Blog posts

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.

The Myth of the Quad II/22/tuner power supply balancing

Submitted by dada on Tue, 02/09/2021 - 15:10

It has been claimed, and indeed I have uncritically repeated it myself in the past, that the Quad 22 and tuners draw heater and HT current from different Quad II power amplifiers, so that the heater and HT loads are distributed.

However reference to the Quad 22 and FM Multiplex schematics shows that this is simply not the case. Instead the distribution is simply such that the heater and HT that comes from the Yellow amplifier goes to power the 22 and also out the Yellow and Black rear sockets, and the heater and HT from the Blue amplifier goes out the Blue socket.

Quad AM2/3 service notes

Submitted by dada on Fri, 11/27/2020 - 14:00

1. Check all resistors attached to the HT line, including the screen resistor to the detector valve, and replace if more than 10% out of tolerance.

2. There are about ten film capacitors. If they are of the red Hunts type, replace them all on sight; otherwise measure and replace when out of tolerance.

3. Valves in these usually last forever: you are unlikely to have to replace them.

4. Carry out the audio-frequency alignment in the manual if you have appropriate equipment, changing the notch frequency to 9KHz as per international agreement in about 1978.