Marantz CD50 Mod that is just stunning

[Chris Daly]

Hi DIYers

Take a standard marantz CD50 , change the two Dual opamps for
opa627/opa627 Duals , or AD825/AD825 on Brown Dog adapters.

Now add a 22K resistor as global feedback between the
two opamps , so the resistor is placed between Pin 6 and 1
of each Dual device, that is Inverting input opamp 1 pin 6
has a resistor of 22k extending to Output of Opamp 2 on Pin 1
In the case of the CD50 Pin 6 of opamp 1 is also the DAC current
output AOR and AOL , and Pin 1 opamp 2 is the final output
before the coupling capacitor.

The arrangement I believe would apply to any inverting
opamp and non inverting follower in CD players.
As I can fathom it ties the two opamps together as
a much larger opamp and forces the 2 opamps to
behave as one. The effect is a very open
sound that convinces me very quickly - that this is about
as good as it gets. it reminds me of some of the best
audio I have heard including the Sondek LP12 variety.

Regards / Chris

[martin clark]

Nice idea.

Just a note of caution - if there is any passive filtering between O/P of opamp 1 and input of opamp 2, you've just undone it! It's unlikely to be a problem with parts like the AD825 and OPA627, but with lesser/standard opamps it could provoke distinctly increased distortion because of the pole added to the response of opamp 1 to force a flat resonse overall.

[Chris Daly]

Hi Martin

Hmm..., if the filtering has been undone, that is then to say
the circuit with additional resistor magically makes the other components dissapear.

A resistor placed as I have suggested would definiately lead- telling the circuit that the new path should go first - which invites discussion about phase. I think the passive components still contribute- my ears tell me there are absolutely no wierd anomalies with frequency.

Its certainly the best improvement I have ever made to a CD player
Regards / Chris

[Chris Daly]

Dear DIY'ers
Here is the amended CD 50 Marantz schematic, showing
analogue I/V stage and placement of 22k resistor.

I simply soldered a resistor under the board as easy
access can be found there to the relative pins.

Cheers / Chris

[Maousse]

hi,

you forgot to upload the schematic

Cheers

[Chris Daly]

Hi

Please find attached schematic as requested showing
22k resistors modifying the feedback path.

Cheers / Chris

[ezkcdude]

Still no schematic.

[Chris Daly]

Hi
Sorry for the image problems , I had to reduce file size
hopefully this time it will work.

Text at the bottom infers 2x opa627 Dual ( on brown Dog adapters )
replaces NE5532

Cheers / Chris

[Mooly]

Erm, not to sure about this one, the resistor has altered the characteristics of the low pass filter and de-emphasis network, have you done any tests comparing an unmodded channel with your mod using a frequency sweep test C.D inc one recorded with emphasis. I am sure it does sound different. The network that is switched by the FET is used for de-emphasis, although I think it was only early (usually Japanese) discs that used this facility. As both OpAmps are configured for inverting mode your feedback is technically positive not negative but it's a bit more complicated than that. Very Interesting. A full analysis of what your mod does is very complex. But don't stop experimenting Will see if I can figure out what it really does.
Regards Karl

[Mooly]

O.K. so what does this mod really do, well it's not as drastic as I thought, but it does alter the response of the filter. The 22K causes a drop in output of around 0.8 db at 1Khz, but it also causes a small rise in output that increases with frequency which begins at around 6.5khz. It is small however, about 0.15db at 10khz rising to around 0.5db at 16khz. Discs recorded with pre emphasis fair much worse, a 0.3db increase at 4khz rising to around 0.8 db at 16khz. There is a small phase shift as well but this is almost certainly not audible. Is the altered response audible, well at any spot frequency I think probably not, such small amplitude variations are hard to identify, but as to whether the increase in overall "energy" at hf is audible, I dont know.
Don't know what I was thinking of saying the feedback was positive earlier on, second opamp is of course a follower to implement the low pass filter. Squinting at the image on screen probably, sorry if it mislead anyone. Well that's my excuse anyway.
Eh, it's all good fun is this
Regards Karl