Bruel & Kjaer Model 2604 Retrofit

[ZHOUJUN]

This is a 1960S audio testing instrument, used as a preamplifier for B&K measurement microphones. The sound is very warm and the frequency response is very wide. I compared it with the Telefunken v72. I found that it has greater gain and more power than the V72. but there is a slight motorboat noise. I modified it to meet the needs of professional recording. After the modification, it is quieter than the V72. I strengthened the shielding of the line, replaced it with a standard XLR interface, and added A JORGEN SCHOU input transformer to match the impedance of different microphones, and the filter input was modified so that I could plug in any filters and compressors in series internally.

 

[gyraf]

I've tried in various ways to get good audio out of these and always failed (yes, I sunk months worth of work into this some years back)

It seems that B&K, perhaps because this is aimed at metering-only, did absolutely not care about noise or distortion if less than a needle-width worth of total contribution.

That said, I still wish very much that it could be possible to up-cycle these beauties - but I also think their modest pricing at the used market to some degree reflects their unsuitability for audio.

Very happy for you if you ended up with a mic-amp that suits your taste - this post just to note that the result may not be for everyone

/Jakob E.

 

[ZHOUJUN]

I've tried in various ways to get good audio out of these and always failed (yes, I sunk months worth of work into this some years back)

It seems that B&K, perhaps because this is aimed at metering-only, did absolutely not care about noise or distortion if less than a needle-width worth of total contribution.

That said, I still wish very much that it could be possible to up-cycle these beauties - but I also think their modest pricing at the used market to some degree reflects their unsuitability for audio.

Very happy for you if you ended up with a mic-amp that suits your taste - this post just to note that the result may not be for everyone

/Jakob E.

The key is the line shielding of the power supply part, as well as the modification of the input part. Currently, my 2604 is very quiet, and the gain is turned on to above 60DB, which is just some white noise. I will record an audio clip.


I recorded a piece of audio. It started with my voice and some computer noise. Then I turned off the 48V power supply of the microphone. The microphone sound will slowly disappear until its original noise is left. I don’t know if this is considered quiet. I think their design fully complies with audio standards. I have actually been trying to think of ways to modify them, and I have failed many times. Of course, not everyone may like such modifications, because there will be some trouble in operation, and the amplification circuit does not need to be changed in any way. , just modified the interface and line shielding, but it did not increase a lot of cost. The accessories used in these antique amplifiers were all very top-notch at the time. The original price exceeded many professional audio amplifiers at that time, and the sound was also great. Because the original fixed gain was too large, It reaches more than 120 DB, causing noise. After modification, the noise control is very good. I tested its frequency response and signal-to-noise ratio. It is the same as its manual description, 10Hz-200KHz, the curve is very flat. I have never seen any I just wanted to share my experience with mic preamps that behave like this, thank you!



View attachment B&K2604.wav

 

[opacheco]

ZHOUJUN,​

I made some work in a couple of these units long time ago (2006), you can see here some discuss about. Check it and let us to know what do you think please.

https://groupdiy.com/threads/pentode-control-gain-by-screen-grid.32289/ (go directly to the message #11)

Thanks
opacheco

 

[NovaPA]

Thank you, very useful note. Several times I came across such devices at local online junk auctions for little money from people who did not know their true use.
And I always doubted whether it was worth buying for myself, whether they could make something useful out of it?

 

[ZHOUJUN]

I did not change the circuit structure of the amplification part and retained the original performance， I did not modify the tubes， I just strengthened the shielding of the power supply part and replaced a shielded power transformer (it will have no impact even if it is not replaced)， I replaced all the electrolytic caps, added an impedance changing transformer to the input section, and changed the connector to a universal XLR jack， The sound it produced was very good， I think the frequency response width and performance exceeded the Telefunken V76M, no exaggeration.

 

[ZHOUJUN]

ZHOUJUN,​

I made some work in a couple of these units long time ago (2006), you can see here some discuss about. Check it and let us to know what do you think please.

https://groupdiy.com/threads/pentode-control-gain-by-screen-grid.32289/ (go directly to the message #11)

Thanks
opacheco

I read all the posts carefully and learned a lot.，I think the key point is not the TUBE, but the design of the circuit and the quality of the components， Thank you!

 

[ZHOUJUN]

Thank you, very useful note. Several times I came across such devices at local online junk auctions for little money from people who did not know their true use.
And I always doubted whether it was worth buying for myself, whether they could make something useful out of it?

If it is B&K 2603 or 2604, it only needs slight modifications, and you can get high use value at an ultra-low price.

 

[ubxf]

Thank you for sharing your experience , is the JORGEN SCHOU input transformer designed for Kristal microphone 200ohm to 125Kohm.
Shielding of the line is the one at the microphone input from the xlr connector ? I have one of these sitting unused in my studio for the last 25 years and I will now try to revive it. Many thanks

 

[opacheco]

ZHOUJUN,​

I don’t have any idea how do you did that!; Could you post a couple of pics of the strengthened the shield of the power supply please?

I changed the gain of the unit because it have a lot gain at all, but I can restored to the original if this will be necessary. Maybe the input transformer (BTW, what exactly transformer did you use?) you used have to do with the isolated from the noise a little; what do you think?

Thanks
opacheco

 

[ubxf]

The one I have is a 2603 but looks pretty similar to your pictures. I see that you put xlr plugs everywhere except for the microphone input.
Could you please share a note on how you rewired this preamp. I'm still not sure I understand the shielding, is it the power supply or the signal lines or both ?

 

[ubxf]

schematic

 

[ZHOUJUN]

Thank you for sharing your experience , is the JORGEN SCHOU input transformer designed for Kristal microphone 200ohm to 125Kohm.
Shielding of the line is the one at the microphone input from the xlr connector ? I have one of these sitting unused in my studio for the last 25 years and I will now try to revive it. Many thanks

The power transformer that comes with the 2603 is itself shielded, so the 2603 only needs to shield the power supply lines, such as the heater line shielding， The input transformer is to balance the access and microphone impedance matching， The difference between them is : 'The frequency response range of 2603 is 2HZ-40KHZ, and the frequency response range of 2604 is 10HZ-200KHZ'，The Telefunken V76M we are familiar with is 20HZ-20KHZ. There is no problem in audio use，All electrolytic filter capacitors need to be replaced, and the coupling capacitors need to be tested.

 

[ZHOUJUN]

The one I have is a 2603 but looks pretty similar to your pictures. I see that you put xlr plugs everywhere except for the microphone input.
Could you please share a note on how you rewired this preamp. I'm still not sure I understand the shielding, is it the power supply or the signal lines or both ?

It is mainly a shield for the heater power supply， It was originally designed to be mixed with other power supply lines, so it may cause interference to the audio signal. There are 4 XLRs in total， If you do not need to insert a filter or compressor in the middle, you only need to use two , one is responsible for input and one is responsible for output， The principle of transformation is not very complicated， The circuit layout and process are very important to determine the success， We must constantly try the best access point of the circuit.

 

[ZHOUJUN]

ZHOUJUN,​

I don’t have any idea how do you did that!; Could you post a couple of pics of the strengthened the shield of the power supply please?

I changed the gain of the unit because it have a lot gain at all, but I can restored to the original if this will be necessary. Maybe the input transformer (BTW, what exactly transformer did you use?) you used have to do with the isolated from the noise a little; what do you think?

Thanks
opacheco

I think the key point is the isolation of the audio signal and the heater line， The input transformer can only better match the impedance, convert the balanced signal interface, and switch the signal input phase, so that the preamplifier can adapt to more types of microphones.

 

[opacheco]

What kind of transformers did you use for?(type and impedance spec for input/output/for filters in-out)??

Thanks
opacheco

 

[ubxf]

It is mainly a shield for the heater power supply， It was originally designed to be mixed with other power supply lines, so it may cause interference to the audio signal. There are 4 XLRs in total， If you do not need to insert a filter or compressor in the middle, you only need to use two , one is responsible for input and one is responsible for output， The principle of transformation is not very complicated， The circuit layout and process are very important to determine the success， We must constantly try the best access point of the circuit.


View attachment 119474

Thank you so much Zhoujun, If I understand correctly you plug your microphone in the amplifier input (just above the filter input) and that line has a mic input transformer inserted in its path ?

 

[ZHOUJUN]

What kind of transformers did you use for?(type and impedance spec for input/output/for filters in-out)??

Thanks
opacheco

我使用的是 JORGEN SCHOU 输入变压器，上面写着 0.32M， 我不知道是否是型号，因为我找不到关于这款变压器的任何有用信息，但卖家告诉我，它是从丹麦的另一台 B&K 设备上拆下来的， B&K也大量使用该公司的变压器。

 

[ZHOUJUN]

Thank you so much Zhoujun, If I understand correctly you plug your microphone in the amplifier input (just above the filter input) and that line has a mic input transformer inserted in its path ?

Yes, the microphone input position is the amplifier input interface, then an input transformer is added to match the impedance, and the selector switch points to the amplifier input.

 

[ubxf]

Yes, the microphone input position is the amplifier input interface, then an input transformer is added to match the impedance, and the selector switch points to the amplifier input.

Great thank you so much