CDP output signal too strong for AMP input??

Hello fellow members !!

I recently purchased a UnicoCD and did notice some unpleasant changes. On highy powerfull recorded music it seems to overdrive the input of the Pathos. My Sphinx Project 2 MKII Preamp comes back on monday or tuesday and i would like to know what i can do to reduce this problem. The problem does not happen with my Rega Planet 2000. My friend told me this


the Unison might have a higher gain, which overdrives the input stage of the power amp


He suggested a passive-preamp in case it wont work with the Sphinx. Are there any other options out there exept buying a new CD-Player? How can i build a passive preamp or how can i lower the output of the Unison?

Thanks

Flo
 
I have experienced the same problems albeit on different models of kit, there are two cheap and easy ways.

If your cd has a remote control, look for the volume controls and simply lower it, works great.

For no more than £40 you can purchase inline Attenuators, a -10 dB reduction usually does the trick and from my experience of the ones i purchased off a dealer on ebay did not detract from the sound quality whatsoever. In fact you will hear a lot more detail with a more equal volume level rather than booming bits that just sound loud and nothing else ! You can get them configured for cd output side or amp input side, i use them on my tuner output and enables the fm stereo side to now sound the way it should, and not like you are listeneing to the radio !

Hope this helps
 
Hi Florian,

Find the input (differential) section of your amplifier / preamplifier.

Find R1 and R2.

Increase R2 to decrease the gain or
Decrease R1 to decrease the gain.

For the best results, slightly change the value for both resistors.

Cheers,
Extreme_Boky
 

Attachments

  • diffsection.pdf
    36.4 KB · Views: 56
It' very dangerous to tent to decrease the gain by this way (i.e increasing the ratio feedback) . For an amplifier , there's a quick way to destroy it .

Hi guytou,

In fact, that is the proper way to set the gain of an amplifier.

It is possible to change the gain other ways - but none apart from the above affects only the gain - they affect linearity as well.

Maybe you've tried and fried the amplifier(s)?

This is an excellent read:
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

Extreme_Boky
 
guytou said:
Likely the amplifier will oscillate & you'll burn the output transistors (at least).


That is, if little or no phase margin is present. Decently designed amps have sufficient margin to increase feedback by a factor 2 or 3.

Indeed, you are right, those amps that don't have phase margin, will oscillate.

That way they disappear from the world, Darwins mechanism works here too :)
 
Right , that means one have to know about phase margin before simply replace a resistor.

Hmmm, obviously you know about phase margin, don't you?

I was aware of the phase margin issues and I never suggested resistor replacement, but slight change in resistor value to lower the gain. Properly designed amplifiers have many mechanisms to control / break the oscillations even if loaded with purely capacitive loads. I also draw the capacitor across R1, but that hasn’t been registered either! (see post #3)

The main reason I suggested to slightly change the values of R1 and R2 was to avoid sound degradation if external units are used to control - reduce the gain. The simpler - the better when it comes to analog amplification, and if we can get all the voltage and current gain by using only 3 gain stages - fantastic! So, I wanted to avoid external units.

I have never had the instability problem with any amp / pre-amp and I have adjusted the gain by slightly altering the feedback ration on many of them. However, since this thread evolved in something that looks like feeding-up individuals egos and not in to attempt to help anyone who asks for help / advice, I'll add that it is better to increase R2 and leave R1 alone form phase margin and stability point of view.

With properly designed amplifiers, change in gain of 10 (20dB) is totally ACEPTEABLE.

There is a lot of info on the net regarding this and I sincerely believe that members should try to provide honest help to fellow members, instead of proving to (themselves?) that they are the most knowledgable on the Planet, while in fact they are keeping the advice needed for themselves and not helping anyone!

Extreme_Boky