Setting up a power amp for modular preamps aka pedals

[comradehoser]

Turning to the kindness and generosity of the PedalPCB community to help me think through some amp-related things.

Here was a simple idea I had since I have built several Sunn Beta preamps (before Sunn decided to Lazarus themselves)--

Use a 100w Class D power amp placed in a homemade enclosure into which I could plug power and audio for two Beta (or whatever) pedal preamps, with a blend for the two signals and a master volume control (maybe redundant). The preamp pedals are definitely capable of line level and maybe a bit more.

I got this icepower 100AS1 board (should have gotten the stereo version, d'oh!) : https://shop.icepoweraudio.com/wp-content/uploads/2023/02/ICEpower100AS1-2-Datasheet_1_6.pdf

It has a mains conversion circuit and an input buffer, so I don't think I need to worry about that. Not sure if I should put in an impedance switch at the end of the line, or if that's a consideration for Class D power amps. It definitely was for my OG Sunn Sonaro.

So, the question is, how do I, in all of my best-understanding-of-electronics-is-color-by-number-pcb-building self best go about the rest? I assume I have to fabricate a way for the signal to be split between the two pedals and then united afterwards, and stick a master volume potentiometer (A100K?) between the blended signal and the power amp in. The preamp volume knobs can handle the balance of the signals.

I am 100% sure there is something I am completely missing and look forward to learning from your advice in my most pitiful noobularity.

 

[jwin615]

Few observations
Better off with a 4ohm load. Only 65W at 8 ohm
Also. At 8ohm frequency response goes up at 10k.
Input Z is 1.6k in non-inverting inputs, workable.
Needs an operating temp of less than 50C
Don't want to run it at 100% for thd reasons. 95% power should be fine.
Noise goes up after 10W, but it's low level still.

Need some form of heatsink. See section 8 and 14 of the linked PDF.

 

[jwin615]

You can probably find a sub$10 heat sink on AliExpress. Not sure if you'll find a 70x140mm one but there are some pretty cheap COB led Al heatsinks abound.

Edit
Lol, here's one. $9

https://a.aliexpress.com/_mL7cF1C

 

[comradehoser]

Thank you very much, jwin!

So, I cannot devise a way to feasibly switch the power amp output to correspond to different speaker cab loads? An amp tech was able to do it on a rotary for my Sunn Sonaro (linear Class A tube amp). Maybe because the baseline output was 16 and he was attenuating from there, as opposed to going up from 4 ohms. [edit: I remember: the Sonaro Dynaco transformer had 16, 8, and 4 ohm taps, and was wired for 16/8, so he just connected the 4 ohm and routed everything through a selector. So I guess in our case, here, the answer is no.]

Also, what is "thd reasons"? How do I determine the percentage power usage, as I am assuming this is not the same thing as incoming signal strength.

Z input should work with pedals, is what I am understanding.

Bolting to a metal chassis [edit: is not a sufficient heat sink, I take it is listed as an option--advisable?].

 

[jwin615]

Thank you very much, jwin!

So, I cannot devise a way to feasibly switch the power amp output to correspond to different speaker cab loads? An amp tech was able to do it on a rotary for my Sunn Sonaro (linear Class A tube amp). Maybe because the baseline output was 16 and he was attenuating from there, as opposed to going up from 4 ohms. [edit: I remember: the Sonaro Dynaco transformer had 16, 8, and 4 ohm taps, and was wired for 16/8, so he just connected the 4 ohm and routed everything through a selector. So I guess in our case, here, the answer is no.]

You should be able to do 4-8ohm loads without switching, just note you lose some wattage at 8 ohm.

Also, what is "thd reasons"? How do I determine the percentage power usage, as I am assuming this is not the same thing as incoming signal strength.

Thd, total harmonic distortion. You should be able to hook a control to act as a master volume iirc. Don't have the time to pull the data sheet back up at the moment wut can look tonight. THD spikes at 100% but it's still pretty small.

Z input should work with pedals, is what I am understanding.

Bolting to a metal chassis [edit: is not a sufficient heat sink, I take it is listed as an option--advisable?]

I think you could get by with a metal chassis alone, but for me(I like to over build/engineer, I'd rather have it on a heatsink with a small fan.
122f is maximum operating temp and you can get to that pretty quick in a sealed box. Especially in a hot club or outside during the day.

 

[comradehoser]

I will look into the heatsinks, for sure. Since I'm designing and fabricating everything myself, though, I should be able to give passive cooling priority in design.

Alright, well, since it's pretty modular, can I just solder everything together and go from there without blowing everything up---any circuits/mechanisms to recommend for parallel wiring split/merge or is really just as easy as dividing a cable before preamp ins, splicing a cable after preamp outs, and popping a A100K (??? no idea on the value) master volume pot after that before the power amp in?

 

[comradehoser]

You should be able to do 4-8ohm loads without switching, just note you lose some wattage at 8 ohm.

Thd, total harmonic distortion. You should be able to hook a control to act as a master volume iirc. Don't have the time to pull the data sheet back up at the moment but can look tonight. THD spikes at 100% but it's still pretty small.

So, switching is possible? IIRC, the Marshall cab I'm planning on using is 16 ohms.

THD= aha.

 

[jwin615]

Datasheet recommended a 4-8ohm load with a nominal of 6 iirc.
16 ohm wouldn't be advisable.
Few class D amps can push a 16ohm load.
I think that there are terminals for a volume control but will have to look more into the data sheet.

 

[comradehoser]

Hey jwin--I will respect the ohm load--I think the Marshall cab actually has a 16 or 4 ohm jack inputs, probably to service stacking, so it's all good.

I think that there are terminals for a volume control but will have to look more into the data sheet.

I don't see anything there. P101 is listed as a "basic control connector," but lists auxiliary voltage [edit: upon reading, they are designating this for "signal front-end" which I assume means preamp], standby voltage, and clipping and protection mode indicators--not really sure of the utility and how to set up all that or what it would interface with other than status LEDs for the indicators.

edit: For example, standby--what function does that serve in a Class D amp? On a tube amp, I understand--cut the signal without the tubes cooling down. But why wouldn't you just turn the amp off on a solid state?

Other than that, I see blocks for mains power connection, input and output connections, and "hanger rail supply" which I am inferring is used to provide power-amp dependent power for other devices (?)

 

[comradehoser]

Alright, well, since it's pretty modular, can I just solder everything together and go from there without blowing everything up---any circuits/mechanisms to recommend for parallel wiring split/merge or is really just as easy as dividing a cable before preamp ins, splicing a cable after preamp outs, and popping a A100K (??? no idea on the value) master volume pot after that before the power amp in?

The answer to my question for posterity's sake is yes, yes you do need something. Splitting and merging signals through two preamps with only Y cables results in a dry and wet output mix, as well some very considerable feedback.

Looking at the 3 channel mixer circuit from PPCB.

 

[jwin615]

Didn't mean to go mia on you there. Been busy couple of weeks.
Are you wanting to parallel the preamps, parallel with blend, or just have them switchable?
For blending. I'd do with both a ppcb splitter and a mixer setup.
Setup the splitter channels for unity gain on each out, no pots needed. Feed each preamp with that. It can be tucked away somewhere quiet. You won't need to touch it once it's in there.
Then. Feed the outputs of each preamp into a channel of the 3 ch mixer. Note. Make sure none of your preamps invert phase or blending will sound like hot trash. If one is out of phase with the other two, I'd take care of that on the input by hacking up one of the slitter channels by swapping inverting and non-inverting inputs. We can cross that bridge if you get there.
Anyway. Take the out out of your preamps, I'd feed each into a double pile switch to act as a channel kill switch with led indicator, then feed into the mixer. You could skip that but then your only way to kill it is turning the pot all the way down on the mixer. If you want to switch channels and keep a consistent/unity level, easier to do with switches.
Then, mixer to amp, of course.

 

[Feral Feline]

I've been fly-on-wall following this thread, but it occurs to me now that maybe some Radial gear (splitters / cab&amp switchers / etc) could/should wind up in the Wish List...

 

[jwin615]

I've been fly-on-wall following this thread, but it occurs to me now that maybe some Radial gear (splitters / cab&amp switchers / etc) could/should wind up in the Wish List...

Check out the Pigtronix Keymaster and the Eventide Mixing link if your looking into re-amping or interfacing pedals with line level(or mic level).
You can do it with radial, and probably of slightly higher quality, but those two open up "pedals anywhere in audio" for decent prices.
I've been after a Keymaster for a while but haven't pulled the trigger yet.

 

[Feral Feline]

I've been looking for a used Keymaster, but people don't want to give them up.

Wasn't aware of the Mixing Link, thanks for the Eventide tip!

 

[jwin615]

I've been looking for a used Keymaster, but people don't want to give them up.

Wasn't aware of the Mixing Link, thanks for the Eventide tip!

Yeah. I don't know if they like them that much, not that many made it into the wild or if it's a never get rid of a swiss army knife mentality, but they are a thin herd.

 

[comradehoser]

Didn't mean to go mia on you there. Been busy couple of weeks.
Are you wanting to parallel the preamps, parallel with blend, or just have them switchable?
For blending. I'd do with both a ppcb splitter and a mixer setup.
Setup the splitter channels for unity gain on each out, no pots needed. Feed each preamp with that. It can be tucked away somewhere quiet. You won't need to touch it once it's in there.
Then. Feed the outputs of each preamp into a channel of the 3 ch mixer. Note. Make sure none of your preamps invert phase or blending will sound like hot trash. If one is out of phase with the other two, I'd take care of that on the input by hacking up one of the slitter channels by swapping inverting and non-inverting inputs. We can cross that bridge if you get there.
Anyway. Take the out out of your preamps, I'd feed each into a double pile switch to act as a channel kill switch with led indicator, then feed into the mixer. You could skip that but then your only way to kill it is turning the pot all the way down on the mixer. If you want to switch channels and keep a consistent/unity level, easier to do with switches.
Then, mixer to amp, of course.

No problems--any input is good input, and I appreciate your time and help enormously. I like your ideas.

Originally, I had wanted to replicate the Sunn Beta, which has two channels that you can adjust independently from clean to massively distorted. So, you could select a, b, or both together--which could be pretty interesting, especially with different pedals or clean/dirty, particularly in a bass application. Apparently in the OG circuit, you needed to adjust channel volumes accordingly to have a proportional mix of clean and dirty, so I was guessing it was a fairly primitive parallel wiring setup, but now I'm guessing there was something there to effect the blend. Now I have to dig up that schematic....

There's also this GCI prepopulated board: https://www.godcityinstruments.com/...roducts/series-parallel-pre-populated-diy-pcb

I actually hadn't thought of something that would bring *any* pedal up to line level, but that's kind of intriguing. I was originally going to stick with "actual" preamp circuits which in principle should get to line level on their own.

So far, I have these candidates to mix and match:

PPCB Model FeT
PPCB Decade (might not make line level)
PPCB Deofol (pretty sure will make it dialed to18v)
PPCB Tyrian/Sanguine and Kraken pedals, maybe
Coda Black Hole (maybe not a preamp)
GCI N.E.W. Apostle (Orange OR120/Matamp GT120)
Aion Sunn Beta Lead
Aion Sunn Beta Bass
PCBGM Sunn Beta Lead/Bass
Aion Traynor TS50b
Aion Diezel VH-4
Aion Ampeg VH 140-C
Aion L4 (Bass) and L5 (Guitar)

Or, I could just keep it simple and just do one damn channel, and swap the pedals. It is modular.

 

[jwin615]

I'm not sure any pedal can get near +4dBu. -10, maybe a few.
No reason to commit to a design. Just get it functional on the bench with a mess of wire and hopefully no smoke. See what works.
Does the amp accept balanced input?(it does)
The amp will accept max +/-8v input which is 20dBu!
I'm sure you could run unbalanced without any harm, but I don't think you'll get near maximum output. To do so, going to need a balancing preamp stage. THCustom had a line driver board, but it's out of stock. There are some on PCBway?
Could possibly power the circuit with the DVDD +/- outs?
But the datasheet notes the AVSS and DVDD power outs a "loosely regulated" and seems they may fluctuate, maybe ties with output? Not sure.
Five fish have a board for $10

FAB3346 - Unbalanced to Balanced Driver Module

FAB3346

www.fivefishstudios.com

But the main site for those seems dormant since 2013

FiveFish Audio Building Blocks

Build. Learn. Make Audio Gear...for Students, Makers, Hobbyists and Professionals

fabmodules.com

Also, a decent looking board on eBay
https://www.ebay.com/itm/1143899935...78wDaq7TP6&var=&widget_ver=artemis&media=COPY

 

[comradehoser]

How about this, now?

Electro-Harmonix Tri Parallel Mixer

3-loop Switcher/Mixer Pedal with Channel Volume, EQ, and Phase Controls and Global Wet/Dry and Volume Controls

www.sweetwater.com

 

[Feral Feline]

Klein Bottle looper-mixer design files | VFE Pedals

This item provides a digital download of a ZIP file with all the documentation needed to build the Klein Bottle. The Klein Bottle was designed for the noise makers, soundscape artists, endless tinkerers and tone wizards. This epic pedal starts by splitting the input signal into three FX loops...

www.vfepedals.com

PPCB Model FeT
PPCB Decade (might not make line level)
PPCB Deofol (pretty sure will make it dialed to18v)
PPCB Tyrian/Sanguine and Kraken pedals, maybe
Coda Black Hole (maybe not a preamp)
GCI N.E.W. Apostle (Orange OR120/Matamp GT120)
Aion Sunn Beta Lead
Aion Sunn Beta Bass
PCBGM Sunn Beta Lead/Bass
Aion Traynor TS50b
Aion Diezel VH-4
Aion Ampeg VH 140-C
Aion L4 (Bass) and L5 (Guitar)

SUNNβEAM V2 – Effects Layouts

 

[Passinwind]

edit: For example, standby--what function does that serve in a Class D amp? On a tube amp, I understand--cut the signal without the tubes cooling down. But why wouldn't you just turn the amp off on a solid state?

Standby allows you to do the power-up with a much smaller/lower current switch, which also allows front mounting without the hassles of running line power around wires and components that may not respond in a great way.

I've done several 700AS1 bass amp builds and one with the 500ASP module. Most pedals will not drive either of those modules all that well, but "good enough" tends to have moving goalposts. Best practice dictates at least 10dB of headroom past the nominal full power drive spec, IME.

For custom cut heat sinks, I've had excellent results with these guys: https://www.heatsinkusa.com/