Su-v55a technics amp cuts off when volume up to 2. 1

That sounds suspiciously like an issue with speaker connections rather than amp output!

There are manuals and specs here including a Service Manual

https://www.google.com/search?q=technics+su-x840+user+manual

..

I have the same TECHNICS SU A900 MKII as you are talking about and it always runs warm I think using 4R speakers instead of 8R speakers should solve the problem and actually I am thinking of switching to 4R as well.

Generally speaking, an amp attempts to protect itself from heat, shorts, overloads and operator exuberance by refusing to turn on or stay on; or it may turn on but produce no audio to the speakers.

Overloads can be from excessive periods of high output or marginally low impedance loading by the speakers; and shorts would be wiring issues or a speaker blowing up.

You should be able to feel if it's hot. WHY is it overheating? Make sure it has sufficient ventilation on all sides and that vent holes are not blocked by dust balls. Ensure the fan (if equipped) is running as designed (some only operate on demand). Clean dust and debris from it.

If the amp comes back on after cooling, you're lucky. They only have so many self-protection cycles in their lives so continuously resetting or cycling their power without addressing the cause can do more harm than good.

If it protects immediately on a cool power up you should disconnect the speaker connections and try it 'nekkid'. If it comes up then diagnose which lead(s) are shorted. If it does not come up the problem is internal and should be left to an experienced and competent hands-on tech.

Check for loose speaker connections at the speaker as another possible root cause for intermittent shutdown.

Check for dry solder joints especially on the main power output transistor.
A blown power transistor will stop the 'Current Drive' from engaging so if the problem you have is intermittent then it is most likely dry solder joints.

Hope this helps.

Let's establish which is the IN and which is the OUT here. The PC is a Line Level source so its OUTPUT would feed one of the amp's Line Level INPUTs.

What's the second amp going to do in your scenario? Something other than the normal stereo channels; something multichannel (if your PC supports it); or just more speakers supported for the PC to feed?

An RCA Y-connector pair would share the stereo output of the PC with multiple Line Level destinations.

If a second amp with its own volume control is intended to share the same two channels with the SU-V500 it could also be slaved off the back of the SU-V500 wih an RCA patch pair exiting the Tape 1 or 2 REC(OUT) jacks. The second amp would receive full-volume audio from whatever source the SU-V500 is playing. Just don't activate the selected Tape monitor on the SU-V500 or you will cut off the source to both amps.

See there is a problem with the volume conroller as it is 18 years old the controller has worn out replace the volume controller with a new one and you will be able to enjoy great sound again

Hi and welcome to the site,

The specifications prescribes:

• 4 ~ 16 ohms if either MAIN or REMOTE is used;
  
• 8 ~ 16 ohms if both MAIN and REMOTE are used at the same time.

Offhand, any 8 ohms speaker rated at 60 ~ 100 watts would be compatible.

Good luck and thank you for asking.

usually meens it is protecting itself from destruction
check your wires and speakers for problems

The most common cause of this is defective coupling caps inside. These allow some DC voltage to pass through to the final amp stage. This DDC voltage is detected by the protection circuit and the unit shuts down. This unit will require service. Please note that this should be a simple and inexpensive repair. The cost should be less than $10 for parts.

Dan

hello. we cant help if we dont know what music source you are using.