Tube preamp into Class D power amp for custom Leslie build?

[guitylerham]

Howdy!

For my current project #20,209 I'm trying to design the amplifier for my homebuilt Leslie that I'll be giving my friend for his Crumar Digital Organ. I've built a lightweight Leslie cab with rotor and horn from fiberglass and styrofoam a la some popular bass cabs. It now needs an amplifier to drive it!

The Crumar organ puts out "+4db signal" that I want to feed to a simple triode and transformer-coupled interstage preamp (for flavor, controllable overdrive/xfrmr saturation, and impedance matching) that then feeds the Class-D power amp, such as an IcePower module. I'm a self-taught researcher/builder with many tube amps, effect pedals, etc under my belt so I'm not new to learning how to do this and implementing this but I could use a little guidance in this new world of tech that I have less experience with.
 
Design parameters:

-Class-D IcePower amp has a 3.3v input max and maybe a 10k input impedance
-Organ outputs +4db = about 1.23v?
-will use only a single dual-triode (if mono, then I'll just ensure minimal gain with the two stages, if 2-ch then only 1 stage per channel)
-Transformer coupled output. Not quite sure what spec xfrmr to use but I assume something smallish. Still need to learn more about that.
-Leslie requires 6db/octave crossover to send 800hz and above to treble horn and 800hz below to bass rotor
-Organ volume knob/pedal will be "gain" control into tube/xfrmr stage, a Master Volume after the OT feeds Power Amp

Some questions:
1) The Leslie has two seperate drivers that need a crossover set at 800hz and 6db/octave to send hi/lo frequency signals to each respective driver. How I do this is still in question but I figure I can either build a 1-ch preamp and run the signal mono all the way to a 1-ch. Power Amp and output into a heavy duty crossover that splits the signal into two before going to the drivers (how they did it originally), or I can build a 2-ch preamp w/2 OT's and a 2-ch Power Amp and put the crossover somewhere before the input to the Power Amp--either after each channel's triode stage and before xfrmr, or after xfrmr and before PA input are my only two legitimate options right?

2) A variable attenuator (MV) after the OT(s), before input to power amp: since we're talking low volts/current, can I just use a simple potentiometer instead of a heavy duty L-pad?

3) My friend wants this Leslie to be able to play clean and loud as well as grind if so desired--irrespective of actual volume. Is a soft clipper circuit something I should be considering as well to help get some of the gritty organ drive as well as apply some limiting/compression when dialed in for it?

This seems like I'm reinventing the wheel a little bit but I enjoy the challenge and potential of successfully designing something that fills a role. It feels like I my process should be designing this from back to front. At full Volume on the organ, I'd like the Output transformer to saturate a little and the triode to overdrive. Overall volume is then adjusted by the Post Preamp MV into the clean Power Amp (3v max).

Seems doable, correct?

[jjasilli]

+4dB = professional line level signal, let's say about 1.5V.  Presumably: i) a Class D power amp will handle line level input; ii) the Crumar organ already has volume & drive controls.  If other effects are missing from the Organ, then line-level effects can be added between the Organ and the power amp.  I think there there are line level effects with tubes in them.

Also there are great sounding amp sim boxes or pedals such as Line6 (Helix), Strymon, Neural, etc. some of which can handle line level input & output.  Some of these also have numerous effects built-in.

Anyway a tube preamp seems to be superfluous.  Generally, modulation effects such as delay, reverb, etc, should come after overdrive / distortion.  For correct impedance for the tube preamp input: i) use a conversion box: line level > guitar amp input; or ii) maybe increase the value of the input grid leak resistor from 1M up to as much as 10M.  At the tube preamp output use a DI box for guitar level to line level.  The tube preamp does not need an output tranny.  If you want a tube output buffer, maybe use a cathode follower output.  Or search this Forum or the internet for a guitar preamp effects loop out, or line out circuitry.


Lots of speaker crossover info on the internet.

[guitylerham]

+4dB = professional line level signal, let's say about 1.5V.  Presumably: i) a Class D power amp will handle line level input

Full amplitude and volume out of the Class D amp won't be reached until you approach the 3.3v max input voltage though, right? At this point, I'm only guessing how loud, say a 250w module into my Leslie would be but if I want to be able to get max volume, my preamp signal would need to be be closer to 3v (which is about 2x the output voltage of the Crumar). I'll absolutely consider the power handling of my drivers before deciding on which power amp module to build with.

Also there are great sounding amp sim boxes or pedals such as Line6 (Helix), Strymon, Neural, etc. some of which can handle line level input & output.  Some of these also have numerous effects built-in.

True, I can't compete with so many of the new tech these days. I was hoping to stick to a homebuilt setup since I'm not looking for any other effects beside tube characteristics into the clean, loud Power Amp. You're right about the Crumar having overdrive effect and maybe that is sufficient just to drive into the Class D. There's is something very alluring about a tube/transformer setup tho... I'm guilty of looking for mojo!

Anyway a tube preamp seems to be superfluous.  Generally, modulation effects such as delay, reverb, etc, should come after overdrive / distortion.  For correct impedance for the tube preamp input: i) use a conversion box: line level > guitar amp input; or ii) maybe increase the value of the input grid leak resistor from 1M up to as much as 10M.  At the tube preamp output use a DI box for guitar level to line level.  The tube preamp does not need an output tranny.  If you want a tube output buffer, maybe use a cathode follower output.  Or search this Forum or the internet for a guitar preamp effects loop out, or line out circuitry.


Lots of speaker crossover info on the internet.

Cathode follower is definitely an option for lowering the output impedance. From reading and intuiting, I know there are differences between capacitor coupling and transformer coupling between preamp and poweramp. I know a transformer isn't necessary but it must impart some sonic uniqueness compared to just a capacitor.

My first iteration of an amp for this Leslie was a reclaimed stereo tube amp where I stuck an inexpensive kit pcb active crossover circuit on the frontend feeding each channel with the crossed over signal. It worked alright. I think with a simple triode stage, there'd be plenty of gain on hand to use a passive crossover circuit on the output of an output transformer too, before sending the two signals to a 2-ch power amp.

Thank you for throwing in you wisdom, jjasilli, it gives me more to ponder!

[jjasilli]

I didn't see input specs for your class D power amp; but are 250 watts needed? 


Some benefits of tranny coupling the preamp output are ground loop isolation, and phase/polarity revetsal.  I don't know if you need these yhings.

[guitylerham]

I didn't see input specs for your class D power amp; but are 250 watts needed? 


Some benefits of tranny coupling the preamp output are ground loop isolation, and phase/polarity revetsal.  I don't know if you need these yhings.

Looking on the data sheets for various modules, they mention max input voltage +-3v so that's all I was going off of. Again maybe 250w isn't needed but comparing to a similar Motion Sound Leslie model with a class D bi-amp, they have something like 130w for the bass rotor and 70w for the treble horn, so 200w total or so. I'm just using that as a framework is all.

Yeah, I don't foresee ground loop issues or phasing either. A transformer or capacitor will sound identical for the most part?

[guitylerham]

After endless researching, I'm starting to lean towards just running the Crumar into an active crossover circuit that will then feed a 2-ch Power Amp. The crossover doesn't add gain so when it's all said and done if I need more gain, I'll address that then.

I was looking for amp simulator pedals and off-the-shelf circuits, maybe something I could rehouse into the same chassis as the Power Amp but I didn't really seeing anything in the appropriate form factor or any pedal I'd want to spend the money on that will just get kept separately inside the Leslie.

So much for recreating the Leslie 145 tube amp but this simpler route will be worth experimenting with.

[jjasilli]

https://www.b3guys.com/product/Leslie-Speaker-Crossover
Leslie just used a passive crossover