Reused Low-Cost Power Amps

These notes are mostly for my own use/curiosity. I'm limiting these notes to class AB and A amps. Once you get into switching amps, things change quite a bit.

The Nelson Pass designed GFA-555 is known for a good but bright presentation. I will … wait for it … pass on that. Other less costly GFA amps are widely available on the used market. The c. 1993 GFA-535 was reviewed in Stereophile.

H/K embraced Otala, et al. early on and have a history of designing “wideband” amplifiers, going back to tube designs. This maybe tips them toward having good top ends. My experience with a receiver has been favorable. But the company seems to have let their quality slip starting in the 90s or late 80s (?), so it's unknown if these amps are credible contenders.

• Manual is ©2000
• 4 channel bridgeable
• 4 x 45 watts @ 8 ohms, 20Hz – 20kHz, <0.07% THD, all channels driven
• 2 x 100 watts @ 8 ohms, 20Hz – 20kHz, <0.07% THD, all channels driven
• Plentiful
• Dual volume control on back (one for each pair)
• Can't find decent reviews

• Manual looks older than PA 2000, shares manual with PA 2200
• One for sale ad claims 1996-97, which is believable.
• 2 channel bridgeable
• 2 x 65 watts @ 8 ohms, 20Hz – 20kHz, <0.09% THD
• 1 x 130 watts @ 8 ohms, 20Hz – 20kHz, <0.3% THD
• NFB: 12dB
• Slew rate: 120V/usec
• Rise time: 1.2uSec
• TIM: unmeasurable
• Can't find decent reviews

• Manual looks older than PA 2000, shares manual with PA 2100
• 2 channel bridgeable
• 2 x 100 watts @ 8 ohms, 20Hz – 20kHz, <0.08% THD
• 1 x 200 watts @ 8 ohms, 20Hz – 20kHz, <0.08% THD
• NFB: 12dB
• Slew rate: 140V/usec
• Rise time: 1.2uSec
• TIM: unmeasurable
• Can't find decent reviews

• Manual is ©2000
• 8 channel bridgeable
• Standard Operation: 8 x 45 watts @ 8 ohms, 20Hz – 20kHz, <0.07% THD, all channels driven
• Bridged Mode 1: 2 x 100 watts plus 4 x 45 watts @ 8 ohms, 20Hz – 20kHz, <0.07% THD, all channels driven
• Bridged Mode 2: 4 x 100 watts @ 8 ohms, 20Hz – 20kHz, <0.07% THD, all channels driven
• Can't find decent reviews

NAD is persistently dogged by reports of reliability issues. Personal experience with a 7020 receiver (ancient!) corroborates this somewhat. It also corroborates that the NAD sound favors a “warm” or thick bottom. This might be expected if the design approach uses large amounts of “dynamic headroom” (as NAD AB amp designs have tended to do): With small power supply caps, supply modulation in the LF will increase, and this will increase the LF THD.

• 1985

• Continuous Output Power (into 8 Ω) : 60 W (17 dBW)
• Rated Distortion (THD 20Hz - 20kHz): 0,03%
• Bridged Mode Continuous Output Power (into 8 Ω) : 100 W (20 dBW)
• Bridgeable
• L/R volume controls on front

• 1985

• 1985-1989
• 100 watts per channel into 8Ω
• Total harmonic distortion: 0.04%
• Slew rate: 25 V/uS
• Apart from input stage, symmetric circuit.
• Uses NJM 2043DD opamp in front end for bridging and soft clip.
• Very favorably reviewed in Audio Science Review.

These are claimed not to have output protection. Otherwise promising candidates.

• 50 watts per channel into 8Ω (stereo)
• Total harmonic distortion: 0.02%
• After the front end, symmetric circuit.

• 100 watts per channel into 8Ω (stereo)
• Total harmonic distortion: 0.005%
• Fully symmetric (“Complete Mirror”) circuit.

Not as popular or having the same buzz as lots of other manufacturers. But the brand intrigues me. They pay at least lip service to wide bandwidth some some/most/all power amps, so are worth a deeper dive.

• 100 Watt - 8 Ohm - 20 - 20000 Hz - THD 0.08% - 2 channels (main)
• Audio Science Review is meh about it.

• 150 watts per channel into 8Ω
• THD: 0.09%

• 105 watts per channel into 8Ω
• THD: 0.09%

• 75 W/Ch (1 kHz, 0.08%, 2 Channels Driven, FTC)

John Curl's involvement with the company lends them some cred. Whether the products not design by JC are to the same standard is not currently known.

• Manual is ©1991
• 50 watts RMS x 2, 20Hz-20kHz, 8 Ω
• THD: < 0.25% at full power, < 0.15% typical levels
• Slew rate: 40V/μsecond
• TIM: Unmeasurable

• Manual doesn't have © date
• 100 watts RMS x 2, 20Hz-20kHz, 8Ω
• THD: < 0.08% at full power, < 0.03% typical levels
• Slew rate: 60V/μ second
• TIM: Unmeasurable
• L/R volume controls on front panel

• Manual is ©2005
• 75 watts rms per channel into 8Ω, both channels driven
• THD: 0.035% at full rated output, 0.025% at average listening levels
• TIM: Unmeasurable

• Claimed to use chip amps (SK…).

• vintage hifi