Blog posts

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.

Quad FM1/FM2 service notes

Submitted by dada on Thu, 11/26/2020 - 11:17

Despite their external appearances being so different, the Quad FM1 and FM2 tuners are internally almost identical.

They are both very well-regarded FM tuners and are still capable of delivering a quality FM sound when in good working order. The guidelines for renovating these are basically the same as for any piece of valve equipment.

Bias Stability in a Quad 303

Submitted by dada on Thu, 11/26/2020 - 11:15

I had set the bias in my own Quad 303 in 2006 or 2007 with the aid of a high-quality Tektronics AA501 distortion analyzer and oscilloscope. This technique clearly shows cross-over distortion spikes when present, so it is easy to arrive at the minimum bias setting required to eliminate cross-over distortion. By this means I usually arrive at somewhere around 10-12mV across the emitter resistors.

Practical issues and benefits of two of the Quad 405-1 modifications suggested in Bernd Ludwig's paper on the Quad 405

Submitted by dada on Thu, 11/26/2020 - 11:11

These modifications are not for the inexperienced or those without adequate test equipment. This article is provided for information only. Use at own risk.

This article is an exploration of the practical issues and benefits of two of the Quad 405-1 modifications suggested in Bernd Ludwig's paper on the Quad 405.