[Hifihedgehog]

*Drum roll*

First off, a huge thanks to Tangent for releasing the board designs. Without a doubt, I cannot thank you enough for this and all your numerous contributions to the world of audio and electronics. Just minutes ago, I got my 10 Pimeta V1 boards in from Maker Studio, neatly and nicely packaged with all the goods. Call me overwhelmingly impressed. For 10 PCBs at around $17 shipped, this is the bargain bin deal of the decade for electronic hobbyists and audio hacking geeks who need PCBs fabricated.





Slightly related, I also got these resistive puppies in the mail today as well. I won't be needing to order parts for a long, long time now:
http://www.amazon.com/gp/product/B00H703OKE/ref=pd_lpo_sbs_dp_ss_3?pf_rd_p=1944687642&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B00FVT8I22&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0997HY0FJ8SGE31JYHXM#productDetails

 

[tangent]

First off, a huge thanks to Tangent for releasing the board designs.

 
I'm glad you're getting some utility out of them.
 

I won't be needing to order parts for a long, long time now:

 
Why such a large value? I don't mean why 1.28 k ct resistors, I mean why 10 MΩ? You have a lot of use for such things?
 
I guess it's one way to create power resistors. Gang up a hundred of those in parallel, and now you have a 100 kΩ 25 W resistor. 

 
Hmmm, "low noise."  Someone doesn't know the Johnson-Nyquist formula, I see... A single resistor creates about an -84 dB noise floor on a line level signal all by itself. Now add a few more resistors, then follow that with some gain stages...we're getting up to LP noise levels now.
 
(Yeah, heresy, I know, never diss the vinyl...)

 

[Hifihedgehog]

 
Hmmm, "low noise."  Someone doesn't know the Johnson-Nyquist formula, I see... A single resistor creates about an -84 dB noise floor on a line level signal all by itself. Now add a few more resistors, then follow that with some gain stages...we're getting up to LP noise levels now.
 
(Yeah, heresy, I know, never diss the vinyl...)

Yes, oh, yes. Scarcely mention distortion or noise and vinyl records or resistors in the same breath and you may become this furry guy's midnight snack. As in, ohm, ohm, ohm... *gulp* *licks lips* Moral of the story: treat this master ohm guard dog with care and kindness and it will stay ohm, sweet, ohm. You have been warned. 
 

 
The funniest thing happened today. Today, my circuits class began building the power supply portion for their speaker amplifiers. With excitement, the TAs were rooting, almost chanting, in going around the workstations saying things like, "This is the most exciting day of the semester. Finally, exploding capacitors! Inevitably, at least one of you guys will accidentally place their capacitors in reversed polarity. Then boom!" Mind you, these aren't those little party popper kind of capacitors that most measly headphone amplifiers sport. These are 5 to 20 farad, jumbo, ACME firecrackers that will literally blow up into the roof if not used properly.
 
And you know what? They were right. Someone did blow one up--the group seated right next to me and my lab partner. Yes, KABOOM! In the wake of the explosion, that pitch black, torn-to-shreds sucker was smoking like a miniature mushroom cloud, so much that they were worried about the fire sprinklers going off. Luckily, no water accompanied this unintentional electronic "fireworks display" but it sure added to the atmosphere of today's class. Popping caps, smoking pots and using opamps. I swear, people sometimes must think we are on drugs or something. Punny, I know.

 

[Hifihedgehog]

Hey @tangent! Just as a quick sanity check, if I were setting up a unity gain loop in a Pimeta-V1 board, I obviously would leave out R3 and jumper R4 in the LR (or left and right) channels. But as for the ground channel, if my R1 were 3.3 KΩ in the LR channels, R1 and R4 in the G (or ground) channel would be 3.3 KΩ as well and R3 would remain open, correct?

 

[tangent]

Sounds plausible.

 

[Hifihedgehog]

 
Day one is done and progress for my first PIMETA V1 board is coming along quite nice. Excluding a few parts here and there, I was able to solder on all the resistors, capacitors, sockets and ICs. It is likely impossible to make out on the board, but those op-amps are OPA627's, nestled securely in their sockets and ready to bring the house down! I also have some AD797's, AD8620's and AD843's on standby for some fun op-amp rolling! Not shown are the buffers, quad-stacked BUF634's and soon-to-be-modified LMH6321's, which will also take their positions in the ranks when the time comes.
 
Still on my to-do list is to get in my four Panasonic FM series 1000 uF 35V capacitors, one ALPS RK097 potentiometer and two 1/8" TRS phone jacks, install those components and then do the front panel work. I must admit that my expertise with front panel work leaves much to be desired so I might rely on a friend or my university shop to pull this final step off. Even it is only plastic, I only have one front panel and one panel only in my Hammond case and I am not about to ruin it by practicing my drilling skills on it. I will leave that battle for another day and for the assorted junk heap of parts I have from years long past.

 

[Hifihedgehog]

The headphone amp monster lives!

 

[Hifihedgehog]

Sorry for the triple and quadruple posts. I am currently listening to my stash of op-amps and I must say, op-amp rolling is no myth. Quite far from it, as a matter of fact, and I can finally confirm all the good things and the bad things that people say about these individual parts, and put in my two cents as well. After rolling a few of them, here is a ranking of some op-amps.
 
1. AD797ANZ: The last word in detail, balance and energy; The AD797 takes things to a whole other level, where bass is fast, punchy and tight as ever, midrange is coherent and emotive, and treble is crystal clear and precise. However, such fidelity does not come without its price, meaning the AD797 is so true and exacting that it brutally reveals all problems in your signal chain and in the original recording down to very last reverbation and electronic buzz, even when having no gain at all. It is a wide-open window to your music, but you may not want to know all the dirty details this op-amp sets in plain view to you. It is analogous to the Sennheiser HD 800, so for those who do not like hearing absolutely everything in such a garbage-in, garbage-out fashion, you may want to try the second place contender.
 
2. AD843JNZ: Fun and detailed; Although with a smidge of blurring here and there, the AD843 is actually a bit more detailed than the OPA627. The only drawback is its very, very slightly less refined nature in the bass or midrange. In its favor, however, unlike the OPA627, there is no slight blanket of warmth over the sound which I consider a huge plus in my book. Bear in mind that, while it is more neutral than the OPA627, it still does not have the laser precise, all-revealing nature of the AD797. For some, this slight deviation of neutrality can be a benefit when you do not want to hear everything in the signal chain, including in the original recording itself. However, for the most diehard purists, this will still be a turn-off.
 
3. OPA627AP: Warm yet accurate; This is a substantial upgrade over the OPA132/OPA134 but, unlike its price would suggest, it is nowhere near as revealing as the AD797. In practice, OPA627 editorializes over hiss, grit and noise, which can be good, but it adds its own take on the music in the process. And yet, it is still detailed and resolving at the same time with a nice hint of crystallity in the upper registers. Think of this as the classic Sennheiser of op-amps, somewhere in-between the HD 650 and HD 600 in terms of tone. It is quite nice in this regard but it lacks that wow factor with genres which rely on drive and punch. As such, it is not my cup of tea. For those who thirst for a romanticized retelling of their music or have an affinity towards tubes, this just might be your thing.
 
4. LM6171/LM6172: Detailed and accurate; I was surprised by this op-amp's ability to project air, space and detail. What was not so compelling was its noticeable lack of dynamics and energy. In a way, it models the classic list of complaints against the K701 and, for some, the DT 880; there is just not enough oomph and verve for musical pleasure. This is essentially the classical side of the op-amp sound coin, whereas the OPA627 is set on the opposite romantic face. If you savor some dynamics and warmth to round out your listening session, the OPA627AP is a better choice. If you crave exactness and detail, the LM617x is the better choice of the two.
 
More coming!

 

[alphaman]

Good to see this thread active again.
 
One of my daily-portable-use HAs is a Pimeta2 -- a unit that has been going strong for over 5 years (some early issues -- see way back in this thread -- but after those resolved, all good and RELIABLE).
 
Some noted tweaks/upgrades I added to Pimeta2:
 

1. tri-color-LED battery condition ckt (a mini daughterboard on Vero)
2. 12v battery pwr using AA NiMH [the AD825 really need more than 9v]
3. AD825 opamps

 
One specific mystery at early (post-build) stage was weird turn-on transients (loud pops) ... search this thread for that discussion. It turned out that two 9v batteries wired in series (18v) was the culprit. I now use either one 9v, per orig. case design, or a small multi-AA external battery box (the AA cells are in series), similar to this...

 
I only use the Pimeta2 for IEMs as I don't think they drive my full-size cans as well as other HA's I currently own.

 

[Hifihedgehog]

Quick question: Is there a low noise and ripple battery board design online that I could use to charge 9V batteries with a wall wart? I have the older PIMETA V1 and I am very much interested in a charging board so I can just plop in the batteries and never have to switch or replace them every 3 hours. I have quad-stacked BUF634 buffer stages which makes battery life very, very short.
 
I must say, by the way, that the sound quality is nothing short of spectacular! With my HiFiMeDIY as source and my Phonak PFE's, I seriously enjoy this loads more than any of my past Objective2's. It's got such energy and drive and PRAT and all those audiophile terms you would associate with perfect synergy. 
 
EDIT: Another quick question: What would cause a PIMETA V1 with quad-stacked BUF634's to get unstable at a certain pot position when using OPA2132's? I am measuring zero DC offset, and I do not have this issue at all with the op-amps listed above.  For some reason, I get a somewhat noisy pot and distortion centered around a certain pot position (roughly 10 o'clock/25%) whenever I use the OPA2132. I have five of them at my disposal and they all behave the same, which is strange for an op-amp which is commonly recommended to beginners because of its renowned stability. Would it be that I need to add the bandwidth resistors on the buffers? Would it be that I need to mod in some 10 pF capacitors? (I am thinking this second case is much less likely, but I thought I'd ask anyway.) Technically, the buffers are still faster than the OPA2132 according to the datasheet, but maybe they need to be faster just in case. It just doesn't make any sense why it is behaving this way.
 
EDIT: I just answered my own question where I asked "Would it be that I need to add the bandwidth resistors on the buffers?"
Per the BUF634 datasheet: "High speed or fast-settling op amps generally require the wide bandwidth mode to remain stable and to assure good dynamic performance."
And per the OPA132 datasheet: "The combination of high slew rate and wide bandwidth provide fast settling time."
That would be an affirmative to that question.
 
The other question is irrelevant, but if I added some 10 pF capacitors to the knock-off Browndog I have, it might very well remove the very very very slight hum in the LM6171 I only hear at eardrum-bursting volumes (and no music playing--heaven forbid I accidentally press play).

 

[Hifihedgehog]

I have found a problem with DC offset in my PIMETA V1's ground channel, and would like some feedback and suggestions. I am measuring 5 mV DC offset from input ground to output ground. Unlike offset of this magnitude in the left or right channels which would be acceptable, apparently, this amount of offset is not acceptable here. See here: http://tangentsoft.net/audio/trouble.html#loudclick (Out of curiosity, why would that be?) I looked at my ground channel where there is a double BUF634 buffer stack and an AD797, and I am at a loss as to what to be looking for. Please help. I can offer more information about my particular implementation as needed. 

EDIT: Additional information for this mystery: This PIMETA V1 is set up in a unity gain configuration. Resistors were hand matched and were already 1% precision resistors to boot. R1L/R/G is 3.3 KΩ, R2L/R is 1 MΩ, R3L/R/G is open, R4L/R is jumpered, R4G is 3.3 KΩ, R5L/R is 3.3 KΩ, R6L/R 1MΩ, R8L/R is jumpered, R10L/R is 1 KΩ, R11 is open and RLED is 10 KΩ. C6G is installed as well, and C6's were soldered on the back side of the boarf to the LR opamp socket's associated pins as well. Just to make this caper even more intriguing, the offset occurs independent of the model of opamp installed. 

 

[tangent]

Unlike offset of this magnitude in the left or right channels which would be acceptable, apparently, this amount of offset is not acceptable here. See here: http://tangentsoft.net/audio/trouble.html#loudclick (Out of curiosity, why would that be?) 

 
The only reason we expect the ground channel's DC offset to be lower is that it's always unity gain, whereas the other channels can be higher, which magnifies any inherent DC offsets.
 
Given that all three of your amp channels are G=1, I'd expect them to all be well under 20 mV.
 

Just to make this caper even more intriguing, the offset occurs independent of the model of opamp installed. 

 
If you're changing OPAG and nothing changes, then the problem almost certainly isn't the op-amp, since there are manufacturing variations in op-amps that ensure variable DC offset. I would measure DC offset across the buffer stack only.
 
I hope you're not changing OPALR and expecting to see a change in the ground channel.
 
Personally, I wouldn't have recommended the AD797, because it's a bipolar-input op-amp, which you were warned about in the parts selection guide.
 
Have you gone through my generic troubleshooting guide?

 

[Hifihedgehog]

Thanks for the reply! Hopefully, these symptoms help me in reaching a valid diagnosis. 
  Quote:

  I hope you're not changing OPALR and expecting to see a change in the ground channel.

Nope. That would be like a dog chasing his tail, with no foreseeable resolution in sight. I am not madly beating my head in agony and bewilderment... yet.
 
Quote:

  The only reason we expect the ground channel's DC offset to be lower is that it's always unity gain, whereas the other channels can be higher, which magnifies any inherent DC offsets.
 
Given that all three of your amp channels are G=1, I'd expect them to all be well under 20 mV.

That is my understanding as well. Following math and real world experience, I would expect the DC offset to increase as a multiple of the gain or thereabouts.
 
Quote:

 If you're changing OPAG and nothing changes, then the problem almost certainly isn't the op-amp, since there are manufacturing variations in op-amps that ensure variable DC offset. I would measure DC offset across the buffer stack only.

Bingo. More precisely, I am measuring across from the ground output to the ground input. So instead, I should measure from the ground op-amp output to the headphone ground output, right?
 
Quote:

 Personally, I wouldn't have recommended the AD797, because it's a bipolar-input op-amp, which you were warned about in the parts selection guide.

  Thanks. Already seen and duly noted. As mentioned, I have this issue with any op-amp in the ground channel, be it an OPA134 or OPA634 or an AD843 or AD823, etcetera, etcetera, etcetera. 

 Have you gone through my generic troubleshooting guide?

 
I have, and I plan on going through the Trace a Signal through the Amp section next. That is where, I wager, I am bound to find the underlying cause(s). As recommended, I will first skip ahead to the buffer section and measure for offset. Much to my chagrin, I may very well have a set of misbehaving buffers on my hands. We will see soon.

 

[Hifihedgehog]

I am getting roughly 24 mV offset from opamp ground output to output ground. Now, for the strange thing. It is the same result regardless of the buffer stack I place there. I also measured -30 mV offset from the opamp. The opamp appears to be overcompensating, and it does this, again, regardless of the opamp being used. Thoughts? I'm near agony and bewilderment. jk. Seriously, this is rather puzzling.

EDIT: From what I am reading around the web, if all it is is only around 5 mV, I shouldn't really be too worried about it. I mean, it is still three quarters of the way from the 20 mV mark. Previously, in a different thread, someone had 8 mV in a similar configuration, only it was when using fully discrete components and it was only in their left and right channels. I am totally willing to live with this seeing how the amp actually sounds so wonderful already--ultra low noise, dynamic and detailed. I just saw the number in reading through your website again and it caught my eye in the reading. If it really isn't a big deal and I am getting what I want, maybe I shouldn't be all too worried about it and I should just move on and enjoy my tunes. I can get all worried about DC offset with the next PIMETA I build from the stack of boards I have. After all, I want to build my little brother an amplifier as a gift and I want to get some more practice in so I can be a headphone amp samurai when I build him the perfect one when the time comes. This one is good enough for me.

 

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