What's the point in output resistors?

[nad]

Hello,

I've pretty much upgraded everything in my NAD C515BEE.

I removed the muting transistors today and found a significant improvement and the only downside is a very low level minor scratchy plip when a disc is loaded and 'play' is pressed and that's it.


In the title.........

While I had the board out I decided to remove/bypass all the resistors and dc blocking caps after the opamps. Jumping directly from the opamp (LME49720HA) outputs to the phono pins.

The output is massively better in every way, it's almost vinyl like and very mellow with far greater dynamics at lower levels and 'less sharp' approach.


Q. Why are the resistors there? I'm wondering if NAD put these components in to simply 'degrade' the output, in turn making the higher models better? I hope not as this would be rather bad of them.

Q. Is there any side effects from removing the dc blocking caps? is any damage likely?


Anyone help?

[nad]

I've replaced the dc blocking caps (Rubycon ZA 25v 47uf) and the sound is still as dynamic but it has reverted back to it 'harshness' and not as mellow. Shall I replace the caps with something of the same value like Black Gates? or shall I change the values?

Anyone?

[sangram]

The output resistors help when driving capacitative cables. A lot of 'audiophile' interconnects actually have a bit of capacitance, depending on their construction and the dielectric, as well as how they're wired up. You could run wihtout them, but it might create a bit of distortion in some situations.

When modding mine, I removed the output blocking caps. The PCM1710u also has some caps in series, I left those in place though on hindsight it may have been better to change them out.

There is no DC on the output, just a few mV and if your preamp has a blocking cap at the input it'll probably be fine. Only one is needed between any two pieces of equipment, though all manufacturers usually put them at both ends.

It's a nice player. I sold mine around four years ago, and as of last reports it was still playing fine though the optics had started to wear.

[nad]

Hello again, should I revert to the original output design but change the resistors for welwyn 0.1% tol and still bypass the capacitors after the opamps? or put the caps back in too?

Appreciate any help.

[Bill_P]

Check the DC voltage at the output and if it is only a few millivolts, leave the capacitors out. In the NAD C542 there is no capacitor at the op amp output. You really should have one resistor at the op amp output, in the 50-100 Ohm range.

[nad]

Thanks BILL_P, just the answer I was looking for.

[mikesnowdon]

Originally Posted by nad

I've replaced the dc blocking caps (Rubycon ZA 25v 47uf) and the sound is still as dynamic but it has reverted back to it 'harshness' and not as mellow. Shall I replace the caps with something of the same value like Black Gates? or shall I change the values?

Anyone?

ZA's can tend to sound as you describe. I once tried them in feedback on an amp and the sound was much like you describe. Changing to film was a revelation!

I wouldn't use them (ZA's or ZLH) in the signal path. They can be good in PSU decoupling though. Blackgates can sound nice (lush and smooth, maybe a tad boring but good clarity) but take an age to fully burn in and once powered down they revert back and need burning in all over again. So if you play music 24/7 go for it!

I'd suggest a Wima MKS2-XL between 3.3 and 10uF (depending on your speakers). These are polyester film caps and should fit on the PCB nicely. People have use'd large MKP's but the size and axial leads make them tricky to deal with. I prefer to use the wima's as they avoid having caps floating around on long leads.

You might like to try Tantalum cap's (10uF) bypassed with a small value (47 - 100nF) Film capacitor. Tants have a warm dynamic sound and the film cap smooths out their slightly grainy treble. This is a cheap way to get better sound than an electrolytic cap. That is if its to your taste. Wouldn't cost much to try it. Nice funky bass and good dynamics from Tants IMO.

Having said all that you are using an opamp that has very low DC offset. Get you meter out and measure DC (200mV range) before the output cap's. If its low enough just remove the caps as they wont be needed. Another thing is that most preamp's have dc blocking caps on the inputs anyway.

Keep us posted!

[nad]

Originally Posted by mikesnowdon

ZA's can tend to sound as you describe. I once tried them in feedback on an amp and the sound was much like you describe. Changing to film was a revelation!

I wouldn't use them (ZA's or ZLH) in the signal path. They can be good in PSU decoupling though. Blackgates can sound nice (lush and smooth, maybe a tad boring but good clarity) but take an age to fully burn in and once powered down they revert back and need burning in all over again. So if you play music 24/7 go for it!

I'd suggest a Wima MKS2-XL between 3.3 and 10uF (depending on your speakers). These are polyester film caps and should fit on the PCB nicely. People have use'd large MKP's but the size and axial leads make them tricky to deal with. I prefer to use the wima's as they avoid having caps floating around on long leads.

You might like to try Tantalum cap's (10uF) bypassed with a small value (47 - 100nF) Film capacitor. Tants have a warm dynamic sound and the film cap smooths out their slightly grainy treble. This is a cheap way to get better sound than an electrolytic cap. That is if its to your taste. Wouldn't cost much to try it. Nice funky bass and good dynamics from Tants IMO.

Having said all that you are using an opamp that has very low DC offset. Get you meter out and measure DC (200mV range) before the output cap's. If its low enough just remove the caps as they wont be needed. Another thing is that most preamp's have dc blocking caps on the inputs anyway.

Keep us posted!

Thank you for your reply, very informative. I've tested the DC output and it's very low (7 mV) so I guess I can just remove the caps.

Thanks again,
Ant

[Pars]

The output stages that I have seen have output resistors (usually 100 ohm) right before the muting transistors or relay. I always thought one of their purposes was to protect the output devices from a short which occurs when muting? I could be wrong

[nad]

Originally Posted by Pars

The output stages that I have seen have output resistors (usually 100 ohm) right before the muting transistors or relay. I always thought one of their purposes was to protect the output devices from a short which occurs when muting? I could be wrong

Sounds like a sensible explanation to me.

Thanks