Fosi BT20A Max Integrated Amp- Vintage Audio Review Episode #208

Vintage Audio Review: Fozzy BT2A Max Integrated Amplifier

Introduction and Overview

• The host expresses gratitude for viewers tuning into the episode and introduces the Fozzy BT2A Max integrated amplifier with Bluetooth, which is provided for review.

• The amplifier comes with a remote control that can manage several preamps, including P4 and ZP3, along with a 48V power supply brick.

Specifications and Features

• The BT2A Max is rated at 180 watts into 4-ohm loads (both channels driven) and has a peak output of 300 watts. It retails for approximately $250 on Amazon as of February 2026.

• Frequency response is specified as ±0.5 dB from 20 Hz to 20 kHz. A user manual accompanies the product, detailing its internal components and controls.

Performance Benchmarks

• Initial testing shows the BT2A Max outputs 5 watts into an 8-ohm load with minimal distortion (THD better than 0.004%) and a signal-to-noise ratio (SNR) exceeding 91 dB.

• When connected to a 4-ohm load, THD remains low at around 0.004%, but SNR drops by about 3 dB compared to the performance with an 8-ohm load.

Power Output Testing

• At an output of about 80 watts into an 8-ohm load, distortion levels remain acceptable (<0.005% THD), while SNR reaches up to 100 dB.

• As power increases towards maximum output (around 100 watts), THD stays below the critical threshold of <1%, although it begins to rise significantly past this point.

Final Observations

• Testing reveals that at higher outputs (126 watts into a 4-ohm load), THD remains under control (<0.02%), but does not meet the manufacturer's claim of delivering up to 180 watts per channel under similar conditions.

• Further tests indicate that while performance metrics are strong, they do not align perfectly with advertised specifications; however, overall sound quality remains commendable throughout testing phases.

Power and Performance Analysis of Audio Equipment

Power Output Measurements

• The left channel power output is measured at approximately 140 watts, with a slight increase in noise level affecting the signal-to-noise ratio (SNR), which drops to around 70 dB.

• Incremental adjustments show power output reaching between 143 to 146 watts, while total harmonic distortion (THD) remains low, indicating good performance despite increasing noise levels.

• At 160 watts, THD is reported at about 0.7%, with an overall THD nearing the new standard of 1%. This suggests that the equipment can handle high power outputs effectively.

• The maximum output is noted to be around 180 watts RMS; however, practical usage indicates a more realistic output closer to 130 watts per channel under load conditions.

• Harmonic analysis reveals that odd harmonics are higher than even harmonics, which may affect sound quality.

Frequency Response and Gain Characteristics

• The frequency response from 10 Hz to 40 kHz shows consistent performance at a gain setting of +25 dB into both 4 ohm and 8 ohm loads.

• At higher frequencies (20 kHz), the device meets specifications within ±0.5 dB for both load types, demonstrating effective frequency handling capabilities.

• A graph illustrates THD versus frequency across various power levels; at lower frequencies (80 watts), THD spikes above acceptable limits but improves significantly at higher frequencies.

• Overall performance is deemed satisfactory outside of extreme low-end scenarios; worst-case THD recorded is still relatively low at high frequencies (0.1%).

• Maximum gain observed was approximately +28 dB with a small input signal of 106 mV RMS.

Damping Factor and System Noise

• The damping factor ranges from about 120 at high frequencies down to around 250, indicating strong control over speaker movement which contributes positively to audio fidelity.

• System noise measurements indicate a quiet operation with worst-case noise levels around -95 dBV when connected to shorted RCA inputs, suggesting minimal interference during playback.

Intermodulation Distortion and Crosstalk

• Intermodulation distortion (IMD), tested using dual tones (19 kHz and 20 kHz), shows results better than approximately 0.7%, reflecting good linearity in audio reproduction under complex signals.

• Crosstalk measurements reveal isolation between channels exceeding -65 dB under test conditions, indicating effective separation that minimizes interference between audio channels.

Tone Control Effects

• Adjustments on bass and treble controls demonstrate significant impact: maximum bass boost reaches +12 dB while cuts can also reach -12 dB at lower frequencies like 20 Hz.

Audio Equipment Performance Analysis

Tone Controls and Frequency Response

• The tone controls can provide about 7 dB of boost or cut at 20 kHz, with minimal impact on frequency response when the tone defeat is engaged.

• Engaging the bass and treble controls shows a more significant effect on signal-to-noise ratio (SNR) than previously indicated, despite initial assumptions about frequency response.

• Switching from tone bypass to using bass and treble controls increased power output from 5 watts to approximately 7 watts while decreasing SNR by about 4-5 dB.

• Adjusting gain back down to 25 dB did not significantly improve SNR; however, it confirmed that tone control affects both frequency response and SNR.

THD and SNR Measurements

• The BT28 Max outputs around 75 watts into an 8-ohm load with a total harmonic distortion (THD) better than 0.0009% and an SNR of over 101 dB.

• A THD versus frequency plot indicates that THD remains below approximately 0.09% across frequencies from 20 Hz to 20 kHz, showing better performance at lower powers.

Dynamic Headroom Analysis

• Based on the nominal power rating of the BT28 Max at 75 watts into an 8-ohm load, dynamic headroom is calculated at approximately 2.13 dB.

High-Pass Filter Effects

• The high-pass filter's effect is illustrated through a comparison of responses with it switched on (red trace) versus off (blue trace), showing similar responses above roughly 300 Hz.

Bluetooth Signal Quality Assessment

• A clean audacity-generated signal measured via analyzer shows an SNR exceeding 129 dB when sent over Bluetooth to the BT28 Max, indicating good quality transmission.

• While Bluetooth connection results in slightly worse THD compared to direct connections, overall performance remains satisfactory with acceptable noise levels.

Frequency Response Evaluation

• A chirp wave file played via Bluetooth demonstrates a frequency response within ±0.4 dB from 20 Hz to at least 10 kHz, suggesting decent fidelity despite potential sampling method issues.

Phase Shift Observations

• At various frequencies (1 kHz: ~1 degree phase difference; 100 Hz: ~2 degrees; and at higher frequencies like 10 kHz: ~30 degrees), minimal phase shift indicates effective signal processing by the BT28 Max.

Rise Time Measurement

• The rise time for output signals is recorded at approximately five microseconds, with oscillations noted as part of switching frequency behavior.

BT28 Max Amplifier Review

Phase Difference Analysis

• The switching frequency of the BT28 Max is approximately 500 kHz, with no phase difference between left and right channels at 1 kHz and 100 Hz.

• At 10 kHz, a minimal phase difference of about three degrees is observed when connected to 8-ohm loads.

Powering On and Functionality

• To power on the BT28 Max, hold down the mode button until relays click; this also allows input switching.

• The unit features a fan that operates quietly, mistaken initially for a motorized volume control.

Thermal Performance

• Thermal images show the amplifier running cool, with temperatures ranging from 115°F to 118°F even under high power levels.

• The unit has functionality for connecting to subwoofers and includes a high-pass filter option.

Gain Characteristics

• Notably, engaging tone controls increased gain by at least one dB, which differs from typical vintage receivers where such changes are less pronounced.

Frequency Response and Power Ratings

• The frequency response is rated at ±0.5 dB across the range of 20 Hz to 20 kHz.

• Rated at 75 watts per channel into 8 ohms (both channels driven), it can reach up to nearly 180 watts into 4-ohm loads with acceptable THD levels.

Listening Tests and Bluetooth Performance

• Weighing under three pounds (33 ounces), the BT28 Max delivers substantial power despite its lightweight design.

• During listening tests using Wilson Watt Puppy speakers, Bluetooth performance was decent but showed some frequency response issues at higher bands.

Integrated Amplifier Performance Review

Initial Impressions and Noise Levels

• The integrated amplifier was tested with a volume control set for 25 dB of gain, revealing no hum or hiss; it operated quietly as predicted.

• While listening to various tracks, the sound quality was satisfactory, with adjustments made to boost bass levels slightly.

Power Output and Thermal Performance

• During testing, SPL levels reached around 93-94 dB with peak power indicators showing over 75 watts but not quite reaching 100 watts.

• The amplifier maintained a maximum temperature of 118 degrees without cutting out, indicating reliable performance under load.

Overall Sound Quality and Features

• The reviewer noted no distortion or clipping during use, attributing this to the post-positive feedback filter circuitry designed to reduce distortion.

• The amplifier is recommended for those who appreciate vintage gear and need a compact solution that can handle loud volumes effectively.

Design Considerations and User Experience

• Aesthetically pleasing design; however, the reviewer mentioned that larger LEDs would improve visibility in bright environments.

• Suggested improvements include adding a loudness button and balance control. Despite these quirks, the channels were well balanced overall.

Minor Adjustments for Usability

• Recommendations included adding red paint to the dimples on bass/treble controls for better visibility and ease of use.