Marantz CD6000 HDAM DC offset

dragonweed · 23rd October 2015, 06:30 PM

Hi there!

My CD 6000 (Not OSE) developed a problem. I use it DC coupled, the output coupling caps bypassed, and recently one channel shows insreased DC on the output terminals.
The good channel is 4.6mV, the other 128mV. It is the HDAM module, because on its inputs there is 1.2 and 1.4mV respectively. The other module is OK. I guess it is the input diff. FET pair what is causing the problem. Is there any idea how can I fix it, without replacing the FET-s?
The module is all SMD parts, and is is a PITA to work on it.

Ken Newton · 07:16 PM

It's not correct to view the channel with the 128mV offset as necessarily indicating that something is wrong with that module. It may simply indicate a very normal degree of D.C. offset due to temperature drift, or aging parts, or some natural parameter mismatch. You are attempting to D.C. couple a circuit that was not designed for that, else, Marantz likely wouldn't have added the cost of output coupling capacitors. I can't think of too many discrete op-amp circuits featuring a transconductance output node VAS stage (such as does this HDAM circuit) that don't utilize either a D.C. blocking output capacitor, or an active D.C. servo circuit.

dragonweed · 07:43 PM

I completely agree with you, it is only my nitpicking stupidity that wants everything perfect... But really: that thing (and all audio stuff) sounds SSooo much better without ANY coupling caps in the signal path. That's why I am trying to get rid of them. Anyway, I will try a small (100 ohm) trimpot between the two sources of the FET-s, maybe it will help. (it did in my power amp, similar diff. ip stage with JFET-s). Luckily the HDAM module -beneath the metal shield- is not smd, but discrete parts composed.

Ken Newton · 23rd October 2015, 08:01 PM

Before you give up on A.C. coupling capacitors, I can report excellent results with Black Gate type 'N' (bipolar) 4.7uF electroytics. Yes, electrolytics. The subjective clarity was much superior to that of some Clarity Cap brand polypropylene film units I had tried there. I doubt that the Black Gate can be beat should you require larger value units, such as the 220uF that Marantz specifies in the schematic. While no longer in production, some stock still remains at a few audio parts suppliers, at clearance prices in some cases.

Also, Nelson Pass reports very low distortion with Elna Silmic II e-lytics, these are in current production, but I have no personal experience with them.

Mooly · 24th October 2015, 11:51 AM

I wouldn't add a trimmer, the resistance will lower the transconductance of the whole input stage. Not a price worth paying.

23rd October 2015, 07:09 PM	#2
Ken Newton diyAudio Member Join Date: Apr 2003 Location: Eastern Pennsylvania	It's not correct to view the channel with the 128mV offset as necessarily indicating that something is wrong with that module. It may simply indicate a very normal degree of D.C. offset due to temperature drift, or aging parts, or some natural parameter mismatch. You are attempting to D.C. couple a circuit that was not designed for that, else, Marantz likely wouldn't have added the cost of output coupling capacitors. I can't think of too many discrete op-amp circuits featuring a transconductance output node VAS stage (such as does this HDAM circuit) that don't utilize either a D.C. blocking output capacitor, or an active D.C. servo circuit. Last edited by Ken Newton; 23rd October 2015 at 07:16 PM.

23rd October 2015, 07:34 PM	#3
dragonweed Banned Join Date: Sep 2010 Location: Hungary	I completely agree with you, it is only my nitpicking stupidity that wants everything perfect... But really: that thing (and all audio stuff) sounds SSooo much better without ANY coupling caps in the signal path. That's why I am trying to get rid of them. Anyway, I will try a small (100 ohm) trimpot between the two sources of the FET-s, maybe it will help. (it did in my power amp, similar diff. ip stage with JFET-s). Luckily the HDAM module -beneath the metal shield- is not smd, but discrete parts composed. Last edited by dragonweed; 23rd October 2015 at 07:43 PM.

24th October 2015, 11:51 AM	#5
Mooly diyAudio Moderator Join Date: Sep 2007	I wouldn't add a trimmer, the resistance will lower the transconductance of the whole input stage. Not a price worth paying. __________________ Installing and using LTspice IV. From beginner to advanced.

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23rd October 2015, 08:01 PM	#4
Ken Newton diyAudio Member Join Date: Apr 2003 Location: Eastern Pennsylvania	Before you give up on A.C. coupling capacitors, I can report excellent results with Black Gate type 'N' (bipolar) 4.7uF electroytics. Yes, electrolytics. The subjective clarity was much superior to that of some Clarity Cap brand polypropylene film units I had tried there. I doubt that the Black Gate can be beat should you require larger value units, such as the 220uF that Marantz specifies in the schematic. While no longer in production, some stock still remains at a few audio parts suppliers, at clearance prices in some cases. Also, Nelson Pass reports very low distortion with Elna Silmic II e-lytics, these are in current production, but I have no personal experience with them.