6ch. Headphone amp repair/upgrade (updated)

[JonnyP]

I have this SM PRO HP6 headphone amp. It works ok but it's a bit noisy. I'd like some advice on repairing it.

There appears to be some burnt out resistors on some channels. The board is marked 10R in theses locations which I assume should mean a 10ohm resistor goes there, but they appear to be 47ohm resistors.

I also see that it has NJM4558 op amps,  when the literature says it's suppose to have NJM4580 op amps.

Since the quality control of SM pro products were always questionable anyways, I suppose it could be that the builder put in the wrong components? or because they decided to put in NJM4558 op amps they changed these resistor values for some reason?

So on to my questions:

1. Should i: 
      A. Replace the resistor with the same value that was installed?  (47ohm) 
      B. Replace the resistor with the value marked on the board? (10ohm)

2. Should put in a resistor with a higher wattage rating? (specs say max 1W @32ohm per ch. )

3. Should i swap out the opamps for different ones (NJM4580, NJM4562, NE5532)

Any help is greatly appreciated.

It should be noted that with the resistor problem aside my plan was to change the all the caps to higher quality ones.

Also I've tried to get the schematic but have been unsuccessful. if anyone has ideas on that please say so.

Pictures:

The insides (I unhooked some of the cables that were in the way)

The headphone output section with burnt resistors

Resistors from another channel (not burnt)

Power transformer. (17v x2/0.45a)

One of the line input sections for anyone interested

 

[NOON]

If you're driving the headphones directly from the chip then definitely use the 4580, but it looks like they're using the 4558 to drive some discrete transistors that drive the headphones, so chip should be OK. You could try swapping to a 5532, but differences will be minimal in this circuit.

Those resistors are well fried! Value shouldn't matter too much between 10 and 47R, but the wattage should definitely go up! Looks like they've been run into either a short or too many headphones in parallel. Replace with whatever you can conveniently get in at least a 1W package, 2W if you can get it.

 

[JonnyP]

Thank you for the quick reply.

Although I don't recall having multiple pairs of headphones on one channel, it is possible that could have happened. And no doubt some sketchy headphones have been used with it. 

I will start by getting some higher rated resistors.

Would there not be any benefit to changing op amps specifically for noise and not so much power or performance?  I only ask because some that I mentioned are noted in the specs to have lower noise where as the 4558 is stated for general use. Also would there be any benefit for changing the ones in the line input section?

Thanks. 

 

[NOON]

I'm no opamp expert, but the 5532AP is my go-to for general audio work.  There are plenty of newer fancy opamps with technically higher specs, but doubt whether you'd notice the difference in a headphone amp. As long as there's enough output current to drive the output transistors you shoud be fine. I'd recommend putting some IC sockets in then you can try whatever you having lying around that has the same pinout and see if it makes any practical difference. Just make sure there's some good local decoupling if there isn't already. (0.1uF ceramics from each rail to GND right near each chip)

 

[JonnyP]

Update.

I swapped the resistors for larger 2w ones.  I measured about 2db (RMS)  less noise. Doesn't sound like much but it is noticeable.

On the first channel I changed the opamps and put in Nichicon caps. Just like NOON said, this didn't make a noticeable difference.

So right now its works fine. The hiss starts around 50% volume but I don't go up that high because it's too loud anyway.

One thing I noticed is that when I switch to  an input that has nothing connected, it is totally silent. My source is Fireface 802 that puts out a clean signal, So is it possible that the noise I'm hearing now is all in the input section? Could the line driver opamp be at max gain and adding noise? (I don't really know how that works) there is no adjustment for the input. How do they typically  choose how much gain the input driver adds?

-JonnyP

 

[radardoug]

If you switch to an input with nothing connected and its silent, then your problem is not in the headphone amp. Try this. Disconnect all inputs. Wind up the gain. Is it quiet? Then your problem is in the stage feeding it. This is called sequential faultfinding.

 

[JonnyP]

Ok. So switching to unbalanced cables gets rid of the noise. Haha.  There is still some power noise when you crank it right up, but a huge amount of the hiss is gone.

Either it was not meant to use balanced or it was poorly designed.

 

[Whoops]

My source is Fireface 802 that puts out a clean signal,

How much signal are you sending into the headphone amplifier?

Try to send it a lot but without overloading the input

 

[JonnyP]

How much signal are you sending into the headphone amplifier?

I have the fireface mixer set to 0db. It has 8 output channels and I have them connected to monitors, stereo system , in and out for tape deck as well as using the outs for reamping or effects. Some balanced some unbalanced all without noise problems. Great interface IMO. I have no complaints about it.

Using unbalanced cables for the headphone amp seems to be the way to go. I've put it all back together and slammed it back in the rack.

I appreciate the help from you guys. This is a really great forum.

 

[Whoops]

I have the fireface mixer set to 0db.

That doesn't tell anything about the signal amplitude you are sending into the headphone amp,
it just says that whatever signal you are sending the fireface mixer is not attenuating or amplifying (unity).

 

[PRR]

> doesn't tell anything about the signal amplitude

Specs for the HP amp say: "Peak input level: +25dB".

Which is a technically bogus spec (dB re:what?) but suggests the input won't overload easy.

And BTW: "Type: unbalanced". You should not be connecting to balanced sources without knowing all the implications. (Which may be unclear with some of the junk sold as "balanced".)