Reference ABX hardware design and implementation

[mcdn]

Simply use two relays, on used to switch, one dummy to simulate switching. X switching would always switch a relay, the used one or the dummy one.

Yep, that's a good idea. Actually removing the audible cue takes more though as a dummy relay, while masking the switching-or-not of the other 4 relays isn't a true substitute. Four dummies might be needed, but at $30/ea that's expensive. I'd prefer controlled muting and randomised switching?

 

[ppierre]

If source switching isn't masked by muting both sources. You can introduce a White channel (W) switching always passing briefly by him :
A -> W -> A
A -> W -> B
...
X -> W -> X

 

[mcdn]

If source switching isn't masked by muting both sources. You can introduce a White channel (W) switching always passing briefly by him :
A -> W -> A
A -> W -> B
...
X -> W -> X

Even the different position of relays within an enclosure can make the switching sounds distinguishable. Our ears & brains are really good at developing on-the-fly filters for cues. But keep the ideas coming! Solid state switches have their own problems but are at least inaudible. Can we reduce the audibility of mechanical switches enough given a heavy box and stuffing? Open questions all round...

 

[somebodyelse]

Mechanical relays do introduce some risk of audible cues. Proposals to mitigate this are welcome!

Wouldn't solid state switching be better? We've a thread covering distortion from speaker protection relays, and the lower measured distortion from solid state alternatives.

 

[mcdn]

Wouldn't solid state switching be better? We've a thread covering distortion from speaker protection relays, and the lower measured distortion from solid state alternatives.

It would! The proposed relays are big and expensive ($30 each) and we need 6 of them. The design parameters are 20A and 80V with 0.1ohm resistance, to cover both low impedance situations like subs and low impedance speakers, and also high output high-impedance speakers like big horns. I'm happy to relax the voltage and/or current requirements based on community feedback, but I think 0.1ohms on-resistance is non-negotiable.

 

[pjug]

It's single ended for the obvious cost reasons, and split between the main line level board and a separate power switching board.

Not to disagree with choices but if cost is a concern would it be better to make the best line level comparator that you can do and not worry about speaker level comparisons for this?

 

[mcdn]

Not to disagree with choices but if cost is a concern would it be better to make the best line level comparator that you can do and not worry about speaker level comparisons for this?

Yes, that's why it's split into two boards. The speaker level/power amp board contains only the relays, associated flyback diodes and coils driver transistors, and an isolated 12V supply. All control is done from the line board.

 

[pjug]

Yes, that's why it's split into two boards. The speaker level/power amp board contains only the relays, associated flyback diodes and coils driver transistors, and an isolated 12V supply. All control is done from the line board.

Sorry if I missed it, but does that allow you to make only the line-level design first if you use semiconductor switching on the line level board?

 

[mcdn]

Sorry if I missed it, but does that allow you to make only the line-level design first if you use semiconductor switching on the line level board?

Yes that's the plan. Solve for line level accuracy and satisfactory ABX protocol support first. Worry about power amp/speaker switching later.

 

[pjug]

You may consider something like AQZ202 as a solid state, silent alternative.

For line level are analog switches/multiplexers an option instead of higher current switches?

edit for the sake of consolidating my (maybe not helpful) suggestions. But another one:
In the other thread it was suggested to just use a preamplifier for matching. How much easier is the project if you offload the task of matching to a preamp and make this only about adding the ABX functionality?

 

[ATTI]

The opamp itself is almost the least distorting or noise generating part of this circuit. Resistors and switches (and their associated capacitance), add more noise and distortion than the OPA1612

If we assume different (or any) op amp has its own writing, own little coloration, not seen at measurements or differs in levels far below hearable treshold, we need to leave untouchable signal path without reproducing or "rewriting" by even so good OPA1612. Isn't that the main purpose of such ABX hardware to give an opportunity somebody to prove hearing abilities to distinguish audio things?
People say they hear how mute swithes sounds, so no any semiconductors or any extra parts. The same good quality connectors and good quality relays. Seems it is one way for some agreement between the two camps on testing conditions.

 

[pma]

If we assume different (or any) op amp has its own writing, own little coloration, not seen at measurements

Based on such and similar assumptions, you will doubt any kind of AB test box and probably do not need to follow the thread. Let the guys to do the job. Such assumptions are contra-productive.

 

[voodooless]

As the main reason for having active electronics in the signal path is level matching, I would still stand by my initial proposal to do this in the digital domain. If DeltaWave is accepted for doing this, then other digital means should also be a valid option.

 

[ElJaimito]

I don't want to rain on anyone's project, but isn't this reinventing the wheel?

Inter alia:

Project ABX

ESP Project Pages - Use this ABX teter to find out if there is *really* a difference between amps or preamps

sound-au.com

This design has NO active components in the signal path.

 

[pma]

Please do not suggest primitive solutions, or the thread will dissolve in nonsense.

 

[ElJaimito]

Please do not suggest primitive solutions, or the thread will dissolve in nonsense.

What is primitive about it? It's a perfect A|B|X box? Honestly, just asking. It works? Adding any active components will lead to challenges to the results, IMHO.

 

[ATTI]

Let the guys to do the job. Such assumptions are contra-productive.

I'm understand it, but how to deal both camps? Otherwise it will turn out to be a device only for objectivists, who don't really need it.

 

[pjug]

I don't want to rain on anyone's project, but isn't this reinventing the wheel?

Inter alia:

Project ABX

ESP Project Pages - Use this ABX teter to find out if there is *really* a difference between amps or preamps

sound-au.com

This design has NO active components in the signal path.

I didn't look at that carefully, but he says an assistant is required.

 

[pma]

Two 10k pots + relay switches - sorry. This is for absolute beginners and not a serious project as intended here.

 

[mcdn]

As the main reason for having active electronics in the signal path is level matching, I would still stand by my initial proposal to do this in the digital domain. If DeltaWave is accepted for doing this, then other digital means should also be a valid option.

Yes and no. Even an a-grade cosmos ADC gets best case -127dB in stereo, has a very low
input impedance, and needs a lot of care and feeding to realise it, with battery power supplies etc. I agree with the principle, but in practice it might just be too hard to standardise. The analog approach is much more plug and play and repeatable. And whatever you or I may think, a lot of people just wouldn’t accept a device for comparing DACs that starts by digitising the signal.