NP Acoustic Transformers

This is a long overdue post which I never had the time to write about. I was hoping to get my measurement gear down to where my system is to take a final FR sweep analysis of my 4P1L PSE amplifier, but never got around to do it.  However, after the recent posts in DIY Audio, it was time I shared the measurements made and my listening impressions of the NP Acoustic Transformers.

I found them long time ago when searching around for transformers. I took the risk and purchased a pair of their 3K2 SE OPT (NP3.2SE18F100) with amorphous core and with very promising specs. As usual, specs can be very tricky in particular when the testing conditions are not close to real life and also when they are not even documented! You will find this to be very common in transformer manufacturers out there. Each transformer will behave differently depending on the driver as well as the overall implementation circuit. 

For this reason, I decided to put the transformer to the mercy of my test bench and here are the results.

Testing the 3K2 OPT transformer

First test was made on a pair of NP3.2SE18F100 shipped by NP Acoustics. I found a slight inconsistent HF frequency response between the pair due to the winding technique which was corrected by NP Acoustics who sent me another pair of OPTs. 

The first step was to measure the OPTs with the LCR:

Transformer A
Lp @1kHz = 31.3 H
Rpp @7.8kHz = 416 kΩ
Lsp @ 7.8kHz = 6.5 mH
Cip @ 7.8kHz= 1.632 nF
Rsp = 117.39Ω
Rss = 291 mΩ
Transformer B
Lp @1kHz = 40.8 H
Rpp @7.8kHz = 438 kΩ
Lsp @ 7.8kHz = 6.13 mH
Cip @ 7.8kHz= 1.480 nF
Rsp = 116.49Ω
Rss = 273 mΩ
 Testing notes
  1. Resistance measured with 4 wire probe
  2. All tests with secondary 0 pin connected to ground / primary negative pin

You can see some difference in the Lp inductance however when testing in real life circuit this doesn’t seem to be the case. Lp measurement with the LCR isn’t accurate as isn’t under any DC current or large voltage swing, so not very representative. 

The leakage inductance and capacitances seem promising which will reflect the behaviour of this OPT at HF subject to the driver.

In my test rig, I used my beloved 4P1L, but only 1 valve. This OPT is better fit for a 300B or 4P1L in PSE, however the below FR tests are representative on what you should expect from this OPT in real life.

Here is the test rig used. Obviously C4 and R2 introduces an LF pole which will impact the LF response of the OPT slightly:

 Now the proof is in the pudding as they say. Here is the response of the OPT with the 4P1L biased at 35mA (rather hot) which will ensure that the ra is about 1600Ω. With the measured Lp, Lsp and Cip you should expect a bandwidth of 6 Hz – 90kHz in theory:

Interestingly the HF response (-3dB) is about 50kHz. If you look at much more expensive OPTs like the Monolith Magnetics S-9 (which doesn’t have an  amorphous core), you would be surprised that the HF response isn’t much higher, however as expected due to its specs the LF is much better:

Now, that is pretty impressive for the price of the OPT I think! Below there are some listening impressions, so hang on.

I’ve been using this OPT in my latest version of the 4P1L PSE amplifier for about a year and I have to say that I’m delighted with its performance. 

Testing the 5K OPT transformer

My next adventure was on the NP5SE18F100 5KΩ:8Ω OPT.

Below are the LCR: measurements 

Lp 38.46 H @ 1kHz
Lsp 9.39 mH @ 7.8kHz
Cip 2.322 nF @ 7.8kHz
Rpp 951 @ 7.8kHz
Rsp 126.68 Ω  
Rss 258

You can see that given the step-down characteristics of this OPT, the Cip and Lsp are much higher than the 3K2 version.  In theory again, the response should be around 5Hz to 107kHz.  

In real life, here is the response with the 4P1L valve. In this case its biased to 30mA but doesn’t make a big difference. The HF is limited to 37kHz:

Here is a comparison with the LL1623, which has a slightly better response, HF up to 42kHz:

I was slightly disappointed with the response compared to a Lundahl given the latter isn’t amorphous. However is still great value for money.

I exchanged several emails with the NP Acoustic team and unfortunately couldn’t settle a clear view on their FR testing protocol. They didn’t take on board my measurement feedback and is slightly disappointing to see the FR specs shown on their site for this 5K OPT. Despite I can’t claim myself an OPT expert, I think they have work to do to improve the winding of this transformer. 

OPT Reviews

A year ago, I shared a pair of these OPTs with my friend Andy Evans. I also asked him for a review, which is posted below, and as said earlier I struggled to find the time to write up this blog post. Obviously because I wanted to measure them on my system, which I didn’t. 

In my view, these OPTs are great. I haven’t replaced them with the Monolith Magnetics as they are reserved for my 300B amp. However, considering the price, they are great value for money. The 4P1L provides a unique clarity which in partnership with the amorphous core OPT brings a level of detail on the treble which is outstanding.  Yet, the bass is powerful and strong. For a single ended amp, I’m impressed how good it is. The 4P1L PSE and these OPTs match really well with the Alpair speakers I have.

Here is Andy Evans’ review from a year ago:

"Amorphous core transformers have the reputation of offering increased clarity and low level detail, particularly in the upper mids and treble. For some this comes at the expense of some degree of harshness or hardness on first listening, but there are many user reports of break-in after 50 hours offering a more smooth perceived response.

My experience was similar to the above. My first reaction was that the NP32 output transformers lacked the overall smoothness and sweetness of the Lundahl LL1664/70mA I have had in my system for around 6 months, and which was my preference from a bunch of other transformers. My output stage consists of two 4P1L DHT pentodes in triode mode, used in filament bias which means a small value cathode resistor with no cathode bypass cap. Output is similar to a 2a3, but gain is around mu=11 which allows me to use a simple 2-stage setup with a DHT 01A Gen 2 preamp as the input stage. This is also in filament bias, and both use choke input filament supplies with Rod Coleman regulators. Despite the 01A tubes dating from the 1920s and 1930s, this is a modern design using advanced solid state designs in the filament supply and the gyrator active load. The combination of the 2 stages offers a very high level of detail and good reproduction of the timbre of acoustic instruments. Timbre is important to me as a professional conservatoire trained musician. Input is Mac based through Audirvana+ software and an ES9023 DAC. Speakers are full-range Mark Audio Alpair 10s in infinite baffle columns.

So for the first hours of listening I found the sound lacked the sweetness I had become used to. Listening material was a lot of opera and vocal music, both classical and also jazz and popular vocals such as modern gospel and singer-songwriters. Massed vocals are quite a severe test of a system’s smoothness and resolving power. Added to which they can show up any resonance peaks or high frequency oscillations. Resonance peaks in the treble are a common downside of single unit speakers such as mine, and this offsets the gain in clarity from not using a crossover. So some degree of resonance can be laid at the door of the system, even though the Lundahl OPTs seemed to mask this.

After a week’s listening every day the strengths of these OPTs became increasingly obvious. The most obvious gain was clarity. Orchestral and vocal textures were so much clearer, words so much easier to hear, that I started to listen at lower volumes. This was quite noticeable. I was getting all the information I was used to at a lower overall volume. This also offset the greater perceived energy I was getting in the treble, which was bothering me at first listen. Whether there were effects of the much reported  break-in happening I can’t be sure – it’s a subjective thing. But for sure, I was starting to get quite absorbed in the new clarity I was getting. Vocal music was a joy with the lyrics clearly audible.   

So would I go back to my smooth and sweet Lundahl OPTs? Well - they're excellent OPTs and superior to most conventional core models, but no, I wouldn’t. In comparison the sound from them is less immediate, more recessed. The detail may still be there somewhere, but it’s less obvious and the temptation is therefore to turn up the volume to hear it better. So I’m addicted to amorphous now. I have to have the clarity, now I know it’s there in the recordings I listen to. It’s too difficult to go back knowing that some of the low level detail is going to go off the radar. These OPTs are a hard master – they show up any faults in your system and demand the highest level of detail and smoothness in the signal presented to them. What you get in return is a level of directness and transparency in the sound that is a whole new experience for users of tube equipment. Yes – it’s addictive and I’m addicted!

These transformers are well made and very attractively finished. They are very much in the tradition of Tango and Tamura, with large square cases and a circle of pins underneath for attaching the connections. Builders of high-end tube equipment now have a new name to add to the select few famous names of audiophile transformers, which have become rarer and rarer to find in recent years. NP Acoustics deserve to do well if they can make equipment of this quality at competitive prices.

Andy Evans MA, MBPsS, PCC

Director: Performance and Media Coaching UK

January 2017"

Well, I hope you find this review useful and hopefully support this great manufacture of hi-fi OPTs. They do a good product at a fantastic price point.

12 Replies to “NP Acoustic Transformers”

  1. Nice work Ale,

    I was wondering why your measurements show a resonance for the Monolith Magnetics transformer and on the side of MM there is no resonance in there measurements….

    Martin Mug

    1. Thanks Martin
      It was a manual sweep that one. Sometimes the inductance of the long alligator clips I used around with the testing play with the OPT reactances and create a resonance at HF.

  2. Dear Ale,

    If you do a (manual) measurement please don’t publish it if you are not sure if the results are worth to publish. I don’t know about the other transformer you measured are at the same conditions but it is very important to know how you measure the transformers. Monolith Magnetics make transformers considered to be the best in the field and it would be nice to compare your results with the results of Monolith Magnetics but it is important that it is a fair measurement.

    Martin Mug

    1. Martin, I think the measurements within the audio band 20-100kHz are accurate. I wish transformer manufactures would be as accurate and as open on their metrics! That is where the majority fails.

    2. It would be nice to do measurements at the same conditions as the the transformer manufacture dos. (what are the conditions MM measures are done?) And it would be more interesting to do measurements in real life (measurements in real conditions for the tube it’s made for)

  3. Martin, sorry my friend but we may disagree here. I’m trying to measure real Life conditions, with a simple circuit which matches 90% of use cases.
    please show to me one detailed data sheet from either Lundahl or MM which show same level of detail.

    1. Ale, real life measurements are my first choice too but if an (long)alligatorclip makes such a differance i have some doubts.

      Btw i made some real life measuments at a LL1664, its measures with a EL34 in triode mode very nice.

  4. Hello again Ale, great to see some real world measurements. These appear to be quite good transformers. Do you know if they do any push pull OPT’s?

    Also how did you contact them? I really like the fit and finish they have. As Andy says in his review, if these guys can make these for a reasonable price then they certainly deserve the business to keep them going into the future.

    I have in mind a push pull parallel 6E5P amp. Maybe with amorphous core transformers and such linear valves in triode it will be too clinical but who knows without trying.


    1. Hi Matt
      They do PP OPTs but I don’t have any reference on how they sound or quality. However, I expect it to be as good or even better than the SE.
      You can contact them via Facebook or their website

      6e5p in PP is really nice. I did some test time ago and was very happy with the results. Worth doing it, you can do a spud amp with a SUT

  5. Hi Ale,
    Here’s the 4th day how NP 3k5 SE 18F80 works in my 300B SET instead of James 6123-HS
    I think it’s too early to give my final impressions.
    Sound is great for now, the difference is just a “second”. In general, it seems to me that my Dynaudio D21/2 supertweeter a little less “lively” than with James.

    Basically, one pair will stay in this amplifier, and the other pair will go to the next amplifier scheduled for my weekend home.

    You are one of the best enthusiasts among DIYers, despite alligator clips 🙂

    Best diying

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