No. I just so happens that 28 V is commonly available voltage among the various desktop power supplies.
Lower supply voltage -> lower max output power. Depending on how loud you play and the impedance of your headphones, the could mean that the amp would clip more often with a lower supply voltage. Thus, one could string together an argument along the lines of: lower voltage -> harsher sound.
BTW: I now have both PCB options of the HP-2 and chassis in stock ... and boxes to ship them in. Yay!
Tom
No tradeoffs for more power. That works for me. I have high impedance headphones and a son who listens to, bass heavy, rap music.
That's great news, I can't wait till the board and chassis arrive...I'm more than a little curious and excited to hear what an amplifier with 125 dB dynamic range sounds like!
That's great news, I can't wait till the board and chassis arrive...I'm more than a little curious and excited to hear what an amplifier with 125 dB dynamic range sounds like!
Good things come to those who wait. Your order shipped this afternoon. Given that most of this week is holidays, we'll see when it gets there. Maybe Friday... Maybe Monday. Canada Post did warn that they're experiencing a higher than normal package volume, so I suggest adjusting expectations accordingly. 
If you feel like it, I would welcome your build pictures, build experience, and experience with the amp posted here. I love to see my circuits built.
Thanks,
Tom
If you feel like it, I would welcome your build pictures, build experience, and experience with the amp posted here. I love to see my circuits built.
Thanks,
Tom
Got the board and chassis yesterday!
The chassis is beefier than I'd anticipated. Especially the faceplate, 3/8" thick aluminium...quite a bit thicker than the Hammond enclosures I'm use to working with.
I'm still waiting on one part from mouser...they'd claimed it would be in today but I see they're saying Jan 10th now. Once it comes in and I start building I'll let you know how it turns out...
The chassis is beefier than I'd anticipated. Especially the faceplate, 3/8" thick aluminium...quite a bit thicker than the Hammond enclosures I'm use to working with.
I'm still waiting on one part from mouser...they'd claimed it would be in today but I see they're saying Jan 10th now. Once it comes in and I start building I'll let you know how it turns out...
Yeah. The 10 mm front is nice, even though it does add cost. It also gives the amp a nice finished look.
Last I checked, it was the power supply connector that was temporarily out of stock at Mouser. Digikey has it... They usually charge $8 for shipping on orders less than $100. Annoying to pay $8 to ship a $2 connector, but it'll get you going faster should you so desire.
Tom
Got my board during holidays and ordered most of the parts last week. I made two separate orders, one from Mouser and one from Digikey as all parts were not available on Mouser.
Regarding the Meanwell charger, Digikey was ok shipping it to Europe so no problems there.
I would like to max out this thing, so I was wondering if there's anything that can be improved vs the stock BOM (space and cost no object)?
Tom you probably wanted to keep the BOM cost in check but it would be nice to know if you think there was a place to splurge, what would it be?
Regarding the Meanwell charger, Digikey was ok shipping it to Europe so no problems there.
I would like to max out this thing, so I was wondering if there's anything that can be improved vs the stock BOM (space and cost no object)?
Tom you probably wanted to keep the BOM cost in check but it would be nice to know if you think there was a place to splurge, what would it be?
To be honest I don't think so. You can always buy more expensive passives (wether it be automotive grade, mil-spec or fancy audiophile stuff). But if you are going for improvements that can be measured I don't think you will be able to find much. Even from a durability standpoint I don't think there is any real improvement to be had over the stock BOM.
+1.
About the only thing I will be trying is a separate linear supply and I personally have my doubts whether it would make a massive sonic difference. The PSRR is uber high. The 1khz spectra and multimode IMD measurements are downright amazing. Understand that the measurements on the HP-2 are quite phenomenal and only bettered slightly by the HP1 (on difficult 32 ohm loads) and by his commercial TCA headphone amp which make obvious sense. In fact, Tom's existing AP gear is having a hard time discerning the differences!
For now, build the board as is because it is one of the more challenging thru hole boards to build anyway with the massive number of parts. Different sized parts would be a shame as the design has been perfected with such an excellent layout. Besides, none of us are privy to the schematic so you have NO idea what the signal path is. If you are going to tweak, try different colored push button on/off switches (stock is green) or different output connector options, or even a different volume control (stepped attenuator perhaps).
I think what you will find is that most of Tom's circuits are just well done. He is one of the finest analog circuit designers in the world and his attention and understanding of PCB layouts is second to none. This isn't surprising given his background. You can of course come up with a better enclosure if you want and that would be lovely to see.
Best of luck,
Anand.
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Yeah. Parts drift in-and-out of stock at Mouser all the time. That's the unfortunate reality in this just-in-time world. Good to hear you were able to collect the parts you needed.
Sweet! Thanks for confirming.
Mount the heat sink to the two OPA1656 ICs using thermally conductive (but not electrically conductive) epoxy. You can find my recommendation in the design doc. Mounting the heat sink this way will have far greater impact on performance than anything else you can tweak.
Some may wish to use a different volume control than the Alps RK271 "Blue Velvet" volume pot I specify. I've left plenty of room in the chassis, so most pots and attenuators with 6 shaft should fit just fine.
I find it hard to improve on the Alps RK271-series "Blue Velvet" pot. You can get better channel tracking by using an attenuator. That said, the tracking error only comes into play at the discontinuities between sections in the pot. I've measured these in the past (for the lower grade RK097-series). I found the tracking errors were only measurable within something like ±1º rotation of the pot. I've never noticed them in actual use of an amp, except a high-gain tube amp I tried at one point, where the volume was always turned almost all the way down.
You could use different opamps, including the various "discrete opamps". Do be careful, though. Some "discrete opamps" have a -40 dB/dec rolloff on their AVOL and are only stable around unity gain. Others have a friendlier -20 dB/dec rolloff. I recommend using opamps with the friendlier -20 dB/dec rolloff. That said, to date I have only seen one "discrete opamp" that performed better (and only marginally better) than the LME49720, so in my opinion using "discrete opamps" is mostly an incredibly effective way of draining your wallet for no added benefit. After all, they are 15-20 dB more expensive than a regular opamp.
BTW: If all the stuff I just wrote about the rolloff slope of the AVOL curve just went over your head (and I wouldn't blame you - stability analysis is a difficult concept even for many engineering students), please don't mess with "discrete opamps" in the HP-2.
The automotive components just give you higher temperature range. -40C to +125C instead of -40C-85C. Big whoop!
There are cases where using automotive components could make a difference. Using X8R rather than X7R or X5R, for example, would result in less variation in capacitance versus temperature. I design my circuits such that these variations in capacitance have no impact on performance (I don't use X5R/X7R/X8R in the signal path), so it's not an issue in my circuits.
Many audiophile components are measurably worse than the corresponding off-the-shelf parts. They're also often 20-40 dB higher cost.
At one point, I measured the parasitics of three 220 nF, 630 V capacitors: 1) Panasonic EF-series ($0.50/each); 2) Solen brand polypropylene ($2/each); 3) Mundorf silver-in-oil audiophile caps ($20/each). The Panasonic measured lower ESR and ESL (i.e. was closer to an idea cap) than both the Solen and Mundorf. The Mundorf had about 10x higher ESR than the Solen.
I have my doubts as well. Using a good regulated linear power supply won't hurt performance and it could be a fun self-contained project, so if that suits your fancy, have at it!
+1
Build the board according to the BOM and get it working. Then modify (if you still have the need/desire).
Those would be my choices for tweaking as well.
Tom
I've yet to find a digital potentiometer that offered as low THD as a regular pot. I've also yet to find an encoder that provided the right feel, but I haven't looked very hard either. The touchy-feely aspect of the volume control is important for the experience of using the amp (which will impact the listening experience through various cognitive biases).
If I was going that route, I'd probably use of of the various volume control ICs. I seem to recall that Cirrus Logic has a high-performance one.
Tom
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PGA4311 (I assume you need 4 independent audio channels when using differential) has 120dB Dynamic Range, 0.0004% THD+N at 1kHz (U−Grade) 0.0002% THD+N at 1kHz (A−Grade). The Cirrus you mention might be the CS3318 (8 channels) being -112dB THD+N and 127dB Dynamic Range (the 2 channels chip CS3310 is noisier).
I had the same concern long ago and found out there's a feature called "High Torque". If so, try Grayhill SERIES 62A,V,D.. Pity the high torque is not part of the parametric search engines at Mouser et al. Disclaimer, still haven't purchased this encoder, still designing my volume controller based on WM8816, encoder torque will be the last part, if money allows.
X5R or X7R in the signal path? I have rarely seen that. I remember from building the HP-1 that you pretty much only specified X7R ceramics with high voltage ratings as decoupling caps, that's why I assumed there is probably no real improvement to be had by upgrading passives. Stepped attenuators are cool, but the 48 Step ones from Elma are so freaking expensive. 180 Euros for a Stereo SMD Version + plenty precision thin film resistors on top of that...ugh.
Thank you all for your opinions and insight, I'll stick to the stock parts sans exotics
It's really great to hear the rationale behind the design and component choices.
I'll see what I can do with the chassis parts first. After I get things running I can build a linear PSU inside the chassis to get rid of the external PSU (for cleaner looks and maybe some sound improvement which remains to be seen).
Anyway, looking forward to getting the remaining parts and start building!
It's really great to hear the rationale behind the design and component choices.
I'll see what I can do with the chassis parts first. After I get things running I can build a linear PSU inside the chassis to get rid of the external PSU (for cleaner looks and maybe some sound improvement which remains to be seen).
Anyway, looking forward to getting the remaining parts and start building!
Here's what I said:
It appears you missed the not in don't highlighted in red. As in: I do NOT use X5R, X7R, X8R in the signal path.
Agree with that. The Elma switches are awesome, though.
Tom
Nice find. Thanks for sharing. I may have to get one of those to see if it feels right.
Edit: Um. Nope! $60-100/each. That's a non-starter. It does give me something to look for in the encoder data sheets, however.
Tom
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I am planning to build the HP-2 as a pre-amp/HPA. I have 2 sources both single ended. What would be the recommended method to switch between the 2 sources. Is it ok to adapt the balanced input to single ended by connecting pin 1 and 3 for ground and use the switch to select between the 2 inputs as designed or should I connect a selector switch to the single ended input only and use the external selector for input selection?
Sweet! That would be a good application for it.
Just connect Pin 1 to Pin 3 on the XLR connectors. Take the RCA shell to the Pin 1/3 and the RCA centre pin to Pin 2 of the XLR connector footprint. Then simply use the on-board input selector switch.
Tom