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Old 2nd June 2015, 08:56 PM   #21
llwhtt is offline llwhtt  United States
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I thought transistors were "three legged fuse protectors".

Craig
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Old 3rd June 2015, 12:17 PM   #22
AngelP is offline AngelP  United Kingdom
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Originally Posted by mjona View Post
Another possible cause of failure although I don't know how common, is through someone increasing the Iq. In that regard r641 and r643 the 910R resistors from the driver emitters are not crossed coupled to each other but to the opposite voltage rail. There is a recent instance of failure due to excess heating, of driver stage components.
Indeed, particularly if they are inadequately heatsinked.

However, just to clarify things: I have been working on an RA-820AX (power amp part schematic enclosed), which is quite different to the more complex RA-820BX4 that ostripper commented on.

But I agree with ostripper's views on the strange design topography of the BX4 - and interestingly, its specs are for a much poorer THD - between 0.1% and 0.3% - or ten times the 0.03% of the AX.

Ok, I know that THD is not the only indicator of an amp's qualities - but still...
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File Type: doc Rotel RA-820AX schematic org.doc (178.5 KB, 263 views)
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Old 3rd June 2015, 12:18 PM   #23
AngelP is offline AngelP  United Kingdom
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Originally Posted by llwhtt View Post
I thought transistors were "three legged fuse protectors".

Craig
LOL
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Old 3rd June 2015, 03:55 PM   #24
AngelP is offline AngelP  United Kingdom
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Default Rotel RA-820AX 2nd stage mod

Looking back at the encouraging results from stage 1 - I find it pretty cool that by (net) removing of three components from an RA-820AX (and a bit of rearranging, recalculations and replacings) you can actually improve THD performance with 10dB.

Bearing in mind that in the rules of the wager I was "banned" from changing any of the Rotel design topography in the power amp audio path, so the 2nd modification stage therefore was set to further steady the current supply to the input and VAS stages by improving the ripple on the DC rails.

First by filtering the input stages' positive and negative supply rails with two 100ohm/100uF/100nF lowpass RC filters and then inserting a cascode transistor in the tail-current source using the filter mod from stage 1.

Does this work? - You bet.

Stage 2 results:

Input stage rail ripple:
Reduced from 30mVac to less than 8mVac

A large reduction in the higher harmonic peaks, THD down to 0.0086% - hey, that is now better than e.g. the Linn Majik specs!

THD+N now dramatically down to 0.0203% but the noise reduction still appeared only on the even 50Hz harmonics, while the odd 1st (50Hz), 3rd (150Hz), 5th (250Hz), etc. - seemed practically unchanged.

Again, these peaks may actually not be coming from the amp, but probably caused by our imperfect RTA measurement setup.

Anyway, there is still quite some way down to the 0.003% THD target - and I am now quickly running out of allowable options.

But.... can it be done? ..... will I win the bet? Ta da daaa...
Stay tuned for the 3rd mod post!
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Old 4th June 2015, 07:27 PM   #25
AngelP is offline AngelP  United Kingdom
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Default Rotel RA-820AX 3rd stage mod

3rd and last mod stage was therefore on the large reservoir capacitors. Rotel uses Rubycon 6800uF's that at first measured sort of ok, with decent low ESR and capacitance only down to 6200uF, probably still within specs. However, Rotel has often had the habit of placing the reservoir capacitors so the 0Vdc pins go to the ground star point on the PCB. This is a known bad idea considering the large charging currents that go through these caps.

The budget allowed replacing the Rubycon's with two standard 10,000uF, low ESR, 105oC Panasonic types. I placed them pins-up, attaching the flat tops down to the PCB with a high strength silicone. Then the pins were connected directly to the bridge rectifier's + and – pins and the ground transformer return pin respectively, with equal lengths of heavy wires.
This is the right way to do it – check out the results:

The 100Hz noise peak dropped another impressive -15dB, making the total 100Hz reduction by these mods over -32dB! That is a staggering 97% reduction from what was maybe a faint audible speaker hum - to absolute dead silence! Even the 50Hz and its odd harmonics finally dropped 5-8 dB.

So, the 3rd mod results:
Input rail ripple down below 5mVac
THD+N value staying at 0.0209% - a bit strange given the above clear improvements
THD now at 0.0039% - aaaghh, so close!

So, I expected to have lost the bet, but before conceding defeat I had yet another look at the RTA setup, particularly the ASUS Xonar U7 USB soundcard we used as front-end. In addition to the 50Hz noise pickup even in a direct loopback there were small visible signals at 1kHz, 2kHz and 3kHz, probably from some internal oscillator circuitry, I don't know.
However, I re-measured the 820AX using a 900Hz instead of the standard 1kHz test tone, and bingo – I got a THD value of 0.0029% Yes! - I won (of course ignoring all mutterings about cheating, etc.)

Also, I then put our best digital storage oscilloscope on the amp output and sync'd the trigger to Line in order to look for the supposedly large 50Hz noise signal – Nothing, zilch – nada!
It must therefore be a measurement artefact in the Xonar U7 setup, and therefore the THD+N is probably better than the last measurement would indicate.


It is times like this you wish you had a GBP 10,000+ Audio Precision RTA ready on the shelf! If anyone interested has one available near Stansted, UK – please do get in touch.


Enclosed are the staged power amp circuitry mods with the resulting comparative spectrum chart showing how Mod 1 “shaved off” the grey distortion peaks, Mod 2 the red and Mod 3 the amber – resulting in the respectable green spectrum.

It is always tempting to carry on modding to try further shaving of the remaining distortion peaks – but also important to know when to stop soldering and start listening - which is next.
Attached Images
File Type: jpg RA-820AX orig schematic.jpg (135.8 KB, 948 views)
File Type: jpg RA-820AX mod 1 schematic.jpg (130.7 KB, 941 views)
File Type: jpg RA-820AX mod 2 schematic.jpg (147.0 KB, 947 views)
File Type: jpg org. (grey), 1st mod (red), 2nd (amber), 3rd (green).jpg (107.5 KB, 862 views)
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Old 4th June 2015, 11:52 PM   #26
mjona is offline mjona  New Zealand
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20 or more years ago now a friend upgraded his system with an early 820 model. His system included some super loudspeaker cable which caused a major failure soon after switching on. A loud hum followed by silence. The replacement also hummed but my friend reacted quickly to turn it off in time.

The cables were junked and the amplifier was O.K. with normal cable. I was unconvinced by this as the amplifier was only marginally stable. You are saying you have had dozens of Rotel amplifiers to repair which raises another question about reliability and whether there is a connection between these two considerations.

I should like to think that Rotel made modifications to the circuit accordingly, but I have my doubts. What can you tell us in that regard.
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Old 5th June 2015, 02:00 AM   #27
anatech is offline anatech  Canada
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Hi mjona,
Some times a shop will see quite a high number of a certain model of equipment if it happened to sell extremely well in that area. This doesn't always mean that there is a problem with the design of that model, but a small issue could then really inflate the number the local repair guy sees.

We have all seen some specific loudspeaker cables that had very high capacitance as well. The victims all had to spend enough money on speaker wires to make them prey. If the phase margin was also a little low, that might be all that was needed. The manufacturing fault in this case isn't as horrible as it first appears.

-Chris
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Old 5th June 2015, 02:10 AM   #28
anatech is offline anatech  Canada
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Hi AngelP,
I am curious about two things. I haven't worked this out yet, but can you be certain that the tail current was not changed by modification #1? Did you monitor the voltage across R607? Also, why didn't you leave the diodes in circuit? They would have made a very low impedance load to any AC ripple that got through the filter you installed. I would think that performance would have been increased. You might have had to block the light from hitting the diode junctions.

If you did match Q601 and Q603 closely, it will make an audible difference if they were not originally matched very well. The improvement can be dramatic in fact. Also, tie those transistors together so they will track temperature better. I use heat shrink tubing and heat sink grease.

Otherwise, you did a really good job of plotting your changes for everyone. Nicely done!

-Chris
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Old 5th June 2015, 02:20 AM   #29
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AngelP, that's a first-rate job of applying a few basic engineering principles in just about the simplest manner possible to get a stunning result.

I wonder if the original designer is still around to see your mods and then go bang his head into a wall.
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Old 5th June 2015, 06:57 AM   #30
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by anatech View Post
Hi mjona,
Some times a shop will see quite a high number of a certain model of equipment if it happened to sell extremely well in that area. This doesn't always mean that there is a problem with the design of that model, but a small issue could then really inflate the number the local repair guy sees.

We have all seen some specific loudspeaker cables that had very high capacitance as well. The victims all had to spend enough money on speaker wires to make them prey. If the phase margin was also a little low, that might be all that was needed. The manufacturing fault in this case isn't as horrible as it first appears.

-Chris
My friend told me the transformer in his amplifier had burned out after setting up in his system straight out of the box.
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