A Headphone Amp Comparator Box

[Freq Band]

This is the same project that is on Rod Elliot's site.....
....but I used a 12v relay rated for lower current (1 amp), and added headphone jacks, instead of speaker terminals. Power is from a 9v or 12v DC wall wart.
http://sound.westhost.com/absw.htm

I'm finishing it tonite, so I'll discover if I can hear a difference between two almost identical amps I built (they use different opamps)...and compare some other amps as well.
Amp Mash-Up.

Outputs will come from my 'puter's soundcard, an RME 9632 hammerfall, with
optional analog daughter board, (4 extra identical in/outs). The output gain
on the amps will be matched by .....well I not sure of specifics yet, I'll post back with those, and tell you my listening results as well.

=FB=

 

[Freq Band]

Build one, I dare you.
You might stop thinking that op-amp choice makes a huge difference....

=FB=

 

[fortney]

I find selecting relays confusing.
Would you indicate what relays you used? And what push button on/off switch?
Thanks.

F

 

[Filburt]

Quote:

Originally Posted by Freq Band /img/forum/go_quote.gif Build one, I dare you. You might stop thinking that op-amp choice makes a huge difference.... =FB=

Do you think that it doesn't?

 

[Freq Band]

Quote:

Originally Posted by Filburt /img/forum/go_quote.gif Do you think that it doesn't?

Yes, it does not make a large difference, based on the tests I conducted.
(No, that was a not double-negative sentance

..this one is. )
I compared three headphone amps, all very similiar circuits....opamp with an IC buffer boosted output. Only the opamps and their associated gain resistors were different. (possibly freq caps as well)
OPA2107, OPA602b, AD823
So far from my testing using this A/B comparator box, I was not able to identify with confidence which amp was being passed through to my headphones.
Two amps at a time were compared.
Volume potentiometers on the amps were adjusted to match each-other based on frequency tones from a test CD.
An rca "y" cable to split the source (and into the A/B box) should work, but my PC soundcard has multiple outputs, so I used those.
Once cabled up, the volume levels adjusted, the pushbutton switch allowed me to toggle back and forth several times...so as to loose track of which amp was the source.
Then I listened, switched, listened...etc.
I did hear what I thought were very subtle differences, hardly worth putting up any argument for "opamp A or opamp B will make the circuit sound notably different" ......or even "a little different".
I just can't anymore, justify spending much money, time, or subjective vigor...in choosing the perfectly suitable opamp that will tweak a circuit into preforming better than with another "suitable" opamp option.
Perhaps thoughtful opamp decisions are to be made based on the electrical suitability, and package availability... seem more important to me now, rather than glorified recommendations found...um...here and there.

Perhaps this is a good thing, in that I can now eliminate "one factor from the factory" and concentrate on other's that may make more of a difference, like layout and grounding......we'll see, as I'll compare those as well, in the future.
I have not tried comparing different circuit topologies.... yet. (been too busy elswhere.) It seems to take an entire evening, or two.
Large difference may be in the choice of circuits, not in the chioce of it's parts......this, I've heard around here before, but now I'll take that into practice, not just to-heart.
=FB=

=FB=

 

[cotdt]

doesn't seem to solve the mismatched volumes problem.

 

[Freq Band]

Which mismatched volumes?
Volume knobs on the amps are adjusted via the test tone cd, measured with a scope or DVM and are carefully marked.
To tell which amp is "on" I unplug one rca cable from one amp, leaving the volume knob alone.
(note, I have now edited my earlier post to undo any confusion, sorry if I was unclear.)


Quote:

Originally Posted by fortney /img/forum/go_quote.gif I find selecting relays confusing. Would you indicate what relays you used? And what push button on/off switch? Thanks. F

A pushbutton switch is used because it will not let you know which amp is being listened to, when switching. Any on/off switch would work, but for example a slide switch might let you know which amp is being monitored, and that is not what this box is really intended for. If you know which amp you are listening to, your eyes will affect your evaluation of sound, yes really.
Example, two different amps are sitting on a table, unplugged......and just by looking at them, and possibly knowing some history behind them, you have already made some decisions about how they should sound.

relay questions, see this thread:
http://sound.westhost.com/phpBB2/viewtopic.php?t=1520



=FB=

 

[chroot]

Excellent work, Freq Band -- if only more people had the skill to make such boxes, 90% of the debates on Head-Fi would be over.

- Warren