convert Quad44 radio input to CD input

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Greetings. My Quad 44 has a radio module which appears to be very similar to the Aux module, though much more sensitive, based on the schematic at http://iquebec.ifrance.com/quadfranco/schemas/schemas44.html

Q. By changing the resistors before the opamp I should be able to change the Radio input into a useful additional CD/DVD player input.

Anyone done this or can suggest optimum values for the potential dividers, I know some Aux inputs can be overloaded by CD-level output.

many thanks
 
Outputs of CD players can have a very low output impedance, often just 100 Ohm. Loading with an equal value resistor to divide the output voltage by 2 (-6dB) is not a good option, because the CD output is forced to deliver a lot of current.

If I look at the schematic of the Quad 44 aux input, it has a series resistor of 390k (R600/601) and a shunt resistor of 100k (R602/603), so it already has an input voltage divider. CD players often have an output voltage that is 6..10dB more than tuners. So if you change R602/603 (or the equivalent in your tuner input module) from 100k to 47k or 33k, you get a nice CD input. This will have a negligible influence on the low cut off corner frequency. That frequency is mainly determined by the 390k resistor and the 100n input capacitor.

Steven
 
Steven said:
If I look at the schematic of the Quad 44 aux input, it has a series resistor of 390k (R600/601) and a shunt resistor of 100k (R602/603), so it already has an input voltage divider. CD players often have an output voltage that is 6..10dB more than tuners. So if you change R602/603 (or the equivalent in your tuner input module) from 100k to 47k or 33k, you get a nice CD input. This will have a negligible influence on the low cut off corner frequency. That frequency is mainly determined by the 390k resistor and the 100n input capacitor.

IMO both the resistor and the capacitor should be wire bypassed and leave just the 100K shunt.

The TLO71 is working as a buffer, with no amplification, and every CD player already has an output cap. The 2 volt CDP standard output should be fine for the IC and for the +/-15v supply.


Carlos
 
Rip out C36/37 and C48/54 and put a wire from the front leg of the first to the back leg of the other performs a little miracle.

I am considering putting a switch on the back for bypassing the tone controls and/or to find better caps (14 of them) for them...

If you do the same, let me know what you think...
 
well I took out the CD/Aux board to see what it looked like. This is a 300mv/500K later model which is explicitly titled CD/Aux.

unlike the AUx circuit on the schematic there is no 390K R600 resistor at all. R602 (the shunt) is 470K.

OK,


however the feedback resistors are slightly different values also (based on the colour codes)

R604 on the output from the Buffer is 1K5 rather than 2K2 (R604). R606 from the feedback to ground is 820 Ohm rather than 3K3.

I understand the change in the shunt but I wonder why Quad changed the feedback arrangement ? Is it becuase they moved from 2 single TL071 to a single dual TL072 ?
 
Hi jives11,

Appearantly there was no need anymore to have the voltage divider at the input, maybe the level diagram for the whole unit has been changed. This had nothing to do with a change from 2x TL071 to TL072. That is probably just a price or availability issue.

The change of the output resistor values didn't do anything with the gain of that stage. Gain of the opamp is still unity. Don't get confused because it looks like feedback resistors to determine the gain. They are not. It is just a clipper! The ouput is taken from the voltage divider R604/R606 and not from the opamp output. Because of feedback the output impedance of that stage is still close to zero. Gain is one because the output of the stage is directly going to the inverting input of the opamp. These resistors are used to fix the maximum output voltage of the stage. While the opamp itself may clip at around 13V peak (for 15V supply), the maximum output is 820/(820+1500)*13V = 4.6V peak. I haven't looked at the rest of the circuit but this has probably been done to protect some CMOS switches that are used for source selection or tone control against too high input voltages. Of course this is a "dirty" clipper, since the opamp may be driven very hard into saturation with subsequent sticking to the supply lines. Well, it is allowed if you are quite sure that the input voltage will never exceed 4.6V peak or approx 3Vrms.

Steven
 
The input buffers on the 44 and later 34s certainly do enable a myriad of input sensitivities and impedances, they even made microphone input modules for the 44 but are the buffers just masking a problem with or easing the PCB layout as well as complicating the signal path ??

http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/QUAD/QUAD-34-Modification/QUAD-34-Mods.html

ACD...

Note QUADs deliberate use of asymetric supplies used on the QUAD34 to give a small +ve offset at the op-amp outputs. Did you account for this when you removed all the capacitors ? Also you quote response to dc after the mod, did you throw out the tilt/bass controls just so you could see your speaker cones waft back and forth once a week when driven from a VLF generator !

More seriously if your QUAD 34 was an early one with single op-amps did you measure the cross talk at any time ??
 
Note QUADs deliberate use of asymetric supplies used on the QUAD34 to give a small +ve offset at the op-amp outputs. Did you account for this when you removed all the capacitors ?

Very old post, but none of this is correct. The asymmetry is to ensure the TL071 clips +ve first, as it will latch up on --ve clipping. Asymmetric power rails do not produce DC offsets at the output. The op amp laws continue to hold. Using an op-amp with lower offset makes it possible to remove the coupling capacitor regardless of the PSU asymmetry. It is possible of course to fix that too, but there is very little point.
 
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