Restoring Rotel RA-810A

Hi all, first time poster, long time lurker.

A bit about me: electronics hobbyist from way back, engineer in my working life. I typically make guitar effects pedals, but I'm big into Hi-Fi. Built a few speakers sets, a portable headphone amp (haven't we all done that?), got a home theatre setup that i'm looking to upgrade to Atmos soon and do acoustic treatment.

I'm after some information with regards to recapping and reviving a Rotel RA-810. I picked this up 'broken' unit for next to nothing on ebay recently, all was needed was a new output fuse. I plugged it in and turned it on. It's a pretty average sounding amplifier. I purchased this to replace the Topping TP-21 amplifier on my computer setup. It's a great sounding amp but lacks bass and volume on my low sensitivity (78db) desktop speakers. With the Rotel, the bass and volume is there, but there's no clarity anywhere. For the price I got this thing for, I'm happy to mess around with it.

I've been doing some reading on capacitor replacement and I'm after a bit more guidance specifically on this model.
So I've come up with a parts list and was wondering if anyone can make some suggestions.
Technical manual with board layout and schematic can be found here:
ROTEL RA-810A SM Service Manual free download, schematics, eeprom, repair info for electronics

I've decided to replace all the electrolytics with with Nichicon FG and KW caps, subject to availability in Australia. The big filter caps on the PS are KW series 6800uF 63V. I'm not touching the phono stage because I won't be using it.

The coupling caps C601/602 are going to change to WIMA MKS caps (man they're expensive!). I know MKP is ideal, but I can't find them. I'm also leaving alone the ceramic and those little blue LED-looking caps which i think are styrene.

Should I change the other coupling caps to film also? I was going to leave them as electrolytic to keeps the cost down. That is, C501/502 between the volume/balance controls and preamp and C505/506 between the preamp and tone stack.

I was going to change the caps in the tone stack, but I leave the tone controls in the middle detent most of the time. Can I get away without changing them?

I also read somewhere changing the bridge rectifier can give good gains in sound quality.

Any tips would be helpful and greatly appreciated. I figure replacing all the caps is a no brainer after at least 20 years of use. Also going to replace the resistors with metal films, they're currently what look like carbon comp. I figure these mods would be a good way to revive the amp and bring it into the new millennium.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Welcome to DIY audio.
Since this a recap exercise, it's often hard to get excited when it's routine work for techs and DIYs enthusiasts. It's probably not a common model and unless you are familiar with it and the likely results, its hard to comment when the approach depends on what will fit and is available to you. I've only seen 2, one in a collection of RA 210,310 etc. models and the other from a deceased estate and like yours, all in need of a service. I seldom hear about these early Rotel quasi-complementary designs outside Oz anyway.

I think that trying to replace every cap in sight is going to be trouble. Tackle the electrolytics first, stay within reasonable constraints by not increasing capacitor values just because you can. Recap one channel at a time and check your progress in stages so that any problems can be retraced and you always have a good channel for reference. Most improvement will be purely by fitting new electrolytics. Brands and grades bring much less, if any benefit to audio in themselves.

The main reservoir caps could be increased a little to 6800uF with some improvement and the quality issue there is ripple current rating - higher the better regardless of brands. The problem will be mounting them, as modern caps will be PCB mount snap-ins, much smaller and you'll need to hunt around for more expensive clamp mount types and even then the diameter will be too small and they need adapter rings or new clamps to suit.

Leave the other values as is or make no more than 50% increases as necessary. Don't use low voltage caps like 6V since nowadays, 16-25V caps are small enough and better performing. After all, some things have improved in the 40 years or so this amplifier has been around.

You don't need to replace ceramics or film caps unless replacements are similar size and lead spacing. Even then, only in locations that process signals. Hole spacings that are different to the original parts are usually a deterrent anyway when the necessary awkward mounting for your fancied types begins to look silly.

Some film caps of up to 1uF could well be used to replace electrolytics, including non-polarized coupling caps. Larger values tend to be huge and become more of a problem as an aerial for RF and noise than any benefit to sound quality, real or imaginary. You can buy polyprop. film caps of 10uF and even larger values but you won't get any worthwhile benefit in that amplifier, at the prices asked.

You can't buy exotics like regular MKP types in hobby shops here, other than as crossover caps for loudspeakers. You need to go to industrial sellers like Element 14, RS or direct to Mouser, Digikey in the US (they are very fast and cheaper if the order size justifies the minimum freight charge) We also have the Mouser agents X-On here which carry selected stock: X-On Australia | Australia?s Leading Online Distributor

Good luck with it :)
 
Thanks for the comprehensive info. On the model, it's not the old 810, this is an 810A, late 80s early 90s. So it seems you're suggesting to replace like for like and not go too crazy.

Amp looks like this:
http://www.hifidatabase.com/static/gallery/1/1791-ra-810a.jpg

Just to clarify, the coupling caps between the pre and power stages are electrolytic, equivalent films are physically huge, so best to just leave them as electrolytic? It'll save me some money too.

For the filter caps, the 6800uF ones, is there a target ripple current value I should be looking at? The Nichicons I selected don't have that data in the datasheet. Whether they're not competitive for that spec, I don't know.

So far the best source I've found for these capacitors is X-on.com.au. good selection, terrible website. Very hard to use.
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Third attempt - I can't seem to reply without screwing up the posting and losing it completely to connection dropouts and other faults but my apologies for confusing the amps. RA810A is quite different - a very basic, cut to bone, generic UK design by the look of the ubiquitous 5532 ICs in the preamp. You could find it in a number of textbooks and papers. Rotel RA-810A Manual - Stereo Integrated Amplifier - HiFi Engine The feedback point after the output protection fuse is an interesting, if cheapo, touch.

Capacitor ripple current in mA is listed alongside the dimensions shown on the Nichicon datasheets. KW are not the most impressive type for a conventional transformer PSU at 3.5A and I would expect 4-4.5A from 6,800uF, so a larger 10,000uF cap or another grade perhaps from Panasonic, United Chemicon, Rubycon, Elna etc. too, could be just as good if not better.
This is not critical unless the amplifier is working hard so the anaemic power supply may never be noticed.

The coupling caps are up to 100uF, so film caps are out. The types fitted are non-polarized Dubilier branded but this example will probably do nicely if you must replace them for some reason - I wouldn't unless they tested well out of spec. UES0J101MPM Nichicon | Mouser
 
Thanks again for replying.

I'll stick to elecrolytics for those coupling caps, it'll save me some money too. I'm confused about something though, you mentioned those coupling caps are non polarised. To me they look polarised. Stripe on one side and -ve symbol in the schematic. Am I missing something here? Or do you mean they can stand to be non polarised in their application in the schematic?

Thanks for the tip on leaving one channel untouched as a reference. Seems obvious now.
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
No, you're right, I think. The symbol, whatever it is, is unconventional and usually it shows as something like a white and black bar, curved bar or even + sign to show polarization. On high magnification, it looks to be a circle enclosing a horizontal bar, so probably that means negative and that accords with connections. I guess it's yet another useless standards revision for some bureaucrat to justify his appointment.

Still, that's one type (UES) I would fit, if I had the option in the range available and they weren't prohibitively large or expensive. I would certainly try to use one of 16V or so as C605 & 606, the feedback caps.

BTW, just noticed that in the options to view parts on a visual menu at the X-on site, you wind up with full width page view of all details, like the full option Mouser site pages only prettier!
 
Still, that's one type (UES) I would fit, if I had the option in the range available and they weren't prohibitively large or expensive. I would certainly try to use one of 16V or so as C605 & 606, the feedback caps.

BTW, just noticed that in the options to view parts on a visual menu at the X-on site, you wind up with full width page view of all details, like the full option Mouser site pages only prettier!

Do you mean replace those caps with films? I'll have a look at the sizes but I'm not expecing something compact. At this stage everything is electrolytic apart from the tone stack which I've decided to change to polyprop.

I managed to find 63V 10000uF Nichicons with 4.69A ripple current, and they're cheaper than what I had already selected. It's a snap-in type so hopefully it works.

Also one thing that REALLY annoys me, is that the speakers stay on when headphones are plugged in. I can't figure out a way around this using a switched headphone jack because the 330R resistors are in the way, so I think I'll just add a switch to the back of the amp to select between the two. Eventually I'll build a headphone amp and run it through the tape output.

Yeah the X-On site presents well, but that product finder is extremely slow and prone to just crashing.

I'm interested in this amplifier stuff, but my expertise lies elsewhere (robotics and automation). It sounds like a rabbit hole. I'd ask why ripple current is the objective for filter caps, but it'll probably just sail over my head.
 
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Mooly

Administrator
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2007-09-15 8:14 am
:) no problem. The main gist of it was replacing the output transistors with FET's and a few little tweaks thereafter to ensure the FET's worked optimally. The FET's need the 47 ohm resistors in series with gate to ensure stability (as do all FET output stages).

Good luck with yours, recapping an old amp is certainly worthwhile.

(Ripple current is the current that flows in the capacitor as you draw energy from the power supply. Big value caps have high ripple currents and vice versa. The Rotel won't come anywhere near approaching the max ripple values of any decent like for like value replacements)
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Do you mean replace those caps with films?
Grade UES is Nichicon's NP audio electrolytic grade.

Also one thing that REALLY annoys me, is that the speakers stay on when headphones are plugged in...
You could replace the output fuse with a speaker relay circuit that drops out when the headphones are sensed upon plugging in. I don't imagine it's all that simple to develop but overall, relay protection - even better, using Mosfet solid state relays, is safer for speakers, thumps upon power up/down and meltdowns too. If you're into robotics, it should be doable.
 
I thought about using relays, but I'd need a voltage in a switched headphone jack to energise the coil. Only audio goes through the headphone jack. That is unless there's a headphone jack on the market that has a sort of auxiliary contact that you could attach a control voltage to.
 

cabirio

Member
2015-09-30 6:43 pm
Spain
Actually I know what I need. A 1/4" jack with 9 contacts. That is, two internal SPDTs. They're sort of out there but I can't find any to buy.

Neutrik and Switchcraft don't make them.
Lumberg makes them: https://www.reichelt.de/Jack-Sockets/LUM-KLBPSS-3/3/index.html?&ACTION=3&LA=2&ARTICLE=116200&GROUPID=5172&artnr=LUM+KLBPSS+3

Re. coupling caps, given that R609 / C605 set a -3dB freq. of 3.39Hz, you can safely replace C501 and C601 with 2u2 film caps with negligible effect on LF response.

C505, with the tone controls flat, sees a load of about 13k6, so here you should use 10u to keep LF response, but AndrewT will tell you that to minimize the distortion introduced by C605 (which has to be large -> electrolytic) there should be a cutoff frequency before the power amp of at least sqrt(2) times the one due to C605. This means that you can use a 2u2 cap for C505 too. The loss at 20Hz wrt stock will be ~0.4dB.

Cheers,

Cabirio
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
The ground connection to the headphones is also made to be broken and sense small currents at virtual ground potential with a series resistor and say, a sensitive latching logic gate.

A switching socket could also be substituted for the existing one, taking care with quality - some are awful as loudspeaker connections but would be fine as relay switches and headphone connections: http://www.altronics.com.au/connectors/audio-video-connectors/?type_1=635mm&pg=16.
Note that this type of headphone adaptor (a series 330R dropping resistor)is typical of old school amplifiers but not the best way to drive headphones - hence much of the rise in popularity of dedicated headphone amps. You may want to reconsider how much effort to put into this part of the project.
 
Lumberg makes them: https://www.reichelt.de/Jack-Sockets/LUM-KLBPSS-3/3/index.html?&ACTION=3&LA=2&ARTICLE=116200&GROUPID=5172&artnr=LUM+KLBPSS+3

Re. coupling caps, given that R609 / C605 set a -3dB freq. of 3.39Hz, you can safely replace C501 and C601 with 2u2 film caps with negligible effect on LF response.

C505, with the tone controls flat, sees a load of about 13k6, so here you should use 10u to keep LF response, but AndrewT will tell you that to minimize the distortion introduced by C605 (which has to be large -> electrolytic) there should be a cutoff frequency before the power amp of at least sqrt(2) times the one due to C605. This means that you can use a 2u2 cap for C505 too. The loss at 20Hz wrt stock will be ~0.4dB.

Cheers,

Cabirio

That's a lot to absorb. At this stage I'll just substitute like for like parts. What is the advantage of the substitutions you are suggesting if the end result is the same? Save cost and space on the board for smaller parts?


Thanks, I should have checked altronics :grumpy:

I am reconsidering it. While I'm not sure how long it will be until I do a headphone amp, maybe I'll just put a switch at the back of the amp.
 
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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
It is an interesting read - never realized the number of influences and resulting off-shoots your original LATFET amplifier design encompassed. I'm still waiting to see what you can get from Hexfets though, given the intervening years and time (ahem) for thinking it over ;)

Thanks Ian :) Despite putting a lot of time and effort into some HEXFET designs, I haven't built any of them, in fact I've actually veered off to something much simpler (Lateral FET) that maybe at some point may get to see the light of day on here.

The HEXFET's got this far, and with a near identical design using the LT1166 bias spreader.

HXF1.JPG

The new latfet design started life as a super simple three transistor (as in three physical devices in your hand) AC coupled amp. It grew to four devices because one device was a darlington and that really compromised the performance. So discrete darlington in the end. Two channels (dual mono) including the PSU and solid state relays + control circuit all fitted on a 6 by 4 PCB :eek: No autorouter here.

This design good though it is, still isn't quite what I'm after though.

PCB.JPG

And my apologies to the OP for the diversion :)