The Workbench

Re: BM 3000-2 recap

Paul — Thu, 04 Feb 2010 21:26:56 GMT

I have replaced the 3 5000uf/70v electrolytics with 5600uf/80v.I think all the coupling caps are tantalum.I plan to replace them all with Wima polyester caps.

Re: BM 3000-2 recap

Stoney3K — Thu, 04 Feb 2010 11:31:24 GMT

Dillen:

Stoney3K, You are absolutely right but the caps in question here are sitting at the extreme
output, in series with the speakers.
The amplifier is running on a single supply so the caps are needed to isolate the DC at the amplifiers
center rail from the speakers. They need to be fairly large in capacity and voltage rating to allow for
a good damping factor (and good bass response) while handling the (potentially full) supply voltage.

A third capacitor is filtering the supplied voltage to the amplifiers.

I suspected as much. In that case, the capacitors are a high-pass filter, and fitting a larger capacitance will give you a lower pass frequency, and probably a more present bass response. This depends on your speakers, though.

The tantalums are probably used all over the place around all of the op-amps and other IC's to filter the power supply lines from signal crosstalk? At least when I'm designing analog circuits that's standard practice.

Re: BM 3000-2 recap

tournedos — Sun, 31 Jan 2010 18:11:54 GMT

pebmrb:

While we are on the subject,I notice that the 3000-2 has a lot of tantalum electrolytics.Do these deteriorate over time and need replacement like the others?

Yes! Aluminum electrolytics usually just lose their high frequency characteristics (ESR goes up). Tantalums do that just as well, but can even short circuit with age or just start acting as resistors.

Those that are on the signal path (usually many on '70's B&O) are best replaced with MKS plastic caps. They won't age and you can hear some improvement in sound as well. Although regular aluminum electrolytics will also do - tantalums were originally used for their small physical size, but the caps have become much smaller during the last decades and this isn't usually a problem anymore.

Re: BM 3000-2 recap

Paul — Sun, 31 Jan 2010 17:51:52 GMT

While we are on the subject,I notice that the 3000-2 has a lot of tantalum electrolytics.Do these deteriorate over time and need replacement like the others?

Re: BM 3000-2 recap

Paul — Sun, 31 Jan 2010 16:05:32 GMT

Thanks guys,very informative.I thought the function of those 2 ouput caps was a bit unique,now I understand.My 3 large caps don't appear to be leaking or bubbling.I have cleaned the sliders and switches as best as I can without disassembling them.It is working and sounding good.And it looks good too!I have a couple of other questions that I will post in another thread.

Thanks!

Re: BM 3000-2 recap

lausvi — Sun, 31 Jan 2010 15:52:06 GMT

Dillen:

Having said that, quite often the original large cans are still found to be in good condition. It's very rare to see them faulty unless they are physically leaking and/or blowing bubbles at the downwards facing end near the solder tags.

I once got for repair a BM3000 which was working in general but had some burnt lamps etc. It was tested before I got it (no lights, supposed to be non-working!) and when I opened it up I immediately noted that one of the big caps was NOT alright.... (see photo attached.) The amp was really working with that cap, but I still found it better to replace it! (I believe the cap was "burnt" by a power supply/rectifier fault as the rect. was replaced with some of it's surrounding components)

Re: BM 3000-2 recap

Dillen — Sun, 31 Jan 2010 15:27:47 GMT

Stoney3K, You are absolutely right but the caps in question here are sitting at the extreme
output, in series with the speakers.
The amplifier is running on a single supply so the caps are needed to isolate the DC at the amplifiers
center rail from the speakers. They need to be fairly large in capacity and voltage rating to allow for
a good damping factor (and good bass response) while handling the (potentially full) supply voltage.
A third capacitor is filtering the supplied voltage to the amplifiers.

pebmrb, If you have suitable instruments, you can measure if a capacitor is good or not, though it can
be a bit hard to measure capacitances this high and their related ESR etc. precisely.
Having said that, quite often the original large cans are still found to be in good condition. It's very rare
to see them faulty unless they are physically leaking and/or blowing bubbles at the downwards facing end
near the solder tags.
If you have new capacitors in the drawer, you can try fitting one of them in one channel and compare
to the original still in the other channel.

Martin

Re: BM 3000-2 recap

Paul — Sun, 31 Jan 2010 09:06:20 GMT

Thanks for the input guys.No Dillen,I don't get you wrong at all and I appreciate the input.I simply want to make sure I am not downgrading from the original design.I have drawers full of standard grade caps,but was buying audio grade caps simply for peace of mind.If I can get some rational advise from some of you guys,perhaps I can start using some of my stock.So far my advise comes from my somewhat irrational audiophool buddies.These guys want me to do exactly what you describe,turn it into something it's not.

It's sounds pretty good right now with original caps and dirty switches!Can't wait to get it up to full potential.

Thanks much.

Re: BM 3000-2 recap

Stoney3K — Sun, 31 Jan 2010 06:08:54 GMT

pebmrb:
I am getting ready to do a recap on my BM3000-2,I have most of my caps.I am using Elna Silmic and Wima MKS,but am having trouble finding "audio grade" caps for the three large 5000uf/70V caps.My question is:Do I really need "audio" grade caps or is this just a gimmic?I have some 4700uf caps and 5600uf but am not sure what level of quality I need here.I want this thing to be at it's absolute best sound quality when it's finished and I don't want to be second guessing my cap choices after it's done.I am anxious to hear what others have been using.

Such large values of capacitors (5mF/70V) are only used for buffering of the power supply after the rectifier. Using audio-grade capacitors there is not very useful, if you really need to do so, you can use the 4700uF or 5600uF types as long as their voltage rating is the same (70V). The 5600uF will give you somewhat better stability on the power supply lines.

Using capacitances like this in the audio signal path would give you either a low-pass or a high-pass filter with ridiculous characteristics. I've seen capacitors of a few hundred uF being used in the DC decoupling of the output amplifier, but that's about it.

Re: BM 3000-2 recap

Dillen — Sun, 31 Jan 2010 04:29:36 GMT

Don't get me wrong but when I hear something like this, it often sounds to
me like someone is trying to upgrade a Ford Mondeo to be a Ferarri.
It won't happen unless you redesign the whole machine and then you can edit out
the B&O logo on the front too as it will end up very far from what B&O designed
and intended.

The Beomaster 3000-2 is a superb machine in its own right.
It was very high quality when launched but it was never built to be
an audiophile receiver.
A chain is never stronger than its weakest link and even if you can do a
lot by replacing old, dried out capacitors, that are not longer up to par,
there will still be many other factors acting upon the sound quality.
The choice of small signal transistors and basically the whole circuit
design will show its limits.
And you can start by eliminating the row of pushbuttons, they are nothing but trouble
when it comes to small signals.

Of course, it also depends on what you are striving for;
A modern sterile and linear or the original warmer and a bit colored sound.
In short, mount what you think sounds best for you and don't blindly trust
expensive components to be better than cheaper.
I've found "computer grade" capacitors very good for the output stages.
If you can find suitable sizes, you can fit 6800uF here to improve a bit on the damping factor
but no need for low-ESR types really since the output stage emitter resistors are the ones setting the limit.

Martin

BM 3000-2 recap

Paul — Sat, 30 Jan 2010 21:50:35 GMT

I am getting ready to do a recap on my BM3000-2,I have most of my caps.I am using Elna Silmic and Wima MKS,but am having trouble finding "audio grade" caps for the three large 5000uf/70V caps.My question is:Do I really need "audio" grade caps or is this just a gimmic?I have some 4700uf caps and 5600uf but am not sure what level of quality I need here.I want this thing to be at it's absolute best sound quality when it's finished and I don't want to be second guessing my cap choices after it's done.I am anxious to hear what others have been using.