Single PA top rather than amp?

[Bill Fitzmaurice]

The values each of the rheostats would have at any given level of attenuation are the same as those for a fixed LPad, which you calculate with this: https://sengpielaudio.com/calculator-Lpad.htm

[warwickhunt]

Anecdotal but I transitioned between full bass rig to IEM (no backline) via using my chosen preamp pedal (Fishman Platinum Pro) into a QSC 10 mounted on a speaker stand to my side of the stage (obviously PA doing all the heavy lifting).  The sounded that I had to work with was far better than I expected and I occasionally default back to this set up when I've doubts about the quality of the engineer/PA to supply a good IEM mix... I've not reverted back to full backline, which surprises me (40+ years of relying on backline, often to carry FOH).  

[Phil Starr]

15 hours ago, Stub Mandrel said:

 

I can see for an 8R system the series rheostat should be 0-8R, presumably linear. The parallel rheostat would, ideally be 0-infinity and 8R at the point where the series one is at its mid-point. That would give three 8R points at max and min.

 

But most rheostats are wirewound and linear, so I wonder what compromise is used.

This below is a simple version of how they work, which is as you surmise. You may need to add in an extra resistor or so to add in further attenuation or limit the range over which they operate. As you can see the example in the picture isn't hard wired so you can make the circuit as you wish. Most commercially available L-pads work at 8 ohms. You've clearly spotted the flaw, there is no way in this circuit that you can maintain a constant impedance. For those who haven't worked it out: If we take the simplest example of both rheostats being 8 ohms. At one extreme the series element is 8ohms and the parallel one is shorting out the tweeter which shuts it out completely. The amp 'sees' 8ohms and is perfectly happy. At the other end of the dial the series element is now zero ohms and the parallel one 8ohms, the circuit is now 4ohms, at the mid position it is 6.67 ohms. All of these are easily driven by an amplifier but they are also part of a tuned circuit in the crossover, often a simple CR circuit so the resonant point of the high pass filter will vary and you'll get non flat frequency artefacts around the now crossover point. It's not an issue if you are simply using the L-pad to tune in a sound you like but if you want to use a modeller to best effect you now have something to notch out to get a realistic model coming out through the speaker. I'd also be suspicious of any cheaply made wirewound product in a tuned circuit having unwanted inductance.

 

[Stub Mandrel]

6 hours ago, Phil Starr said:

This below is a simple version of how they work, which is as you surmise. You may need to add in an extra resistor or so to add in further attenuation or limit the range over which they operate. As you can see the example in the picture isn't hard wired so you can make the circuit as you wish. Most commercially available L-pads work at 8 ohms. You've clearly spotted the flaw, there is no way in this circuit that you can maintain a constant impedance. For those who haven't worked it out: If we take the simplest example of both rheostats being 8 ohms. At one extreme the series element is 8ohms and the parallel one is shorting out the tweeter which shuts it out completely. The amp 'sees' 8ohms and is perfectly happy. At the other end of the dial the series element is now zero ohms and the parallel one 8ohms, the circuit is now 4ohms, at the mid position it is 6.67 ohms. All of these are easily driven by an amplifier but they are also part of a tuned circuit in the crossover, often a simple CR circuit so the resonant point of the high pass filter will vary and you'll get non flat frequency artefacts around the now crossover point. It's not an issue if you are simply using the L-pad to tune in a sound you like but if you want to use a modeller to best effect you now have something to notch out to get a realistic model coming out through the speaker. I'd also be suspicious of any cheaply made wirewound product in a tuned circuit having unwanted inductance.

 

 

Thanks Phil, that was what my back of the envelope suggested. I did wonder if there were some 'kludges' applied to approximate a non-linear effect.

[Chienmortbb]

I design and install high voltage audio systems, 70V and 100V. I would use an L Pad of some type if the customer wanted a local volume control, but I would not consider a HiFi speaker with one fitted. DSP does allow the designer of a PA top to tailor the sound for the common scenarios found in the real world.

 

However, my original point was that most of the passive designs for bass guitar that include switched, or variable tweeter attenuation are there because a proper crossover has not been designed and/or a really cheap and nasty crossover has been installed.

[PaulFenderJazz]

On 04/04/2025 at 14:47, dave_bass5 said:

I sold my cab when we went in to lockdown. Last year I was rehearsing with a new band on bass, and used my Yamaha DBR-12 active PA cab. No issues at all seeing up with the band, and it felt a bit clearer than I had been used to with a 2x12 (a very good one though). 

I was driving it with just a BDDI or Zoom B-2 Four. I wasn't expecting FRFR, It was just all i had to hand, but id like to go this route again when I find another band to play bass with. 

I was thinking of going a similar route, as I’m also looking to get a back line solution for playing with a band again, but am also wanting a PA solution for potential solo acoustic guitar/singer performances and I’d rather not duplicate gear. Would a pedal board preamp work straight into an active PA speaker as a solution for bass? TIA

[Wombat]

Not a cheap solution but we have a pair of Bose F1s which ‘the whole band’ go through and I no longer need an amp.

Had some nice comments about sound too from other bassists who have seen us play - when I was expecting ‘why haven’t you got an amp’.

This is for a ‘pub situation’ so I can’t say (yet) if any of the ‘bassness’ would be lost in e.g. a larger hall.

[warwickhunt]

14 minutes ago, PaulFenderJazz said:

I was thinking of going a similar route, as I’m also looking to get a back line solution for playing with a band again, but am also wanting a PA solution for potential solo acoustic guitar/singer performances and I’d rather not duplicate gear. Would a pedal board preamp work straight into an active PA speaker as a solution for bass? TIA

 

Not all pedal preamps are created equal!  I've got several preamp pedals that happily drive an active speaker QSC K10 but I've recently become aware that not every pedal will be guaranteed to do this.  I know the word 'impedance' is the incorrect term to use but effectively the output of the pedal needs to have enough juice to drive the amp section; you'd think being a preamp that this would be a given but apparently not!  

[PaulFenderJazz]

5 minutes ago, warwickhunt said:

 

Not all pedal preamps are created equal!  I've got several preamp pedals that happily drive an active speaker QSC K10 but I've recently become aware that not every pedal will be guaranteed to do this.  I know the word 'impedance' is the incorrect term to use but effectively the output of the pedal needs to have enough juice to drive the amp section; you'd think being a preamp that this would be a given but apparently not!  

Ah, I wondered that! Is there any guidance on what to look for in a preamp? In my last acoustic group I went from my pedal board via a mixer to the PA, maybe a small (eg 2-3 channel) mixer would solve the problem ??

[warwickhunt]

34 minutes ago, PaulFenderJazz said:

Ah, I wondered that! Is there any guidance on what to look for in a preamp? In my last acoustic group I went from my pedal board via a mixer to the PA, maybe a small (eg 2-3 channel) mixer would solve the problem ??

 

I wouldn't jump to go down the mixer route yet as I've found most preamp pedals (I have 5 and all do) will drive an active PA cab.  I'm sure someone cleverer than me (i.e. all of the above contributors) will know how many mV etc you need.

 

[PaulFenderJazz]

4 hours ago, warwickhunt said:

 

I wouldn't jump to go down the mixer route yet as I've found most preamp pedals (I have 5 and all do) will drive an active PA cab.  I'm sure someone cleverer than me (i.e. all of the above contributors) will know how many mV etc you need.

 

Brilliant, great advice, thanks🙂

[tauzero]

10 hours ago, warwickhunt said:

 

Not all pedal preamps are created equal!  I've got several preamp pedals that happily drive an active speaker QSC K10 but I've recently become aware that not every pedal will be guaranteed to do this.  I know the word 'impedance' is the incorrect term to use but effectively the output of the pedal needs to have enough juice to drive the amp section; you'd think being a preamp that this would be a given but apparently not!  

 

If the active speaker can take both mic level and line level inputs then any preamp pedal should drive it.

[warwickhunt]

Just now, tauzero said:

 

If the active speaker can take both mic level and line level inputs then any preamp pedal should drive it.

 

I always thought so but I think there was a thread where a certain preamp pedal couldn't provide the appropriate level!  

[Al Krow]

We put our full band through a pair of RCF 912As, without needing a sub, and they provide us with a very decent bass response.

 

A single 912A should therefore be able to very comfortably handle bass if you want to use it as a 'FRFR' powered speaker, and will cost a fraction of the price many of the more traditional separate amp & cab set ups that many BC'ers use. But perhaps the better comparison is with something like a Markbass CMD 121P combo, in which case the price differential is not so large, particularly if you are comparing used prices.

Edited 6 hours ago by Al Krow

[PaulFenderJazz]

9 hours ago, Al Krow said:

We put our full band through a pair of RCF 912As, without needing a sub, and they provide us with a very decent bass response.

 

A single 912A should therefore be able to very comfortably handle bass if you want to use it as a 'FRFR' powered speaker, and will cost a fraction the price a many of the more traditional separate amp & cab set ups that many BC'ers use. But perhaps the better comparison is with something like a Markbass CMD 121P combo, in which case the price differential is not so large, particularly if you are comparing used prices.

A good comparison: the Markbass CMD 121P is a very good benchmark in my view

[Al Krow]

1 hour ago, PaulFenderJazz said:

A good comparison: the Markbass CMD 121P is a very good benchmark in my view

 

Yes, and the Fender Rumble 500 is also much loved by BC'ers too!

[Al Krow]

23 hours ago, PaulFenderJazz said:

I was thinking of going a similar route, as I’m also looking to get a back line solution for playing with a band again, but am also wanting a PA solution for potential solo acoustic guitar/singer performances and I’d rather not duplicate gear. Would a pedal board preamp work straight into an active PA speaker as a solution for bass? TIA

 

Just seen you're looking at something for vocals also and want to avoid gear duplication...in which case the single PA top / FRFR option may well be the better option for you if you're looking for something that covers all the bases (excuse the pun!)

 

Do you have PA already? If not, there's a whole separate rabbit hole for you to dive into e.g. Compact budget PA set-up to put bass through (without back-line) - I ended up ditching my backline for a pair of RCF 912As, but there's plenty of other options! And many of those PA speakers have been widely used as FRFR speakers by BC'ers: the RCF 732A seems to be a popular option and would handle vox better than my 912As due to better mid range horns.

[PaulFenderJazz]

1 hour ago, Al Krow said:

 

Just seen you're looking at something for vocals also and want to avoid gear duplication...in which case the single PA top / FRFR option may well be the better option for you if you're looking for something that covers all the bases (excuse the pun!)

 

Do you have PA already? If not, there's a whole separate rabbit hole for you to dive into e.g. Compact budget PA set-up to put bass through (without back-line) - I ended up ditching my backline for a pair of RCF 912As, but there's plenty of other options! And many of those PA speakers have been widely used as FRFR speakers by BC'ers: the RCF 732A seems to be a popular option and would handle vox better than my 912As due to better mid range horns.

Thanks, I was actually thinking potentially at a compact PA as I don’t have one yet: eg a Mackie Thump, Bose S1, or similar. Thanks for the link I’ll check out that thread🙂

[Phil Starr]

On 08/06/2025 at 09:43, PaulFenderJazz said:

Ah, I wondered that! Is there any guidance on what to look for in a preamp? In my last acoustic group I went from my pedal board via a mixer to the PA, maybe a small (eg 2-3 channel) mixer would solve the problem ??

What you are looking for is gain. A microphone puts out a few thousandths of a volt, a bass pickup a bit more, pre amps usually give you a bit of gain. 10x is not unusual in which case your few thousandths would become a few hundreths of a volt.

 

Power amps tend to be looking for around 0.7 volts so little mixers or even big ones will put out at least this much and a bit more. Not all power amps are equal so some need a bit more than 0.7V which is usually described as line levelor 0db.

 

Most of the multi-fx units will give you plenty of gain and amplify your bass up to line level and more. Many active speakers can be switched to take a mic level signal and you should be able to just plug your bass in and even have to turn it down a bit. But not all separate fx boxes will add enough gain to drive line level and not all active speakers will be guaranteed to have a mic level input (most do) so you will have to check.

 

Your 2-3 channel mixer will give you all the gain and impedance matching you need but probably won't be necessary.

[Greg Edwards69]

On 01/04/2025 at 09:08, Graham said:

I've been using an Ampeg SGT DI pre-amp for home use, as I'm not in a gigging band at the moment. I really like the sound of this pedal and when I do get back to playing live I'd like to have an amp behind me, to just transparently make the pre-amp louder

 

If I swapped my amp for a single PA speaker as a FRFR, could I get enough volume to amplify my SGT pedal  instead of using a traditional amplifier?

 

My amp is a EBS TD650, into an EBS Neo 2x12, when I was gigging I rarely pushed the volume past 9-10 o'clock. Before the TD650, I had an HD350 and whilst I had to to turn the volume control further, it was rarely past 11 o'clock; I think I went to 1 o'clock once - outside with no PA support.

 

Would something like a QSC K12.2 give me enough volume to replace the head and 2x12, even with half number of speakers? Or is this a fools errand?

 

As long as you have realistic expectations, you should be fine. Gigging with one of these is akin to a high powered 1x12 bass combo, such as the Markbass CMD121 mentioned above. If you can gig with one of those, you can gig with a high-powered 1x12 PA speaker. I gigged for several years with a Headrush 1x12 speaker without PA support, before going direct to PA with IEMs, and I used a markbass 1x12 cab with different heads before that.