Rohde und Schwarz Begrenzungsverstärker ABR / U23

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Falk

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Jun 30, 2013
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101
@gyraf : what a great hint with the PF86. Thank you so much. Again. I ll go this way. Could find a good bunch of Siemens PF 86 them for 6€. I bought a few and will try them.
@jensenmann : Thanks for sorting this out per PM.
@Heikki : Totally right, this all works flawless. The point is the JJ 806s itself. They are noisy more often as accaptable and I am still trying to keep the cost of the project low as possible. Out of 8 tubes 24€ / piece 2 were not useable because they were noisy.
 

Falk

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Jun 30, 2013
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101
Good evening,

today I found time to build the regulated PSU I planned during the past weeks. Works great at least for the hour I tested it. The hum is gone now.
The PSU provides 250V DC, 70V DC, 6,3V DC and 12V AC to the circuit.

The hum is finally gone (50Hz lowered by 15dB) now and overall noise is lowered about 6dB. RMS noise in the my mastering chain is about -90dB FS RMS.

I think this is where I can get it and I will not try to make it any better regarding noise and hum. Should the input and interstage transformers pick up too much hum from the power transformer when stacked, I will think about shielding the audio TXs again.

Here comes a comparative spectrum analysis between the unit with old linear PSU and new regulated PSU:
Bildschirmfoto 2021-11-01 um 14.19.04.png Bildschirmfoto 2021-11-01 um 14.18.40.png

Here comes two pictures from the PSU and todays test:


IMG_0733 Kopie.jpg IMG_0731 Kopie.jpg

I will soon build the second PSU and put both PSUs in the devices and further test everything. Once everything works I will design new audio boards with all the new features and corrections needed to make the unit perfect.

I think in a year or so it could be ready to make a single run of PCBs for those who supported this project and are interested in building one themself.

Best,
Falk
 
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JMan

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Oct 26, 2019
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Same here! A year will give me some the time to clear my backlog of half-finished projects anyway.
 

Falk

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Jun 30, 2013
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101
Dear gentlemen,

today is a great day because I could fix a good bunch of minor bugs. This moves the project forward a lot in my head.
- I could reduce the overall distortion in the sub bass by fooling around with the input transformer wiring and zobel network
- I fixed the voltage to have the ratios correct. The unit now allows for a more or less perfect 6:1 4:1 3:1 2:1 and 1.5:1
- I meassured the release and attack settings and can now define the final values. Minimum attack is 8ms due to the anode resistance of the 6AL5 rectifier
- Attack settings are: 8ms 12ms 25ms 50ms and 100ms
- Release settings are 50ms, 80ms, 100ms, 200ms and 400ms + the chosen attack time (in the final unit you can jumper if the the attack is added to the release or not)

I could really need your advice on one thing. It would be great to be able to match the gain of the sidechain. In which stage of the 6SN7 (ECC40 in original schematic) would you add a trim resistor to be able to match the sidechain gain? Or would you rather go for matched tubes?
 

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gyraf

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Isn't this what trimmer R63 is for (regelspannung-verstarkung=sidechain gain), like a threshold trim? Or do you need ratio trim as well?

/Jakob E.
 

Falk

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Jun 30, 2013
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101
Hi Jakob, thank you.

I use R65 as threshold potentiometer on the front panel. This works fine and can be easily used to match side chain levels.
But it would be great to have exact same settings.

Solutions could be:
- a threshold fine adjust
- matched tubes in the side chain
- or an adjust for the gain of the side chain amplifier tube

But as I am writing I thing the best would be to have a threshold fine adjust.

I dont need an adjustment for the ratio - this luckily works great as it is.
 

capacitor

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Apr 25, 2017
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Hi, Falk! I can send you some pretty decently matched 6SN7, if you wish to give them a try. I have a couple of different versions to choose from. Please send a PM.
 

gyraf

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If you use potentiometer R63 (not R65!) for frontpanel threshold, then you could probably just add a trimmer in series at the top end of this, and adjust until you have the same level at both tops of threshold pots. Something like half the value of the pot for a trimming range of half the maximum threshold sensitivity..

/Jakob E.
 

capacitor

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Apr 25, 2017
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I can send a pair of 6SN7 that are as mismatched as possible, as well :) Most of this collection are Baldwin or RCA clear-top (but it probably doesn't matter since they're in the sidechain).
 

Disco Volante

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Jun 20, 2021
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Been following this thread on the side. Great work, good fun building good machines!

Anyway, if you find you need shielding cans I'd be able to lasercut and TIG-weld you some from mild steel (6-8-10mill?), with threads or lugs for mounting, holes for grommets. I'd just need a good sketch with dimensions... I have a small metal-fabrication business besides building speakers and repairing all manner of old gear.
I'm regularly in Berlin so they could travel in a (heavy!) handbag.

It does seem like you've got it working very well already, just thought I'd chip in to the project.

Best of luck, Viggo
 

Falk

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Jun 30, 2013
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101
Good Morning,

so here we are. I think we got it. The machines work perfect now. RMS noise at unity gain is at -92dB(FS). The only way to further improve this would be shielding of the input and interstage transformers. @Disco Volante if you would be up to make 4 housings of the same type, this would be great. I ll drop you a message and we can sort this out. Thank you so very much.

The side chain gain issue could be sorted out with a trimmer. @gyraf - thank you! So actually there is no need for matched tubes anywhere in the machine. @Heikki was totally right. There is a small bump at 26kHz but I like the sound as it is. The added shimmer is great.

I will design the final revision main pcb at the beginning of next year and build it. Alongside I will make a short documentation what should enable experienced DIYers to build the unit themself. I will do 20 to 30 pcbs next year and give them to those who are interested and contributed here.

Please write an email to: [email protected] and I ll get back to you as soon as possible.
The pcbs will come with Telefunken / Valvo / Siemens NOS EF83 tubes but there will not be a kit or something. Just a the main pcb and psu pcb, a mouser bom, a schematic and a short documentation. Of course we can open a support thread here.

The following features are implemented:
- circuit based on the R+S U23
- Vari Mu Stage equipped with EF83 as used in the later versions of U23, gain stages equipped with EF86, EF806s or PF86 (can be jumpered) on board, Sidechain Amp with 6SN7GT and EB91 / 6AL5 rectifier
- 4 edcor audio transformers per chanel
- 2 hammond chokes
- regulated PSU with DC filament supply
- calibration mode (to calibrate the vari mu stage), amp mode (compression bypass), compression mode
- 5 way ratio switch (6:1; 4:1, 3:1 2:1 1.5:1)
- Threshold
- Gain Make Up
- 5 Way Side Chain Highpass
- 5 Way Attack
- 5 Way Release
- 100uA GR-Meter
- two units can be linked
- NO hard bypass on board
- Power transformer must have the following secondary windings: 225V 150mA; 9 - 12V 5A for Filament; 12V 1A for calibration

I will make new audio samples this weekend.
Best,
Falk
 

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Murdock

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Jan 28, 2015
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Wow,great project!
Seems like i‘m a bit late to the Party .
The only thing I could contribute would be some mumetal sheets which you could put inside the steelboxes disco volante will make to make them even more effective.
But-92dBFS is already plenty quiet
 

Disco Volante

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Jun 20, 2021
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32
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Sweden
Sure, as I said, make a sketch (dimensions, hole placement, thickness, etc), take a picture of it, I can convert it to metal housings. Just don't have the special tools for mu-metal, but almost any other metal I can work with. PM me.
This is an amazing project, I'd love to add something if I can.
 

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