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Old 21st October 2009, 02:28 PM   #1
Mooly is online now Mooly  United Kingdom
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Default Updating an old Rotel RA820A to use HEXFETS's amongst other mods :)

Many years ago I bought a new Rotel RA820A for a second system, and it was a great little amp. Well it's been bagged up and unused for many years so thought it about time to see what if any mods could be done to this old classic. I couldn't even come up with the correct circuit but found an RA820B was near enough even if that did have multiple outputs and single opamps. It's a classic design, very simple, beautifully built, the PCB layout is a model of good design, and it's very easy to work on.

How good is this amp ?

Well my points of reference have moved on over the years and I have come to favour a very "distinct musical sound". Current reference for those that haven't seen it is still the affectionately named "Mooly Amp" which still sees of all comers from a subjective point of view.
My MOSFET amplifier designed for music.


So scoring the reference as 10 I would say realistically the RA820 is a 6.5 It's a bit like swapping your "reference" CD player for a £60 model. It's that subtle (not so subtle even) difference that means everything. It lacks the ability to portray width and depth to the sound stage and can "shout" and sound a little hard. I would call it very two dimensional, almost like a cardboard cut out. The speakers used were B&W 703's which are a pretty demaning load with a 3 ohm impedance minimum. But that has to be put in context... this is a 25 year old "budget" amp although at the time it was the amp of choice for many a system.

Music used was Nicola Benedetti's new CD "Fantasie" and the Ravel "Tzigane" track a trully ravishing piece that shouldn't sound at all harsh or have you heading to turn down the volume. The solo violin should really sing out... every nuance reproduced. If it sounds harsh or unmusical you just aren't hearing it properly.

Next up Andras Schiff and some Mozart sonatas recorded in the 1980's on the "London" label. A very "warm" sound.

And finally Alfred Brendel and Bernard Haitink with the Beethoven "Emperor" concerto. Brendels piano has a wonderful "pearly" tone that seems to really shimmer, and again there should be no harshness.

So on to the amp. It seemed an intriguing possibility to replace the output transistors with HEXFETS... and as I had some... so a pair of IRFP240/9240 were fitted to each channel. Direct replacement and no problem with pin outs.

The print that went to the base (now the gate) was cut and 47 ohm stopper resistors fitted.

The driver stage should have a bit less work to do now so increasing the value of R625/6/7 and 8 from 220 ohms to 680 ohm seemed reasonable.

These are the ones that are the emmiter resistors for the drivers.

The HEXFET's need more bias voltage than bjt's, around 4 volts plus per device so that meant altering the vbe multiplyer. R619/620 was reduced from 2k2 to 330 ohms. Bias was set at around 100ma this falling slightly after it had warmed, and was stable enough that I felt there was little point in
altering the vbe generator to more accurately match the HEXFETS. The important thing is that it overcorrects, and under stress testing with the heatsink very hot, reduces the bias to around half that value so thermal runaway is avoided. It's one area that could usefully be improved though.

There is no output inductor or zobel network on this amp. There is however a 0.22 ohms series resistor to the speaker outputs. Curiously the NFB take off point is at the speaker end ? ... well maybe not so strange, but using the self inductance of this resistor to isolate the HEXFETS from capacitive loading seemed a good idea and this was also something I did on the "Mooly" Mosfet amp. It was a technique that I picked up on that was used by the late John Linsley Hood in several of his designs too. Easily accomplished here, just a cut in the print and a very short link to the other end of the resistor.

The DC offset of this amp is appalling, around 80mv so while that was acceptable 25yrs ago it isn't now. The R820A has unbalanced resistor values at the differential amp input (Why ?) so a good fiddle would have been to tweak the input resistor to bring the offset down. I tried and it worked a treat but with one problem. The Rotel has a passive tone control and if this is in circuit then the response of that stage is critically dependant on the input impedance it works into. So that idea wouldn't do. I came up with adding a fixed 3meg resistor from the regulated (now 15 volt) supply to the "inverting" side of the long tailed pair... well actualy the top end of the cap in the feedback arm... same ish difference.
I also swapped the input transistors for some higher gain matched 2SA872's
That brought the offset down to around 4 mv worst case.

Oh yes the 15 volt rails. Well they were 22 (I think) but I swapped zeners D901/2 to 15 volt so I could use different opamps if I wished... 22volts is outside manys range.

The stock opamp/s are NE5532... arghhh so in went an OPA2604... just the one, the other is for the phono stage and I don't possess any vinyl. I also fited 6pf caps across the feedback resistors of the opamp as there was considerable overshoot on squarewave testing at low volume. I also fitted 43 ohm series resistors with the opamp outputs (easy as there are wire links that can be removed)

What else, oh yes, I swapped all the electroylitics for decent commercial grade parts (low esr types) and used quite a few Panasonic "ECA" series I think from memory.

SO WHATS IT LIKE...

Well certainly different to how it was before.
At low to medium volume it now has real ability, the sound stage does have a lot more depth, in particular the detail seemed better, however on turning the volume up (loud) the sound stage seemed to harden a little and close in. It also seemed to better capture that wonderful piano tone of Brendels, something which totally eluded it before.

If I had to put numbers in I would say it was nearer to an 8 or 8.5 now, a very significant improvement and you had to go to pretty high levels before the sound stage hardened up.
It was an interesting "exercise" non the less although ultimately the reference MOSFET amp has no worries, it's still my "ultimate" reference for musicality.

And more than that... I couldn't live now without at least a remote volume contol Now there's an idea, I'm struggling trying to learn PIC programming with a PICKIT2 (how hard can it be to get an LED to light up) lol so the Rotel might be a useful experiment.

If you have made it to the end here "thanks for reading"

Last edited by Mooly; 21st October 2009 at 02:39 PM.
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Old 22nd October 2009, 07:24 AM   #2
Mooly is online now Mooly  United Kingdom
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This is the RA820B circuit which is broadly similar showing the main points of the mod.
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File Type: jpg Rotel 820b.JPG (140.0 KB, 531 views)
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Old 22nd October 2009, 04:46 PM   #3
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Very interesting project for me.
Please excuse my following questions:
1) why a pair of hexfets and not a pair of better and more powerful bjt ? some 150W complementary pair from let's say toshiba (or even sanken)?
2) why not to try to feed the differential pair directly from the volume pot (with some values mods) instead of passing through a op-amp ?
3) have you thought about a power supply update ? bigger caps, more VA transformer ?

I am very interested in upgrade a little but sane integrated and to use it with a pair of high efficiency speakers I have in mind
I prefer the low power high current amp with high eff speakers approach.
Thanks a lot and kind regards,
gino
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Old 22nd October 2009, 05:00 PM   #4
rdf is offline rdf  Canada
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Mine is an 820BX doing service as a headphone amp and larger, new vintage PS supply caps made a big difference. I think they're at 15,000 uF Nichicons per rail now. Great topic.
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Old 22nd October 2009, 05:16 PM   #5
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Hi,

To answer your questions
Why HEXFETS... because I think the "ease of drive" with FET's is a big plus. It gives the driver a much easier time generally. To just replace the outputs with another bjt wouldn't have achieved as much... they still take a lot of driving. The FETS also don't suffer from gain droop (transconductance with FET's) as do bjt's. Now if I had a spare pair of laterals lying around...

Why not feed the volume pot directly into the amp ? Because I essentially wanted to keep the amp in some sort of original condition... and this has all been done as much as anything as a bit of fun... and it's certainly made a huge difference to the sonics.
The OPA2604 is an Opamp whose distortion characteristics are very desirable for audio... the even harmonics predominate... what I thought of trying, and it's easily implemented is changing the configuration to inverting. Although the absolute phase would be reversed it's only a a matter of swapping speaker leads around.
Reverting to the original NE5532 is interesting... it lacks "musicality". Statements like that may make some cringe but it's true

PSU. The original caps are 6800mfd and IMO up to the task for an amp of this output. Same goes for the transformer, it's a good quality part and is silent in operation.

Do you have an 820 ? Remember the circuit shown is for the 820B not the A

Well worth doing anyway... you might be surprised
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Old 22nd October 2009, 05:29 PM   #6
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Thank you so much for your very kind and valuable reply !
To answer your question, no I do not have an 820
Nevertheless I have destroyed a RA-930AX (whose pieces are still on my table) in an attempt to lower the power stage gain.
My idea was to drive directly the power stage with a tube headphone amp that I have and whose sound I quite like, but the gain of the latter is about 10 times.
So my idea was to lower tje gain of the power stage to use it like a power buffer.
I replace the feedback resistors to get a gain of about 4, the bias changed abruptly and all went in smoke !
That stupid it's me ...
A very superficial approach I understand.
But all started from the idea to use a high gain headphone amp as a line stage
Is easy to find a good sounding HP amp ... even a tube based one.

I will follow your 3D with the greatest interest.
Thank you very much indeed.
Kind regards,
gino
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Old 22nd October 2009, 05:43 PM   #7
rdf is offline rdf  Canada
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Hi Mooly. Don't underestimate the remarkable advancements in electrolytic caps over the past 25 years. Same goes for the coupling and decoupling. Well worth the small outlay. My 820BX doesn't use an opamp front end, it's close to the B version in your schematic.
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Old 22nd October 2009, 05:46 PM   #8
Mooly is online now Mooly  United Kingdom
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A pity about your 930. If you altered the feedback to give such a low gain it almost certainly caused the amp to go unstable. All the frequency compensation would need drastic alteration, and I suspect it all wouldn't perform quite as well as you might think sonically.
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Old 22nd October 2009, 05:50 PM   #9
Mooly is online now Mooly  United Kingdom
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Hi rdf,
It was really all an exercise in using what I had available... I know modern electroylitics have moved on, and in fact all the signal electroylitics were replaced as mentioned together with the regulated supply decoupling.

It all performs remarkably well.
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Old 22nd October 2009, 06:37 PM   #10
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Ooops, see that now.
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