Rohde und Schwarz Begrenzungsverstärker ABR / U23

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jans

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

Falk

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Jun 30, 2013
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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

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Jun 30, 2013
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93
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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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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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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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

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Jun 30, 2013
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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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Oct 10, 2004
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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
 

Falk

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Jun 30, 2013
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93
Good evening.
Its been a long time with no news because parts took ages to arrive. I have been waiting for 5:1 XSM transformers to replace the 1:1 WSM on the output. But there has been no improvement so I ll stick with the 1:1 WSM. In fact the 1:1 is simply better.

In this mood of disappointment I tried the 5:1 XSM in the position of the 4:1 WSM in the side chain which cured the high pass behavior of the side chain.
The unit seemed to have a 90Hz high pass built in. That has not been a flaw to me because the compression was just right. But if desired the unit does compress equally at lower frequencies in the side chain too from now on.

I still have some distortion issues with electronic kick drums. But I am on it and will figure it out sooner or later. I still need to change the input impedance from 4k to 10k. Is there anything wrong with rising the impedance to lets say 20k?

Also I read the thread about the PM 670 which Michael Kingston improved. My attention was mostly attracted by the PSU. I am pretty sure I will make a regulated PSU with a dc filament supply. I put together parts from Mr. Kingstons schematic and some other schematic I found. It culminates to the schematic attached. Any ideas on this on? I have no idea how a good PSU should be but the Spice simulation looks as if should work :)

It is great fun to have that super heavy choke and regulator tube on the PSU but also its just expensive and a regulated PSU will improve the noise level.
Has anyone an idea for a power transformer that could deliver 120mA @ 225 - 250V and 4A @ 12V ?

I ll keep you posted.
Thanks in advance,
Falk
 

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gyraf

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4007 diodes are only rated 1A, you need much beefier - perhaps 5400-series?

I don't really like the overkill of the 3x 6800uF primary reservoir capacitors C1-3-4 - these drag unnecessary current spikes from transformer, possibly noising all around. And as we're actively regulating anyway, there's no benefit from reducing ripple from very small to extremely small. Would make sense (perhaps) in full-unregulated supply.

rule-of-thumb :1000uF per Ampere - make do with a single 2200 or 4700uF

I don't like the way the +68V is generated from the +250 - a transistor fail might be catastrophic (depending on what it's used for)

Has anyone an idea for a power transformer that could deliver 120mA @ 225 - 250V and 4A @ 12V ?

You will want to talk to Tomasz Lachowski from TOROIDY.PL - Ask for a quotation, tell him you need it for sensitive audio, and he'll dimension it at selected cores, low core saturation, magnetic AND electrostatic shielding. Say hello from me :) - I'm a long time fan of their work

If you're heating at 12VDC, you'll probably want some 14-15VAC from transformer

/Jakob E.
 
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Falk

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Jun 30, 2013
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93
Dear Jakob,

I don't like the way the +68V is generated from the +250 - a transistor fail might be catastrophic (depending on what it's used for)

Do you mean to better have a drop with a resistor before? Or better having a dedicated supply voltage (winding) to generate the +68V?

Thanks again for the valuable feedback. I appreciate your advice.
Falk
 
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gyraf

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I looked into the schematic again, and it's safe enough as-is - there's no reason to change. The 68V is just for varimu screen grid, so it don't mess with the real world

( reason for dislike is that I've seen +48V for phantom derived like this some times, and it horrifies me..)

/Jakob E.
 
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