Presonus HP4 modifications

This was my first major modification project, something I’ve been meaning to post about for the last couple of years––it’s morphed over time, I’ve added and removed components as I learned from other projects. The Presonus HP4 is an excellent and powerful headphone amp, and can be found for cheap on Ebay, Reverb, etc. I think I bought this one for about $20 because was missing its power supply or something. Sounds great stock, delivering a whopping 130mW of power to each channel, and there are two designs that I’ve seen: an older version (the one I have), pictured first, which uses six quad MC33079 op amps (DIP package), two for the input and monitor output buffers, and one for each channel (two op amps paralleled per channel)…


Below, the newer version with four quad MC33079 (SMD), two again for the input/output, two for gain stages to headphone amps, and a discrete transistor stage to drive headphone current (I don’t know what the transistors are, so there’s no accounting for noise performance, etc.). Not my picture, taken from this thread:img_3902_edit

The MC33079 is fine, old design meant for audio. Like I said, this thing sounds good already, it doesn’t need help… but OBVIOUSLY I had to find out for sure!

Anyway, I was inspired by this thread, where someone modded the newer version.

Stock capacitors are all cheap Chang caps: 47uF/35V bipolar at the input and monitor output; 1000uF/25V and 47uF/25V in the power supply; and (at least in my older version) local bypass caps in the headphone amp section, 10uF/25V on the power rails (+/- 15V!) and a 470uF/25V cap on each channel that seems to decouple headphone ground from main amplifier ground.

Replaced the input and output bipolars with 47uF Panasonic SU-A, bypassed on the other side of the board with WIMA MKP 0.47uF (0.1uF would be fine). Because the original chips were DIP, I took the opportunity to design and order some custom dual soic to DIP adapters (which I believe anyone can order as such from Aisler), replacing input buffer with OPA1612. I could have used OPA1642 as well, and perhaps it might have been preferred; there is some justification for a FET-input op amp like this in the input buffer position since the input impedance is 10kohms. This chart from TI demonstrates why, having to do with voltage noise versus current noise. Using OPA1612 was maybe an unnecessary expense, but it sounds great all the same.

I soldered four 100uF/16V Nichicon R7 organic polymer caps to either side of the 0.1uF ceramic bypass caps already installed around the input/output buffers, just to offer some more isolation from the power supply. Note the orientation. These caps have extremely low inherent resistance (ESR) and can deliver current very quickly to the op amps when needed.


In the headphone stage I used my soic/DIP adapters again, installing two OPA1688 chips per channel. A relatively new design from 2015, the 1688 is designed for headphones (esp. mobile applications like portable DACs, hi-end smartphone amps), putting out 75mA of current per channel, meaning paralleled as they are here each headphone channel is getting 150mA. Excellent specs overall, very low THD, etc. You could use other more exotic op amps here with crazy bandwidth and warp speed slew rates, but honestly for headphones there isn’t any need, and in fact there can be significantly diminishing (or negated) returns if you put the wrong op amp in the wrong circuit. This offers a classic and realistic explanation of all this stuff. I think the OPA1688 sounds great and plenty transparent enough for reference use: RME’s ADI-2 Pro uses six of these in each headphone amp, three in parallel per channel.

img_0147More power supply decoupling on each channel, originally 10uF/25V, now 100uF/16V Nichicon R7 which barely fit (also added 0.1uF ceramics on underside of board––Presonus omitted this in their design, which is strange, since this is standard practice). Ground decoupling (big cap in center) is Panasonic FC, 470uF/25V––probably could be anything, doesn’t really touch anything other than ground, essentially isolating amp ground from headphone ground. Also pulled original resistors on either side of op amps, originally 120 ohms each (the old standard for headphone outputs), and replaced with 1 ohm resistors. Some resistance is necessary to isolate op amps from capacitive load of headphone cable to avoid oscillation, plus as short circuit protection, but lowering output impedance will make these amps more usable for low impedance drivers like IECs and earbuds. General guideline is to have 8-10x lower output impedance (amplifier) than input impedance (headphone) to avoid any wonky distortions in frequency response––in other words, amp with 4 ohm output impedance shouldn’t drive any lower than 30 ohm headphones, 38 ohm amp no less than 300 ohm headphones, etc. Presonus says HP4’s output impedance is 51 ohms, meaning 400 ohm cans and above will be linear and anything below that will have lumps and bumps in the frequency response. You can read about this here.

Very simple PSU, two regulators, 7815 and 7915, 1000uF/25V and 47uF/25V per rail, used Panasonic FC, but again probably could have used most anything modern and reliable (low ESR, high heat tolerance, e.g. 105ºC)––Nichicon, Panasonic, United Chemi-con, etc. Hot-glued caps together because tall ones were wiggling around a bit.img_0148

And finally the underside, showing WIMA MKP bypass caps and ceramic 0.1uF decoupling. img_0149

The whole shebang!img_0151

Like I said, this amp is already a great deal, and these modifications really open it up because the basic elements of the design are very good. The sound is clear, clean, vanishingly quiet, incredible separation and clarity of voices, amazing perceived sense of space around and behind my head (listening with HD600), and absolutely no apparent personality of its own. What comes in is what comes out. The OPA1688 sounds spacious, if I were to put a word to it.

(6) 47uF/35V Panasonic SU-A ($0.63/ea)

(6) WIMA MKP 0.47uF (or 0.1uF) ($0.29/ea)

(2) 1000uF/25V Panasonic FC ($0.79/ea)

(2) 47uF/25V Panasonic FC ($0.29/ea)

(4) 470uF/25V Panasonic FC ($0.60/ea)

(12) 100uF/16V Nichicon R7 (or other low ESR organic polymer) ($0.72/ea)

(8) 0.1uF/100V ceramic capacitor ($0.49/ea)

(6) dual SOIC to DIP14 adapter boards (~$10 + shipping, maybe $20 total, so worth getting a bunch in one order)

(4) OPA1612 ($6.41/ea) [or OPA1642 ($2.08/ea)]

(8) OPA1688 ($1.56/ea)

(16) 1 ohm 1% 1/4W metal film resistors ($0.18/ea)

Total = about $80 with shipping and taxes (here’s a Mouser project for anyone who is interested). Again, that’s for this older version with 6 quad op amps. If you modify the newer version it’ll probably be about half that, since you won’t need the adapter boards and fewer capacitors (anyway, I added the four extra for PSU bypass at the input/output buffers, as earlier described). Furthermore, the newer version uses only quad SMD op amps, which limits your options, but ultimately forces you to save money: I’d use OPA1644 for front end and OPA1604 for headphone amp buffers. You can probably get a used HP4 for about $50 (or less), which will probably put you out about $120-130 for the whole project. Conversely, if you consider the cost of a CMOY project, which can easily run up to $50 for lower performance (wonky split 9V rail, for example), $80-90 for Super CMOY (designed around the OPA1688), or $129 for the O2 headphone amp (which puts out 140mA/channel, versus 150mA in my modded HP4), this is a pretty great deal for four extremely powerful headphone amps. Of course, this assumes you are the kind of person who delights in voiding warrantees and has the equipment to do so…

As a final note, I should mention that this design leaves two op amp channels in the input/output buffer section unused. The monitor output of the HP4, a useful feature for using the device as a monitor level control, is ‘impedance balanced’, meaning the ‘ring’ connection on a TRS plug is connected to ground through a 51 ohm resistor––practically speaking, it’s an unbalanced connection. It’s possible I could use that second channel to create an inverted signal and fully balance the output, but I decided to let this rest for now… An idea for someone else!

Rokit 5 G3 Limiter and Stand-by/Mute removal

Finally I got around to figuring this out. Had to poke around a little bit.

Ran across this schematic in the land of diy forums. It’s for the G2 Rokit 6 amps, which had the notorious black goo of death problem, so totally different design (well, probably not that different… these budget companies never really redesign their stuff if they don’t have to, unless they can make it simpler/stupider), but it struck me that their limiter was using just one bipolar transistor right after the input buffer, so I went looking for lone transistors near the input, and lo and behold, there is only one (Q5, just above the right-most large ceramic capacitor).


Oscilloscope revealed bits of signal being leached off, so it seemed I had found the culprit. Removed Q5, but both amplifiers suddenly muted… hm.

Traced signal from top-most pin of Q5, found it running to the ‘mystery chip’ just above, U5. Looks like it’s some kind of gate circuit: when it receives enough input voltage at pin 5 (lower left pin), seems like it opens pin 4 (upper left) and passes 5V, which is what the amp circuits need to unmute and shut off standby (datasheet for TDA7296 actually specifies 3.5V minimum to disengage mute/standby function).

This reveals a couple of things about KRK’s design. First off, the limiter is running all the time. Even at relatively low levels it seems to be leaching signal, and globally for both treble and bass. Secondly, the mute circuit depends on the output of the limiter in order to engage, so the limiter needs to be operating rather aggressively in order to unmute the amplifiers. This is why, when I removed Q5, the mute circuit wasn’t seeing any input. I’ll say again: IF YOU HEAR ANY SOUND, THE LIMITER IS LIMITING, A LOT. 🤦‍♂️

So… remove the transistor (hot air is easiest), remove the IC, and short pins 3 and 4 as shown (I used a little piece of wire clipping).

KRK mod detail

Now the amp engages immediately, without any lag. There are electrolytic caps decoupling the input to the mute/standby pins in the amplifier, so there won’t be a big pop. Can see the voltage at pins 9/10 of TDA7296 rising and falling gently with on/off.

Things sound ‘snappier’, more precise––these are some real speakers now! This project has officially reached it’s zenith. EDIT: I’m shitting my pants at how amazing this sounds. All my previous mods really seem to have come to life now, the low end is incredibly clear and integrated (it’s like all the overtones just lock), I find I’m listening with the volume a bit lower, and both speakers sound more balanced (the right speaker always felt just a little softer, and they’re ‘matched’). My reference recordings sound uncanny. And just over a little transistor…

*PROCEED AT YER OWN RISK* I listen primarily to Classical stuff, I’m not pounding EDM or anything. I assume this circuit is here for a reason.

If you want someone to do this for you, check out this guy: Used to be a head engineer for Black Lion Audio, studio monitor ‘enhancements’ are his specialty. I just use the TI SoundPlus series of opamps (which I think sound great, enough of an improvement), based on their functional equivalency to what’s on the board (FET/bipolar, comparing input impedance, etc) he actually hand selects different opamps for their unique flavor. If my amateurish experimentation got me this far, just think what a real pro can do!

Coming up: RME UFX mods, Presonus DP88 mods, Behringer ADA8200 (re-)mods, Yamaha TG77 mods

KRK Rokit 5 G3 Modifications

This was not my first major modification project (my first was an old Presonus MP4 headphone amp, AMAZING, which I might write about another time), but it was significant insofar as these are my studio monitors, and I basically wasn’t using them. They’re just not clear enough. Muddy and incoherent, kinda bland, can’t hear anything clearly, tried some corrective EQ, only mitigates. I primarily work with classical and acoustic stuff, so quick and snappy good. I know the Rokits are favorites of EDM folks, I don’t really care about that stuff (sorry), but these were my first monitors and I didn’t know any better…

Anyway, I’ve been interested in audio modification for a while, and the arguments that swirl around them––rather than constantly buying new stuff, buy used stuff on its way to the garbage bin, pimp it out, and end up with a device that rivals high-end gear. It makes complete sense to me that a company with a profit margin, like KRK or Presonus or M-Audio or RME, would cut corners where they think it’ll be least noticed by 95% of folks in most normal-use situations (i.e., a singer-songwriter at home on their bed crooning into some crappy Chinese mic). It shouldn’t be wrong to “tune” your equipment to sound the way you want.

To me the mindset is very much like what people think about in the instrumental world. I grew up with a Yamaha G2 grand piano, a rather petite instrument, about 5’7″ or so. Yamaha makes spectacularly reliable actions, but the sound usually leaves something to be desired. The hammers were rock solid (not in a good way), so when I was in college we had a piano technician replace them with a set custom made for the piano by Ari Issac in Toronto. The new set completely transformed the sound––there was color, finally, shading, resonance, a dynamic range (especially soft)––and over a decade later those hammers continue to break in, revealing new sounds I never thought I’d hear from such a small instrument. Recently I had the bass strings measured for replacements. Yamaha is not known for making great bass strings––they sound a bit like surgical tubing drawn taut. I haven’t yet heard the results, but I will when I return home to visit my family for the holidays. Anyway, modification is a very musical thing to do, so I don’t think people who are interested in modifying their audio equipment are somehow phoolish to think they can eke out every last drop of good sound. Your milage may vary, and that’s ultimately all that matters.

Okay, if you pay $50 for one capacitor made with raw sheep’s milk or whatever, you are an idiot.

Anyway, I wasn’t using these Rokits much, plus the warrantee was out. What could go wrong?


Nothing wrong with them. They work just fine. This is the internal amp I was going to modify. The actual amplifiers are two integrated circuits (TDA7296, one for tweeter, one for woofer) pasted to the heat sink in back, the rest of this circuit is power supply, input buffer, and crossover. Power supply in the back (large capacitors and rectifier), input section in the front right using bipolar caps and two NJM4580s, crossover section in the middle front left uses three TL074 (and the green mylar caps). The yellow caps up front appeared to be for the corrective filters on the back, and I never use that function, so I figured I’d leave them alone.

Only way to remove the amplifier board was to desolder it from the input PCB. Definitely ripped a pad here, but it wasn’t a problem to repair later (just used a jumper or something).

[There’s also a “mystery circuit,” seen just to the top left of the input board that appears to supply 5V for a digital… something. Wasn’t connected to anything, so I wonder if KRK has another line of bluetooth products that use this amplifier board? I couldn’t find any. I left it alone. EDIT: if you check out my next post about this, I discovered this is part of the mute/standby and limiter circuit, both of which I figured out how to disconnect!]


The electrolytic caps are all easy to replace. They all have capacitance and voltage ratings written on the case––you can go bigger on both, just not smaller, but remember you’ll have to deal with larger cans. The mylar caps are another matter. They didn’t seem to have much of anything written on them, plus everything was bathed in a disgusting sticky glue which seemed to obscure any writing that may have been there. I used a capacitance meter to measure each cap after I removed it.

(apologies for the chicken scratch, and no offense to chickens)


Couldn’t be exactly sure what the value was of each cap (they have a tolerance that usually is within 20% of the stated value), but rounding to the closest available value on Mouser or DigiKey usually revealed what they must have been.

Another view of the mylar caps (green).


To remove the op amps I very gently grasped them with a pair of pliers and used a hot air rework station to melt the solder––add some flux paste around the pins to lower the melting point of the solder. There’s no need to pull, the chips weigh nothing, they basically float up off the pads the moment the solder melts (which usually takes about 5-10 seconds).


Use anti-static tweezers instead of pliers. In retrospect this just looks dumb.
Use anti-static tweezers instead of pliers. In retrospect this just looks dumb.

Clean board (all the SMD resistors, diodes, capacitors are left in place, of course):

Not a whole lot left, is there…

Now had to remove all that glue:

Co-op memberships are good for something.

Added extra supply rail decoupling, 0.1uF ceramic capacitors from each power rail to ground. Seemed there was one ceramic cap between the rails for both input opamps, extra won’t hurt, and would certainly help prevent stability problems when I put in the faster opamps. I used OPA1642/1644––FET input opamps, safe replacements for NJM4580. The OPA164x family is, according to TI, descended from OPA2134 (a popular chip among modders, since it can pretty much replace anything without causing issues), with lower self-noise and current consumption.

The basic guideline for choosing replacement opamps is pretty simple: you can generally use a FET input opamp to replace anything. The quietest opamps out there, speaking in terms of their inherent self-noise, are bipolar opamps (OPA1612 is TI’s flagship, but the NE5532 is the ‘classic’ example), but they tend to be a little fussier and will oscillate if they’re not situated in a complimentary circuit. A lot of this has to do with opamp’s inherent resistance to current, or input impedance, which relates to the bias current of the input transistors in the chip. FET opamps use FET transistors at the input, which require very little bias current, so the input impedance is very high, 10s of megaohms. Bipolar opamps require more current flowing across the input transistors, so their input impedance is necessarily lower, 20kohms or so (some bipolar opamps like MC33078 or NJM4580 have higher input impedances, 200kohm to 5megaohm range, but these are are chips I tend to replace…). I could have used the very quiet OPA1612 (20kohm input impedance) at the input here, since the input impedance for this circuit is only 10kohms. Provided the power supplies are properly decoupled (as I’ve done), it would probably be stable, but the problem is that this chip only has low self-noise at relatively low impedances. The higher the input impedance, the more noise at the input, and extra expense becomes self-defeating, as you can see in this graph.

Screen Shot 2019-05-13 at 12.03.48 PM.png

At 2kohms and below, the bipolar chip (in this case, the OPA160x series) has lower self-noise than the FET input OPA164x, but above that, as the current across the input transistors drops, the chip becomes noisier. So using a fancy chip like OPA1612 in this kind of setting, at $7/ea, doesn’t really make sense.


So, onwards, I replaced all polarized electrolytic caps with Panasonic FR, and bipolars with Panasonic SU. I kept all the values the same, although, come to think of it, I might have done well to at least double all the signal path caps, especially because I bypassed every one with a polypropylene film capacitor (also Panasonic, though I can’t remember which type). Mylar crossover caps were also replaced with Panasonic polypropylene. A couple values weren’t available on Mouser, so I used a Vishay polypropylene cap. Were I to do this again, I’d use WIMA MKP/FKP, I just didn’t know where to look when I did this.

IMG_20171112_194219 (1)
Using cheap parts compounds cheapness… they’re smaller, and can fit on a smaller board, and smaller board = <$
Modded gear always *looks* better, and that’s all that matters, obviously.
IMG_20171112_200106 (1)
Hot glued capacitors together so they don’t vibrate. Still not sure how I fit everything…

So I plugged it in and… no smoke! So how does it sound? I placed both speakers side by side (as pictured at the top of the page), one modified and the other stock, plugged them into the two outputs of my interface and routed an identical mono mix to both (I think it was of Mitsuko Uchida playing a Mozart concerto). “Panning” back and forth between the channels, the difference was NOT subtle. One gave the impression of having a wet towel wrapped around your head, the other didn’t and sounded like music. In fact, more than that, in the modded speaker I could perceive a sense of horizontal depth even with a mono signal, like distance front-to-back between instruments on the stage. That surprised me. So yeah, it was worth it. I modded the other one promptly.

These are the 5-inch speakers, so they still have a limited low end. I’m not sure increasing signal path cap sizes would have helped that much. I understand there is a limiter circuit built into this amplifier somewhere that could be disconnected (EDIT: I did it). Again, I don’t listen to EDM or anything with super heavy bass that could blow out the voice coils, so I’m not worried about damaging the speakers, plus I’ve heard of folks doing this in the Yamaha HS8s to great effect. Finding it would be a matter of poking around with an oscilloscope, so maybe I’ll do this at some point.

So how do they compare to other more expensive speakers? I don’t know, I don’t own anything else (I compared them to some Mackie 828s, which got smoked by my KRKs, but that was unfair because the Mackies are inherently slow and noisy), haven’t done a side-by-side with someone who does. [EDIT: you know who has is this guy, who used to be a head modification designer at Black Lion Audio. He mods these speakers professionally, and others, has done comparisons with hi-end stuff like Genelec and says these modded KRKs hang right in there, and represent the absolute best value in terms of the sound you get!]