Rubicon 355 rising frequency response

Happy New Year all!

Further to this thread: Rubicon 355 positive rail voltage on case?

I tested the amplifier by driving a sweep through it and watching the oscilloscope.

With all the filters off I set up a reference voltage of 1Vrms at 20kHz and dropped the voltage so as to not have the infrasonic filter affect the starting point.

Dropping the frequency down showed a gradual, I think more or less linear, drop in voltage down to 800mV at 100hz.

This is almost 3 dB and something would appear to be up.

Would one start by tracing the signal from the RCA jacks into the OP amps or is there something simple it is more likely to be, like a recap?
 
Last edited:

Perry Babin

Member
Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
FAR too many people replace caps for no reason other than they want to feel like they're doing something. I've tested a LOT of caps because I saw so many people saying that it made a huge difference and found VERY few defective. There are some amps that either used bad soldering practices or were unlucky enough to pick a poor quality cap for production but those are a tiny minority.

Follow the circuit to see where you're getting the loss. That would be the right way to troubleshoot the problem, not blindly replacing parts that are perfectly good.
 
Fair comment, I mainly play with home Hifi from the 70s and see bad caps more often in those than 90s car amplifier. Force of habit I suppose but I do have access to a proper Sencore LC tester for finding the genuinely bad ones.

I suspected a capacitor as the frequency response increased rather than fell away with Frequency, I wonder what sort of fault could cause this type of malfunction. I have never seen it in any amp before. :)
 
Good thought, thanks Perry.

I can’t imagine Soundstream building a weird response like that into their amps either, it makes it sound splashy and nasal. The FFT function on my scope isn’t sensitive enough to get a handle on distortion short of clipping and I don’t have an audio analyser but the Sony XM3040F, for example, sounds much better despite not being anywhere near much to buy back in the day.

It has high pass filters only on the main channels and no EQ of any other sort. The shall have a poke around and see what I come up with.
 
Driving the same reference voltage into the inputs:

Channel 20Hz:200Hz:2kHz:20kHz
1 (6.10) (6.25) (6.58) (6.70)
2 (5.90) (6.05) (6.17) (6.08)
3 (6.44) (6.59) (6.91) (6.95)
4 (6.34) (6.48) (6.63) (6.52)

It seems that Channels 1&3 have the issue.

Going by the schematic they don't seem to share any components in the preamps side.

I'll try and follow the signal through.
 
I think something is up as I measured it at around 2.5dB the other day. There is something that isn't right about the sound, it's hard to put my finger on it. I'm going to make some measurements at the Op amps and try some freeze spray to see if anything is dodgy.

This was the amp where one of the rectifiers was short to the the case, I'm wondering if it has suffered an impact in the past.

I've noticed that the 2200uf 16v capacitors get warm at idle whereas the rail caps do not, the transformer too; the amp has me worried! :D
 
If the caps are very close to the toroid, may be that the eddy currents generated in the caps because of the surrounding magnetic field, creates leakage currents that heat up the cap. So it may or not be a fault per se.

But in the schematics posted below I could't see any 5532 opapmp, only some specialized IC's with the circuitry inside (Commonly called jungle) plus some power TIP transistors.

Opamps offset may be caused by several items, particularly the IC's itself, resistors and capacitor (Altered or with leakage respectively).