gyraf

Re: Rohde und Schwarz U23
« Reply #40 on: August 17, 2020, 07:29:16 AM »
Falk,

I would aim for first getting rid of all traces of hum, that part is possible.

From there it would be a matter of IRL evaluation what noise distance is really needed. Depends on quite a few factors, including how much of the noise gets modulated by GR, what it's sweet spot is, etc.. I'm not holding out on you, it's just something that can't be quantified in absentia.


Oscar,

Unfortunately no info at all about the transformers or their parts.. And your inbox is full..

Jakob E.
..note to self: don't let Harman run your company..


Falk

Re: Rohde und Schwarz U23
« Reply #41 on: August 17, 2020, 11:30:07 AM »
Hello,


@Jakob
thank you so much for giving me this motivating critical view.

I just turned my two power transformers (one for the 4A  of initial filament current and one for the 250V and 12V) 90° and the hum is almost gone. I could also try to isolate the Input TX and Interstage TX + cables since they are sensitive when I touch them.

The good news is: Noise is at -88dB FS now.  Thats almost fine for my mastering use. I am not doing any music that would need a dynamic range like this.

I attach a photo of the noise spectrum.
The 50 Hz is from mains TX the 160Hz comes from the heater TX. 

Apparently my two ground planes aren't a problem. How do you gentleman layout pcbs? Would you recommend to have one ground plane on the bottom or do you fill everything apart from the traces with a ground plane?

Best regards,
Falk
« Last Edit: August 17, 2020, 12:05:02 PM by Falk »

Michael Tibes

Re: Rohde und Schwarz U23
« Reply #42 on: August 17, 2020, 02:44:54 PM »
I've been following this thread loosely. In case you haven't seen it yet: I did some more digging now and came across this company http://vacuvox.com/. They build replicas with some enhancements. They linked a review of their older unit here http://vacuvox.com/wp-content/uploads/2018/10/Vacuvox-U23M-Promo.pdf and the measured noise spectrum there looks very much like yours...

Nevertheless I believe it should be possible to get rid of the hum.

Michael (also from Berlin  ;))

opacheco

Re: Rohde und Schwarz U23
« Reply #43 on: August 17, 2020, 05:27:44 PM »
Oscar,

Unfortunately no info at all about the transformers or their parts.. And your inbox is full..

Jakob E.

Jacob,
That's a really shame!....It will be a great help have some measurements for real transformers used in the U23 from a real unit!! but this is more difficult than find a original unit  in good shape!

I will check my inbox thanks!(you have my email address if will be necesary at any time!)

Thanks a apprecite your help and time so much!
opacheco
I love the Vaccum Tubes Sound!!!

EmRR

Re: Rohde und Schwarz U23
« Reply #44 on: August 17, 2020, 05:59:16 PM »
Two thoughts.

The IT with separate resistive loading of each coil is interesting.  I've only seen that in European designs, never an American one I can think of. 

An input pad like that is common when the unit is used in a bridging scenario with a an unknown number of other parallel amps.   Besides establishing bridging Z, it also gives isolation.  Allows use of a more common step up transformer, and can be removed if bridging isn't needed, as opposed to amps with dedicated bridging transformers that only work in that service. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

opacheco

Re: Rohde und Schwarz U23
« Reply #45 on: August 28, 2020, 10:08:12 PM »
Two thoughts.

The IT with separate resistive loading of each coil is interesting.  I've only seen that in European designs, never an American one I can think of. 

An input pad like that is common when the unit is used in a bridging scenario with a an unknown number of other parallel amps.   Besides establishing bridging Z, it also gives isolation.  Allows use of a more common step up transformer, and can be removed if bridging isn't needed, as opposed to amps with dedicated bridging transformers that only work in that service.

EmRR could you talk us about the Bridging Input Section type used in this unit please? What it does exactly? How this works?

Thanks in advance!
opacheco.
I love the Vaccum Tubes Sound!!!

Falk

Re: Rohde und Schwarz U23
« Reply #46 on: September 02, 2020, 05:38:08 AM »
Hello everybody.

@Michael Tibes, thanks for the Link it was a great motivation to work on the SNR performance of the Unit.
Attached is a screen shot of the noise spectrum. The dB scale equals a dB(V) reading. So my unit is a good bit quiter in the low end than the unit from the comparison above. But its also noisier in the highs freq range.

The noisefloor of the unit in my mastering setup is -88dB(FS) RMS now.  Thats completely fine for me. I will shield some more cables and I am sure it will improve the performance.

Also I ordered a custom power transformers. This will be another advancement too.

« Last Edit: September 02, 2020, 06:38:24 AM by Falk »

Falk

Re: Rohde und Schwarz U23
« Reply #47 on: September 02, 2020, 01:13:57 PM »
The machine is almost working as I wish for it to be. I corrected the frequency response today. Before and after in the plot attached

echoplex

Re: Rohde und Schwarz U23
« Reply #48 on: September 03, 2020, 05:54:45 AM »
Hey Falk,

I'm impressed with what you have reached in the meantime.
-88dB(FS) RMS is pretty amazing and should suffice for even mastering applications.
And then the frequency graph you posted.. it's pretty amazing as well.

I rarely feel so hyped about a project on GDIY as I do with this one.
Please keep us updated and please remain so motivated.
This is really the most interesting project 2020 and I would love to see it proceeding.
Also eager to hear some samples and to see the PCBs :)

cheers

gyraf

Re: Rohde und Schwarz U23
« Reply #49 on: September 23, 2020, 12:33:48 PM »
bumpin' for support and interest.. you can't leave us hanging like this :-)
..note to self: don't let Harman run your company..


Falk

Re: Rohde und Schwarz U23
« Reply #50 on: September 23, 2020, 12:43:43 PM »
Good evening gentleman,

I havent had time during the past weeks to go on with the project. I am also waiting for parts to arrive.
I am not satisfied with the SNR yet, so I redesigned the whole pcb and psu.

I keep you posted as soon as there are news.

@echoplex and @gyraf - Thanks for your support and motivation.

Falk


 

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