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Re: Gorgeous 1967 Bruel & Kjaer All Tube Swept BPF In Action

Posted: Wed Aug 26, 2015 11:33 am
by HideawayStudio
OK I think this is now a full blown addiction!... I have been collecting and using vintage test gear as secret weapons for my sound design for several years now but specifically I've fallen hopelessly in love with 60s era Bruel & Kjaer gear - the looks, the build quality and that sound!! I now have 5 of these monsters fixed up and running in the studio with several more now in the sidelines awaiting attention...

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Here is a gorgeous 1960s Bruel & Kjaer all tube stepped 1/3rd Octave band pass filter in action that I've just returned to life to join the others. The filter is simply being fed with a single sustained Solina like string pad and a series of quite haunting harmonically related pads result as the frequency control is moved between 5 different settings. The filter has around 50dB per octave slopes at their flank but according to the manual max out around 120dB/octave at the crest! The resulting audio sounds very different to the original source material.



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This unit is particularly heavy for its size because it consists of a whole bank of passive L/C resonators consisting of a large choke for each band with laminated iron cores and an array of capacitors to tune them.

The unit was originally designed as an Audio Frequency Spectrometer but as with several select pieces of vintage Bruel & Kjaer gear it proves to be an interesting gadget in the studio in creative music applications.

Re: Gorgeous 1967 Bruel & Kjaer All Tube Swept BPF In Action

Posted: Thu Aug 27, 2015 1:36 pm
by HideawayStudio
A WORD OF WARNING ON 60s/EARLY 70s TUBE ERA BRUEL & KJAER UNITS:

In my rapidly growing opinion, having worked on many tube era Bruel & Kjaer units now, it is not safe to use these units unmodified in the studio. They were built specifically for audio analysis in the lab and not audio processing/creative use in the studio in a line/instrument level signal path. Many of these units including the Type 2112, 2107 and 2603 were all therefore designed to output up to 100V into less than 75 Ohms to a chart recorder!

As I have recently found to my detriment, when I blew up an input channel on one of my little digital mixers with my 2107, unmodified units can produce transients of over 80 volts on switching filter modes etc.

What I have started doing with these units to overcome this is to pull the cathode follower tube in the final stage and tap the output signal from blocking capacitor on the output of the preamp tube in front of it which effectively runs at line/instrument levels via a high voltage blocking capacitor and 1MOhm leakoff resistor to ground. This results in a much more studio friendly output which is in fact also a little cleaner too. The unit runs considerably cooler like this too. The downside to this is that the VU meter is disabled. From the front panel you’d never guess (other than the needle not moving!) and all other functionality is untouched including the neon input signal overload lamp.

Another thing I do is disconnect the Ext Filter Input socket (which is actually an output) on all B&K units that have them as they are not DC blocked and carry around 80 volts DC on them which will blow an input channel strip to oblivion. This anomaly must have caught out many engineers throughout the lifetime of these units. In fact many units have failed because of burnt out resistors in series with the input amplifier cathode follower that drive the ext filter input socket.

I have performed these mods now to my 2107 and 2112 and several others. It is not a difficult mod to do but in light of my recent incident with one of these I highly recommend it at the sacrifice of the VU meter if these units are wanted for creative use in the studio.

Re: Gorgeous 1967 Bruel & Kjaer All Tube Swept BPF In Action

Posted: Thu Aug 27, 2015 4:02 pm
by HideawayStudio
The Gorgeous 8" Diameter Illuminated Stepped Tuning Dial on the Type 2112
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Re: Gorgeous 1967 Bruel & Kjaer All Tube Swept BPF In Action

Posted: Thu Aug 27, 2015 8:24 pm
by nathanscribe
They're lovely 8-)

If someone wanted to buy one of those little vintage test oscillators to do some Radiophonic-style faffing, is there anything to look out for?

Re: Gorgeous 1967 Bruel & Kjaer All Tube Swept BPF In Action

Posted: Fri Aug 28, 2015 11:39 am
by HideawayStudio
nathanscribe wrote:They're lovely 8-)

If someone wanted to buy one of those little vintage test oscillators to do some Radiophonic-style faffing, is there anything to look out for?
There are two old oscillators I can recommend for this sort of thing..

The Bruel & Kjaer Type 1022 BFO "Wobbulator" which is a direct descendant of the model used at the RWS...

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It is unusual in that it covers the entire audio range in one tuning control sweep and has a built in LFO which can be used to create anything from a vibrato effect to a full on dub siren warble. There is also a socket permitting external FM but unfortunately the mating plug is rather difficult to find. Please note, depending on age, this model may be either tube/valve or transistor.

...and the Farnell FG-1.

The FG-1 is a very cool signal generator for creative studio use in that it is a particularly flexible solid state analog oscillator with internal and external AM and FM modulation functions and goes way beyond the audio range so can be used to create superheterodyne effects and can even generate AM at radio frequencies. I used one of these once to transmit speech and receive "distressed" audio on an old valve AM radio whilst interfering with the signal using a second RF oscillator - the effect was really cool.

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The FG-1 can also create oscillator frequency sweeps, tone bursts and can be modulated exponentially or linearly. The sweep generator CV is available at an output jack a lowish audio rates so you even have access to a sawtooth wave which is unusual for a signal generator (most are sine / pulse / triangle only).

Just be warned that Bruel & Kjaer made an awful lot of gear and there is such a strong family resemblance it is very easy to get models confused. From experience so many of them look tempting but I have found only a relatively small number of models are suitable for creative studio use so you need to be a little careful.

Re: Gorgeous 1967 Bruel & Kjaer All Tube Swept BPF In Action

Posted: Sat Aug 29, 2015 2:51 pm
by nathanscribe
Interesting, thanks. Seems there's a Farnell on eBay right now...

Re: Gorgeous 1967 Bruel & Kjaer All Tube Swept BPF In Action

Posted: Fri Nov 18, 2016 8:14 pm
by InnerGeist
I'm trying to work with some of these now. Each one is a little bit different. It seems each one here has different component layouts. I can see a large Jenson electrolytic capacitor that is leaking fluid on one, a burnt large brown (resistor?) component on another, and I'm sure very similar issues on the other 2. Is it a good idea to replace these large capacitors near the power transformer as a first step? I also am interested in the modification to tame them for studio use. I'm trying to identify the cathode follower tube in the final stage, and locate blocking capacitor on the output of the preamp. If I can identify where these are, I can get set up to make some progress. Anyone up for helping with this?