QUAD 405 Lower input sensitivity to 1.5V

[355F1]

I bought this 405 which was overhauled about two years ago and is working wonderfully. All electrolytic and ceramic caps have been replaced with higher quality electrolytic and film capacitors, it was also completely rewired with thicker wires for better current carrying in the power supply and output lines, and with audio type coax cables for audio signals. The op amp ICs were replaced with Burr Brown audio quality op amps ICs but the input sensitivity has been left unchanged.

What cap or resistor do I add, value and where to solder it?

Thanks much in advance.

 

[roclite]

Hi, I have refurbished and modded several Quad 405 and 303. The quick answer to your question is to replace R4 (1k6) to 4k7 and C2 (100uF) to 33uF. For the very best source of information for this modification and the location of the components to change, google "Keith Snook"

 

[355F1]

Thank you. According to the 405 schematic I noticed that R1 is 10K not 1.6K per board number M12368 ISS 5 & 6. ISS 7, 9 & 10, same board number shows 22K.

Do I still use the 4.7K resistor that you suggested?

Cheers,

 

[roclite]

R1? You mean R4. The gain setting resistor values are different depending on the exact Quad 405 model issue as you have noticed. As long as you correctly locate R4 and C2 and change to suggested values, you should reduce sensitivity to 1.5V. This will only be true if R6 (feedback resistor) is 22K. I don't know what version you have so you need to refer to Snook for guidance.

 

[355F1]

I apologize, my mistake, I should have written R4.

Nevertheless. R6 is 330K in all versions but issue 9 is 130K.

How do I distinguish what version do I have, by looking at the serial number?

 

[355F1]

I have found this article from Bernd Ludwig

http://www.desmith.net/NMdS/Shack/Bernd Ludwig 405 Modifications V13.pdf

 

[roclite]

Yes, Bernd is another good source. The serial number should tell you but you still need to check the feedback resistor around the op-amp because that sets gain too. If you study Bernd and Snook info you should be able to find the components to change and their new values. Good luck.

 

[355F1]

Chaps,

Finally, I have found time to work on my 405 lowering its input sensitivity.

I did a search and found only one capacitor [33mF] brand, KEMET, at Allied electronics.

Also, what kind of resistors [4.7K] should I buy, carbon or metal, Ohmite, Vishay Dale, NTE or?

Since I just need only two of each it may be hard to find a supplier selling in small quantities, the time of Lafayette, Allied, etc., where you found everything but the kitchen sink is long gone.

Cheers,

 

[ejp]

The gain setting resistor values are different depending on the exact Quad 405 model issue as you have noticed.

No they aren't. They are R6 and C4 for the audio gain and R4 or C2 for the DC servo gain. It does not vary by 405 model.

As long as you correctly locate R4 and C2 and change to suggested values, you should reduce sensitivity to 1.5V.

No you won't. That only adjusts the DC servo gain. The audio gain is determined by R6 and C4.

This will only be true if R6 (feedback resistor) is 22K.

R6 is 330K, not 22K, and you will only get 1.5V sensitivity if you alter R6 to 1/3 its value, or 100K, and alter C4 to 3x its value, to 150nF.

The quick answer to your question is to replace R4 (1k6) to 4k7 and C2 (100uF) to 33uF.

No it isn't. That changes the DC servo gain by a factor of 4, not 3, moves its operating frequency, and does nothing to the audio gain.

EJP