Marantz HDAM tweak

[GaryKrant]

Hi all: I'm not sure if this topic has been already written, fi so you'r welcome to blame me! Some time ago I started tweaking my Marantz CD6000. At this time (call me lazy) the completed roadmap is:

- Solving CDR errors as described on service manual (a must-be)
- Improving headphone output since all tests are done with my Senns HD-650. I replaced stock NM4556A Op-amp by a NE5532 that supports 300 Ohm of output impedance, the same that my HD-650. Good improvement.
- Improving active low pass filter replacing stock NJM2114 op-amp by a OP275
At this point the sound is extremely detailed, open, fast, but in my opinion too much balanced to the high band, lacking in mids and bass, too much harsh.

Today I was loosing my time reviewing the CD6000 schematics when I put my attention on the two caps an the HDAM output. I supose they are there as DC coupling capacitors, but two electrolytic side-by-side instead of a non-polarised one? Seemed strange to me.

In CD6000 these two serie caps are Elna Silmic 220 microFarad joined side by side by their negative pin. I had some .47 Audience Auricaps and I've replaced both Silmics by only one Auricap (so one Auricap in each HDAM output).

At this point, and after three break-in hours, I have to admit an audible improvement in the signal quality: part of highs brightness has gone, but now mids and bass are balanced, sweet, and the harsh is gone.

Im quite sure this is not a subjective opinion and I would like to know the opinion of those that have tried a similiar approach. Technically the change is far from being a 1:1, in fact the original series capacity was 110 microFarad (two 220 in series) and now it's 0.47, I don't know how it can internally affect to the HDAM work, so your technical advices would be really appreciated.

[wwenze]

A non-polar electrolytic capacitor is made with two polar ones in series, but negative leads connected together like what you saw.

But ELNA Silmic for signal coupling? Eww. Film is good. The drawback of the reduced capacitance is the bass will be cut-off at a higher frequency, .47 might be too little but not a problem if you didn't notice less bass, but I'd suggest 2.2 at least.

[GaryKrant]

Thanks for your reply wwence. The fact is that bass is performing MUCH better than before now that caps are in their 16-hour uninterrupted break in period. Definitely the used cap size (,47 microFarad 200V) is all except the right one, but they were the only high quality ones I had to perform the test.

I'm trying to expand which were my objectives with this test: as I said before, after installing the OP275 the sound improved in terms of detail and dynamics, mainly due to the incredible gain in highs. My overall opinion was, as I said, that the sound was clearly unbalanced to highs, lacking in mids and with a poor, excesively discrete, lazy bass.

After replacing the stock Silmics caps the sound is more balanced, mids are rich and bass is deep, well driven and defined, but I have noticed a slight loss in highs. Summarizing I have to admit that now the sound is better.

But at this point I'm considering if I can combine the current mids and bass with the previous highs. So, I would like to know, as an absolute electronics newbie, what I've really done changing caps in terms of electronics.

I attach the HDAM electronic schematic, the big grey area is the HDAM itself, in the small grey area are the two original Silmic caps and two resistors that, together seem to be a kind of filter.



I've found people claiming that, as DC blocking, these caps are doing very few job because the measured dc values are too slow (in order of milivolts) and that they can be removed. I'm not going to do it, I'm quite sure that Marantz guys are not stupid so if they are placing two caps there is because they are required.

So do you think these caps are really acting as DC coupling caps or they are performing other tasks? I'm confused because, as wwence said, two Silmics acting in the place of a good film cap seems strange.

[Luke]

They are dc blocking and minor gains can be had by removing them. Back to back makes them bipolar, and Im not sure a film is better, try it and see.

[GaryKrant]

Well, entering in the field of tastes and perceptions, I'm quite sure that sound has been improved in the terms decribed: a light loss in highs with a gain in mids and bass, what leaves a balanced, defined, open and fast sound.

Maybe the result of this tweak was very dependant of cap brand and model, in this line I have to say that Auricaps are mainly appreciated for their neutrality, so I supose that the current result is the real sound that CD6000 in able to generate.

As I said I really don't know, that's the reason of wanting to know the opinion of other people that had made a similar change. At this point I consider it an important improvement over the above described player (stock CD6000 with OP275 replacing the original NJM2114 op-amp in the DAC output active low pass filter)

[maudan]

Did you measure the DC offset on the output of the HDAM stage?
If the offset is less than 10mV you can bypass the capacitors and have a real improvement. The offset on my CD6000 is very low, few millivolts, I removed the capacitors and it works nice.

regards

[GaryKrant]

Hi Maudan: No, I didn't. As I said previously, and from the point of view of a perfect electronics ignorant, I think that if designers placed a DC coupling there they had a solid reason. Maybe now the DC offset was low but a time based component degradation, or punctual situation, could increase it damaging other parts ... or maybe no.

Maudan, have you performed other changes in you CD6000 output stage? How did the cap removal affect the sound in your case?

[maudan]

Well, capacitors for blocking DC are placed for safety reasons. The designer cannot know at the beginning to which kind of amplifier will be connected.
Or it is better to say the designer doesn't know if the amplifier input can tolerate or not a DC component; some amplifiers have capacitors in the input stage and some other not. For this reason and to ensure an universal capability the CD outputs are always equipped with capacitors, sometimes good (like the Silmic of the Marantz) and sometimes very bad.
Your consideration about the degradation is good too, in theory the HDAM is a push-pull stage and his output has to be close to the ground level, if any component suffers for a damage there will be serious risks to have the total DC voltage on the output.
Trusting in the reliability of the components I've removed the capacitors and I also trust in the input of my tube preamplifier. If a problem will occur (I hope not so!) I will experience an increase of distortion but no damages on the tube.
Therefore the removal of the capacitors is up to you and your amplifier input.

About other modification, working are in progress, I'm just at the beginning since I'm very busy, and up to now I've tried just "passive" modification.
Are you sure about OP275 as filter stage? I've tried it but I cannot feel the difference if compared to the original OPA.
Another modification I can suggest to you is the removal of muting transistors and some other resistors placed around them. I don't have the complete list now because I'm at office. Anyway on the forum you can find a lot of threads about this.

Bye

[GaryKrant]

Originally Posted by maudan

Are you sure about OP275 as filter stage? I've tried it but I cannot feel the difference if compared to the original OPA.

Yes, I'm sure of the improvement: dealing with objective data, consider that OP275 is 50% faster than stock op-amp (in terms of slew rate, 15 vs 22 V/microsec).

But, to be honest, I'm sure that it's not the optimal filter configuration. I consider the active RC low pass filter and the HDAM output stage as the main objective to improve 'cause I think a lot of work could be done there, and that's the reason of this post.

Please, correct me if I'm wrong: at the end an analog conversion must be done, and it can be done inside or outside the DAC. I agree that a good DAC performing the analog conversion internally is cheaper -from the CD manufacturer point of view- than a DAC that requires an external conversion (more parts), but magic doesn't exist and if a good internal conversion can be done then that conversion can be performed externally too.

I've read about many tweaks on Marantzes but the most, IMHO, driven in a paranoic way: the biggest caps available, dampening everywhere, better rectifiers, extreme shielding... then you read the service manual of reference devices and see that many (not all) of these things are not performed by serious manufacturers in serious devices.

Then you read about Heart tube mods and see that they use the RC part of the low pass filter prior to send the signal to their tube stage (I hope that they use the differential DAC output instead of treating it as single ended using GND instead RON or LON), and when reading the reviews you find that, again, there's a problem with highs...

I'm confident with the HDAM implementation, it has been used in many successful Marantz devices, but perhaps some of the internal passive components could be upgraded, have in mind that CD6000 is now 11 years old and many technical upgrades have been made in these last years.

But I'm sure that, prior to that, the key is improving the low pass filter, maybe with a fully passive 3th-4th order LC implementation, maybe designing an active low pass filter based on a tube instead of an op-amp. As I said I'm an absolute electronics ignorant, so the only thing I can do is reading, learning, then asking and finally testing.

[maudan]

You are not so newbie as you-re saying. Your considerations are pretty correct there are just some points to be clarified.
In the Marantz CD6000 the Digital-to-Analog conversion is performed inside the SM5872 chips but, since they work in Pulse Width Modulation, there is the needing for an external accurate filtering. Using output stage the filtering problem is "transfered" since the intrinsic capacitance of the vacuum tubes act as lowpass filter.
I suggest you to visit this site Ray's Audio Page. Ray is a smart guy and always available to assist people. You can find a lot of threads from him about Marantz players.

Bye