Hi:

I have a beomaster 8000 that I just recapped. It sounds much better with new capacitors installed, however I the digital noise from source switching, volume control, frequency change is now more prominent. 

For example, changing the volume, you can hear the attenuator "step." 

I checked all grounding, resoldered the ribbon cables but so far I am unable to find the fault. The problem seems to be coming from board 4 (tone control module) as the noise is present even after removing the audio output from the pre-amp (board 1).

If anyone has experience with this please advise.

Thanks,

Derek

It's actually very common to hear the volume steps and it's one of the weak points in the BM8000.
Imagine a sine wave and then introduce a sudden change in volume. Depending on where on the waveform
the volume change happens, it will break the waves continuity and the resulting flank will be amplified and if it
happens at a fairly high volume setting and in an "unlucky" spot on the waveform, you can often hear it.

The BM8000 has an adjustment for this, it's on the tonecontrol board if I remember correctly.
You will probably not be able to eliminate it completely, but you should be able to locate a "best" setting.

Actually proves that less signal detail is lost after recapping...

Martin

Hi Martin:

Thanks for your reply. After posting last night, I read upon the "analog offset" adjustments and adjusted accordingly. I was able to lessen the steps but not too much. You are correct in that you can't get rid of it.

Having spent a great deal of time on the BM8000 now from redoing the displays to recapping, I've come to know it quite well. For 1980 technology it was indeed VERY advanced (optical encoders, active filters, digital attenuator/electronic switching...) but I think the engineers got carried away. With tone controls out, the shortest signal path goes through 5 op-amps!

Recapping definitely improves the sound. The sound stage widens and you hear improved highs. After this I took the project further one more step.

After consulting with Die Bogner (other Martin), I upgraded the op amps to OPA2134. This brought much greater detail but makes the BM8000 sound brighter which analog sound people may not like. I added ic sockets so I can always restore the old opamps.

Even after all this however, the BM8000 is still a noisy unit. When you crank the volume all the way up you can hear noise and some crosstalk through the electronic switch. Overall though it is still a nice piece of B&O history and worth restoring.

Derek

Can you tell me what op amps are in the signal path? I'm currently rebuilding a BM8000.

There's a lot of OpAmps in there and they are practically all in the signal path.

I also tried the OPA2134 OpAmp swap but came to the same conclusion, you did.
It sounded great at first but I got tired of listening to it and ended up putting the old ones back.
I think the sound got too sterile for my liking. I enjoy the original warm, soft yet powerful sound, also
being a person who actually likes to switch Loudness on, it suits my speaker setup and livingroom fine.

Martin

For a "warmer" sounding opamp, try an OPA2604. I have used both the 2604 and 2134 in a couple modded tuners, and there is definitely a sonic difference. Interestingly, an older tuner (Kenwood KT-7500, heavily modded w/2134) and a newer tuner (Audio Dynamics T-200, mild mods w/2604) wound up sounding identical, to me. It sounds like the 2604 would go very well in the Beo.

Jeff: in my 8000 mostly

TL 072CP
uAF 772TC

1 NE5532 in the phono stage.

NE5532 would also be a good op-amp for "warmer" sound. These are used extensively in Studer tape recorders.

auric:

Jeff: in my 8000 mostly

TL 072CP
uAF 772TC

1 NE5532 in the phono stage.

NE5532 would also be a good op-amp for "warmer" sound. These are used extensively in Studer tape recorders.

You have to be careful about replacing opamps with a completely different internal design, though. This goes especially between the likes of TL072 (which is a BiFET opamp) and NE5532 (which is bipolar). Both are very good audio opamps when used properly, but may fail badly if you try to simply interchange them.

This is mainly because of the vastly different characteristics of the input stages; the FET inputs have practically zero input bias current (in the order of few tens of picoamperes), while the bipolars don't - especially the NE5532 has an input bias current of almost 0.5 mA. Thus dropping a 5532 into a very high impedance source might even disturb the operation point, but most importantly its bigger input noise current will create a lot of noise in a high impedance circuit. The noise characteristics of NE5532 make it mostly suitable only for circuits that have a source impedance of 10 kohms or less.

The BiFET opamps also have much better DC offset specs; if you drop bipolars in their place, you might even need to introduce some new decoupling caps in the signal path (B&O designs usually have them all over the place already, though )

Some of these imperfections might actually even sound good, but it is then pure luck instead of the result of using a "better" opamp.

TI itself has recommended TLE2072 family to be used in place of TL07x if higher specs are needed. Less noise, but the slew rate is even bigger, and I guess those would then sound even more sterile.

Huh! Interesting posts. I read all the time about Burr Brown chips having a characteristic dark sound, and was thinking about swapping in some OPA2132's to get a more laid back sound from the BM. From the research I've done, B&O used some pretty nice OpAmps for ~1980, maybe I should re-think replacing them.... I definitely wouldn't want a brighter sound - my B&O speakers are on the lean side as it is.

As far as slew rate, I always thought faster was better. More accurate tracking of voltage swings, or no? What about OPA 2227's? Their slew rate is quite a bit lower than the NE5532's, LF353's, or TL072's, at 2.3 v/us. Anyone tried those?

With Beovox's, you pretty much have to have the loudness on in my opinion, unless they're stuck completely in the corners of the room. I really am convinced B&O didn't compensate (or did so minimally) for the 2pi - 4pi radiation transition in response. I still like 'em, they're just lean to me.

Jon