Improving -11db THD on a Rotel RA-840BX4 55watts amplifier

[EricZenviewStud]

A little video on a simple significant THD improvement on a Rotel RA 840 BX 4 !

Brings the amp to newer THD standard from its original -70 db to -81db.

It also cures the overheating of one channel versus another, on this older generation amplifier, I touch also briefly on why this amps fail in time…

The reason for my post : longer life for old Hifi gear, and this cheap amp sounds even better molded !

Ps: to create some turmoil , I don’t like to change caps (like most others do) without factual reasons : out of specs etc…rather test than change could be my moto

Cheerios

Eric

 

[Jim Taylor]

Welcome to ASR, Eric! Hope you enjoy being here!

 

[EricZenviewStud]

Thanks Jim

 

[AnalogSteph]

Here's the classic diyAudio thread that inspired this:

Improve a Rotel amp THD by 20dB!

Question: How good can an old Rotel get? I have had dozens of Rotel amps through the repair workshop over the years, and this RA-802AX came in as a “speaker killer”. It turned out to be fully working – except it had a solid +16Vdc on both Left and Right speaker outputs. In a Rotel, the 12A...

www.diyaudio.com

It may have been a good idea to offset the spectra in the before/after measurement as you can't really see what it's like after.

The level of power supply harmonics is still concerning to me. Are these real or is there a ground loop in the measurement setup?

 

[EricZenviewStud]

Here's the classic diyAudio thread that inspired this:

Improve a Rotel amp THD by 20dB!

Question: How good can an old Rotel get? I have had dozens of Rotel amps through the repair workshop over the years, and this RA-802AX came in as a “speaker killer”. It turned out to be fully working – except it had a solid +16Vdc on both Left and Right speaker outputs. In a Rotel, the 12A...

www.diyaudio.com

It may have been a good idea to offset the spectra in the before/after measurement as you can't really see what it's like after.

The level of power supply harmonics is still concerning to me. Are these real or is there a ground loop in the measurement setup?

Hi Steph,

You are right ! ,but I think I might have a capacitor(s) problem !

If I am right it will be a perfect example where they need to be changed ! (Magnificent occasion to do a before and after!)

I haven’t had time nor the parts but I’ll look into it.

Some other posts on dyaudio showed similarly shaped noise but tamer…
Now figure wise it still is -65 db down at 100Hz , I measured -64,9 db at 100HZ…

Anyhow, it inspires me to look if the « rumble » can be diminished too , I soldered rapidly to extra 50v caps on each side in parallel with the original large caps,( which on the schematic say they should be 10000 uF ,when they really are 8200uF) and all my figures improved somewhat with still better THD results, around 14-16 db at low power, where my extra caps play a role,I’ll do a follow up when I get the parts needed.