The discussion started with the first article in the Netherlands about the revision/upgrade and modification of a Quad 33 and 303 by Ruud Janssen. See appendix 1
This upgrade, apart from the standard replacement of all the electrolytes, was based upon the increase of the power supply voltage to 16 volt, reducing the gain of the output amplifier stage, to cope with high level inputs, and increasing the gain of the PU amplifier to compensate for the lower gain of the output stage. Also the power supply voltage for the PU amplifier was kept at the same level by changing R300. Although the series resistors in the PSU are decreased when you install a 16V zener, this is still one of the disadvantages of this type of regulation, a PSU with a high internal resistance. This gives poor load regulation and higher crosstalk figures.
Considering the input signal levels, I built 33’s with 7812 and 7815 regulators, (check the remarks about the 7812/7815 below) There is more headroom with a 15V supply, correct. So with certain inputs, there is an advantage.
There is a big difference between the standard (12V) zener based supply and the regulator based supply, but the regulator based supply is difficult to sell in a DIY upgrade kit. Experienced DIY users can do this job, also it can be done in one of our shops.
The reason for keeping the PSU voltage at the same level for the PU amplifier was the following: To preserve the noise level produced by the first BC109 transistor on the PU board, Ruud Janssen stated that the DC bias current should not change. This looks like a valid argument, but increasing the gain of the PU amplifier to compensate for the lower gain of the output amplifier increases the noise level of the PU stage.
The decrease of the gain of the output amplifier is no guarantee that a high level output from a CD player or other source will not overstress the input amplifiers on the tape adapter board. All the input signals, except the tape input are routed via the input amplifier on the tape adapter board. This is even more a consideration when someone uses the modified PU input selector board for connecting a CD player, trough the PU amplifier with the normal PSU voltage setting.
My own observations:
R300 together with C313 forms a low pas filter to reduce the hum and noise in the PU stages. The -3dB point is around 3hz. By putting in a 7812 or 7815 in the PSU, this filter is no longer necessary. Keep the capacitor in place and replace R300 by a wire link. This also reduces PSU induced crosstalk. If you are consequent replace resistors R305/308 and R315/316 to maintain the DC bias current settings and the voltages as mentioned in the diagram, if you are a purist also replace all the other resistors concerning the DC bias settings of the other stages when you are applying a 7815 regulator or the 16V zener option! The 7812/7815 will not
work in all situations, in some cases the input voltage of the 7812/7815 will not be sufficient. The LM293
7ET-15 low voltage drop regulator is a little bit more expensive, but performs very good under all feasible conditions. So we advice to start with the LM2937ET-15.
In the diagram you will see the placement and the pin layout ( 1,2, and 3) of the regulator.
Replacing R305/308 and R315/316 is a better option than increasing R300 when you want to reduce the PSU voltage of the PU amplifier. Increasing R300 also means increasing the PSU induced crosstalk, but also a unwanted feedback effect between the two collector resistors of each channel but also between the channels. All four resistors are fed trough R300! Because the filter formed by R300 and C313 also stabilize the voltage at R305 (10,25V) and the rest, the effect will be small, but it will not be zero!
There is also a positive effect, the filter formed by R300 and C313 will have a lower -3dB point when increasing R300.
In the table the values are given for the three power supply voltages, situation one: keep R300 in place and situation two: replace R300 by a wire link.
Keep the DC PU settings:
PSU Voltage, Value in Ohms
Remove R300 (replace by wire link)!
12V, R305/308=100K, R315/316=4K4
15V, R305/308=133K, R315/316=6K5
16V, R305/308=140K, R315/316=7K2
Instead of reducing the gain of the output stages I use a resistive network to adjust the input sensitivity, like Quad themselves does concerning the tape input. Reducing the gain of the output amplifiers helps, but increasing the gain of the PU amplifier does not help. If you don’t need all the inputs, i.e. incidentally a turntable, and don’t want to connect the FM2, FM3, FM4 or AM3 tuners to the preamp then adjusting the gain of the output amplifiers and the PU amplifier is a valid option.
So to sum up: my revision advise for experienced users. Remember, there is nothing wrong with the standard upgrade, these are extra's.
If you use Quad tuners, leave the gain settings as they are. If you use other high output equipment, change the gain of the output amplifier boards. Use a LM2934ET-15 volt regulator in the PSU, remove all the unnecessary components, also the extra supply for the switching of secondary equipment. Because of the regulator, the caps in the PSU can be of relative low value, there is also the extra 100uF on the PU board. Replace R300 by a wire link in the PU amplifier and replace R305/308 and R315/316 to maintain the DC settings. Change R101/102/103/104 to 47K, this gives a more or less standard impedance of the PU input. Replace all the electrolytic capacitors. Connect a CD player to the Tape input, 1 Volt setting.
The Ruud Janssen 33 upgrade and revision in a nutshell. (See the Quad-links in the left column for the original text - in Dutch). This is the basis for our standard upgrade kit.
Replace R500/501 (120 Ohm) by two 28 Ohm resistors. Replace MR500 by a 16V 250mW zener.
Replace C500/501 with radial capacitors of 4700uF 25V, maximum dimensions: 35 x 17mm, replace C502 with an axial type, 1000uF 25 V (30 x 16mm). Replace MR501/502 with 1N4002 diodes. Remove C503/504, R502 and MR503 (My advise).
Output amplifier boards
Reduce gain, from 5 to 2,4
Change R411/411 from 1800 Ohm to 1000 Ohm, change R412/412 from 470Ohm to 1200 Ohm
C401/401 2,2 uF 25V
C405/405 47 uF 25V
C406/406 22 uF 25V
C300/303 47uF 25V
C301/302 100uF 25V
C307/308 47uF 25V
C311/312 47uF 25V
C313 470uF 25V
Change R300 from 560 Ohm to 1500 Ohm
(I don't agree with this value, to low!)
PU Adapter board
Replace R101/102/103/104 with 47K to change the input impedance from 68K to 47K.
Use M1 position of the adapter board to compensate for the gain reduction of the output boards. Otherwise change the values of R105/107 for the left channel, and R106/R108 for the right channel, to change the gain of the PU amp. No values are specified, only that the sum of R105/107 and R106/108 should stay the same for a proper RIAA correction. A lower value of R107 will increase the gain. A gain setting above 78x is not recommended.
Optional: replace all the resistors in the RIAA section and the first transistor stage with low noise types.
C5/6 100uF 25V