Neurochrome HP-22 - The $90 high-performance DIY headphone amp

[tomchr]

It's great as long as you don't have to drive in it. I a moment I will excavate my car so I can go for a grocery run. Getting out of my neighbourhood is the tricky part. The main roads are clear. The side streets will probably be cleared tonight or tomorrow.

In other news: The HP-22 boards have arrived at my local UPS pick-up point. I expect to start shipping them tomorrow.

Tom

 

[tomchr]

Final boards are in.

Tom

 

[Bioguy]

Just finished my HP-22 today, fun build!

 

[Bioguy]

The prep work was the longest part of the build, cleaning, measuring and pre bending parts, but after that everything fell into place with no unexpected pitfalls

 

[Bioguy]

Measurements are a challenge for me, I’m still putting together a basic lab bench, but I managed. The build documents require a test tone at a specific output voltage, I had to jerryrig it but managed to get close (I had 520mV instead of 500mV)The observed output was near the expected.

 

[tomchr]

Fantastic! It looks like it came together nicely for you. Thanks for sharing.

Tom

 

[DualTriode]

Great questions actually. Thank you.

The OPA1622 is able to provide more current with a positive swing than with a negative, so not surprisingly it clips sooner on the negative rail than the positive when forced to current limit. The clipping behaviour is clean, though.

View attachment 99078View attachment 99079

The behaviour with asymmetrical power supply is clean as well. Once you get down to +1.5/-12 V the amp will turn off as its enable signal goes low. I ran it at +2/-12V and +12/-2 V in the plots below. A 9 V battery will provide around 5.5-6 V when fully discharged, so there's plenty of margin there.

View attachment 99080View attachment 99081

The reason the HP-22 provides a larger pop than what you see in the data sheet is because the data sheet figures show the behaviour when the enable line toggles while the power supply remains stable. My graph shows the behaviour when the power supply is turned on and off. I did fiddle quite a bit with the delay on the enable line and what I show was the best performance I could get without getting into elaborate circuit design.
Start-up and shut-down are surprisingly hard things to get right. While at TI I spent quite a while designing a power-on reset circuit for a digital circuit. It's one of those "can't you just use an RC?" types of circuits that ends up being a few weeks spent simulating the circuit over various corner cases (process, voltage, temperature) to ensure that it produces a reliable reset, even on a power glitch or brown-out. Oh, and please fit it in zero area, because it's only active once...

Why two PCBs? Because the OPA1622 is impossible to solder by hand. OK. Maybe not impossible, but darn close. The device measures 3x3 mm and has 14 pads connecting to it underneath. If you have a solder paste stencil, solder paste, and some proficiency using those, you can solder it yourself. I have done the occasional toaster oven reflow (or hot air reflow) for prototypes. But it is not something I would want to support via email. So I leave the hard part to TI.
I did consider having the builder remove the 3-pin terminal block from the TI eval board and connecting the two boards by wires, but decided against that. First off, the lead-free solder TI uses is pretty hard to work with. The EVM is a through-plated board (four layers, I think, possibly two) and if you don't have the ability to heat up all three terminals at once, it's pretty hard to get that connector off. And good luck supporting that by email when someone pulls the through-plating out with the connector. Secondly, the footprint required for the wires would interfere with the RCA connectors. I really like the compact nature of this amp and didn't want the 'shield' to become too big. It's really no big deal to connect two wires to one of the terminal blocks. I use 20 AWG wire. It fits just fine.

I don't have a standard enclosure in mind. I'll leave that as an exercise for the reader. I'm sure Hammond has something that'll work. I'll probably search a bit and see if I can find something that'll fit. I will provide a drill template so you can at least get the holes in the right locations.

Tom

Tom,

Nice Keysight O-Scope.

For the purpose of playing with the toys.

I have your HP-22 headphone amplifier running on my bench with your power supply.

I just ordered a Texas Instruments INA1620EVM board. The plan is to make wire connections between this different Headphone evaluation board and the HP-22.

I will post AP plots for fun.

Thanks DT

 

[Kadent]

@tomchr Pardon the resurrection of an old (but good!) thread. However it is that time of which I am getting slightly bored with my current headphone amp, and considering building the HP22.
A few questions I had in mind, of which possibly others may be thinking as well. One requirement in my application is that my headphone amp must also contain variable pre-amp outputs of at least 1.8-2.1v. I figured the gain of the HP22 is near 3 or so, which means if I have a fixed voltage of 2 (from a DAC for example) it would be quite hard to get 2 back out of it. I've used some voltage dividers before in terms of a series of some resistors for subwoofer applications, however I am unsure if this will effect the performance of the HP22 board.

One other question that I had was the use of different op-amps on the blue input board. The LME49720 is quite stellar if used right, however I've always been curious of the different sound each op-amp has. Especially those fully discrete op-amps. Even though the difference of rolling them cannot be measured (from what I saw by others) they claim it sounds different? At this point we're not not listening to the music, we're listening to the equipment. But it still intrigues me in the name of science

Kaden

 

[tomchr]

The HP-22 has 10 dB gain (so 3.16x), though, you could pluck a couple of resistors and get unity gain.

You can use discrete opamps if you wish as long as they're stable at whatever gain you choose.

Tom

 

[tomchr]

BTW: I have a video where I show how to build the HP-22:

Tom

 

[Kadent]

The HP-22 has 10 dB gain (so 3.16x), though, you could pluck a couple of resistors and get unity gain.

You can use discrete opamps if you wish as long as they're stable at whatever gain you choose.

Tom

Would it still be possible to have a gain of 3.16 at the headphone out and unity gain tapping off the unbalanced output of the HP22 to the input of the evaluation board?
I assume that would make more sense as to modifying the evaluation board which I would guess is already in unity gain.
Kaden

 

[tomchr]

You could do it by hacking the eval board. That's not going to be super fun unless you have a pair of hot tweezers, though. I forget if the resistors are 0603 or 0402 sized. If the latter you could entertain removing them by heating up both ends at the same time with a regular soldering iron. Then solder on different resistors. You'd have to swap four resistors and possibly two capacitors.

Tom