Active Crossover System Design 419 This can be countered by using opamps in parallel to increase the drive capability, assuming you're not designing to the absolute minimum cost. Two opamps working together allow the circuit impedances to be halved, giving us another 3 dB improvement, while four opamps allow them to be halved again, giving 6 dB less noise. This is going to be about as far as it is economical to go unless you're designing really gold-plated gear, so we have a possible Johnson noise improvement from Low-Impedance Design of 16 dB. Johnson noise is, however, only one component of the circuit noise, the other two important contributions coming from the voltage noise and the current noise of the active devices. Reducing the circuit impedances reduces the effect of current noise--proportionally this time, as the current noise only manifests itself when it causes a voltage drop across an impedance. Voltage noise is a tougher proposition to reduce, the options being a) shell out for quieter and more expensive active devices; or b) make use of opamps in parallel again. If two opamp stages of the same gain are connected together by low-value resistors (say 10 ) then at their junction you get the average of the two outputs, so the signal level is unchanged, but the noise drops by 3 dB (1/2) as the two noise components are uncorrelated and so partially cancel. Four opamp stages give a 6 dB improvement. This technique obviously goes extremely well with using opamps in parallel to allow circuit impedance reduction, and can make for some very neat and effective circuitry.