Pass Balanced Monoblocks

I'm a novice builder, having successfully completed one ACA on my own. And it sounds great. I really enjoyed building it, and I've been reading lots and lots here to learn more about this hobby. It appeals to my inner need to ponder multi-variable problems and tradeoffs. I'm now considering my next project, and here are some of the parameters I have in mind:

1) Monoblock amps; I've always wanted a pair of big monoblocks.
2) Balanced inputs: I want to run the amps near the speakers, potentially with long(ish) runs from the pre. I'd like a "true" balanced design to take advantage of common mode noise reductions. My current pre amp (Schiit Freya) has balanced outputs, 600 ohm output impedence.
3) Boards available from I feel like I can build off a schematic but I am a guy with a soldering iron, not a designer and most definitely not an EE. I need to work from an established BOM. I want to use the available PSU and soft start boards from the store.
4) Output in the 50-100W range, I don't want to do cascoding or anything exotic (see #3) but I'd like a little more headroom than the ACA offers. I'd like to build with some degree of conservatism and not push any component too hard.
5) Budget. I'm willing to make the investment in the chassis (either 4U or 5U as needed) and other stuff to make this happen.

Speakers? Well, I'll be getting new ones soon (in the process of moving). I'm interested in going to high-efficiency, either horns (Klipsh/JBL etc) or full range drivers like Zu or similar. I'm also toying with the idea of using an active pro-x-over like an Ashly xr-1001 to run stereo subs (another reason to want balanced connections) down the road. So, I don't need a ton of power, but I feel like I'd like to try something more than a pair of ACAs would offer.

Musical tastes: wide variety but I really enjoy live concert recordings, the Dead, DMB etc, solo piano, blues, solo guitar, also hard bop jazz, some vocals. Very little orchestral.

Knowing all that...what boards should I be looking at? It seems like the BA-3B is an option and I've seen threads on the F5, but making a balanced version of the F5 looks perhaps more complex. What about the Aleph J? Welcome any thoughts or advice from the learned members.
Yes to the Aleph J

Skip the soft start boards; they aren't necessary, and will make your build needlessly complex. Instead, use the simple thermistor soft start scheme on the primary side of the power transformers. This is documented in the illustrated build guide.

Consider a proper dual-mono power supply. With that in place, you will effectively have a pair of monoblocks. Use a pair of Antek AS-3220 power transformers, and bias the output stages until the heat sinks are good 'n warm.
Consider a proper dual-mono power supply. With that in place, you will effectively have a pair of monoblocks. Use a pair of Antek AS-3220 power transformers...

I'm new to all this so pardon me for asking, but what is it about a dual-mono power supply that turns an amplifier-per-channel into a monoblock? I thought the fact that there was an amplifier-per-channel was what made it a monoblock.

This question and another I saw today on the whammy thread, "what is bias?", got me thinking that the website could use a glossary section that explains terminology and concepts specifically in the context of audio amplifiers.
To have a complete amplifier per channel, one needs to build a stereo amp with separate PSU for each channel, or to build completely separate monoblock amplifiers for each channel. This is what an Aleph J looks like with a dual-mono power supply. Even though this is in a single chassis, the only thing in common between the two channels is the power cord. The power switch is a double-pole, single throw type.


  • Aleph J internal.jpg
    Aleph J internal.jpg
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I still don't understand. In that picture it looks like you have 2 XLR ins and 2 RCA ins, neither connected, which indicates to me that it is a stereo amplifier. You've set up a dual power supply in it, which makes me want to call it a dual-mono stereo amplifier. But the context of the original question was about one amplifier enclosure and associated power supply per speaker with a single XLR in for each. In that context, your reply seems to state that you would need to run dual-mono power supplies for each enclosure, with one power supply for each +/- rail of the XLR signal, in order for it to be considered a monoblock.

I think the question boils down to this:

Is it a monoblock because there is one power supply per speaker (what you have there)?

Or is it a monoblock because there is one power supply per rail? That would be a single power supply for an amplifier for a single-ended RCA signal and a dual-mono power supply for an amplifier for a single balanced XLR Signal.

Or is it a monoblock because the amplification circuit per speaker exists in a separate enclosure, regardless of the power supplies to rails ratio?

I want for terminology that clearly identifies each. Like is there a term that indicates that you have a balanced amplifier for one speaker in an independent enclosure with a dual-mono power supply, either in the same enclosure or in a separate enclosure? Or do you just have to word it out like "I run a pair of balanced monoblocks in separate enclosures with independent, external dual-mono power supplies." Does a setup like that make it more of a monoblock than what you have in your Aleph J?
For context, I've toyed with the idea of stuffing 4 whammy power supplies in one chassis and using an umbilical to connect it to another chassis with 2 whammy boards, each acting as preamp for a balanced XLR signal, and I was wondering what to call it. So questions like this have been on my mind recently. Stereo monoblocks with dual-mono power supplies?

I kind of like "Frankenwhammy".
Joined 2004
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Monoblock is a single channel (single speaker) chassis. Can be balanced or single-ended but still a monoblock.

Stereo is 2 channels in a single chassis, no distinction on PSU separation.

Dual mono is as TungstenAudio describes - stereo amp (meaning 2 speakers powered by a single chassis) but with completely separate power supplies per channel. To turn this amp into 2 monoblocks simply requires an additional chassis and IEC plug/switch.

The ACA is a stereo single ended amp but people take a single 2 channels of ACA and feed it with a balanced signal (1 channel with +/- and gnd input) and turn a stereo single ended amp into a balanced monoblock.

Monoblock is a reference to the PSU per channel separation. Balanced or unbalanced refers to the input/output signal - 3 wire is balanced where + and - are the same signal in opposite phase.
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Additional clarification - you can have an amp like the Aleph J and other Aleph variants which is what is called a differential amplifier so it has the ability to amplify a balanced signal (V+/V- and GND) but you can still feed it with an unbalanced (V+ and GND only) signal. In that case you connect V- to ground.

This is why the Aleph J has both XLR and RCA inputs but you do not connect both of them at the same time. One or the other. If you are using the RCA connections then you need to connect a jumper from V- to ground.

To really screw people up, you can install V+ rca, V- rca and feed it balanced signal using 2 rca connections instead of a 3-wire XLR but electrically it's the same.

There is nothing stopping you from connecting a stereo aleph J like an ACA and feeding it with a balanced signal but you don't need to so just don't. The Aleph has the ability to amplify a balanced signal for each channel already while the ACA doesn't.
Thanks to twitchie for the assist. :)
That picture of my Aleph J was taken during initial bias setting, before I had hooked up the inputs. I'm still just using the RCA inputs, as I don't have a proper balanced preamp yet. I included the XLR connectors because it's much easier to do that at the beginning. With single-ended inputs, the IN- connection on the amp PCB is tied to GND. If and when I get around to using the balanced inputs, I'll need to re-wire things a bit.
Thank you Zen and Tungsten for the advice. I'll explore the Aleph J in most detail.

I like the full dual-mono idea. Other than the cost of another chassis and some minor parts...why go dual mono as opposed to monoblocks? Is there a way to mount a single channel that would take advantage of the heat sinks of a full chassis? I don't know if there's enough thermal connection for the heat to migrate naturally to both sides, maybe I could tie them together with some copper strapping. With two boxes I'd have more room for a bigger power supply. Maybe I'm over thinking this....:D
To have a complete amplifier per channel, one needs to build a stereo amp with separate PSU for each channel, or to build completely separate monoblock amplifiers for each channel. This is what an Aleph J looks like with a dual-mono power supply. Even though this is in a single chassis, the only thing in common between the two channels is the power cord. The power switch is a double-pole, single throw type.

Tungsten - can you please explain how the transformers are connected to the PSU? It looks like you used toroids with dual secondaries...if you are doing a dual-mono set up would it be better/easier to use a transformer with a single secondary? Your layout looks good and this will be a useful reference for me on how to lay out the chassis. Thanks!
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Good morning Guys, I’m pretty much a newbie here. I have a little experience, learning a lot gaining more with future projects. I’ve successfully completed an F5T with the help from the group on this forum and love it. I’m in the process of building Wayne’s BA2018 Linestage but blew up the Muse’s VCM board and I’m waiting for a new one to arrive any day. With that being said, I’m not trying to go off topic because I bought boards from the store to build the Aleph J. Can the Aleph J be built as a monoblock and utilize both channels into one chassis doubling the 25wpc output? If even possible, building a dual monaural amp is probably not a good idea because trying to incorporate (4) boards into a single chassis would’nt work do to size constriction and dealing with the heat. I’m concerned the Aleph J may not powerful enough for the speakers I plan to pair it with especially since they’re nominal impedance is rated @ 4 ohms with 90db sensitivity. Any thoughts?
You are confusing mono blocks and bridged amplifiers. A mono block just means a single channel of amplification with its own power supply and chassis. Basically a stereo amp split into two chassis, theory being that the separate power supply means better power to each channel and the physical separation allows you to put each amp closer to the speaker, meaning shorter speaker wires. Bridged is a whole different animal. A dual mono means putting two separate power supplies into a stereo amp such that both channels are essentially isolated beyond the AC power switch. This is how I built my Aleph J in a 5U chassis and it made things tight. If I were doing it over I’d probably build two separate monoblocks in smaller 3 or 4U chassis. Again, not bridged just building the amp to the standard design. In terms of whether 25W will be enough….even on 90db speakers I think you’d be ok unless you have a huge space or want to listen really loud. What kind of music etc?
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@JMacII, thank you for the reply. I wouldn’t say my listening area is huge. It measures 18’x26’ with 8’ ceilings. I mainly listen to jazz (Diana Krall, Norah Jones, Tracy Chapman type stuff) and rock (Steely Dan, CSN, Doobie Bros etc). Sometimes I’ll kick it up a few notches with some heavier stuff but I have other systems for that if I’m in the mood to go crazy. From my listening position set of power meters on an amp of mine, it seems like most of my listening peaks at about 30w maybe slight less. With a db meter at my listening position, it Max’s @ about 90db averaging in the 82/84 db range. Btw, I have no idea if this means anything at all.

Isn’t there a difference between bridging and paralleling the outputs on an amp? My understanding bridging increases wpc but loses output current? Paralleling outputs increases current but not wpc ? Is there a way to achieve both Aleph J?
To hit 90db peaks at 4M with a 90db speaker should take about 16W if my math is right. You’re listening pretty loud if you’re consistently seeing 30W peaks. Or your amp meters are off. You can look up the math, but each 3db increase in SPL takes double the power. So 90db 1w/1M, takes 2W for 93, 4W for 96, 8W for 99, 16W for 102db etc. then you have to subtract dB for distance. 4M is about -13db I believe. Anyway, the Aleph is well suited to the type of music you listen to at volumes I’d think would be typical.