RCruz said:
I would suggest that you bypass (short out) the capacitors on the CD and connect it to the pre-amp without your amp connected (or disconnect the speakers), then measure if you get any offset on the pre-amp / amp output. Chances are good that the pre-amp got input capacitors, if not you won't damage anything with the speakers disconnected. This way you can save the losses of a capacitor in the audio line.
Regards
André
Yes, the best cap is NO cap 
On a related note, I just installed a discrete DC coupled I/V stage in my Rotel 855 (TDA1541a DAC, I used rbroer's less simple stage here on diyaudio). The offset is typically less than 1mV with the DC servos. My preamp is a Counterpoint SA5.1 which being tube does have coupling caps. I also use a DIY Gilmore Dynamic headphone amp which is fully DC coupled. This is connected to the preamp tape outs, which have only gone from the input, a 20K resistor and thru the selector switch before hitting the outputs.
If I have the volume all the way down on the headphone amp, it is basically loading the circuit with a dead short (low ohms... whatever the TKD pot is at its lowest setting). Is there anything short of putting in input caps on this that would prevent it from loading the circuit when not in use? Perhaps use an input relay?
Thanks for any suggestions... sorry, I know this isn't Marantz specific and I don't mean to thread crap, but with the discrete output stages here, you guys may have run across this issue before.
Chris
On a related note, I just installed a discrete DC coupled I/V stage in my Rotel 855 (TDA1541a DAC, I used rbroer's less simple stage here on diyaudio). The offset is typically less than 1mV with the DC servos. My preamp is a Counterpoint SA5.1 which being tube does have coupling caps. I also use a DIY Gilmore Dynamic headphone amp which is fully DC coupled. This is connected to the preamp tape outs, which have only gone from the input, a 20K resistor and thru the selector switch before hitting the outputs.
If I have the volume all the way down on the headphone amp, it is basically loading the circuit with a dead short (low ohms... whatever the TKD pot is at its lowest setting). Is there anything short of putting in input caps on this that would prevent it from loading the circuit when not in use? Perhaps use an input relay?
Thanks for any suggestions... sorry, I know this isn't Marantz specific and I don't mean to thread crap, but with the discrete output stages here, you guys may have run across this issue before.
Chris
Pars said:Yes, the best cap is NO cap
If I have the volume all the way down on the headphone amp, it is basically loading the circuit with a dead short (low ohms... whatever the TKD pot is at its lowest setting). Is there anything short of putting in input caps on this that would prevent it from loading the circuit when not in use? Perhaps use an input relay?
Chris
If the pot is wired correctly it will not influence the input impedance, can you send a diagram? I would also like to see the diagram of your dc servo.
Regards
André
The headphone amp is this one:
http://headwize.com/projects/showfile.php?file=gilmore3_prj.htm
It is built on a www.headamp.com commercial PCB (back when he sold PCBs as well). The pot is wired correctly... input jack signal to pot input pin, wiper to amp, ground pin to ground (TKD CP2500, same pinout as an Alps RK27 Blue). IIRC I also have a 475K resistor to ground, but if the pot is turned all the way down, it dominates the impedence, so near dead short. No series resistors or anything.
Additional info on the amp here:
http://www.djgardner.com/headphone/gilmore/
Servo schematic is here as well. Or did you mean the servo on the I/V stage?
That is here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=22542
I am using the same component values for this as the Gilmore servo, except the resistor feeding the DC back is 100K.
http://headwize.com/projects/showfile.php?file=gilmore3_prj.htm
It is built on a www.headamp.com commercial PCB (back when he sold PCBs as well). The pot is wired correctly... input jack signal to pot input pin, wiper to amp, ground pin to ground (TKD CP2500, same pinout as an Alps RK27 Blue). IIRC I also have a 475K resistor to ground, but if the pot is turned all the way down, it dominates the impedence, so near dead short. No series resistors or anything.
Additional info on the amp here:
http://www.djgardner.com/headphone/gilmore/
Servo schematic is here as well. Or did you mean the servo on the I/V stage?
That is here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=22542
I am using the same component values for this as the Gilmore servo, except the resistor feeding the DC back is 100K.
Hi Chris
From what I can see the input impedance should stay at the same value as that of the pot because you are only driving the gate of a fet (high impedance) from the wiper. If I understand correctly your problem is when you turn the volume down (wiper grounded). If so, I think your wiper and input of the pot is swapped or you have a short between the wiper and input.
Regards
André
From what I can see the input impedance should stay at the same value as that of the pot because you are only driving the gate of a fet (high impedance) from the wiper. If I understand correctly your problem is when you turn the volume down (wiper grounded). If so, I think your wiper and input of the pot is swapped or you have a short between the wiper and input.
Regards
André
Right, I have redone my discrete output boards and this time I designed the layout myself, only to allow use of smt resistors btw.
It works quite well except for a "tape hiss" sound. This is quite loud and clouds the detail which I know is there. I think Brent also experienced this at some point too. Any ideas anyone?
Oh, and the boards are fed from 7812/7912 regs so nothing special there.
Cheers,
Lee.
It works quite well except for a "tape hiss" sound. This is quite loud and clouds the detail which I know is there. I think Brent also experienced this at some point too. Any ideas anyone?
Oh, and the boards are fed from 7812/7912 regs so nothing special there.
Cheers,
Lee.
Thomo said:
Hi Lee,
Is the hiss also present when the inputs from the DAC are not connected? If so, it could be the current sink that is noisy. It is a fact that some noise is present without signal (also with the original opamp filter), but normally this is not audible because it is muted. It should not be clearly noticeable however, only with the volume on max. and with your ear next to the tweeter (that's the case here anyway).
Ray
