Thursday, May 21, 2020

Topping D90 MQA DAC


Topping is well known for their low-cost audio electronics, ranging from amplifiers to DACs and headamps. They have become increasingly upmarket and their top-of-the-line DAC, the D90 was released last year. 

The buzz around the D90 was centered around the decoding chip used, the Asahi Kasei AK4499. The DAC chip alone does not maketh the final output. The quality of the power supplies and the output stage play a part too, not to mention the implementation of the digital receiver, clocks etc. 

The AK4499 is Asahi Kasei’s premium offering in their Velvet Sound range. It is described as a 4 channel switched resistor DAC with top-notch measurements like a dynamic range of 137 db (stereo) and THD+N of -124 db. 


The D90 MQA DAC is an update of the D90 DAC with MQA decoding and a change of the USB chip from the XMOS XU208 to the XU216. The rest of the features list of the D90 MQA reads like any top-shelf DAC, including fully balanced outputs, DSD 512 decoding, I2S input and dual Accusilicon Femto clocks. The best feature is left for last - a retail price comfortably below USD 1,000 (USD 799.99) !

In a nod to modern conveniences, the D90 MQA includes a Bluetooth receiver, with support for Bluetooth 5.0 and LDAC/AAC/S-BC/APTX/APTX LL/APTXHD protocol. In other words, you can stream 24 bit / 96 kHz music with the right source. If you believe that a receiver and antenna is the work of the hi-fi devil, you can disable the function from the front panel. 

The rear panel has all the usual inputs, including Toslink and AES. I2S input is in the form of a HDMI socket, and the pin settings can be configured to maximise compatibility with your source. 

Both single ended and balanced outputs are provided. The RCA jacks are spaced quite close, so be careful if you have cables with very large connectors. 

Looking at Topping’s internal pictures, a single power transformer is used in the linear power supply (voltage is easily switchable by the user), with lots of goodies like Nichicon FW power supply capacitors, TI OPA1612 chips for I/V conversion, and healthy voltage regulation (including 6 independent regulated power supplies). Bear in mind that all this is packed in an enclosure that is small and light enough to be held in the palm of your hand. The Topping's external dimensions are 22.2 cm x 16 cm x 4.5 cm (W x D x H), and the unit weighs 1.4 kg.

The Topping is a fully balanced design, with two outputs from the AKM4499 DAC chip paralleled to achieve better performance. The AK4118 receiver chip supports up to 24 bit / 192 kHz sampling rates through the Optical, Coaxial and AES digital inputs. USB and I2S inputs support up to 768 kHz sampling rate, as well as DSD 64 - DSD512. The XMOS XU216 chip contains two xCORE tiles with up to 2000 MIPS processing power (double that of the XU208). You may also choose to have simultaneous output from both the RCA and XLR analog outputs, or only have one output active.

The digital volume control can be operated in 0.5 db steps, which permits you to skip a separate pre-amplifier if required. The Topping also has a choice of six FIR digital filters for PCM and two settings for DSD.  A plastic remote control is also provided with close to full functionality.

I liked the front OLED panel, which uses large fonts that can be seen across the room. All necessary information is displayed, including the active input and output(s), sampling rate, volume and whether the stream decoded is PCM, DSD and MQA processed.   

MQA - A Primer

MQA, or Master Quality Authenticated is a process that was established by Meridian Audio, the well-known British hi-fi company. MQA allows high-resolution audio to be packed into a compact data stream. It also digitally fingerprints files so that you can be assured that your file has not been tampered with in any way.

MQA files are backward compatible with any equipment capable of playing back red-book CDs or digital files, and can be encoded on popular files formats like WAV, FLAC or ALAC. In a process that is termed as "Audio-Origami", the high-resolution signal is folded into either a 44.1 kHz or 48 kHz sampled format depending on the sampling rate of the original file. Lossy compression is applied to the ultra-sonic frequency range (which has little amounts of energy). The most practical benefit to this is reduced data bandwith required if you are streaming (e.g. from Tidal), and much smaller file size.

MQA is also able to do digital compensation to the encoded signal based on the recording equipment. If your DAC has a full MQA decoder, there is also further compensation for the playback device.

The first level decoding (MQA Core Decoder) of the signal enables the signal to be unfolded into a high resolution stream (MQA Core Stream) of either 88.2 kHz or 96 kHz. You may use Roon, or any other compatible software decoder to extract the MQA Core Stream. 

The Topping is a full MQA decoder which will perform the full unfolding, authentication of the stream and do file and DAC specific compensation to achieve the highest possible sound quality.  

I high recommend watching this video on "The Hans Beekhuyzen Channel" which explains how MQA works. 

(To be continued ....)   

Sunday, May 3, 2020

Bottlehead Crack and Bottlehead S.E.X. Headphone Amplifiers


COVID-19 and the lockdown making you sing the blues? Maybe it's time to get out your soldering iron, and start building a kit that you had your eyes on? That's exactly what I did - my poor Bottlehead S.E.X. kit has been collecting dust since I ordered it sometime last year. 

Bottlehead is an established company that has been offering vacuum tube kits to audiophiles since 1995. Their product range covers a wide variety of tube equipment, as well as a speaker kit. 

While kits are not for everyone, I encourage every audiophile to at least try assembling a kit at least once. A lot can be learnt from the experience, including soldering, basic circuit design, patience, and the ability to use swear words in ways that you never thought possible. 

On a more serious note, immense satisfaction can be obtained from building something that you can truly call your own masterpiece. Photos of builds around the world show that audiophiles are a pretty talented lot, with some builds exhibiting build techniques and wood / metalworking quality that can put even professional companies to shame.

As an educational process, I probably learnt more from DIY work than anything else. Mistakes in assembly or poor construction technique will teach you many things, including the importance of good grounding practice, and the importance of proper wire routing.

Bottlehead kits are far from being the cheapest on the market, but their builds are guided by very clear instructions with lots of pictures. They also have a very supportive online forum where you can find a lot of tips and suggestions for modifications. If you encounter problems, the folks there are helpful and willing to go the extra mile to ensure your build is successful.

Introduction to Crack 

This type of Crack is legal around the world and no need to risk your liberty or health to enjoy this. 

The Bottlehead Crack 1.1. OTL Headphone Amplifier kit is described as a Level 1 kit, perfect for first time builders, involving an evening or two of build time. The cheeky name is a common trait across Bottlehead products - they believe that their products involve feelings of euphoria!

The circuit of the Crack is simple, with a low parts count. The power supply is a C-R-C-R-C design, so no chokes to complicate your life (and more importantly, bump up the cost of a starter kit). A lone 12AU7 tube is directly coupled to a similarly lonely 6080 tube used as a cathode follower. As both tubes are dual triodes, each triode section handles one channel. 

There is no output transformer (OTL means Output Transformer Less). Tube designs typically need an output transformer to match the high resistance of the tube output stage, to the low impedance load being driven, e.g. a loudspeaker or headphone. The choice of the 6080 tube (which has a relatively low plate resistance), allows the omission of an output transformer provided that the matching headphone has a high impedance. The Crack has an output impedance of 150 ohms, and is recommended to be paired with headphones of 200 ohms or higher. Parts quality is decent with nothing fancy save for the ultrafast rectifiers. The two most obvious cost-saving measures are the electrolytic output coupling capacitors and the cheap carbon potentiometer.

The Crack can also be upgraded with the Speedball upgrade kit, which is a constant current source supply for the plate of the 12AU7 tube, and the cathode load of the 6080 tube. A similar upgrade kit is also available for the S.E.X.   

Introduction to S.E.X.

This is a family-friendly blog, and this is one three-letter word that can be legally enjoyed with the whole family. Like Crack, S.E.X. is a two-stage circuit. Version 3.0 of the S.E.X. uses a 6FJ7 tube. If you have never heard of the 6FJ7, that makes two of us. The 6FJ7 is a dual triode with dissimilar sections fitted in a Compactron envelope. Described as a TV oscillator tube, this is not a tube that you will be fighting over with other audiophiles.     
The S.E.X. is a substantially more complicated build compared to the Crack. I took more than four evenings to complete the kit, taking my time to carefully check the instructions, and building the kit at a leisurely pace. The S.E.X. has five transformers, consisting of one power transformer, two output transformers, and two plate chokes. Unlike the Crack, the S.E.X. is able to power sensitive speakers, with 2W on tap. 

After completing the kit, I admired my extremely skillful soldering, powered up everything, and music flowed forth like voices from Heaven. The truth of the matter is that I could not pass the resistance tests as set out in the instruction manual and had to go to the Bottlehead Forum for help. Bottlehead's Paul Birkeland noticed almost immediately that one of my ground wires was not connected and all resistance tests passed after I fixed that. Interestingly, I had no plate voltage on one channel. Checking all connections carefully with a magnifier and torchlight showed that one of my connections had not yet been soldered. Some of my sloppier solder joints were re-flowed and everything was good after that, or so I thought.

I switched on the S.E.X. only to be greeted by loud popping and static from one channel. This was puzzling since I was pretty sure that all issues had by now been addressed. After swapping interconnects, I realised that the sound would stop when I gently tapped the chassis near the left tube. I removed the left tube and after a few sharp taps to the envelope (please do not do this to a hot tube), it was quiet - time to look for a replacement tube.      

Build Experience

I bought the Crack pre-owned, and it came with the Speedball upgrade already installed. So, my experience relates to my build of the S.E.X. kit.

Thanks to the excellent instructions and well-thought-out kit, assembly of either kit should be achievable, even for the novice who cannot read a circuit diagram. However, understanding the circuit makes things a lot easier, as well as good soldering techniques. Neither of these kits is in my opinion suitable for a person with no soldering experience, nor a basic understanding of electronics. My suggestion is for the inexperienced to do some basic reading, and practice stripping wires, trimming parts and soldering some waste or spare components on a practice board.

The instructions are also catered for the world of Imperial measurements. I had absolutely no idea what a number #4, #6 or #8 screw size is, although to give Bottlehead credit, some of these parts are listed in the instructions with metric dimensions.

It also pays to be very attentive when reading the instructions. As multiple connections may be made to a single terminal hole, you have to make sure that you only solder the point when asked to do so. Despite taking my time, I made this mistake a few times. Thankfully the terminal strips have plenty of space for fixing these mistakes.

Due to a mistake made in stripping the supplied ethernet cable (used for signal hook-up), I had to use some spare Canare star-quad microphone cable to run from the RCA sockets to the potentiometer. Otherwise, I used all the parts as supplied. The carbon potentiometers could have better tracking, and show noticeable channel mismatch at very low volume levels. At practical listening volumes, the channel tracking was acceptable.  

Sound Quality

In today's episode of Spy vs. Spy, will White Spy (S.E.X.) or Black Spy (Crack) emerge triumphant? 

To give both spies a level playing field, only two high impedance headphones were used - a Sennheiser HD6xx (300 ohms) and a Beyer DT-880 (250 ohms). Both spies were sent on physical training camps and a few simple missions to ensure that they were in tip-top condition. 

Black Spy by virtue of its creamy and wet midbass will win over a lot of fans. I thought it would make the Sennheiser sound too dark, but it actually worked quite well. For the brighter sounding Beyer, it added fullness and heft to the sound. Black Spy was not the most meticulous or detailed of spies and glossed over some details. However, if you have an aversion to thin, bright and etched recordings, Black Spy will be your best friend.

The Crack is not tonally neutral. It adds a pleasing warmth to your recordings and takes an edge off all the nasties. Bass control is a bit loose and if you favour fast-paced rock or EDM tracks, this is probably not the best choice for you. 

White Spy, on the other hand, is leaner, faster, and a lot more detailed. The immediate thought that strikes you is, "Where did the bass go?". The bass is taut, tuneful, and very controlled with no flab. I actually appreciated the detailed and articulate bass lines of the S.E.X. 

In similar fashion, vocals are more open and textured, with a deeper insight into the recording. The neutral approach also means that your enjoyment will be dependent on the quality of your recording. My only criticism of the S.E.X. is that I did find myself missing the bass impact of the Crack. A half-way house between the two would have been perfect. I will revisit how the S.E.X. performs after installation of the CCS upgrade kit. 


Which kit is best for you ? My thoughts on this :-


  • First-time builder
  • Preference for a warm and forgiving sound
  • To be paired suitable headphones (>200 ohm impedance)

  • Second-time or advanced builder
  • Preference for a faster and more neutral sound
  • To be paired with a variety of headphones, including low impedance models
  • To be paired with sensitive speakers 

A select few listeners would prefer the Crack over the S.E.X. I personally think the S.E.X. is worth the extra cost for a more sophisticated sound, better transparency, and more flexibility with partnering equipment. 

Bottlehead Crack
USD 315 or 430 with Speedball Upgrade

Bottlehead S.E.X.
USD 539 or 574 with C4S Upgrade

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