Nelson Pass DIY H2 "injector" for loaner tour

I will not go into great detail but would encourage anyone wanting to play with this to read through the article that Nelson published on his little project that allows you to inject a little second harmonic distortion into your audio signal.

Nelson saved me from having to breadboard this project by giving a bunch of these away as a Christmas giveaway through DIYA. So a big thanks to NP for his generosity and enabling us to have some fun.

I installed the H2 into an old CD ROM case with an SMPS that is regulated to 24V, added an additional filter for good measure and a small pot to allow for volume adjustment ( so that it can be used as a preamp ).

I have added test points on the front and drilled holes aligned with the trim pots to allow the T2 voltage to be adjusted without opening the chassis. I couldn't find my nylon flat screwdriver so I shaped a tool out of a trusty bamboo chopstick which works very well.

Not pretty but functional and should be fool proof enough to allow most people to try this out.











added an ultra bright white LED so that it is easier to see the trimmers



The graph below indicates the anticipated % H2 ( percentage second harmonic ) that will be injected into the signal based on the voltage measured at Test point 2 (T2) relative to the pinch off voltage for the specific JFET.



The JFETs used in this build have a pinch off voltage of 2.81V , thus null ( ie no distortion ) is approximately 17.5V , 1% H2 is close to 16.1V and 2% is 15.5V

Just because shipping is what it is , we are going to keep this to the United States. I will let @ChaChaRealSmooth decide if this is to be part of an official SBAF tour .

I have not thoroughly tested the unit out yet but it works. I will not be responsible for anyone doing something negligent and blowing out an amp / speakers / headphones. There is a power up thump. I would recommend powering this unit up first before switching on the amp / plugging in headphones

Way to use this.

1. without anything connected , power up the unit and measure the T2 voltage
2. adjust voltage to your desired H2 % using the graph above to direct you ( these are approximations which I think are close enough to play )
3. connect it all into the music chain and see what you think.
4. I would not suggest adjusting H2 "in realtime" - shut the power / headphone amp down first , you can leave the H2 preamp powered and reset H2. Then power up the power amp / headphone again.

Don't forget to read the article and the changes to the music if positive phase H2 versus negative phase H2 - with speakers this would mean swapping the positive and negative wires , not so easy with headphones that have plugs unless you own Joe Grado HP1 which has a phase switch on the earpiece.

anyone interested in the loaner tour can post interest here - that will give @ChaChaRealSmooth a sense of level of interest. If it is only one or 2 people, I will just box it up once I am sure it is bomb proof and send it out.

I will include a cheapo multimeter , assuming not everyone has one lying about

..dB

 

I interested but I’d have to try and finish with it by ~Dec 26 as I have lots of travel in the near year.

 

I am heading out of town for 3 days, so lets see how much interest there is and I can have it ready to ship by the end of the week.

Aegir suddenly has the impression of deeper bass presence?! I noticed this with my tube pre / buffer too.



This is by default negative phase second harmonic which NP describes as ""a deeper soundstage with improved localization than otherwise"

..dB

 

Nelson has long advocated for some negative second order. I wonder if this is psychoacoustic or if it compensates for transducer distortion.

 

Very cool, dB! Would like to be on the list...thanks!

 

Looking forward to hearing your thoughts. Timely with the posts of audibility of distortion too.

.. dB

 

Likewise, that's an interesting project right there.

 

Hello All,

Received @dBel84's harmonic generator. Thanks for sharing the fruits of your labors, dB!

Before hooking it to my system I put it on the test bench. The results are interesting and may be unexpected...

• Left and right channel are not symmetric. Left channel needs to be about 1V higher than the right.
• Turning the right-channel trimpot CCW results in distortion dropping then increasing again right at the end. Left channel is monotonic all the way to the end.
• H2 is (mostly) dominant, but H3 and other harmonics are also within audible range.
• H3 actually dominates when turned all the way down.
• Intermodulation distortion is also quite significant. The 'effect' is a combination of all the distortions.
• Note right channel measurements indicate 179.5 degree phase shift.

Here's what I found for calibration settings (volume pot set at approximately unity gain):

Minimum distortion

Left: 16.46V = 0.18% THD, the minimum is at one extreme of the trimpot.
Right: 15.42V = 0.17% THD, the minimum is not at one extreme of the trimpot.​

1% THD (not quite 1% H2)

Left: 15.58V.
Right: 14.56V.​

2% THD (not quite 2% H2)

Left: 14.99V.
Right: 14.04V.​

Plots

Minimum distortion Left


Minimum distortion Right


1% THD distortion Left


1% THD distortion Right


2% THD distortion Left


2% THD distortion Right



IMD Tests
Fundamental at 1kHz @ -3dBV (0dBr)
Secondary at 60Hz @ -15dBV (-12dBr)

IMD test Left


IMD test Right


listening impressions below.

 

Imagine this being implemented like the old BBE button on old Aiwa stuff!

 

Thanks for putting it through its paces. Fascinating to think that parts variation resulted in such a significant variation as the Jfets were matched by Nelson to try and avoid this exact phenomenon

At least @CEE TEE won't have to guess where to set it up.

.. dB

 

@skem
Other than level, does the shape of the distortion plot change as you adjust the trimpot? It'd be interesting to see this plotted out if you had a way to add a third axis representing the trimpot.

 

Listening impressions
system: Allo DigiOne -> Metrum Adagio -> Pass XA25 -> B&W 801 Series III. This is with both speakers hooked up in-phase but reverse polarity

classical orchestral, conventional Redbook recording

At 2%: sounds quite rich but timbre is noticeably off. Depth is very much present.

At 1%: sounds flatter. I don’t like it as much as 2% although in some way it’s cleaner/drier. Strings are a bit better defined.

Bypassed: Timbre is clean and strings are more detailed. Stage is still flat but I can hear the recording venue better. The flatness now seems like distance from the stage rather than artificially flat.


Studio vocalist over light instrumental

Bypassed: Vocal very crisp. Imaging perfect.

At 1%: imaging more diffuse. Sounds more recorded but not bad. Plankton gone. Still enjoyable.

At 2%: recoding now sounds like a good shellac rip. Poor imaging, no plankton, and transients sound more like pops. Still, has an ease and prettiness to it. I prefer 1% to 2% in this case.

Solo piano sonata
This a dynamically compressed recording with poor naturalness.

At 2%: Rich and intoxicating. Timbre not there but if you don’t pay attention easy to get lost in the music.

At 1%: Not as engaging. Timbre not much better really. Really kind of meh.

Bypassed: Sounds more muddled and quieter. On the basis that the harmonics are giving a bit of extra energy, I pumped up the volume. Now more engaging but something more challenging about it. Louder in a less pleasant way.

Solo Piano #2

bypassed: This is an excellent recording. Very natural timbre and realistic stage. Delicate throughout.

At 1%: Still pretty but does not have the timbre (piano rings) or staging it had. Now a slight downgrade. Complex passages now garbled.

At 2%: piano rings as in 1%, staging lost. But I like it more than 1%. Smoother. Overtones on complex passages sort of run together in background with fundamentals still relatively clean. Lacks drama of that a big piano can give in a stark setting.

Solo Viola Gamba
This is an excellent recoding by M•A Records.

At 2%: Emotional and engaging. Can’t hear the room but I like it.

Bypassed: Now I can hear the room. Dramatic. Dynamic. Not exactly easy listening: Macro-dynamics seem more intense. Bass is cleaner.

The Who: Baba O’Riley

Bypassed: Sounds like crap. Mushed up, digital. Must be a bad recording.

At 2%: Still sounds like crap. More forgiving somehow. Boring.

Daft Punk: Doin’ It Right

At 2%: definitely not crisp. Harmonic and fun.

Bypassed: less harmonic and less pleasant. More definition in bass and in cymbals (or whatever they are. Synthals?)

Conclusions
Except for the very best recordings, a 2% distortion (dominated by H2) seems to make listening easier and more enjoyable. The cost is you lose any semblance of the recoding venue, some timbre, and most of the staging. It can become a garbled mess in busy music. At 1%, I feel the gains often don’t outweigh the cost. If I had a harmonic distorter in my system, I’d probably set it at 1.7% and would like an off switch for those times when it hurts.

I’ve learned something tonight and [maybe] understand better the tube loyalists. Thanks @dBel84 !

 

Can you clarify? The shape seems to be mostly the same except at low levels where it becomes H3 dominant. See plots above. Maybe I don’t understand the question.

 

Sort of like a waterfall plot... to visualize the changing shape as you turn the trimpot.
x = frequency
y = % distortion
z = trimpot setting

 

Yes I could make that, but it would be tedious requiring dozens of measurements at different frequencies and settings—so I’m not going to do it. Sorry.

note: there is a 1 to 1 correspondence between trimpot and THD%. It may not be linear with trimpot resistance, but it doesn’t really matter since the trimpot can be set anywhere. So in a sense what you’re asking for can be done in just two dimensions.

More interesting may be a 3Dplot of %distortion vs. frequency vs. amplitude of the fundamental.

The IMD in this circuit is really unfortunate. I’d love to hear something with straight H2 and virtually no IMD.

 

@skem Do you know what the 2nd harmonic distortion profile looks like for the XA25 - nevermind, google answered my question. I was curious if the XA25 had inherent second order distortion adding to the H2 injector effect.

Did you try switching polarity of speaker cables at all? I have not done that kind of experimentation yet.

Thanks for taking the time to share your detailed observations, this is similar to what I experienced when adding in the korg nutube infront of the "SissySIT" - just too much mush

..dB

 

I didn’t switch polarity. Maybe tomorrow.

 

A brief test with normal polarity. This seems to improve localization (stage) and reduce some of the unnatural ringing/echo quality of piano I was mentioning. Seems to make vocals more forward and present. Potentially much better outcomes with it this-way around, but I don't have the time to make a full assessment.

Ok, just read what polarity reversal is supposed to do (after listening) and I see my impressions are roughly the right effect. But it suggests something is backwards. Reversing the polarity for me seems to give me the “positive phase” effect, as described by Nelson. I guess it depends on (1) how @dBel84 wired the circuit and (2) unknown phase of original recording.

EDIT: Looking at my bench measurements above, I see the phase angle is -180° out of phase, so does that mean the polarity is reversed inside the unit? Hrmmm