DOD SR460 headphone amp hiss

[camarada78]

i got a DOD SR460 headphone amplifier from the late 90s laying around with little use. Its a six channel single input monitoring 19' 1u rack.

Sometimes i actually use it but people always complain about excessive hiss on it. Even a HA400 nameless aliexpress behringer clone have less hiss, a lot less hiss (cheap useful stuff btw).

The DOD uses all JRC 4050 on the master section and 2073D on the channels out. I've tested changing the 4050 to opa2604 and got no improvement on hiss so i will roll back to the 4050 to not waste the 2604 on it. I have no replacement (yet) for the 2073D to test yet.

It is a design problem or aging components? Is it worth modding or reccaping?

thanks in advance, have a good day

 

[cyrano]

The numbers, from the manual:

Input impedance: 40K balanced - 20K unbalanced
Outputs: 6 TRS 1/4” stereo
Minimum headphone impedance: 32ohm - All Outputs - Single output is capable of 8 ohm load.
Maximum Power output: 1 watt stereo per channel
THD : 0.75% @ 0.250 w
Power Requirements: 120V AC 50-60Hz 20 watt max.

I don't know this unit, but don't you always start by checking/recapping the PSU?

And since I have a bad case of ACBR (anything can be repaired), I'd take the time. But that's just me

 

[camarada78]

I'm all for repairs and refurbishing too that is why i am asking here :-D
The powersupply is very simple. It uses a half bridge rectifier and a few electrolytics and three 7812. i added aditional 100nF on the 7812 outputs and i'm about to replace the electrolitics. I will post un update here

 

[ccaudle]

It is a design problem or aging components?

Hiss is usually a design issue. Aging capacitors would be more likely to cause hum (power supply caps), or scratchy pots (coupling caps).
Hiss is wideband, and is usually related to resistor values and gain staging in the design.
Do you have a schematic available? If not you will likely have to trace out the circuit and write down the component values. My first suspicion would be higher than necessary resistor values in the feedback networks, and higher than necessary gain overall.

 

[camarada78]

is there a good replacement for the NJM2073?

 

[camarada78]

update:
i've replaced some of the broken NJM2073 with modern TDA2822 that supposedly are pin-to-pin compatible but now i get a strong hum on all channels.

 

[camarada78]

View attachment 109683

if i wrote everything correct, this is the schematic for the output channel of each headphone out.

 

[camarada78]

 

[ccaudle]

That device is capable of 1A output current, and is configured for a gain of 38.
I assume you have to keep the level controls turned way down for normal listening? Not surprising it would have poor noise performance.
That is referring to wideband noise (hiss), the hum is something else.

 

[camarada78]

the hum is because the internal circuitry of the 2822 is way different than the NJM2073 (alhough they have the same output pins).
the njm2073 circuit is original like that. not my build.

 

[ccaudle]

the hum is because the internal circuitry of the 2822 is way different than the NJM2073

I think that assertion needs more evidence to go along with it. The devices are basically power op-amps (op-amp style device, but with high current output capability) with internal feedback resistors. The example circuits given in the STMicro TDA2822 datasheet and the NJR datasheet for NJM2073 are essentially the same. They are marketed as used for the same applications. Quiescent current draw is specified the same for both devices.
There is no obvious reason that one should be more sensitive to either power supply ripple, or incident weak magnetic fields from a power transformer.

the njm2073 circuit is original like that

Understood, my comment was just my best guess to your original question:

It is a design problem or aging components?

My answer is that hiss is usually a design problem, not caused by aging components.

 

[camarada78]

the power supply uses a 7812 for +12 volts without much additional filtering. I even added a 100nF to see if gets a little better. without success.

 

[Beatpeat]

the power supply uses a 7812 for +12 volts without much additional filtering. I even added a 100nF to see if gets a little better. without success.

Any News? Did your get rid of the hiss? I have the Same Unit humming and looking for a mod… Cheers Peter