REW Update

[trobbins]

John updated the beta version - may be some benefit, so worth a check of the changes imho.

 

[ruffrecords]

John updated the beta version - may be some benefit, so worth a check of the changes imho.

I just got an email about this. He has added a dBu and dBV scale with calibration to the RTA. Perfick! Also dBA weighting to distortion for the marketing department.

Cheers

Ian

 

[alexc]

Awesome!

 

[ruffrecords]

Awesome!

Yes, I am very pleased. I have sent him a couple of donations. Well worth every penny.

Cheers

Ian

 

[alexc]

The beta REW addresses all my concerns ..  has the 'input overload warning' opt out (thanks!) and a huge number of added functionality.

The dBu handling and calibration capability is top shelf! 

Even more info in the 'heads up displays' of distortion and rms input aw well.

Wow what an upgrade  8)

 

[ruffrecords]

Wow what an upgrade  8)

I agree. It is a great product for audio testing now. Better IMHO than any of the other PC based ones.

Cheers

Ian

 

[alexc]

I did the REW calibrate procedure with my bench MAudio Profire 610  today ...

All went well ..  took a couple of goes to line everything up.

Here's a handy link for dBu conversions .. there may be some out there who don't use this ...

http://www.sengpielaudio.com/calculator-db-volt.htm  Thanks 'Tontechnik-Rechner - sengpielaudio'

I used a CRO to measure the output voltage of my audio interface 'balanced out'  .. 

I measure the xlr  'hot' pin to ground with the cro, an 'unbalanced' measurement for brevity. 

The audio interface balanced line outs have voltage waveforms which are symmetric about 0V and in opposite phase  ..  as one might expect - but always one needs to check. So if measured differentially, there is a +6dB level shift.

Also, there is a small amount of 'loading' that is common when measuring the audio interface output voltage when 'looped back'  (ie. output connected to input) - I do all my voltage measurements that way (rather then not connected/open load)

So - did the loop back with one xlr shell open, measured 'phase-to-ground' off the CRO, in 'peak-peak' volts,  converted to RMS volts (div 2.83) and entered that into the cal dialog.

If all goes well, then that's that  .. almost  .... it took a few goes to get it right ...  no matter. It does work!

There are two other places where the calibration factor can be entered  ...  in the sig_gen 'readback' field  as well as the RTA 'readback field'.  One needs to make sure they both end up correct as you work thru your own cal procedure.

Here's a snap of the first baseline after the cal ...

 

[alexc]

The above snap is the '0 dBFS' loopback scenario ...  maximum output and input levels.

That is a hum floor of -120dBu and a margin of +129.7dB  ...  of course the thd is a little higher at this maximal level.

By the way, it's great that REW lets you work right up to 0dBFS now ..  
.... big plus for those 'maximal levels' measurements ..

and of course, calibration.

......

It turns out that my interface has a 0dBFS = +9.67 dBu. 

This is a little different to my previous measurement of +12.7 to convert -  still looking at that ....  approx 3dB diff in my 'pre-v5.20beta'  calcs!

...

Anyway I triple checked,  and it all lines up as it should ...  in both dBFS and dBu, both sig-gen and signal return as well as my CRO and some conversion math.

Thanks double plus Mr REW.

And just like that, a huge step forward! Direct dBu measuring with no 'fussing or fighting'.

...

Here's a 'nominal +4dBu' scenario ...    THD is well down on the previous 'max level' scenarios as expected.

Hum floor at a touch below -120 dBu with a signal margin of +125 dB at super low THD and very low noise.

 

[alexc]

It's good to note the basic accuracy is fantastic - I get limits of 0.1 dB across the inputs, outputs of the signal and reference channels.

...

The usual 0.5dB 'factor' between the 'rms input' heads-up display and the RTA dB cursor measurements remains ..  this is not an error, but an 'rms vs peak-frequency' kind of thing ...  others can explain it better than I 

It also manifests in the 0.5dB difference between the 'levels window' display for 'out'  level vs  the 'in' and 'ref' levels.

Interestingly, the new 'peak sample dBFS' reading kind of  indicates that 0.5dB 'factor' ..  if you do the math with the dBFS/dBu conversion.

There is also a factor in the 'hum floor' line depending on whether there is a peak signal present, or not ..  I call it a 'hum floor crest factor'  ..  and it can range from 0.5dB down to 0.1dB.

...

It's good to note all these things are self-consistent and very high quality! 

So much so, I'd like to see  the 'dB measurement cursor' that you can 'nudge'    ...  to half the current increment. 

You use the '+' or '-'  key stroke  to 'nudge' the dB scale horizontal measurement cursor and get around 0.2dB increments, as far as I can get.

I wish there was a 'shift +' /'shift -' key strokes combos that would shift it half of that to 0.1dB  (100m dB)

The underlyings already support it (0.1dB); I suppose the RTA window dB cursor will soon catch up.

Today's cal is the first time I've noticed it! 

....

I have to do some rearranging of my windows, however ..  with the improved sig-gen window, the larger size needs to be accommodated so I don't lose my 'levels window' to the overlapping and such.

Never happy, are we  ;D ;D)  But I say that with respect and awe for the updated  power of REW.

Another feature or two and I'll have to get a modern hi-performance  interface ..  and really REW into hyperspace.

 

[alexc]

Nearly have my bench 'tube rig'  reset  for the latest and greatest REW tests. 

Hopefully I'll get some readings on my latest 'tube choices'  soon ... I'm using my 'remote psu' unit for powering.

First up is to check :

-  a couple of pentodes including the noval  e83f in se
-  some moderate powered finals, including  the  octal 7591a  in pp

It's a real luxury to have a bench 'tube test rig' with a remote psu of regulated hv and well filtered dc heaters.

It's like an air-conditioner for the soul  8)  Working now on a 'mk2'  bench  'tube test rig'    remote psu ...

Including an easily variable Vreg, seperately mains-fed heater-transformer and  an external 'varactor' transformer unit  for the HV    ...  to vary my primary ac around  the nominal  240Vac  ... 

I'm quite excited to finally get me one of them rotary transformer units for my bench!

It's getting to that point now, what with the fine-fine  measurement capabilities of 'REW'  for audio DIY testing.

pic ... a bit blurry, but you get the drift.  I'll do a better one shortly, along with the diagram of my 'hv probe'.

 

[alexc]

Here's the signal unit of the 'tube test rig'  ..

It has a basic 10W pp output to voice-coil transformer (deluxe style), an Edcor 10K/600 plate-to line output transformer [on the under-side - they are great performers and cost effective  ..  but could use an aesthetic do-over!]    ....  then a little DI traffo at the front end,  from a cheap tube mic ..

*And* a  small-ish  se  plate-to-voice coil    output traffo - an old telefunken 12K prim  radio unit  .. good for a watt or two  of old time se fun 

There's a bunch of tube sockets for octal, noval, loctal and 7 pin (septal ?)  ....  some are in pairs, for pp and vari-gm blocks , some are singles for class-a blocks etc.

I also have some jacks for input, insert/return, output plus a pair of xlr connectors.

...

Everything one needs to do some entry-level  tweazing of some basic tube modules ..  it's made from materials and parts recycled or left over from other things  ... the alu was a previous attempt (#3) of a little amp head;

I'm just making up some wood frame for it to sit on and similarly for the new bench psu - the timber is from an old futon!

 

[Brian Roth]

I should RTFM.

Does REW have an X/Y mode to do  Lissajous measurements for tape head alignment or monitoring the "phase" when cutting lacquers?

Thanks!

Bri

 

[alexc]

Not that I know of - I don't think it has a 'cro' mode of operation.

 

[alexc]

I have my tube test rig prep'd  for some REW'ing ...

e83f pentode, medium height 9pin, 300mA heater at 6.3V and plate,screen limits of 2.1W and 0.35W respectively - pretty standard spec except for the larger envelope - a small signal pentode.

....

First test is simple - triode mode with 275V supply and 100K resistive plate load driving an easy load, say 1Mohm.

I have some cathode voltage trimming with a pot+resistor, some bypass cap jumper-ing too.

Driving with a variety of means .. including with a debalancing traffo or not ...  and a few different ground schemes till I get the stage working happily.

---

I'm hoping for a good measurement turnout ...  It is Christmas after all.

but realistically, some of the popular modern hi-gain types can be very hard to beat ...

These were around 10usd each in a sleave of 5 - early sixties I would say. [could be wrong - this kind of thing was well sorted in the mid 50s I think]

Pretty standard fare  from them in that day ...  looks to be superb construction inside ...  mesh plate with substantial structures .. I can just picture the workers painstakingly doing their bit to build up the whole.

In the days that I recall, it was always the women who were the best at soldering and wiring ..  let alone micro-brazing and such

If I were a tube, this would be it.

...

I'm kicking off a new round of REW snaps  ...  in dBu  and with my refined 'annotation' 

.. so people with less browsing time can get something useful from them.

Tally ho!

 

[alexc]

and a good pentode pic 

 

[alexc]

I have my first results for  the  small signal pentode  E83F  .. 

I'm running it in triode mode with a B+ of 280V regulated, dc filtered heaters, remote psu 2m away

I have a plate resistor of 13K5 at present and a variable bias resistor.
Also, 470K grid leak resistor, 2K2 grid stopper and 2K2 screen stopper.
Driving an edcor xsm 10K-600 output transformer terminated with 2K2 across the secondary

It's running a plate current of around 6mA and a screen current of around 1.3mA for a total cathode current of 7.3mA -  less than half the rated power or so.

....

SO ...  varying the bias to +4.1V gave me the lowest THD with a nominal 1Vpp input signal, sine 1KHz

THD was very low (no nfb)  at 1.15% nearly all H2.  Hum floor super low -120dBu. Hum margin -119.35dB  note - this was due to excessive LF rolloff - now corrected for -113dBu hum floor

Which is (still) s really good - especially so given I haven't yet got it humming at it's best ....  I need to vary B+ and the plate resistor

The input and output transformers together are adding around 0.1% of thd  ....

....

This small signal pentode (2.1W plate) has a high gm at 9000 mA/V  and it shows in the very low noise. 

Of course, the remote, regulated psu is really important - this one (eby kabota hv reg pcb) is great - easy to vary the Vreg with a trimmer and no issues of any kind so far.  I've standardised on these, so a few more on the way!

If I tried this with an 'amp head' type build with just choke-cap filtering it would be at least 12 db worse hum floor, all the time ..  bottom line - a  case of significantly higher B+ ripple *plus*  likely  em-induced hum into the signal transformers *and* ground lines  ...  from the psu.

And anything less than a 47K plate load resistor - in a similar circuit  in an 'amp head'  -  is asking for hum.

....

Gain is reasonable, even in triode mode with this low-ish plate loading. I'm still tweaking on that, so I'll wait for more data!

I definitely haven't noticed any sensitivity or undue microphonics - rock stable hum floor. 

Even with my odd-wooden-blocks 'wobbly' test rig  But the wiring is solid ..  I mean, the psu is 'way away' !

I haven't even optimised grounds or anything yet, and there's a few things to route better - but this is mostly affecting the 'mids'.

ed.  (so far! - I have to check I'm not 'band -limiting'  the bottom end with that quite small coupling cap  ..  till tomorrow, it's a possibility

Update : sure enough, my low freq extension *was* being curtailed by too small of a coupling cap from the plate to the edcor xsm 10K-600.

I checked the freq resp of a range of values ...  I'm settled on 4.7uF ...  a big wima

I now get 20Hz at -2dB or so and 22KHz is less than -1dB - so the response is now good!

My noise floor is now -113.4dBu with +28.3dB gain ...  (worsened 6dB from previous low-rolled-off 'prelim'  measurements)

....

Early indications are showing this tube performs better than anything yet I've tested, and by a *modest* margin.

Margin over hum is +116dB - previous best (JJ 5751) was 105dB margin with similar distortion  ...  in an 'amp head' type build

[discounting the 12 dB of worsened hum margin with 'amp head' vs 'remote psu'  .. brings things (re.  prev best)  to within  1dB of today's result - very nice --  but until I do the 'head off' testing between the 5751/6922 historical standouts and newcomers, who knows wth ? ]

When I push it to 3% THD, the hum margin rises to  122dB.    Thats for a 2Vpp input, x26 gain, 52Vpp at the plate.

It all amounts to  'less sensitive' than a 5751 stage ..  with super low noise  ..  sparse harm.dist. spectra, albeit with slightly increased thd  ...  with 'that  diff' made up of  ..  increased h2  .. arguably a good thing  8) 

To the tune of only 1dB and maybe 0.3% thd  diffs  at the end of the day, over the prev. personal best  ..

Still it is significant, especially given my 'prototyping' build qualities   ...  and in it's way, rewarding the right. efforts

....

As to the audible diffs, not so much, really - although I haven't yet 'listened'  ...    that's another thing altogether.

Ask someone with good hearing!

...

But all very interesting to figure, no doubt.  If this world truly is a 'sim' wth does it mean to chase 'tube stuff' seven decades after it's heyday ? Who knows.

....

This tube shines as an single-ended  output tube, no doubt ..  I suspected as such, given the obvious construction quality and the high gm.

And it performs pretty well as a first-stage tube too!  Plus anything in between, I would say.

The onset of distortion should be gradual ...  this tube likes to be biased somewhat cool  ...  +4.1V bias for 1Vpp input signal showed a thd minima. 

A good thing too (cooler bias) .. this tube likes to run a decent amount of current (>10mA) given half a chance.

The challenge is to reduce the currents whilst increasing gain without losing the thd or it's spectra ..  along the way.

.. it's a 5-way tweaze where the hours fall away as you try to find your current 'best'. (and take a snapshot!)

So I'll spend some time tweaking in triode mode for max gain, min current  etc  ....  and after all that ...  with a really high quality output transformer. Much better a one , than I should  hope for.

Then continue with some pentode tests - I think they will be very good results.

Finally, I plan to try the 'enhanced triode'  or 'screen drive'  mode ....  where the grid g1 is grounded and signal is applied to grid g2. This tube has a reasonably high gain[g2g1] which is a parameter of interest for doing this.

 

[alexc]

And the practical application of all this is *of course*  tube limiters

Having an extra 12 dB of hum-free-ness  'in hand'  to 'spend'  on even more excessive gain-reduction, is ..  well, it is.

After I get thru the characterisation of this nice tube, I'll be looking at the 'remote cutoff' version  ... the 6eh7 in typical diff. stage with varying bias-es. It's a very interesting one, for stupid high starting gm with a truly panoramic slope to the deck.

I've done a few vari-gm tube stages now and I've recycled a few too!  I'm looking for this next one to really shine. 

This 'tube test rig' is just the thing to grind on tube functional blocks ... some good results also motivates to chase the next level of esoterica.

Like cathode follower vari-gm stages, grounded grids, constant-current-sources, active loads *and* aikido cancellations!

 

[alexc]

More grinding on the E83F pentode in 'triode mode' ...  with a 4u7 cap to edcor 10K-600

Increasing plate load resistor to 47K is good ..  gain increases 1 dB or so, bias  down from the coolest 4.0  ..  thru 3.5  and to this +2.5 V  ..  for a nominal 1Vpp signal to grid.

THD is now at 1.53% nearly all h2  - the increase (around 0.4%)  is due to the now-properly-extended low end.

Margin over hum is 116dB rising to 120dB at 3% thd

Increasing  B+ supply voltage doesn't change much the figures ...  decreasing it increases THD.

Increasing beyond 47K isn't doing much good ...  increased gain is offset by maintaining bias voltage.

So it looks like this is the best I can do right now - about right for a 'medium mu tube'  ...  in this operating scenario    mu=49 approx  (gain x26)

E83F in triode mode still wins 1st Place for margin-over-hum at 'low THD'      - even though the hum floor is 'only' -113.4dBu and the THD is 'moderate' at 1.5% mostly h2.    (x26 gain)

Low freq extension is -2dB at 20Hz and the high is better at -1dB at 22KHz  - a good baseline turnout.

...

I think 'ideal' for this config  is -120dBu hum floor, thd 1% h2 and 125dB margin over hum with +/- 1dB 20Hz .. 22KHz.

I doubt I'll ever get there, for a simple grounded cathode gain stage  'gca'  -  but I'll keep on trying.  A l.o.l (labour of love)

It wasn't that long ago that I would be capering with joy at the figures I've just measured.

Such is the impact of REW Audio Analysis  wizards' tools.

The maxim 'measure, then measure, then do it again'  does apply ...  to rein in the early results' exuberance envelope.

Then wait another day .. and 'do it all over again, this time flip-flopped'    ...  (and with tracability!)

...

Whilst I still have a few tricks to try here, my next  'new exotic choice' would be the C3g tube ... it's kind of grabs attention in the sims sims. But it's crazy exe (pensive) ..  so pass. 

I think I'll try a few others instead ...  6EJ7 grabs me (very similar to the e83f), as does 12BZ7 (of citation fame), EF80  (popular in german stuff)  ..  and of course, the venerable  usa great  ...  the  6AU6.

And I'll be comparing pentode performance of this e83f with my more usual choices ...  6AK5 (great for mics), 6BC5 (unglam little tv pentode)   

and  ..  the  humble  6J13P (russkie ef86 type), my previous best small  signal  pentode  ..  which is now  teetering precariously from it's exalted and high 1st place ....  but not until I do some proper tests.



....

To finish up testing on this mode of operation, I'll check my batch of 8x  E83F  to get a 'feel'  for the 'spread' of key performance indices

(kpis - used to do this alot back in my Quality Assurance Engineering days in biotech!)

....

Now for some wire - jagging  ...  for sensitivity checks, particularly on the input/output wiring and routing/locations. This has always been fertile ground for the grinding.

In this case, however, I don't expect any improvements - they're mostly 'tapped out' and 'simplified away'  .. in my 'Tube Test Rig'.

'Remoting, regulating and dc filtering heaters'  psu neatly eliminates most of the usual dross. 

Now it's mostly about  'input signal - output signal couplings' and  'short signal paths'  where some gains can be made.

It's certainly a novel (for me)  and a very pleasant place to be.  I'm sure this is,  I want,  going forward!

I would expect a righteous layout could do a little better ... maybe a dB or three for a real wiz.

 

[alexc]

And here's the 3% spectra ...  quite nice.

 

[alexc]

Tested up all 8x e83f ...  these are  all holland made branded amperex (x5) and sylvania (x3).

spread of  thd was tight ...  the one I was using turns out to be the worst of the 8, at 1.5%

I get a range of 1.17% .. 1.29% with two out-riders at 1.45% and 1.5%

Gain, spectra and hum floor are all virtually identical.

So, I have a close matched set of 6  with really low thd  ...  and a matched pair with a bit more.

I sound a bit like tube 'flanders' I think!  (glass half full)

I'll continue testing with the 1.29% tube  ...  a nice little reduction in thd ..  for a little modest judicious 'tube rollin' selectionism.

My new baseline  1.29% h2 with 116dB of margin over hum.

...

Right then ..  now to reconfigure for 'enhanced triode' mode  

Also described as 'screen drive'  ..  and with some sand-state support,  the term  'crazy drive' has been used by some

I'll be playing around with this kind of circuit ..  thanks kindly to 'Black Tom' for this interesting implementation!