SSL2 with REW

[mediatechnology]

WRT to polarity-reversed analog loopback producing similar results the conclusion I might draw is that one of the converters, either ADC or DAC, dominates distortion generation with the other having a less significant contribution.

[Tubetec]

My loopback cable was phase reversed all along , I made another xlr barrel adapter that I can add in to reverse it back to normal and sure enough there is a very small difference .

I wasnt getting very good results from monitor(line out) looped back to input , 20db high spurs popped up out of the noise ,
what I did do was set up REW to send a differential out on Headphone L/R , much much better results , virtually no noise spikes ,
and any noise that was there was reduced another small bit with coherent phase from output to input .

Theres something flawed about using the same interface for both input and output , you always have the possibillity of unwanted interactions within the box via the PSu rails, stray capacitances can cause some erroneous noise floor readings also.

A sepparate A/D and DAC is one obvious way to improve matters , maybe even two PC's dedicated to the job ,
REW beta versions are now including an API , with the pro version liscence you can control two instances of the software , one on the output and one on the target pc .

[mediatechnology]

I recently updated AudioTester and it looks like it now supports, via ASIO, the ability to use two different devices for conversion.

Using the Focusrite 2i2 I see some low level crosstalk between the output and input. I see this primarily as a limitation for testing common mode rejection.

I would like to use the Focusrite as a generator and the TI PCM4222 evaluation board as the A/D. The TI has a full 124 dB DR whereas the Focusrite is about 100 db DR.

There definitely is an argument to be made for using separate devices. If I ever upgrade to a COSMOS A/D it will be a requirement.

The 20 dB spurs that you saw using the line output of the SSL in loopback seems really high. Were they even harmonic?

The Roland Quad Capture has high second harmonic and because of that I don't use it much for test. Its SPDIF input however is very useful to receive the TI PCM4222 output. Before upgrading to a laptop I had to use the Emu 0404 to receive the PCM4222.

[Tubetec]

The noise spurs pop up even in the absence of signal and even with the line out volume reduced all the way ,
the fundamental is at 1.2khz with smaller amplitude higher 2nd.3rd,4th 5th harmonics ,

Using the Headphone out the magnitude of these spikes was very much reduced , balanced or single ended output made very little difference to the noise spikes , but I did observe the distortion figure was more than cut in half in balanced mode .
Theres usefull options in the generator panel in REW , check second output and phase invert , you can easily then change from balanced to single ended by toggling output 2 on and off .

Ive been keeping an eye out for a Sound blaster X G6 DAC as a portable option , they seem to go fairly cheap now ,it gets around 120db S/N , maybe a handful more with a good quality 5v supply .

I found a PFC choke from a threadmill psu , built on an iron transformer core , measures a fraction of an ohm , its also gapped ,very much like an LF choke
I tried it in both the positive and ground wire of the USB 5 volt rails but no matter what i did the USB wouldnt power up , it wasnt due to voltage drop I dont think . so thats a bit of a head scratcher .

[mediatechnology]

My hunch is the 1.2 kHz spike are USB-related.

[Tubetec]

I just wanted to add a short update here as it ended up buried in another thread ,

I didnt like the 4K mode on the SSL2 , it creates a spray of high level harmonics upto 9th ,
I decided to excise the 8065 op amp , the extra distortion is now gone , the unit still passes signal and the low cut element still works as it should .
For me its a very worthwhile mod with minimal risk of damage to the pcb as no soldering is required , it takes only a few moments to cut out the offending part using a razor blade .

Soundwise , its like a fault has been cleared , its less fatiguing on the ear , while still pushing the mids/hi's forward due to the low cut , instantly more usable than before .

Remember , measure twice cut once , make sure your familiar with the circuit before going ahead with the surgery ,
the other thing is the leds in the 4K switches are permanently on as a result of the op amp being removed .

https://www.ifixit.com/Teardown/Solid+S ... own/135354

[mediatechnology]

Send some analog loopback FFTs when you get a chance.
From your description sounds like a veil has been lifted.

[Tubetec]

I cant provide any graphs of the distortion pre mod at this stage , maybe its for the best ,
suffice to say it looked bad , distortion factor jumped from around -100db in non 4K mode to around -30db relative to fundamental or more depending on record levels in 4K .
At lower listening levels it gave the impression of added edge but at higher levels the artifacts became more obvious and more subjectively annoying.

It does have the effect of compressing the peaks and pulling down any transients , but its very obviously distorted .

With the Opamp removed the distortion and noise are identical to the normal mode appart from a minor difference due to the EQ curve .
Basic noise measurement figure pre and post mod dont reveal much of a difference either , but the ear could easily hear poor CMRR causing spurious noise to be injected on 4K mode .

As it is I considder the 4K mode near enough unusable or perhaps only very ocassionally where lo-fing is intended , by contrast with the mod its 100% usable as an additional proximity filter, on any source with any mic, reducing LF content and chances of digital overloads on tape .

I'll do another set of curves very soon ,

[mediatechnology]

I was wondering how the FFT looked with it out of circuit.