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
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
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.
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.
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
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
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
Will see if I can figure out what it really does.Regards Karl
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
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
CD80 too benefits from this mod
The Marantz CD 80 similarly benefits from this modification
The CD80 layout is similar but offers as part of
its original circuit DC offset cancellation, and has single type
opamps rather than duals.
A 22K resitor placed between AOL input to opamp 6385
Pin 2, ( a common NE5534, which can be changed for a opa627
or AD825 ) to opamp 6387 Pin 6 ( also a NE5534 change this too for better sound ). Then connect similarly AOR - this time its
opamps labelled as 6384 and 6386 Pin 2 to Pin 6 ( remember these are single type opamps )
There is a bit more room on this circuit board, I attached a
resistor to pin 6 of the opamp labelled as 6387 then used some
shielded wire extending this to resistors 3455 for Left channel AOL
and similarly Pin 6 of the opamp labelled as 6386 this time to Resistor 3454 for AOR - effectively the same point as Pin 2 but easier to access
I grounded the shield of the shielded wire to a gnd connection tab
located conviently close by.
I will get to an image soon to assist.
The Marantz CD 80 similarly benefits from this modification
The CD80 layout is similar but offers as part of
its original circuit DC offset cancellation, and has single type
opamps rather than duals.
A 22K resitor placed between AOL input to opamp 6385
Pin 2, ( a common NE5534, which can be changed for a opa627
or AD825 ) to opamp 6387 Pin 6 ( also a NE5534 change this too for better sound ). Then connect similarly AOR - this time its
opamps labelled as 6384 and 6386 Pin 2 to Pin 6 ( remember these are single type opamps )
There is a bit more room on this circuit board, I attached a
resistor to pin 6 of the opamp labelled as 6387 then used some
shielded wire extending this to resistors 3455 for Left channel AOL
and similarly Pin 6 of the opamp labelled as 6386 this time to Resistor 3454 for AOR - effectively the same point as Pin 2 but easier to access
I grounded the shield of the shielded wire to a gnd connection tab
located conviently close by.
I will get to an image soon to assist.
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