PLX-1804 Parallel

[Lowtonejoe]

Apparently the kids at QSC don't have any issues with the jumper, which you correctly identified as being topologically identical to a Y cable.

*snicker*

You said 'topologically'.



Joe.

 

[zombywoof5050]

From what I've gathered....as long as you are feeding your single poweramp input via a balanced connection, then it's ok to jumper the two 1/4" inputs (but be sure and use TRS cabling as in the picture posted earlier), and there will be no reduction in input gain to each channel.

But on the other hand, it doesn't work well to do this if you are driving your single poweramp input via an unbalanced connection (like with an Alembic F-2B, like Passinwind mentioned). The only way you could do it would be to use an actual Y cable (1/4"), otherwise you would need special cabling to jumper from the unused XLR input to either the XLR or the 1/4" of the other channel (or you could use a 1/4"/XLR adapter to feed the XLR input, but it wouldn't be a true balanced connection).

When splitting an unbalanced input into two (by either using an actual Y cable or jumpering unused inputs), you are effectively sending only half of your input signal to each channel. This is why, if you are using an unbalanced preamp (such as the Alembic F-2B), and you want to drive both channels of a poweramp from a single input, you really should choose a poweramp that has a 'parallel mode' allowing you to acheive this internally without having to use a Y-cable or 'jumpering inputs'. I have an Alembic F-2B (which only has unbalanced outputs), and I've experienced this issue.

 

[Growly Lytes]

So if i use the 1/4' out of the Sansamps into the XLR input of the QSC with the jumper in the unused channel 1 & 2 for parallel output it will not work & i will lose input gain ? It must be the Balanced output of the Sansamp is what your saying ?
The Sansamp `1/4' output is 1Mohm` & the `XLR output is 600ohm ?
My first poweramp i used the 1/4` output of the Sansamp to the XLR balanced input of the poweramp then flicked the switch to parallel it worked fine. The channels were internally connected.
How do i get to the bottom of this,is it going to work & how do i make it work ?

 

[Lowtonejoe]

So if i use the 1/4' out of the Sansamps into the XLR input of the QSC with the jumper in the unused channel 1 & 2 for parallel output it will not work & i will lose input gain ? It must be the Balanced output of the Sansamp is what your saying ?
The Sansamp `1/4' output is 1Mohm` & the `XLR output is 600ohm ?
My first poweramp i used the 1/4` output of the Sansamp to the XLR balanced input of the poweramp then flicked the switch to parallel it worked fine. The channels were internally connected.
How do i get to the bottom of this,is it going to work & how do i make it work ?

Are you using the pedal or the rackmount preamp?



Joe.

 

[Growly Lytes]

Ive got both Rackmounts the RBI & RPM hooked-up in parallel.This thing about the input impendence miss match is sort of making feel like it wont work now ?

 

[monroe55]

Does this help?

Joe.

QSCPLX1804 owner too.
This diagram is exactly what i do using an Ampeg SVT IIP preamp. Am I missing something here? Am I sacraficing power or anything from my amp by running it this way? Am I endangering my equipment in any way? I like to think I have @900 watts available into my 4ohm cab on one channel and @600 watts into an 8ohm cab on the other channel. Not that I'd ever need all that power of course.

 

[Growly Lytes]

QSCPLX1804 owner too.
This diagram is exactly what i do using an Ampeg SVT IIP preamp. Am I missing something here? Am I sacraficing power or anything from my amp by running it this way? Am I endangering my equipment in any way? I like to think I have @900 watts available into my 4ohm cab on one channel and @600 watts into an 8ohm cab on the other channel. Not that I'd ever need all that power of course.

God i love it when positivity comes round , its like the Sun Rising Up In The Mornin`
I have to go with this PLX-1804 because the next one is nearly double the price of this one & exact same wattage....
1804 = $1599
3102 = $2999
The only difference between them is the switches & filters but for power they put out the same , input sensitivity is the same too.I use Parallel so i really cant afford going for the 3102 because of the extra bells & whistles.I just need the power really & for it to work as you do with you SVT IIP.
Thanks for the fresh air Monroe55 !!

 

[greenboy]

Note the emphasis.

One very valid reason for not wanting to do that is that it'll half the input impedance of the amp. Often times not a big deal, but with something like Alembic F2B it could be a deal breaker.

Now look again.

One very valid reason for not wanting to do that is that it'll half the input impedance of the amp. Often times not a big deal, but with something like Alembic F2B it could be a deal breaker.

A statement that is being qualified. I bet there is some informational value in that ; }

 

[Bob Lee (QSC)]

When splitting an unbalanced input into two (by either using an actual Y cable or jumpering unused inputs), you are effectively sending only half of your input signal to each channel.

Uh, no.

 

[SactoBass]

Uh, no.

Growly Lytes: Bob's response is right on the money. (Uh, he works for QSC, so he would know).

Jumpering the inputs on the 04 series QSC amps as shown in the diagram from the manual is EXACTLY the same as flicking the "parallel" switch on the 02 series QSC amps. No loss of power. No impedance mismatch. No issues WHATSOEVER.

 

[zombywoof5050]

Uh, no.

I'm was just repeating what the guy at Crown Tech Support told me when I explained how I was trying to operate my Alembic F-2B with one of Crown's Drivecore XLS amps. He said this reduction in signal level will not happen if I had a balanced connections from my preamp out into the poweramp in.

I'm sorry if this sounds disrespectful of your expertise, but 'uh no' just sounds smug and is not really much help. How about some more info? Are you telling me that there should have been no reduction in signal level at all? Because I can tell you for sure without a doubt that my signal level was reduced considerably.

Is it possible that the Crown amps suffer from this and the QSC amps don't?

 

[Bob Lee (QSC)]

Are you telling me that there should have been no reduction in signal level at all?

I'm saying that parallel inputs each get the same voltage--i.e., "half" doesn't go to each one. Please don't advise people that.

Because I can tell you for sure without a doubt that my signal level was reduced considerably.

Alembic preamps have low output signal capability and extremely high output impedances (perhaps they're still anticipating that the pres are used as extended front ends to regular heads for passive instruments?), so reducing the input impedance causes more loss in the output impedance. That is not the same as "half goes to each input."

Is it possible that the Crown amps suffer from this and the QSC amps don't?

Probably not, but amps with higher input impedances will not cause as much loss in the preamp's output impedance.

 

[Passinwind]

Alembic preamps have low output signal capability and extremely high output impedances (perhaps they're still anticipating that the pres are used as extended front ends to regular heads for passive instruments?), so reducing the input impedance causes more loss in the output impedance.

That's true only for the F2B AFAIK. The F1X uses a buffered opamp output and doesn't much care what sort of impedance you ask it to drive, at least IME.

The F2B was originally meant to drive tube power amps, I think. So a typical passive input of a bass amp will work pretty well too. When I bench tested one a year or two back it was immediately obvious that it didn't like loads of <10K ohms, just as the Alembic website clearly says.

 

[zombywoof5050]

Thanks, Bob and Passinwind for the extra info.
I guess it's just the Alembic F-2B that suffers from this.
Sorry for spreading misinformation.

 

[SactoBass]

zomby: to your defense, you were only sharing info relating to what you personally experienced. Nothin wrong with that!!

I can tell you that I have learned *ALOT* from TB'ers (especially from people like Bill FM, Greenboy, Alex Claber, rpsands, KenJ, and many others).

Keep on

 

[Bob Lee (QSC)]

That's true only for the F2B AFAIK. The F1X uses a buffered opamp output and doesn't much care what sort of impedance you ask it to drive, at least IME.

The F2B was originally meant to drive tube power amps, I think. So a typical passive input of a bass amp will work pretty well too. When I bench tested one a year or two back it was immediately obvious that it didn't like loads of <10K ohms, just as the Alembic website clearly says.

I think the old F1 was like that, too. I remember some years ago trying to help a bass player troubleshoot his rig, and I was surprised to learn about its output configuration--a low output voltage and a high Z.

 

[zombywoof5050]

That's true only for the F2B AFAIK. The F1X uses a buffered opamp output and doesn't much care what sort of impedance you ask it to drive, at least IME.

The F2B was originally meant to drive tube power amps, I think. So a typical passive input of a bass amp will work pretty well too. When I bench tested one a year or two back it was immediately obvious that it didn't like loads of <10K ohms, just as the Alembic website clearly says.

Right.
When I first got my F-2B back in '82, I was pairing it with a Crown DC300A II which seemed to be fine back then.

Years later when I wanted more power, I did my homework when searching for a new poweramp. Since Alembic recommended one with an input impedance of at least 10K ohms, I wanted to make damn sure I was covered, so I ended up getting a Carver PM1400 with its whopping unbalanced input impedance of 25K ohms. On top of that, as soon as I received it I internally switched the input sensitivity from 1.5V to 0.775V. Problem is, now when looking at new lighter weight and shallower poweramps, I can't seem to find any with similar specs , so I'm probably destined to always be a little let down when comparing these new breeds to the Carver. And since I like to run in parallel mode, jumpering two 10K ohm unbalanced inputs is like connecting to a 5K ohm input (I hope I have this right) which is not going to work well at all with the F-2B.

I guess if I'm going to try and go lighter/shallower with my poweramp, while still using the F-2B as a pre (which I still love dearly), I'll probably need to get some sort of booster/converter to boost the output of the F-2B and convert it to balanced. For now I'll just keep using the big heavy Carver with the F-2B.

Sorry, Growly Lytes, if I took your thread off track and caused you any alarm.

 

[Growly Lytes]

Im trying to find out as much as possible about the 1804.I wrote to QSC , Bob Lee & TB. TB has helped the most so far.I sent Bob a PM & he replied thats great too !!
So i geuss the RBI & RPM will have no problem driving it then.
The Sansamps both have the same `1/4' output @ 1Mohm` & the `XLR output is 600ohm.
But Input impendences are not the same.....
RPM Input impendence = 4.7Mohm
RBI Input Impendence = 1 Mohm
I think they should drive this amp after reading some of the comments i was a little worried but Bob cleared that up.Im not going to lose signal level by jumpering the inputs so thats one step in the right direction.No voltage lose & no halfving the signal.....execellent news,
Thanks.

 

[Bob Lee (QSC)]

I think they probably mean the output can drive a 1 M&#937; input impedance. No one in his right mind would make an audio device with a 1 M&#937; output impedance.

High impedances like 1 M&#937; and higher are typical of inputs designed for passive instruments. That's mainly because with their pickups, volume and tone controls, etc., such instruments have pretty high output impedances of like 5K or 10K, maybe even higher.

 

[Growly Lytes]

I think they probably mean the output can drive a 1 M&#937; input impedance. No one in his right mind would make an audio device with a 1 M&#937; output impedance.

High impedances like 1 M&#937; and higher are typical of inputs designed for passive instruments. That's mainly because with their pickups, volume and tone controls, etc., such instruments have pretty high output impedances of like 5K or 10K, maybe even higher.

Im Really Sorry Bob i just checked over what i wrote & yes its not 1Mohm its actaully ......
1Kohm output for 1/4` & 600ohm for the XLR
Sorry mate !
May i ask Bob which of these Preamp outputs should i use with the PLX-1804 ???
1/4` or XLR ?
Thanks