Help Me I'm Stupid (c521i Upgrade and Capacitor Questions) - diyAudio
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Old 26th June 2013, 03:47 PM   #1
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Default Help Me I'm Stupid (c521i Upgrade and Capacitor Questions)

I'm looking to try some cheap upgrades for my NAD C521i. I feel that most of these questions will already have been answered somewhere, and that place is probably obvious, but I've spent a good while looking now and aren't getting anywhere. This is due to a combination of either not being able to find the info or not being able to understand it. So permit me if I may to splurge some questions for you. I'll probably add more as they come back to me, unless I get no replies, in which case I'll probably just feel embarrassed and wander off.

A lot of my questions are based on general internet research and this thread:
NAD C521 Mods help
I understand that I'll get the best improvements in terms of cost/ease/benefit from upgrading the analogue stage and the diodes on the power supply (to Vishay 11DQ10 Schottky type things). Please correct me if I'm wrong. I have already removed the muting transistors.

1. The output levels are a little high and I'd like as clean and direct a signal path as possible. I suspect that my cunning plan to get rid of one or both of the opamps is really dumb, can anyone confirm?

2. I have removed C368/9, was this a bad idea?

3. I have removed C362/3, was this a bad idea?

3.5. If I replace the above caps with something better what are the suitable types of capacitor and where can I get them cheaply in the UK?
I know that PIO is preferable to electrolytic for the application but they seem pretty expensive (5 plus each) and non-polarised which can't be correct for replacing a polarised right?. I'm wondering if there's another type of cap which would be suitable for this application and represent a middle ground. Also would the suggested cap types be good for any application in the CPD signal path or does one need to select carefully?

4. As I mentioned I'd like to lower the output levels a little. The obvious solution seems to be to replace the resistors in series with the output to a higher value, but I'd rather not have them there at all. What, if any, would be the impacts on sound quality for these options?

5. I was typing a load of questions about specific parts but I'll cut to the chase and ask what in the analogue section I can rid of? I notice caps and resistors going to ground but really I'd like to cull anything which isn't absolutely necessary.

6. After culling, which components (excluding DAC and clock) would benefit from an upgrade and which would make no difference?
In the thread I linked there's mention of the input and output caps but if I'm honest I don't know which they are.
I'm not keen on touching the DAC or clock at the moment as they are relatively big jobs for a bear of little brain and I understand that what is around them is just as important as which model they are.

nb I've not attached the service manual as I suspect it's against the forum rules, let me know if there's an acceptable way I can link or attach it.

Last edited by MrLem; 27th June 2013 at 05:33 PM.
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Old 27th June 2013, 10:15 AM   #2
DF96 is offline DF96  England
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Quote:
I notice caps and resistors going to ground but really I'd like to cull anything which isn't absolutely necessary.
You can be reasonably certain that all components are necessary; if not, commercial pressure would have them removed. To modify and improve a circuit you need to understand that circuit even better than the guy who originally designed it. Most DIY tweakers don't seem to understand this. Instead they follow fashion and each other.
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Old 27th June 2013, 12:30 PM   #3
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Quote:
Originally Posted by DF96 View Post
You can be reasonably certain that all components are necessary; if not, commercial pressure would have them removed. To modify and improve a circuit you need to understand that circuit even better than the guy who originally designed it. Most DIY tweakers don't seem to understand this. Instead they follow fashion and each other.
Well, I reckon understanding what that part of the circuit does and establishing that it performs a function in isolation which degrades sound quality and serves a purpose not needed by the user is sufficient. The muting transistor removal seems to be a good example of that.

The other caps I've removed I believe degrade sound quality and are only needed if the amp they're running to doesn't have DC protection. However if I'm honest I'm not 100% sure I have that right, and would certainly want a better understanding before doing any more, hence my post and my consideration of replacing them with higher spec equivalents.

I think if I bought second hand and especially if I factored in my time I could certainly get a better player cheaper and with less risk than doing a lot of mods. However, for me and I'm guessing for a lot of DIY tweakers it's a hobby, a way of learning interesting things and having a weird sort of fun.

Last edited by MrLem; 27th June 2013 at 12:39 PM.
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Old 27th June 2013, 04:30 PM   #4
Bill_P is offline Bill_P  United States
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1. The output is 2.2 Vrms at 0 dB. That is standard for a CD player. Your system has excess gain that should be corrected outside the CD player. The op amps in the CD player provide filtering for the DAC output and should not be removed.

2. C368 and C369 can be removed (replaced with a wire) if there is no DC voltage at the output of the CD player.

3. C362 and C363 are part of the output filter and should not be removed.

4. See item 1 above.

5. There isn't much to get rid of in the analog section. Maybe the output coupling cap if there is no DC offset, and maybe one of the two muting transistors. That's as far as I would go.
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Old 27th June 2013, 05:29 PM   #5
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I had a feeling I did bad when I took out c362/3 so I'll pop those lil fellers back in, or possibly look at PIO as smaller is cheaper and it's only two.

I did check for DC on the output before and after I pulled the caps and it seemed clear. I had an inkling about the opamps doing a little more than boosting the voltage, so thanks for confirming. Individual components in series I get, but my current knowledge peters out just after darlington pairs. I think it's about time for me to learn a little more electronics so I can understand the interactions in these circuits better.

As for the output levels, I'll look at external attenuators if it's unwise to change the values internally, although I may also see about replacing the series resistors with high quality ones of the same value.

I notice that bridging the sockets where c368/9 sat with wire would short c379/382. I assume that means they will also be redundant and may as well be removed at the same time?

Thanks for your help.
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Old 27th June 2013, 07:15 PM   #6
Bill_P is offline Bill_P  United States
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C379 and C382 can be removed if C368 and C369 are shorted. There are two 150 Ohm output resistors in series. You can remove one resistor (short it) and its associated muting transistor. One 150 Ohm resistor should be left in the output path as it improves op amp stability when high capacitance cables are connected to the CD player output.

The attachment shows a comparison of several NAD CD player analog sections.
Attached Images
File Type: gif NAD-CD ANALOG CIRCUIT.GIF (24.2 KB, 67 views)
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Old 27th June 2013, 07:36 PM   #7
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Thanks again. Excellent diagram for me to study.
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Old 29th June 2013, 07:04 PM   #8
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I've now made the suggested changes, and corrected my mistake (feeling very foolish for doing something with understanding it). The diodes and mains cable are next. The mods really did make a huge difference to my ears. For the first time since I've rebuilt my system I'm happy with the sound so I wanted to say thanks very much for your assistance.
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Old 24th November 2013, 01:55 PM   #9
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Default my nad c521i mod

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Click the image to open in full size.Still messy.
Sounds brilliant.

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Old 24th November 2013, 02:54 PM   #10
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