Power Amps...

[agedhorse]

Again, this is pretty much the what I'd expect between a pre & power amp: a quasi-linear response bordering on unity gain. Were you anticipating or desiring something different?

Riis

Unity gain? Amp wouldn't get very loud if the gain was in fact unity (Av = 1).

 

[Zooberwerx]

Unity gain? Amp wouldn't get very loud if the gain was in fact unity (Av = 1).

I was typing faster than my brain was dictating. I meant to address "gain structure". Good catch! Other than that, would you say the power amp response is WNL's?

Riis

 

[agedhorse]

WNL? Am I missing something?

 

[Zooberwerx]

WNL? Am I missing something?

Within Normal Limits or as expected.

Riis

 

[RobbieK]

Were you anticipating or desiring something different?

Umm, I'm not really sure where your questions are leading... As I mentioned, the amp cuts it for most gigs, but sometimes, on a deep stage, where I'm 2-3 paces further from the rig, and I turn it up just a little more, it quite quickly sounds compressed and the treble rolls off. Of course that could also be due to my speakers, or also the acoustics of these rooms. Perhaps I'm standing in a weird place where the reflections are out of phase or something. The room I played on saturday night was being renovated, and even though the stage is really big, the room itself was only two-thirds as big as it was last time, with a temporary wall across the back.

Hopefully with a better z match the amp will work more efficiently. I'll just have to do a few more gigs with it I think.

 

[agedhorse]

Within Normal Limits or as expected.

Riis

Ahhh, that's not an abbreviation that I have encountered or use myself.

It might very much depend on the speakers, especially how wide and deep the regions of sub-nominal impedance are. It's also possible that a "repair" or "upgrade" was done using incorrect parts, resulting in a cabinet that is not what the specification indicates. I do see this often enough that it's part of the troubleshooting tree.

 

[RobbieK]

Well the rig was much improved this weekend. Especially last night where it was pushed harder. Still a bit embarrassed I didn't think to check the specs before posting, but fixing the impedence match seems to have done the trick.

Thanks to all who chipped in!

 

[murphy]

There's nothing wrong with your amp, it's a great amp. The problem is with the preamp, which does not adhere to any modern standard for sensitivity and output impedance. In fact, it did not adhere to many of the older power amps as well. This has been a big issue for decades.

This is directly from Alembic's own website:

The signal loss of the passive tone controls and the volume control is made up by the second stage, also a common-cathode vacuum tube stage. The second stage plate circuit feeds the output jack. Since the output impedance of the tube circuit is fairly high, the power amplifier(s) should be located adjacent to the F-2B so that short connecting cords can be used. While the output circuitry of the F-2B can satisfactorily interface with the vast majority of power amplifiers, the F-2B does not work well if the power amplifier input impedance is lower than 10 K Ohms or if the power amplifier has very low sensitivity.

The PLX preamp, IIRC is about 10k ohms (single ended input) and is a lower sensitivity compared with power amps of 3 years ago which were often 500-750mV for rated output compared with the more typical 1.4-2V today. IIRC, there was a mod (maybe a factory mod) for adding a low impedance line driver to the output to get around this issue. I know I have done this a few times for touring clients and it was not difficult for a well equipped service shop. I believe the newer F-1X has the line driver installed from the factory.

The money I would have saved myself had I known this...first blaming my new speaker cabs...then my new PLX ......replaced everything. but wish I could go back and see what it would really sound like with this line driver and my Sadowsky 410 cab