Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
« Reply #180 on: February 22, 2021, 04:55:18 PM »
One thing that occurred to me earlier is a solution (modification) to help a little with potentially too low an inductance in that interstage.

Of course strapping the input valves as triodes would lower the source impedance to the interstage and relax the inductance requirements, but say you wanted to mostly keep the pentode characteristics.
Well, an ultra-linear connection would get the anode impedance down a bit, as well as lower any distortion.

If the main anode voltage is, say, 265V, a pair of simple resistive dividers from the EF83 anodes to ground.  Lets pick 165K on top and 100V on bottom for ease (scaling them up would lessen any extraneous anode load though).  At the mid point, feed that into the grid or gate of a triode or Mosfet and tie the triode/Mosfet's cathode/source to the EF83's G2 with a bit of current care of a 33K to ground.  Triode/Mosfet anode/drain is tied to actual B+.

This would be about a 37% ultra-linear feedback and would lower the anode impedance.
There is precedent for this in a compressor, albeit using cascodes rather than pentodes so it's not totally off the wall I don't think.  Values need to be adjusted and a cap possibly added, but hopefully someone gets the point.

Anyway, just a thought I had.
« Last Edit: February 22, 2021, 05:17:13 PM by Winston O'Boogie »
D. J. H.


Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
« Reply #181 on: February 23, 2021, 02:15:11 AM »
Triode or pentode with screen feedback might introduce new problems. With pentode assuming the negative feedback lowers the plate resistance significantly. With triode or screen feedback pentode the changing plate resistance from low to high impedance needs to be considered. In gain reduction how much will the changing plate resistance affect frequency response? Is the sidechain sufficient for triode or screen feedback pentode? How much more control voltage will be needed and can the sidechain provide it?

I think lowering the plate resistors to 1.5k was the smart move. Gain goes down in the the gain reduction stage with 1.5k resistors, but it doesn't really matter because there's plenty of gain after it.

Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
« Reply #182 on: February 23, 2021, 08:28:22 AM »
Thanks Heikki.   It was more a thinking out loud for future experiment than suggesting  Falk do this on his current unit.   
I take your point about changing anode impedance, although it's something that happens with 90% of the variable mutual conductance compressors we discuss on here, and to a greater extent than I'm suggesting from a 30+ % of ultra-linear fb.
Edit:  Side chain voltage -  looks like, with 100V on G2, then -20V is the maximum before it poops out anyway.  Probably not a compressor for deep gain reductions whichever way it's cooked.   OK for mastering etc of course. 

I checked my scribbles from messing with the Edcor in this type of use - I had a 10K across the full primary and a zobel on the secondary.   

Cheers  :)



« Last Edit: February 23, 2021, 06:54:04 PM by Winston O'Boogie »
D. J. H.


Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
« Reply #183 on: February 23, 2021, 06:33:45 PM »
Good Evening.

Sorry for being so awfully less responsive but the studio work keeps me busy. Thank you so much for making this thread flourish. There will definitely be some compensation for all this sooner or later. I will need some more time testing but the last mastering sessions felt great and the customers didnt feel too much distortion anymore.

@Volker.. totally missed your message. Regarding the mu metal housings I had a tough one. I got a quote for about 400€ for ONE housing from the Netherlands. I have another for 10 housings for 900€ from France and a third for 10 housings for 600€ from England. Honestly thats too expensive for a DIY project which was aimed to build the compressor as cheap as possible. I will keep myself busy on finding a solution for this.

Aside of that: Input TX is 10k:10k, Interstage is 10k:10k, Output is 600r:600r, Sidechain is 600r:2k4
The transformers are the biggest compromise in this build but for the price they do a great job. This build was never aimed to become a boutique compressor rather than a unit that works well for what it is. It has some weak spots but its too early for me to evaluate if they really bother me or if they become the weak spots that every good friendship brings.

Best regards,
« Last Edit: February 23, 2021, 07:11:26 PM by Falk »


Re: Rohde und Schwarz Begrenzungsverstärker ABR / U23
« Reply #184 on: February 24, 2021, 02:34:34 AM »
You may not NEED mu-metal casings at all, unless you are physically limited to a certain amount of space in your build.  Remember, the main benefit of mumetal shielding is that it has some 6-10 times the magnetic softness (or magnetic conductivity) of standard soft iron

..which in the case of shielding only means that you have shielding efficiency of 6-10 times the same amount of iron. This off course paramount important for tight fits like Danner-casettes with integral power transformers - not necessarily for us

..which again means: Just put in 10-times the thickness of cheap soft iron and you're golden IF you have space for this

I have had really good results just getting some 8-10mm iron sheet lasercut to profile fitting the transformer, then stacking a handful of these with a top and bottom "lid" of same material. Yes, gets heavy - but do we care?

For smaller transformers; in building supply stores you can get off-the-shelf soft iron pipes in short lengths, threaded and with fitting end caps. Yes, you probably can't ship this solution internationally for obvious reasons  :o but it works really nice as well..

/Jakob E.
« Last Edit: February 24, 2021, 02:38:50 AM by gyraf »
..note to self: don't let Harman run your company..


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