Power Amps...

[Downunderwonder]

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 colourscheme in the last paragraph doesn't work in the Dark Universe.

''

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."

 

[agedhorse]

the colourscheme in the last paragraph doesn't work in the Dark Universe.

What part doesn't work? What's the "dark universe"?

 

[murphy]

Any way....all my amps sound phenomenal now...D-800 ...or Demeter VTBP201 driving my Magellans power amp or my Yorkville AP1200
Very happy

 

[Downunderwonder]

What part doesn't work? What's the "dark universe"?

The forum display mode which shows a black screen with white text and greyscale doodads renders the quoted section a very dark shade of grey on a near identical shade of grey backing.

 

[RobbieK]

Thanks heaps for all this info!

which PLX are you using?

dip's

It's a PLX 1804. No longer made. Has no dip switches that I know of. None that are external. My cabs are all 8ohm, and I've never run either side lower than 4ohms.

Carvin may not be available to him.

Yeah, here in Aus, Carvin is relatively unknown. But there are a couple of distributors listed on the Carvin site. None in my city though...

Alembic F2b

The input sensitivity of the PLX series requires more voltage from a preamp than many other power amps.

Amp's sensitivity controls should be all the way up f

PLX preamp, IIRC is about 10k ohms

These are all good points. If memory serves, my valve pre has a cathode follower at its output, but I still think 10K would be too low. You are right, the qsc has only a 10k input z. I think this may be at the heart of the problem. (The Aussie Monitor had a 25K input z.)

My solid state preamp of choice is custom made, and it has an opamp at its output, but I just checked, and the master vol is after this, and typically I run it at 12 o'clock, so this would greatly increase its output z. I will lower the pot value from 10K to 5K, and also try it this weekend at a higher setting, even maxed, with less gain at the front. If this fails, I'll buffer the vol control with another opamp.

I run the input on the qsc at full - always have. Voltage from the pre is not a problem, all knobs at 12 o'clock and the rig is pretty loud, and sounds great. It's only when I push it further than this, that it starts to sound compressed and weird. As I sing, on big stages I'm a couple of metres from the rig, and that's when I have to crank it a little more, especially with certain drummers we work with.

Yeah, I'll play around with impedence matching and report back. I'm in a pretty big room again this saturday...

If I'm still struggling, I'll definitely check out some other amps as suggested, or go for another QSC. It's been fine otherwise.

Huge thanks to all!

 

[agedhorse]

The forum display mode which shows a black screen with white text and greyscale doodads renders the quoted section a very dark shade of grey on a near identical shade of grey backing.

I wasn't aware of this, does underlining work better?

 

[agedhorse]

Thanks heaps for all this info!





It's a PLX 1804. No longer made. Has no dip switches that I know of. None that are external. My cabs are all 8ohm, and I've never run either side lower than 4ohms.


These are all good points. If memory serves, my valve pre has a cathode follower at its output, but I still think 10K would be too low. You are right, the qsc has only a 10k input z. I think this may be at the heart of the problem. (The Aussie Monitor had a 25K input z.)

My solid state preamp of choice is custom made, and it has an opamp at its output, but I just checked, and the master vol is after this, and typically I run it at 12 o'clock, so this would greatly increase its output z. I will lower the pot value from 10K to 5K, and also try it this weekend at a higher setting, even maxed, with less gain at the front. If this fails, I'll buffer the vol control with another opamp.

I run the input on the qsc at full - always have. Voltage from the pre is not a problem, all knobs at 12 o'clock and the rig is pretty loud, and sounds great. It's only when I push it further than this, that it starts to sound compressed and weird. As I sing, on big stages I'm a couple of metres from the rig, and that's when I have to crank it a little more, especially with certain drummers we work with.

Yeah, I'll play around with impedence matching and report back. I'm in a pretty big room again this saturday...

If I'm still struggling, I'll definitely check out some other amps as suggested, or go for another QSC. It's been fine otherwise.

Huge thanks to all!

If your tube pre is a copy or was "inspired by" the Alembic, it's likely that there is just a common cathode stage driving a high value pot hanging right on it's output. That's the "kiss of death" trying to drive 10k.

A 10k pot driven by an op amp won't be bad at 12:00 position, the output impedance would only be the high side of the voltage divider into the amp and if the pot is 10k, that would be roughly 5k source driving a 10k load and while you will lose some level it won't be anything like 50k source driving a 10k load with the 50k source also being frequency and distortion variable.

I would learn more about your tube pre's output section and maybe there is an easier solution.

 

[Downunderwonder]

I wasn't aware of this, does underlining work better?

Nope.

It doesn't happen often. I think it must be a dark coloured original text on top of a misplaced quote box. Instead of flipping near black to white it leaves it in its original colour. Normal black text is lacking a colour attribute and the Dark Universe converts it to an appropriate white or grey.

 

[agedhorse]

Nope.

It doesn't happen often. I think it must be a dark coloured original text on top of a misplaced quote box. Instead of flipping near black to white it leaves it in its original colour. Normal black text is lacking a colour attribute and the Dark Universe converts it to an appropriate white or grey.

I just underlined normal black text, sure glad products I design don't have a mind oft heir own and randomly re-invent the way the world sees them. That would drive me mad.

 

[Zooberwerx]

The 1804 is kind of a strange agent: can't be bridged and you have to use an external 1/4" > 1/4" or XLR > XLR jumper cable for dual-mono operation (the 1/4 & XLR inputs are wired in parallel). If given the choice, I'd run a 1/4" cable between the preamp and 1804 and use an XLR > XLR jumper for dual mono...the XLR / DI outputs found on many preamps are not always line level and intended for sending a signal to the board. 1/4" output is your safest bet. Here's the manual:

Invalid Link Removed

I'm scratching my head with regard to the preamps as you've had similar experiences with 3 different units. Quick question: can you get any of the front panel LED's to light up when fed a signal?



Riis

 

[RobbieK]

A 10k pot driven by an op amp won't be bad at 12:00 position, the output impedance would only be the high side of the voltage divider into the amp and if the pot is 10k, that would be roughly 5k source driving a 10k load

Not a log/audio taper though, more like 8K for a typical "A" taper. The manual says the signal source impedence should be less than 600 ohms. So, yeah it's time for a post-volume-pot opamp voltage follower I think!

If given the choice, I'd run a 1/4" cable between the preamp and 1804 and use an XLR > XLR jumper for dual mono.

With respect, I think that would mean the pre has to essentially now drive a 5K Z. (Happy to be shot down on this if I've missunderstood...)

I see from the manual, that the xlr inputs have a 20K Z, but that's only for a balanced signal. My pre only has an unbalanced output. I've also noticed the manual recommends earthing the spare (inverting) pin if you use the xlrs with an unbalanced signal. I've been using the phonos, but was planning to make a patch lead for the xlr ins.

I probably should have read the manual in the first place...

 

[murphy]

I just underlined normal black text, sure glad products I design don't have a mind oft heir own and randomly re-invent the way the world sees them. That would drive me mad.

worked fine for me also...I am on my laptop...not my phone

back to PLX.....I was not happy with my ART tube EQ preamp driving it also...although memory is starting to fade.....but just remember my PLX never sounded as good as my led sled amps

 

[RobbieK]

Well I spent the evening installing a simple opamp buffer after the volume pot. I also installed a locking output jack and made a new patch lead so I could use the plx's xlr jack. (I have had a normal jack-to-jack patch lead work loose on a gig, and it's not easy to get my hand in behind there.)

It was too late to try her out at any decent volume so I'll test it tomorrow. Then properly when I'm gigging on the weekend.

Still kicking myself for not realising the amp had a really low input z. It's even written on the back, next to the input jacks...

 

[Zooberwerx]

With respect, I think that would mean the pre has to essentially now drive a 5K Z. (Happy to be shot down on this if I've missunderstood...)

I see from the manual, that the xlr inputs have a 20K Z, but that's only for a balanced signal. My pre only has an unbalanced output. I've also noticed the manual recommends earthing the spare (inverting) pin if you use the xlrs with an unbalanced signal. I've been using the phonos, but was planning to make a patch lead for the xlr ins.

I probably should have read the manual in the first place...

The 1804 will accommodate a standard TS cable as the sleeve acts as the "jumper". Since this is now in place and the source signal, I don't believe any further modification of the XLR > XLR is required when sharing the signal with channel B. I would, for kicks & giggles, run each of your pre's as I've described just to determine whether the 1804 is "seeing" line-level signal via the LED's.

Riis

 

[agedhorse]

Even 8k (which is why I said "roughly" is probably ok. That's a far cry from 50k. Buffering the output will be better (I always do), but be sure to add a small build out resistance to insure stability into a capacitive load (some power amps have a small cap right across the input, generally not a good design element)

 

[BassmanPaul]

Amp's sensitivity controls should be all the way up for most of these applications.

That's how I run all of my power amps.

 

[RobbieK]

yep the pre already had a build out resistor. it is 220 ohms.

unfortunately the amp is mounted backwards so it's hard to keep an eye on the lights while playing, but typically, the sig led is lit fairly constantly, the -10 led flickers with harder playing, but I don't see the clip light even if I strum the open strings. I don't think the amp is being overworked, or speaker efficiency is an issue.

with the opamp the pre now has an output z much lower than the recommend 600ohms, so I'll throw a patch lead and an extra speakon in the gig bag and try the dual mono, one cab per side thing as well this weekend.

 

[Zooberwerx]

yep the pre already had a build out resistor. it is 220 ohms.

unfortunately the amp is mounted backwards so it's hard to keep an eye on the lights while playing, but typically, the sig led is lit fairly constantly, the -10 led flickers with harder playing, but I don't see the clip light even if I strum the open strings. I don't think the amp is being overworked, or speaker efficiency is an issue.

with the opamp the pre now has an output z much lower than the recommend 600ohms, so I'll throw a patch lead and an extra speakon in the gig bag and try the dual mono, one cab per side thing as well this weekend.

Well, this is pretty much what I'd expect to see. This is with your preamp volume at noon-ish, maybe? If so, can you clip the power amp by diming the the preamp volume? You can do this without speakers attached (...say the people who live next-doors).

Riis

 

[RobbieK]

can you clip the power amp by diming the the preamp volume?

With gain, eq, vol all at 12 o'clock, I can't get a clip light, even with a heavy slapping on the E. As I mentioned, with speakers, this would be pretty loud. If I then roll the vol up around 2-3 o'clock, I can get the clip light to flicker with a heavy slap on the E.

 

[Zooberwerx]

With gain, eq, vol all at 12 o'clock, I can't get a clip light, even with a heavy slapping on the E. As I mentioned, with speakers, this would be pretty loud. If I then roll the vol up around 2-3 o'clock, I can get the clip light to flicker with a heavy slap on the E.

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

Riis