Yet Another diy Sum Bus

[alexc]

And the kicker is that a couple of months ago, when I started this build, I thought the preliminary 0.002% THD result was really good  ;D ;D

Since then I've spent a lot more time measuring various discrete and integrated solid state gain stages .... I *was* mostly accustomed to tube amp stage THD  measures.

Now, my opamp test rig, with debalance-balance stages is showing 0.00023% at +4dBu (returning an additional +6dB of drv134 level shift and a further +1.6dB at the drv134 due to 10K-ish loading ) which is basically no added THD over the measurement channel!

Interestingly, I was able to measure an improvement in the debalancing stage with the LME49860 dual-opamp over an NE5534 dual-opamp .. of about 10 pips  ie. down from 0.00033 -> 0.00023. 

Not a heap but still .. significant ... for the extra 3.00usd for a *genuine* us sourced part.

Similar result when I patched a variable gain stage, unbalanced, into the opamp test rig ..  an improvement of around 10 pips over the standard 5534.

...

To go lower than that I need to buy a new Motu audio interface - they have a new one now which has awesome double plus good performance ..  the Motu 624    For the 800usd or so street price, it quotes super performance.

SO, I appear to have found my analog boundaries!

...

AND  for some real giggles, I tested the digital interface on the Motu with a Finaliser Plus into REW.

Kind of gets things into perspective ....

 

[alexc]

Of course, patching in the Finalizer analog I/O brings it all crashing back to earth ...  pretty lacklustre and quite a bit worse than the Motu analog IO THD at a much lower level to boot!

Pretty good hum floor tho' 

Actually it's a little unfair of a test at -20dBFS nominal ...  with higher level, the reported THD drops a little, provided one stays clear of 'onset of clip', either in the measurement channel (Motu) or the device-under-test (Finalizer).

I didn't bother snapping that one - it  would improve maybe 10 pips or so at nominal +4dBu levels.

 

[alexc]

Here's the pics ...

 

[alexc]

and rear ..

 

[alexc]

and inside ..

 

[alexc]

and here is my 'op amp test rig'  - it has 2 channels, each with :

- balanced inputs and outputs
- debalancing module using 2x ne5534 type dual opamps
-  switchable inverter + non-inverting gain stage module using 1x ne5534 dual op amp
- variable attenuator + non-inverting gain module
- balancing module with drv134

and a stereo led meter 

I use it for investigating my system/measurement channel lower boundaries of THD, noise etc as well as for experiments in residuals and so on.

These are all dirt cheap ebay modules and are all pretty high performance. With LME49860 opamps installed, the performance is equal or better than my Motu828 Mk2 audio interface measurement channel.

8)

...

I've wired up the test rig to act as the Lawo module; using this test rig, I have verified that the summing bus arrangements are *not* causing the excessive THD in my Lawo Sum Bus build.

Which is good news! The pcbs, mix resistors  and precision 'build out' resistors I am using on the sum bus are not causing any grief.

The test rig easily gives my THD in the 'triple 0' range, which is as it should be. 

Ergo, the Lawo module  and/or my mods  is the culprit.

...

Now I need to determine if it's the Lawo's :

-  input transformers
- 'virtual earth sum amp'
-  loading effects that I may have inadvertently introduced by my mods to feed headphone amp/direct outs
- limitations of unipolar supply
- issues with my mods to the variable gain arrangements
    ie. mute switch, stereo ALPs pot + a pair of 'trim' pots in the sum amp feedback path


I'm pretty sure the Lawo back end is OK because I have duplicated it with the additional 'Direct Out' module I added and get identical result.  The Lawo back end is a T-filter feeding a non-inverting buffer + output transformers.

--> not the mods to the variable gain arrangement  : replacing the lot with a resistor makes no difference
--> not the unipolar psu : increasing (and decreasing) supply from +24V makes no improvement

Tally ho!

 

[alexc]

Next in line for my summer of diy ...  refurb of a pair of 8" speakers ...  a pair of cheap 8" JBL 'Radiance Series' model xv55.

The best thing to be said is the boxes are pretty decent - heavy, veners and grills in good condition etc.

Very large and solid for an 8" woofer + paper tweeter.

Unfortunately, the sound is pretty poor - the tweeters were horrible and the driver, whilst not bad for a cheap unitm, was pretty lacklustre. Thin stamped , lightweight frame but a decent motor structure and cone - all in great condition.

I think it was an early 90s affair.

SO - OF COURSE, first I change the tweeters to some titanium dome units.

That certainly gave the top end a lift . too much! 

Next, I put in a pair of Peerless Tymphany Woofer 8inch SLS-P830667 drivers - a real nice unit indeed! Much more solid frame structure ..  sort of 'sculpted stamped basket' ... heavy but not so massive as a typical cast  frame  basket.

ANd so, they finally gained an authoritative voice! 

WHile I was there, I changed the crossover 'lytic for a big ass film cap and added a tweeter attenuator network od -2dB.

The result is really good    For an end cost of around 330usd they are now sounding great and still have decent cosmetics.


These guys move up a couple of positions in  my 'speaker league chart'  ...  they're now beating out my customary Vifa 8" driver + silk tweeter boxes which were something like 250usd.

I have a liking for 8" driver in a largish box.

....

Now I need to add a similar -2dB tweeter attenuation network to the Vifas    They are too trebly;  the upgraded Vifa silk dome tweeters are a tad too sensitive.

I prefer to 'print' the 'hf taming'  in the crossover as opposed to eq'ing at the monitor amp. I think it colours the result less.

SO, now I have my eq flat and my speakers well-balanced  - when I turn up the power, they really 'buck' at the bottom end  rather than 'sizzle' at the top end.

....

Next speaker job is a full custom build   boxes, crossover and driver choices  ....

 

[alexc]

Here's a pic of the small sum bus in it's natural habitat ..  my lounge room

All's well that ends well !

 

[alexc]

And the big sum build in my music room ... now a master sum for a yami 03D digital desk and my own neve style sum bus build

It's chilling there at 0.002% THD ... but surely does sound great!  It has improved my neve style sum bus with a much nicer psu rail too. (I made the psu capable of powering itself and the neve unit - big improvement over cheap wall wart!)

 

[alexc]

And a little closer ....