The amplifier, whether integrated or basic, is the heart of your system. It is the engine that drives your loudspeakers, quite literally. When a unit needs attention, any service required is done correctly, without reservation. This includes using ONLY genuine components, never on-line or eBay purchased Chinese counterfeit devices to save a buck, and no re-stamps or generic replacements. The original issue is addressed, and as standard practice, everything is verified as being proper. This includes DC offset, if adjustable, which is set to exactly 0mV, and allowed to idle for 45 minutes to confirm stability. If non-adjustable and there is no more that several mV of offset, we will match the differential pair to achieve this.
Afterwards, the amplifier section is CORRECTLY biased to eliminate all crossover notch at the transition between device polarities (push half of signal switching to pull half of signal). This is done into a 4 ohm load at 10kHz with one watt of output. This impedance resolves, visually on the oscilloscope, any crossover notch, regardless of the impedance connected in normal use. The frequency of 10kHz is also being used being as harshness from under biased output stages almost always manifests in the 10th octave of human hearing 10kHz-20kHz. It is set to the point where the output is free of crossover notch, and locked in. Just enough forward bias for a clean output, and a cool running output stage. This all applies to direct coupled output stages. In older capacitor coupled output designs, the AC balance of the output stage is set using the permanently used impedance (usually 8 ohms) for symmetrical full power clipping, performed at 400Hz and again at 1kHz, which is extremely important. This allows both halves of the signal to clip at the same point during full power transients. Bias is set in a traditional fashion as outlined above for direct coupled output stages. After confirmation of all functions, designed output power is verified.
The pre-amp section, if its an integrated, is tested for every single function and adjustment. All inputs, including phono confirmed as operating as designed. The phono section is also verified as having correct RIAA equalisation curve.
Valve (tube) amplifier are always returned to their exact original state, including removing ANY improper mods or so-called “upgrades”. A lot of times you’ll hear folks say its normal for a valve amplifier to have some hum. This is false, and your unit will be silent upon completion. Nothing short of exact original operation is the only way its done at Stereo Rehab.