Rohde und Schwarz Begrenzungsverstärker ABR / U23

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jans

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Joined
Feb 21, 2013
Messages
12
Very excited about this project. I checked the files, already sounding amazing. Well done, very impressive.
 

Falk

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Joined
Jun 30, 2013
Messages
91
Dear Jans,

thank you. Since I recorded the files I learnt a lot about this device and took it some steps further. I am still waiting for the new parts to arrive. I already designed a new revision of the main boards and will build them until mid of this year. There are still some details to be figured out but its going to be a really nice compressor.

Best,
falk
 

Falk

Well-known member
Joined
Jun 30, 2013
Messages
91
Good Morning everybody. I am working on revision boards of the units and still try to cure everything that I notice. I invented that small circuit to for the rectified side chain signal. Its super simple and uses a voltage divider R3 / R4 to change the ratio. It works super and as intend.

BUT - in the middle postions when R3 ≈ R4 a 50Hz noise in the audio signal increases by 6dB. (side note: its always R3 + R4 = 1Meg)
The Values of R1, R2, R3, and R4 are all changed by switches.

Does anyone has an idea on this?

Thank you,
Falk


IMG_9952.JPG
 

Heikki

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Joined
Jul 31, 2008
Messages
227
Location
Finland
High impedance node will easily pick up hum and buzz. Try moving the "to grid" wire a bit or routing it differently, it might help or make things worse.
 

gyraf

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Jun 4, 2004
Messages
10,163
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Aarhus, Denmark
Agree, at center of pot you get the highest source impedance. Perhaps try shielded cable here, shield grounded to ground at the divider?

Ideally, any hum introduced here should end up cancelling when added evenly to the two differential grids. Perhaps your balance adjustment is a bit off?

And again, agree: Ratio is just gain/attenuation of control-dc AFTER rectification, threshold is gain/attenuation of signal-ac BEFORE rectifier

/Jakob E.
 

Winston OBoogie

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Joined
Jun 3, 2004
Messages
2,466
Location
UK.
Agreed too. -6dB spot is the highest Z so, shielded cable, playing with the routing of that line, and making sure the audio balance is good.

6AL5 is nearby and AC heaters or? On a PCB or twisted pair? Careful routing of that 6V3 line too if so.
 

Falk

Well-known member
Joined
Jun 30, 2013
Messages
91
Thank you very much for your feedback. I will try and see if I can manage to get it better. The cables to the ratio switch are not shielded so this is going to be the first thing I will check. The dc control voltage is fed through the input tx secondaries center tap. The secondary does not measure ideally symmetrical. In the revision boards I could integrate an option to feed the voltage through resistors instead of the transformer which would improve symmetry. On the other hand it would mean to lose the RL-lowpass.

The 6AL5 is not really near to the cables. All heaters are 6.3V AC with twisted pair. I just realized that there is another 12V AC line on the same multi pin connector that goes to the grids. I ll check if the hum disappears if I disconnect the 12V there. This 12V line feeds the grids in balancing mode.

Other than this there will be 15K:600r output tx´s coming end of the month. This will actually be one of the last modifications.

If this project is going to become DIY I am sure you would appreciate to have a regulated PSU instead of the old fashioned tube PSU right? Is there any regulated PSU draft that could provide 250V and 63V?

Noise is pretty even at -90dB(FS) only 50Hz peaks at around -85dB(FS) when ratio is in the middle position. I use the units almost every day since half a year and they do a great job.
 

jensenmann

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Joined
Oct 10, 2004
Messages
2,618
Location
Karlsruhe, Germany
Good Morning everybody. I am working on revision boards of the units and still try to cure everything that I notice. I invented that small circuit to for the rectified side chain signal. Its super simple and uses a voltage divider R3 / R4 to change the ratio. It works super and as intend.

BUT - in the middle postions when R3 ≈ R4 a 50Hz noise in the audio signal increases by 6dB. (side note: its always R3 + R4 = 1Meg)
The Values of R1, R2, R3, and R4 are all changed by switches.

Does anyone has an idea on this?

Thank you,
Falk


View attachment 80697
If you look at it as a (high impedance) conductor loop then it´s clear that it will pick up magnetic field. Check your layout so that the area inside the loop is as small as possible. That will lower the induced voltage according to the law of inductivity Uind.=dPhi/dt= d(B*A)/dt
 
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