convert Quad44 radio input to CD input

[jives11]

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/quadfranc...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

[cunningham]

If the input device is an op-amp, then the impeadence is high. Just make a voltage divider with two resistors that of which both in series equals the output impeadence of the CD player. Probably about 2KOhms or so.

[Steven]

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

[Upupa Epops]

Steven is correct. Change also this bloody TL's .

[carlmart]

Originally posted by Steven
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

[carlmart]

Originally posted by Upupa Epops
Change also this bloody TL's .

Indeed that might be a good move. Being singles you can use better chips, like OPA627 or similar.



Carlos

[Jan Dupont]

I have modified my QUAD 34 some time ago.
One of the things done was removing (shorting) all DC blocking caps in the signal path, and changing orther caps to new and better types.....
The result: The was expanded to go from DC to app. 300.000 Hz +/- 0.5 dB, and a all new clear and dynamic sound

[Zombie]

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...

[jives11]

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 ?

[Steven]

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