Quad 306 sensitivity mod C4 question

[Francesco De Nigris]

Hi everybody,
when modding a 306 to lower input sensitivity should C4 be changed along with R13? If only R13 is changed my guess is that the frequency response should change in some way. What's the mathematical relation between those two components that should be used in order to calculate the value of C4 according to R13 changes?
Thank you

 

[Arkless Electronics]

Don't touch C4 or R13!

If you change R1 and R2 to make them both 10K this should approx halve the sensitivity.

 

[Francesco De Nigris]

Don't touch C4 or R13!

If you change R1 and R2 to make them both 10K this should approx halve the sensitivity.

So you suggest halving input signal instead of altering the preamp section... this could be done with a 25k trimpot in order to get more flexibility. or better yet with a double stepped precision pot. What's the advantage of doing so in your opinion?

 

[Arkless Electronics]

So you suggest halving input signal instead of altering the preamp section... this could be done with a 25k trimpot in order to get more flexibility. or better yet with a double stepped precision pot. What's the advantage of doing so in your opinion?

There's no "pre amp section". The way you are proposing to do it messes with the negative feedback and hence also the efficacy of the loop compensation and the stability of the amplifier. It may well oscillate, or have bad ringing on its square wave response, or upset the balance of the current dumping bridge and cause distortion...

 

[Fatmarley]

Strange - I replaced R13 as suggested by Dada electronics (can't remember the value though) and I didn't notice any problems.

 

[Arkless Electronics]

Strange - I replaced R13 as suggested by Dada electronics (can't remember the value though) and I didn't notice any problems.

I did say "may".... certainly many parameters of the amp will be optimised for the amount of loop feedback it uses and so even at best it's a bit of a dodgy way of doing it.... especially as you would be increasing the feedback to reduce the gain!
I didn't know that Dada suggest doing it this way and it could be that they have thoroughly tested it for all the things I mentioned and in the case of the 306 all is OK, but increasing the feedback reduces the stability margin and without also changing several small capacitors in the compensation scheme one could expect at least some ringing on square waves and a possibility that it is no longer unconditionally stable and that certain speakers or even speaker cables may cause it to oscillate.
The way I suggested has no such risks.