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Improve a Rotel amp THD by 20dB!
Improve a Rotel amp THD by 20dB!
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Old 30th October 2015, 12:48 PM   #111
Dave S is offline Dave S  United Kingdom
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Quote:
Originally Posted by RussellKinder View Post
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.
Stan Curtis was the original designer of the Rotel 820 series. He has quite a cult following:
Stan Curtis, Engineer, my clients past & present

I remember trying to improve the sound of my 820 and it was easy to make it sound a lot worse.

Looks like some of the Cyrus stuff, which gets good comments on this thread, might have been down to him as well!
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Old 1st November 2015, 12:32 AM   #112
sgrossklass is offline sgrossklass  Germany
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Slew-induced distortion tends to set in long before you actually hit slewing limits, so it's certainly a good idea to have about a factor of 10 to spare. Then even a headphone amp can use 3 V/µs, and 1 V/µs seems way too slow for a speaker amp.

Some excellent results there (with a relatively modest amount of parts changes no less), though I wonder where the improvement in noise would be coming from, as I'd expect that to be dominated by the preamp section. The tone controls may worsen it though, so maybe they were simply turned off. Speaking of those, watch out for scratchy bass pot when throwing out C601/602.
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Old 3rd November 2015, 10:31 AM   #113
AngelP is offline AngelP  United Kingdom
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Oops.... firstly, my apologies to Stan Curtis, all Rotel affectionadas and a note to self:

Never try to read a slew rate from the oscilloscope trace late in the evening with tired eyes!
I re-measured both a RA-830AX and a RA-831AX mkII this morning and the stock slew rate is indeed about 10 - not 1 V/us.

The modded 820AX is still a bit faster and it is not ringing on square waves. But, I also broke my own rule to change only one thing and only in one channel - then measure, and so on. And now, after half a minute of circuit warm-up I have an irritating 38MHz / 40mVrms sine on all rails - which I need to find the root cause of. So, I need to isolate one channel, re-trace and revert my recent VAS mod steps and try again.

Oh, the joy of modding!
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Old 3rd November 2015, 11:39 AM   #114
sgrossklass is offline sgrossklass  Germany
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38 MHz? That must be something fairly local. Probably in the output stage, at least there aren't too many other things that would take half a minute to warm up. How does quiescent current behave over this time, does it go up or down?

With your change in compensation you've also changed VAS output impedance, which may be the problem here. Maybe try the other, only slightly worse-performing setup?
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Old 3rd November 2015, 12:42 PM   #115
AngelP is offline AngelP  United Kingdom
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Yep, it is a small, but stable 38MHz sine on top of all traces and literally measurable everywhere. The bias dc remains stable at 4mV / 18mA. I tried to turn it down to zero - without any effect on the RF sine.

I then tried to turn off the amp and power just the preamp from external +/15V supplies - all stable. Then, I powered the entire amp from a +/- 27Vdc lab supply - dc current normal at 41mA, but the 38MHz sine still appeared.

So, as you say it must be local and the most probable culprit is of course the VAS stage, as the last mod was to remove its heavy lag compensation. The 2SC1941 heats up to about 40oC within half a minute after turn on.

Not sure what you mean about scratchy bass pots when removing the input DC block capacitors C601/2 (they are already long gone in this amp). Scratchy pots are always bad, but both ends of this pot are (almost) at dc zero?
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Old 3rd November 2015, 01:06 PM   #116
anatech is offline anatech  Canada
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Hi Per,
Keep an open mind. Either channel could be setting this oscillation off, but the one warm output transistor looks guilty on the channel likely to do this. Make certain the heat sink is either grounded directly to the chassis, or through a capacitor, 1 nF ought to do it.

If an amplifier is going to be unstable, you would normally see ringing in the output on fast wave form edges. But at 38 MHz, it almost has to be a driver or output transistor. Sgrossklass has that exactly right.

Any control that can have a DC component across it will often be noisy - even if the control is perfectly okay. Tone controls are normally high impedance, so a good type of capacitor shouldn't cause any problems. If you can run without capacitors - great. Put coupling caps in if you do end up with some DC offsets or scratchy controls.

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Old 3rd November 2015, 02:55 PM   #117
AngelP is offline AngelP  United Kingdom
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Thanks to Dave S for the link to Stan Curtis' pages. They were new to me and I particularly enjoyed his "Safari" articles:

Stan Curtis, Engineer, articles from HiFi Critic magazine
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Old 3rd November 2015, 06:20 PM   #118
Dave S is offline Dave S  United Kingdom
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Originally Posted by AngelP View Post
Thanks to Dave S for the link to Stan Curtis' pages. They were new to me and I particularly enjoyed his "Safari" articles:

Stan Curtis, Engineer, articles from HiFi Critic magazine
Yes he's a great character (maybe not the world's best business man :-)).

In amps with no high voltages around I use the "capacitive finger test" to locate the likely source of the oscillations. It's very simple:- just place a finger on the tracks of the pcb and watch the scope, you can usually make the oscillation get worse or disappear by placing your finger in a particular location. If the "hooting" gets worse you need to consider why capacitance across the traces makes things worse and damp it somehow, if it gets better just add a few pF in the same place. I am not saying this is the ideal fix, but it shows you where to look and gives some clues as to why.
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Old 19th November 2015, 12:13 PM   #119
AngelP is offline AngelP  United Kingdom
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Default Rotel Mod Stage 5: -28dB THD reduction achieved! Can we go to -30dB??

I finally got back to the RA-820AX and undid the most recent VAS modifications. The MHz oscillation disappeared duly, never to be seen again. Bowing my head in shame, I can't rule out that it most probably may have been yours truly that had produced a late night dry solder joint somewhere on the PCB or in the airwires. Anyway, by then I had received some rare “old stock” of TL011C, TO-92 Current Mirrors (CM). This product has been discontinued by Texas for quite some time, but as it is not easy to find through-hole CM components these days, I decided to give them a go and to replace the input collector resistor R605 with the TL011C and …... it didn't work. Amp output hovering about 11Vdc, so clearly the CM didn't work properly.

I then resorted to read the the instructions (the datasheet) and realised that TL011C is a Wilson CM with an extra transistor in the mirror path, so it needs at least 1.2V across it to turn on, and the original R605 only has about 0.9V from the VAS and its emitter resistor. So I needed another diode voltage drop in the VAS stage and replaced the 2SC1941 with a ZX603 Darlington – that didn't work either. Too much current gain?

Ok, back to the trusted old 2SC1941 which I fitted with a glued-on 2SC1845 emitter follower connected as shown in the schematic. Surely, you would think that trusted solution would finally work?.....No way, Jose. I tried one of the other TL011C samples – same result. Perhaps there is a reason why these components were discontinued? I can't explain it.
So, I decided to make my own CM modules. Done on a piece of SMD stripboard holding a NXP BCM61B and two 100E 0.1% emitter resistors, it fits nicely in the place of the R605. And, finally: YES!
1) Amp stable, offset down to 1mV
2) Slew rate over 15 V/uS with no overshoot or ringing on a 20 kHz square wave
3) Bandwith: 3Hz to 153kHz (+0/-3dB)
4) THD now at 0.0012% - that is a massive -28dB improvement over the original spec. - or in other words the THD now down to less than 4% of the original.
The Stage 5 CM mod (light blue trace) seems to reduce the even harmonics, whereas the abandoned Mod Stage 4 (dark blue trace) T-shaped local Miller VAS feedback did its magic mainly on the odd harmonics. So, if there is ever to be a Mod Stage 6 it should probably be to combine the two.

I say “if ever” because I realise that I by now may have gone OTT on this project and may be boring the life out of other people with my marginal modification madness. For that I do sincerely apologize.

However, I always try to reach the goals I set myself – and I definitely don't like losing bets. But I do hear the choir asking “how does it sound?” so perhaps this is the point for the amp to finally leave the workbench and be embedded into the audio rack?

Cheers,
Per
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File Type: jpg ra-820ax mod improvements (5th stage).jpg (80.5 KB, 498 views)
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Old 19th November 2015, 01:57 PM   #120
anatech is offline anatech  Canada
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Hi Per,
Congratulations to you for pursuing the problem to its source and fixing it! You really improved this amplifier by a huge amount. I think you should definitely post further on how it sounds along with any other changes you decide to try.

When something doesn't work due to your own mistake, it is easy to throw in the towel and walk away. But going back in and trying another approach shows tenacity and determination. The educational value to yourself is far greater than what you posted, because you learned a bunch of little things along the way too. The mark of a really great technician.

So yes, I have to ask in all seriousness ... how does it sound? And the next question for you. Will you apply this change to some of your own sound equipment? In other words, is the improvement in sound quality worthwhile to perform these general changes to your own gear?

-Chris
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