentode

8-ohm output loading
Cyan line: 60W
Blue line: 30W
Magenta line: 10W
Red line: 1W

Ultralinear

8-ohm output loading
Cyan line: 60W
Blue line: 30W
Magenta line: 10W
Red line: 1W

Chart 4 - Damping Factor as a Function of Frequency


Damping factor = output impedance divided into 8
Magenta line: Ultralinear
Red line: Pentode

Chart 5 - Distortion and Noise Spectrum

Ultralinear

1kHz signal at 10W into an 8-ohm load

 

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