A Very Compact Hybrid Amp

[fishski13]

any ideas on the BOM $$$.

 

[cfcubed]

Quote:

Originally Posted by fishski13 /img/forum/go_quote.gif any ideas on the BOM $$$.

I believe we have a pretty solid BOM now & I've placed an order based on it that will allow me to build a complete amp using the prototype PCB (due in a week or two).

It's looking like the cost of a completed amp w/AC adapter and case should come in at roughly $125 USD (plus various vendor shipping costs & any taxes).

I guess the other question on peoples minds is its performance sound-wise, and this should start to be answered by the proto PCB builders in the coming weeks. But my impression from listening to the POC is that builders are going to be pleased. And the amp is quite capable of driving my low-Z D2000s as well as my hi-Z DT-880s, contrary to my experiences with say, a typical opamp buffered SOHA.

 

[m0b1liz3]

Quote:

Originally Posted by cfcubed /img/forum/go_quote.gif And the amp is quite capable of driving my low-Z D2000s as well as my hi-Z DT-880s, contrary to my experiences with say, a typical opamp buffered SOHA.

Wow, this is exactly the type of project I was hoping for my D2000s. I wanted something compact that doesn't take up much space on the desktop, sounds great and can work on 110/240v with a wallwart. I built a Millet Starving Student to that end but this sounds like it would be a step up from that.

Any idea on how hard this build will be? Sounds like you will be cramming alot of things into a small board. However I might be interested in prototyping this...

 

[runeight]

There is nothing unusual in the build, but there are a lot of components on this tiny board. If we follow the usual procedure (resistors and diodes first, small caps and transistors and opamp sockets, chokes and bigger caps, etc.) the build will be straightforward.

The main challenge will be that there are so many devices packed so close together there will be lots of opportunity to put something in the wrong place. So a slow and careful build process will be the best way.

And, of course, I'm saying this without ever having built one so I really don't know for sure.

 

[cfcubed]

Quote:

Originally Posted by m0b1liz3 /img/forum/go_quote.gif Wow, this is exactly the type of project I was hoping for my D2000s. I wanted something compact that doesn't take up much space on the desktop, sounds great and can work on 110/240v with a wallwart.

Sounds like a great fit... Tube rolling should be fun & easy too as there are no pots to adjust and with off-board heater switch won't need to open the case. Cost of the amp should be nice too, the small size means reduced case & PCB costs and features like e12 & discrete buffers are onboard.

Quote:

Originally Posted by m0b1liz3 /img/forum/go_quote.gif Any idea on how hard this build will be? Sounds like you will be cramming alot of things into a small board. However I might be interested in prototyping this...

"Sounds like you will be cramming a lot of things into a small board" - I think this sums it up pretty well

I believe runeight did a remarkable job WRT PCB layout, as expected, but there are a lot of components there and as I said, about 400 holes to fill. We did try to "reuse" parts where possible to reduce the total # of unique components to mitigate this. One will want to stick to the BoM (e.g. using 1/8 watt resistors, etc.) and we have only 1.1" from surface of PCB to bottom of target case top (watch electrolytics).

Casework involves off-board wiring of pot, jacks, heater switch and optional power switch. As heatsink room is tight on the 24V reg, might want to case-mount that too.

Runeight is handling the distribution of these initial prototype PCBs, so I suppose "he's making a list, checking it twice...."

Next up for me is heater supply...

N.B. runeight's post just happened too.

 

[nsx_23]

Would this be too difficult for somebody looking to step up from a CMOY to construct? No SMD parts makes this project very attractive to me.

 

[adamus]

looks doable to me. There are plenty of 3 legged beasts to get mixed up. Other than that its through hole paint by numbers...

giev it a go, plenty of people on here that would help you. But i would say go for a design that is well tested, this will be in protoype.

 

[nsx_23]

The PIA for me is sourcing parts, especially tubes.......

I wouldn't mind trying out a prototype though. Doesn't look too difficult, and I'll be sure to get the parts the right way round before I solder

 

[cfcubed]

The Heater Supply

Using a 24VAC wallwart to attain a higher 85V B+ (as runeight discussed) coupled with support of 6.3V and 12.6V heated tubes presented an early hurdle for the project. With no center-tap, this meant deriving these low voltages from an approximately 35VDC supply. A transportable amp implies being carried about, and this posed both size and heat constraints. Conventional linear regulation, such as a power resistor followed by a linear regulator, could not be employed.

Our solution is below…. A switch-mode heater supply, which as far as I can tell, could be a first for DIY headamp projects (although not without precedent for DIY audio). Reasons for this may be: 1) common, low-cost switcher implementations (without ripple filters) are less desirable for audiophile applications; and 2) early, lower-frequency devices require larger passive components and could present issues too close to audible range.

Edit: We found that C3H needed to be a larger value to eliminate very faint hum when using 6.3V/high draw tubes w/efficient cans.

My desire to avoid SMD solutions, especially for prototyping, drove us to National’s (150kHz) LM2595 step-down (buck) switching regulator. Listening to this implementation in the POC with ripple filter (L1H / C1H), I was unable to detect any audible differences between SM heater and use of a battery for heater supply. The significant ripple reduction we see through use of a post-ripple filter is similar to that seen in figure 17 on page 20 of the LM2595 spec sheet

National’s Simple Switcher tools were used to drive associated component values and selection. The design uses 0.8A parts and supports up to 600ma 6.3V or 12.6V heaters. In feedback, R1H (9k) alone yields 12.6V and switching R3H (7k) in parallel with R1H yields 6.3V. The LM2595 seems a good fit and requires no heatsink in our implementation. Builders will want to follow the BoM components here, e.g. use of low impedance caps, and proper choke and diode values and ratings.
Higher frequency (260kHz+) SMD switchers could be considered in a future revision to save even more space.

This heater circuit coupled with runeight’s use of capacitance multiplier filters in HV and LV sections makes for a small, high-quality power supply.

 

[Bismar]

Just out of curiosity, seeing as your going for compact here. Why not take a leaf out of AMB's book and have two pcb's stacked on top of each other?

Certainty looks possible with the height of some of the caps in the rendered picture.

 

[runeight]

Bismar, we actually talked about this. It's a good idea. But since we were able to fit everything onto the 120mm x 75mm board after a bit of work, we didn't try to squeeze any farther.

However, who knows what may happen in the future . . .

 

[Bismar]

Cool, a cube 75 x 75 x 75 would be utterly cool though

 

[cfcubed]

Quote:

Originally Posted by Bismar /img/forum/go_quote.gif Why not take a leaf out of AMB's book and have two pcb's stacked on top of each other? Certainty looks possible with the height of some of the caps in the rendered picture.

We sort of let Hammond Manufacturing drive the PCB size

Their cases are reasonably priced and available, but not that diverse in the small sizes. Also, there was/is new ground here we'd like to proof and using a single PCB is easier and less expensive.

WRT to cap heights, we only expect .1" clearance from the tops of C5P & C10P to the bottom of the target case top. There are also off-board components that need clearance (pot, jacks, fuse) that may present a challenge in front panel placement WRT symmetry as it is.

But, if there is sufficient interest & satisfaction with the amp we can look to further optimize the board (e.g. board mounting some things, more room for coupling caps, etc.)

 

[nsx_23]

With the casing, do you expect it to be difficult, or are you planning on doing some custom front panels kinda like AMB does with the mini3? Heck, a full custom case would be very nice.

I'd love to see a group buy for this. I'm really, really interested.

 

[cfcubed]

Great to see/hear the interest! We're hoping it'll nicely fill a niche in current offerings...

WRT front panel/case offerings, guess this can be considered after the initial amps are built/listened to/etc. Plan to have first full build on proto PCB in 2-3 weeks. More will be known then about the looks of the front panel.

Interest, experiences, opinions and suggestions from the community can then drive things... Things like board-mounted mini-jacks & pots facilitate standard front panels & simplify the build but can also limit options (esp in such tight quarters).