Gain of a Quad 33

Submitted by dada on Fri, 08/27/2021 - 13:46

It is sometimes stated that the gain of a Quad 33 is about 5, given by (R411+R412)/R412, or 2270/470 ~= 4.8, because the voltage divider formed by R411-2 provides negative feedback around the tone control section.

The truth is a little more complex.

  1. R412 is in parallel with the series string consisting of R8=100R, C5=100uF and half the 1K balance control, or another 600R (at centre balance poition).
  2. So the effective value of the bottom leg of the NFB voltage divider is not 470R but 470R || 600R ~= 263R.
  3. So the gain of this stage is not about 5 but nearly 8, actually 7.844.
  4. However by this time the signal hasĀ  been through three emitter followers: Tr200, Tr400, and Tr402, each having gain of ~0.9, or a little less in the case of Tr400 due to voltage division by R404-5, i.e. 0.9*4700/(270+4700) or ~0.85.
  5. So the total gain through the 33 at maximum volume is given by 0.9*0.85*0.9*7.884 ~= 5.4.

When a tape input is selected, Tr200 is not in circuit, so this is multiplied by 1/0.9, giving ~6.

This analysis also shows why C5 is important to the signal quality: it is a partial emitter bypass capacitor forĀ  R412 and therefore Tr402, and is therefore in series with the signal. It also shows that the 33 balance control works via negative feedback, not just as a passive shunt to ground.

All component designations refer to the left channel.