Monday, January 02, 2006

Quad 34 input sensitivity

Thomas had a (very usual) problem with the input-sensitivity of his 34.

He wrote:
"Als hij op CD-speler staat, moet ik hem half open zetten om voldoende volume te hebben. Op radio krijg ik hem dan weer niet stil genoeg (dus stand 1 is dan al te luid). De CD-speler is een quad 66, tuner voorlopig een ouwe marantz maar dat moet op termijn een FM4 worden."

"When it is set to CD, I have to put the volume half a way to get a reasonable volume. Set to Radio, I can't get it silent enough (position 1 is already too loud). The CD-player is a Quad 66, the tuner - for the time being -an old Marantz, but in time that has to become an FM4."

This is a typical problem with Quad-equipment, indeed. Quad defines "line-voltage" as 500 mV while most other manufacturers apply +/- 1,2 V today (and even at the time Marantz defined it this way...).

The standard input-sensitivity for the 34 is 300 mV (CD) and 100 mV (!) for the Radio (Tuner) input.


However, the CD-input can be easily adapted by changing R135/R136 to the correct value. The original Quad "flags" (see the picture on top) are not needed, good quality resistors (metal-film) will do. The Tape in- & outputs are adaptable the same way.

Unfortunately for adapting the Radio input some soldering is necessary as no variable sensitivity is foreseen by Quad.

The X1 / X2 "flags" are interconnections between the tape-output and the CD-connector (in order to use this one as a second Tape-connector).

The best solution for Thomas is to connect the old Marantz to the Tape-input I guess, adapting the sensitivity of it, until he has his FM4.

BTW. On the picture we also see the antique TL072 Opamps that should be exchanged for Burr Browns if we want the same dynamics "modern" endstage amplifiers like a revised 405 or 606 can provide.

Stefaan

2 Comments:

Anonymous Jacques said...

The input sensitivity on the Quad 34 matches the output from the FM4 corresponding to the modulation of BBC Radio 3, which is very cautious and conservative, to say the least. The advantage is that with the not-too-high quality of our cable distribution here around Ghent, BBC3 still comes reasonably through. The broadcasts are, of course, for the partisans of BBC3 :-)

Note that I had the same impression as Keith Snook about the weak stereo channel separation on the Quad 34 except with the 3 mV Disc input (the only input which I have 'upgraded' with new op-amps). There may indeed be an issue with the PCB design of early 34s.

Best regards,

Jacques

4:04 PM  
Blogger Khuong Anh Tuan said...

Tuan,
Thank for your comments, so please tell me the best value of resistors that will replace the "flag" in CD input? 10k or higher?

Thank you,

3:50 PM  

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