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.

Note on de-emphasis in HH Scott FM tuners and MPX

Submitted by dada on Mon, 08/16/2021 - 14:31

All the HH Scott stereo tuners and the 335 MPX unit use a constant-k, or in later models m-derived, π-section filter to accomplish both de-emphasis and suppression of 19KHz pilot, 38KHz switching, and general 23-53KHz stereo DSSC information. Legend has it that it took a roomful of MIT students a week to come up with the filter, using slide-rules in 1960.

Quad 44 and the IEC Amendment

Submitted by dada on Mon, 06/28/2021 - 16:20

The 1976 'IEC Amendment' to the 1954 RIAA phono equalization curve is a controversial extra zero with a time constant of 7950uS, which causes a loss of -3dB at 20Hz, increasing at lower frequencies at 6dB/octave. The reasons behind its introduction are mysterious, nobody apparently having asked for it, and many manufacturers have never implemented it at all. As an LF or rumble filter it is rather useless, being only first order, and it cuts into the music as high as -1dB at 40Hz, or the bottom note of a double bass, and starts to cause phase errors from this frequency down.

Quad FM1 or AM half-moon chassis rods

Submitted by dada on Mon, 04/19/2021 - 17:01

I'm having a short run made of the 1/4" chassis rods that are used in the FM1 and 'half-moon' AM tuner. They tend to break off at the threaded part that screws into the front panel. I am making all five different types: the four types of various lengths that are tapped at one end and threaded at the other, and the other type that is tapped at both ends that is found in some FM1s.

Audio whistle in an FM4

Submitted by dada on Fri, 03/26/2021 - 15:24

I am indebted to my colleague John Matheson for some of this information.

Some FM4 tuners exhibit an audio whistle. This has been traced by John to a square wave appearing at pins 2 and 3 of the TL074 op-amp, which operates into a capacitative load provided by the 100uF bipolar smoothing capacitor across the L tuning reference voltage and ground.

The Myth of adding extra capacitors to a Quad II

Submitted by dada on Fri, 03/26/2021 - 14:54

Internet lore, repeated by respected colleagues, has it that when renovating Quad IIs and replacing the coupling capacitors to the KT66 grids, an additional 33pF capacitor should be added from the anode of each EF86 to its grid. This local negative feedback around these valves is intended to compensate for the loss of about 30pF leakage capacitance to ground in the original coupling capacitors, which, it is alleged, was designed into the circuit.