Measurement Software - 'Room EQ Wizard'

[alexc]

Now, it would be a waste to have just one signal routing on an esoteric amp such as this ...

The 'Balanced Direct to Driver - Edcor 600/15K traffo debalancing' routing option (top panel TRS and switch)  is the most hi def.

That gives me a hum floor 10dB better - comes in at -94dBu  with the finals at full rated power - which is the best I've achieved on this platform.

The signal-hum margin comes in at >95dB. THD at around 0.6% os so at the grids of the finals, nearly all H2.  At the speakers, it comes out as  <1% THD for 'near full rated power'  8)

........

Next up is the 'Instrument Direct' - (front panel TS with top panel switch)  - which bypasses the EQ+makeup + v.e.m stages.

Comes in a -90dBu hum floor and again > 95dB signal to hum margin. THD around 1%.

..........

Followed by the 'Aux 1, Aux2, Instrument Mix' (front panel TS with top panel switch) - starts at -84dBu, rising with the level of any of the 3 inputs.

Typical usage results in hum floor around -80dBu with 3 sources mixed. Still very low <1.5% THD.

Of course, with both the Aux's going strongly, and the Instrument+EQ preamp banging away, together like, ..  the noise floor creeps up and up ..    but so does the signal, of course, so you turns it down some .. and so on. Until you don't anymore.

.....

So - pretty good performance considering all that's going on in here!  I'm happy to leave that all as it is for this iteration.

I can use for instruments, aux effects and/or and a mono 'tape feed'. Flexible without sacrificing the higher performance 'direct options'. Sounds great with some drums mixing in - all of the 'single instrument tracks' from Cubase sessions sound jim dandy thru this amp ..  so, in a way, it's a 'space-saver' right ?

One amp, a notebook and an audio interface. Good to go!

 

[alexc]

NOW - for the last part before buttoning up - the 'transformer balanced output'. 

The speaker terms are generally around 3 .. 10Vrms in usage.

I have the 6u8a as tube position #4 for this - last time I used a white-cathode-follower which was nice but ideally I need a little gain plus a 'level' control would be nice too.

So - imma knock up a couple of options, including a 'source' switch - selecting from speaker terms  or  6SN7 as the source for the traffo balanced line amp. Kind of throw some plaster and see what sticks.

I need the noise floor to be similar to the speaker terms .. around 82dBu or so  in the worst case.

.....

Of course, everything is more difficult in a tube mixer - this 'pa style' instrument amp with 'routing options'  and 'traffo  bal line amp'  has been a real expertise extender.

It is a lead-in to one of my next builds - a tube mixer based on the  '4chn One-Bottle  pcb'  - from a really thorough GroupDIY guy. (thanks again!)

I will be doing the line amps in haufe 5532-traffo balancing modules, adding a pair of mix busses from same modules

External PSU,  regulated and clean like. 

This and subsequent builds like it,  I hope will be like going to be like a distant, noise free planet!

We shall see.

4x mic/line/DI inputs, 4 direct outs, 2x mix outs;  traffo+tube front end,  chip+traffo back end  - what could go wrong??

In the words of The Small Faces  - ' it's all too beautiful' 

....

Lets see how I go with this last stage - I'm getting weary of this build!

Here's the current inside shot with all the switches and jacks and stuff - one one little socket left,  ..  free and defiant .. surrounded by a sea of widgets, grudgingly brought to heel 

NOT for long!

 

[alexc]

and the front end    ...  soon I'll print off the labels ...  but ..  not  .. just .. yet  .

 

[alexc]

Oh yes - my 'pre Room EQ Wizard' worst case acceptable  noise floor was around -78dBu, as the crow flies.

I think it's fair to say that REW has lifted my game pretty significantly.

About 5dB minimum, but really more like 12dB ->  a combination of quieter/louder/less THD , all at the same time.

I'm seeing figures really suggesting the next level of performance,  if built pared-back VTV style, could be seriously crazy 

Of course, one needs to be able to substantiate such measures - so cross checking pc based audio-analysis applications with CROs, DVMs, matching transformers, attenuation networks, 'noise free chip amps'  and the like, is essential and never ending!

I'm confident my figures represent reality reasonably well and I do keep good records as I go ...

As soon as this unit is done, I will put together a new probe/atten box and try to get to the 'next level' of measurement  confidence as well as quantifying (hopefully) my present measurement inaccuracies and such.

I'm also thinking of a new interface box - my Motu 828MKII was old before I got it ..  and it's even older now!

Once I understand a bit better my sources of inaccuracy with the time honoured tools at hand,  that is.

 

[kambo]

some great build
5751 vs 6sn7 : which one was giving you more 2nd harmonics, and less 3rd...
i have the impression of 5751 is, but wanted to make sure... if 6sn7;s are good too, i have bunch of them to mess with!
cheers

 

[alexc]

Thanks!

6SN7 is the king of medium-low mu tubes, imho.

It has very low THD, < 1% is pretty easy to achieve, nearly all H2. Also, the larger socket size makes low-noise building somewhat easier than smaller socket formats - seperation of components/wiring is your friend!

Also, it is the only tube Ive found where nos can be significantly better than current production.

The 5751 is a different beast, of course, being a high mu - the current production type I have tried is very hard to beat. 

Low noise most definately, and the THD can be made most all H2 whilst still providing >95 signal-hum margin.


Both are capable of great performance - THD profile is largely a question of what amplitude of an input signal you want to play with. The higher the input signal, the more THD you can expect and more H3+ harmonics you will find.

The transition from nearly all H2 to significant H3+ can be smooth, like the 6SN7 and 5751 or a bit more abrupt, like the AY7.

Me personally, I find nirvana in the 6SL7 - 6SN7 combo for the octals, and the 5751/AY7 and 6DJ8/6922 in novals.

 

[kambo]

thanks!

 

[kambo]

hey alexc,
many thanks for the 6SN7 tip... i just slapped a 2 stage 6sn7 in to SRPP,
OMG, sound amazing  ;D ;D ;D ;D

edit: here is the awesome 2nd and 3rd harmonics

not to worry about the hum on graph, cables are all over place and super microphonic tubes...
when i connect a mic, it dips anyway! edit : this is more than 70db of gain

edit 2 : max 67db of gain : actually i just made it little over 70db

 

[alexc]

Yep - the 6SN7 is a nice tube, alright. In my amp, I have a 'direct mode' into the 6SN7 driver for the single ended 6550s finals.

With a step up traffo for debalancing from my Motu, I can get upwards of 95dB signal-hum margin and still have nearly all H2.

The sound is scary realistic. In the future, I will be doing a 'minimal' 6SN7 type build with a 'plate choke' in lieu of a plate resistor and a nice 1:6 step up traffo at the input.

Pretty sure that will be as good as it gets, preamp-wise.  I'll use regulated psu and single-ended output traffos, probably 'Powered by CJ' with his '6N1P DI' model.

I plan to match that with a discrete power amp using Douglas Self's 'blameless' pcbs and some diy biamping speakers 
BUT I have to begin recycling my earlier units for parts  - 'what goes around comes around'.

Too many builds!

 

[alexc]

That's one of the positives with quantitative thd profiling .... gives you the motivation to put in some more *informed * effort and get to the next level of audio performance.

Only made possible, for *ahem* low cost, in the diy community quite recently as these things go. Certainly as far as top quality capabilities go, it's only been in the last few years that I've had access to thd measures.
.....

Room EQ Wizard really works well - enough so that I have to upgrade my interface hardware and probe-test interface hardware 

The responsiveness is all there, the functionality .. functions .. and it is perfectly happy with asio hardware, and at high sample rates.

I've only found one or two things I consider need 'fixing'  - that's huge, in my experience with such things!

So, with some good test tools, should be good to 'see' into the >50Khz range. 

I'll know when I get there

 

[alexc]

Last stage went quite well, after several tries, that is  ..

I ended up once again with a push-pull cathode follower stage as the bigSEamp traffo balanced line amp stage. 

Line amp output level is set by the main amp's speaker terminals level, and a +6dB switch which simply switches the main/CT primary winding of the edcor xsm10K/600 traffo.

Works well, and is simple. There  is very low hum floor, very low intrinsic distortion and it faithfully captures the speaker terminal signal without loading it down too much by virtue of the very high input impedance.

The CF I variant I chose is from the 'DOAC Limiter by Analag and Silent Arts',    ......  done on tube position 4, a nos GE 6U8A pentode/triode 9pin.

It's an interesting circuit : ac-coupled pentode cathode follower stage with a triode active load.

The lower triode samples the psu noise and does some cancellation.  Similar to configurations from John Broskie with his 'aikido' offerings.

Although I haven't 'dialled in' the noise null (some fatigue setting in!), I can attest to the performance, noise wise.

I tried a few different things - unfortunately I tried the nos rca 6AU8 - similar to the above mentioned ge tube, or so I thought. Except that it includes, no extra cost, 20 to 25dB 50Hz hum! I sh*t you not. Sample size of 2, both NFG with respect to hum. ac heater noise at 50Hz.

So, I returned to my previous fav, the ge 6u8a  tube and bingo - all is well! 

...

Here's the spectra, first for the main speaker outputs, set up for 9Wrms into 8ohms ..    which is close to maximum lowTHD triode mode for 150mA SE output traffo.

 

[alexc]

And here is the cathode follower output of the above.

The distortion is a little higher at the CF output, could be the bias values I chose, or the xsm traffo or the tube itself.

But it's pretty good.

 

[alexc]

So - that's it for the bigSEamp   Buttoned her up and returned to service.

Even mananged to finish dampening the interior of the big 15" speaker box (open back) with some heavy acoustic speaker box insulation matting.

That made quite a difference to the speaker box low end capability - more solid than the previously resonance prone pine box.

 

[alexc]

All that's left to do is make my new passive probe.

So imma do that now. 

I have a cro probe, some bnc and xlr chassis connectors, an aluminium box and a rotary switch.

Inside is a 600V 4uF cap in series with the cro probe, a 3M3 resistor shunting that to ground then an attenuation network with a couple of divide ratios on the rotary and a 10K -ish load across the hot and cold terminals of an XLR output.

Some seperate ground terminals for the 'circuit under test' and the cro and a 'lift' kind of thing for convenience.

Also an XLR inpout connector for the output of my 'load box', a 100W dummy 4ohm/8ohm dummy.


That should make things tidier for future thd work 

 

[trobbins]

Stray capacitance to ground from that large cap?

 

[alexc]

There's a few large caps! 

Which do you mean?  The big ass can caps, or the large-ish resevoir caps or the decently large CF output cap?

 

[kambo]

got some 5751s today, they are nice  8)

 

[alexc]

Yep - I have several spare ax7s now destined for psu regulator duties, since I have replaced them with the jj 5751.

I couldn't quite get them to out-perform the eh ay7, but it was close. The smooth transition to higher thd orders sealed the deal for me.

I kept the ehay7 in the 'aux mix' stage because of it's ability to take hot levels at quite low THD - good where one is less likely to overload the grids.

....

My next instrument amp, a 12W push pull octal affair - 6sn7/7591a power amp and for preamp-age,  an rca special red metal envelope  pentode.

The line up is rounded off with  a 6sl7 for mix/eq makeup duties. Line amp will likely be a 6u8a again, but with a 6dj8 to provide some gain for a level control and feedback.

Sort of a small, push pull version of the big fella above and with regulated screens.

....

I've grown well contented with the quietness of my amp optimisations, and the proper thd profiles I now have, thanks to REW 

I think it is going to be difficult to top the previous ones, but that's the 'name of the game' after all - pushing one's own boudaries!

 

[kambo]

just received my NOS 1950's Sylvania 6sn7 pair
holy_grail, they are smooth!
i had 1960's RCA's but these 1950's Sylvania's are really something_something!
i probably should have kept it quiet , as the price is keep climbing!
gonna order another pair for the other mic_pre  ;D

 

[alexc]

They do indeed rock measurably better

I have my 50s chrome tops as driver in a single ended 6L6 stereo amp, which does around 4Wrms at low thd.

I'm only using one triode of the pair, so its a bit of a waste - the sylvanias have super close matching within the same envelope .. as well.

I definately hear more detail over the ri tungsol and can discern a different 'timbre' than the rcas.

The smallSE amp is good for that sort of thing - no feedback, one stage only before the final - the chrome top. 

Tube variations are not 'minimised' as they are in amps with more stages, local and/or global negative feedback.

Next thing to do is to add plate choke instead of resistor for it. I figure the smallSEamp is the best chance to actually hear if there is much difference!

Looking for a reasonably priced good quality set  .. 

In the meanwhile, I'm taking a break from the testing while I finish up the first of my 'modular' psu units and kick off the second.

That should allow some real tube fun downstream. Off board psu for the next phase of stuff.

Can't wait to get some measurements of my newest 'throw togethers'  There'll be preamps, eq, limiting, summing and power amping  plus  vintage driver , bi-amped monitors  ;D ;D 

I'm putting together an all-tube, all-diy 'room' 

Looking to beat the measurements off my recent benchmarks -  I'm confident I'll get some really good results  8)