Articles, Reviews and Measurements of Audio Products (As an Amazon Associate I earn from qualifying purchases)

In a recent review of this phono stage, there was some discussion of the approach taken for measurement and interpretation of phono stages. This post is a follow up with additional measurements which I (and other designers and users of phono equipment) find necessary for evaluation. This a "in addition to" rather than "instead of" for Amir's review, but I'd like to not only present measurements, I wish to also go into the whys and hows of things which are peculiar to the genre.

And I'll start things out by thanking Amir for sending the unit to me, as well as @dinglehoser who volunteered his unit to let me dissect its performance a bit.

As usual, my test setup is centered on an APx525 analyzer, but I also pressed into service a Hewlett-Packard 3466A volt-ohm meter and a Kikusui COS6100M 100 MHz scope.

Let's start with the basics. I...

This is a review and detailed measurements of Klipsch PowerGate Multifunction streaming power amplifier, DAC, Bluetooth and phono preamplifier. It normally costs USD $499 but for some reason it is on Amazon for USD $150 including Prime shipping! This puts the PowerGate in direct competition to SMSL AD18 and Topping MX3.

The front panel of the PowerGate is plastic and not all that nice looking:


The buttons and rotary control have good feel though. Strangely the LED bar showing the level only goes 2/3 of the way up on the left and not all the way around.

There is a headphone jack which I will be testing in the review.

The back panel shows a very feature rich set of options...

This is a quick overview of how class D amplifiers operate. Note the “D” does not mean “digital”; it is simply the next letter in order as standards bodies enumerated amplifier types (A, B, C, D, E, F, G, H, I, etc.) It takes an analog, not digital, input and produces a pulse width modulated (PWM) output. I apologize in advance for the length.

The figure below is a simplified schematic diagram of a class D amplifier. It is a rather busy diagram, so we’ll step through each piece from input to output. First the input signal, Vin, is applied to the input buffer (shown as an operational amplifier, opamp). The negative input of the opamp receives the feedback signal (Vfb) from the output (Vout). The output of the buffer is the difference between these two signals, providing a modified signal to the rest of the amplifier. The feedback signal allows the amplifier to correct any errors at the output, improving linearity (e.g. lowering distortion) and stabilizing the circuit so part...

This is a review and detailed measurements and comparison of SMSL AD18 DAC, Headphone Amplifier, Power amplifier and Bluetooth Receiver. I will be contrasting its performance against the Topping MX3. I purchased the AD18 in February of last year through Amazon. Seems like the price has gone down to $140 and it now includes Prime shipping. Topping MX3 is still cheaper at $130 from Amazon.

The look of the front panel is not bad:

I left the screen protector on so that if I sell it, the next owner can have the satisfaction of taking it off.

I wish the volume indicator was displayed in large font instead of the small values in top corner.

The rotary encoder for the volume control feels wobbly and cheap. And it has very sharp burr around that machined silver ring. The Topping MX3 one is worlds better in...

This is a review and detailed measurements of the Topping MX3 DAC, Bluetooth, Headphone Amplifier, and Power amplifier. You even get a remote control with it. Despite all that functionality it costs just USD $130 from Amazon including Prime shipping. I bought mine from Massdrop 10 months ago. I don't know what I paid but likely less.

From the outside, the MX3 looks quite attractive:


It has the same design language as the Topping DX3 Pro but even more attractive with that slanted logo.

The volume control is digital and very sensitive.

There is a ton of functionality here which I won't go into as there is a good review of it from Z which I will post after this review. Suffice it to say, it is very feature rich and well designed for such a bargain product.

Everything is powered by an external laptop sized switching power supply:


Not only is extra expense incurred in getting a...

This is a review and detailed measurements of Amazon Echo Link Amp power amplifier, DAC and network streamer. I purchased this from Amazon for USD $299 and it arrived yesterday.

While rather featureless from front, the overall feel of the plastic and case is quite good:


When you rotate the volume control white LEDs light up to show a coarse volume control. They time out very quickly though so I could not capture them on the camera.

Not sure what the hole is for. Maybe it is for a remote control but none was provided.

The back panel is the more interesting bit:


In addition to the power amplifier, we have a nice little DAC and pre-amplifier here. There are digital inputs in the form of both Coax and Toslink S/PDIF. And of course we also have streaming capability with the Ethernet jack and wifi.

Surprisingly we also have S/PDIF and Toslink output for digital connectivity to other...

This is a review and detailed measurements of the Ayre CODEX balanced DAC and headphone amplifier. It is on kind loan from a member. The CODEX is a current product but seems like it was released 3 or so years ago. It costs USD $1,795.

I am puzzled by the look of the CODEX as it doesn't match the rest of the Ayre products:


If you are not familiar with Ayre, it is a US company and is probably best known by its late founder, Charles Hansen. He has been active on forums for years. The company is in high-end audio business so my expectations were high on this DAC and headphone amplifier.

I was pleased to see a set of balanced XLR connectors on the unit and inclusion of mains power supply despite its rather diminutive size:

Then again, closeness of the mains input gives me some pause regarding its noise immunity for the RCA outputs.

In use the CODEX runs very warm. The marketing material...

This is a review and detailed measurements of the Acoustic Power Labs APL1 digital audio signal processor. It is on kind loan from a member. The APL1 comes in a number of formats. The one under review is the automotive version and costs 510€ or USD $570. This is for the bare hardware. Software that let you program its internal "FIR" filters costs 262€ or USD $295. I am surprised that given the cost of the hardware, the software is not bundled in for free. For this testing, I only focused on the measurements of the hardware as the software seems non-trivial to use.

The automatic configuration of APL1 is nice and slim with phoenix connectors for power:

The LEDs change color depending on where you leave the slider switch. In my case I set the switch to bypass.

Here are the back connectors:

We have analog RCA input and output. In addition there is also S/PDIF and Toslink for input and...

The Classé Sigma SSP is an AV-preamp targeted for audiophile music lovers who don't want to setup 2 different systems (one for audio, one for home cinema). It is optimized for very high quality on the stereo channels and somewhat less performance for the remaining surround channels: the stereo channel has both one balanced input and one balanced output (XLR) while the remaining analog IOs (2 x stereo in, 7.1 out) are single-ended (RCA). The specs are as follows (selected):

This is a review and detailed measurements of the Cyrus soundKey portable USB DAC and headphone amplifier. It is on kind loan from a member. It costs USD $98.99 from Amazon with prime shipping. It comes in a few colors.

I like the flat form factor with detachable USB cord:


I received two short USB cables, one of which you see in the picture above. I don't know if that is how Cyrus ships it or the owner sent it me as such.

There are no volume controls so you would be relying on your software player for that.

One of the differentiators for the soundKey is low power consumption according to the company. I will be measuring and reporting on that later in the testing. For now, there was no rise in the temperature that I could feel on the unit.

Audio Measurements
As usual, we start with our dashboard view of 1 kHz tone:



Typical of these small dongles, there output is too low to use the soundKey...

This is a review and detailed measurements of the Lounge Audio LCR MKIII Phono stage (amplifier). It is on kind loan from a member. The LCR MKIII costs $300 from the manufacturer direct.

The LCR MKIII is a very simple, typical DIY box:

A touch of class is intended to be provided by that lexan sheet with blue LED. It doesn't do much for me especially since the lighting is not uniform.

The back panel is simple too:

Since this is a moving magnet only phono amplifier there are no switches or controls. We have our simple RCA in and out plus a ground lug. Power is provided through an external AC adapter which is not a bad idea to keep mains inductance from the transformer leaking into sensitive amplifier stage.

As the name technically implies, this phono amplifier sets itself apart from others by using an inductor ("L") to implement RIAA equalization instead of just using capacitors. In...

This is a review and detailed measurements of the Lake People G109-S headphone amplifier. It is on kind loan from a member. It costs USD $520 as of this writing but I see it has been offered on Massdrop for lower price.

Lake People is located in Germany and designs and builds its products there. Alas, the industrial design is ordinary and doesn't impart what I associate with German engineering:


I saw the anniversary edition (?) with gold letter and larger feet and that looked better. I know enclosures are expensive but over $500, I like to see a more professionally designed writing, logos, etc.

The volume control is analog but has notches in it. There are two headphone out jacks but they are identical (unlike some others where one is lower gain). I would have wished one was 3.5mm to accommodate IEMs without adapters.

Missing from the unit is any kind of external gain control. If you open the unit which requires removing...

This is a review and detailed measurements of the vintage Dynakit Dynaco Stereo 70. It is on kind loan from a member who bought it used online and repaired it. Previous owner hand upgraded it but could not get it working.

Given that it is 60 years old, one can forgive the rust and dust:


Some of the online samples I have seen are a lot more crusty.

The unit I received came from matched pairs of tubes from Apex. While each pair had the same parameters, the ones for each channel varied quite a bit. As you will see, this is reflected in the measurements.

This unit has an upgraded driver board:


There is a single bias control for each pair of tubes which is not optimal. I went ahead and adjusted both for 1.56 volts as stated in the manual.

I am not sure what output transformer tap is used. I tested it as is with my 4 ohm load. Hopefully the owner will advise what it is wired for...

This is a review and detailed measurements of the Audioengine D3 portable DAC and headphone amplifier. I bought this refurbished from the company direct a year ago. I think I paid USD $99 for it but now see it new for the same price from Amazon including shipping. A member wrote to me asking if my version was 2 or the original. His has v2 after it, mine does not. I contacted the company and just heard back that the new versions have lower output impedance than what I measured. They do not note any other changes.

The D3 is one of the more substantial thumb drive DACs with chunky and solid metal enclosure:



On the back side there is full set of regulatory certifications including CE and FCC which I was pleased to see. And yes, the CE logo conforms with the EU graphics standard. Audioengine is big enough company that likely did go through proper testing. The metal enclosure definitely helps with any radiated emissions standards.

As soon as...

This is a review and detailed measurements of the Schiit BiFrost audio DAC. The unit is on kind loan from a member. The one I have doesn't have USB input so not sure if that is an older version or current??? Schiit website shows USB input as standard but then again there is some wording regarding optional USB module. Regardless, as best as I can figure out, the cost for BiFrost is USD $399 plus shipping.

Note that I had previously measured the Schiit BiFrost Multibit. This is the non-multibit version which is based on AKM DAC chip.

Not sure what is to say about the look of Schiit other than if you have seen one, you have seen them all:


Since there is no USB input, the momentary switch toggles between Toslink and S/PDIF. I used the latter for all of my testing.

The back panel has nothing exciting...

This is a review and detailed measurements of the Meier Audio Corda Jazz headphone amplifier. It is on kind loan from a member. The Corda Jazz costs USD $355 from the company direct.

Given the inclusion of an AC mains power supply, the Corda Jazz feels quite hefty for its size:


No, you are not seeing double. I actually received two identical units for testing!

I find the graphics preminative and unattractive.

Other than the usual low and high gain, there is a cross-feed button which I did not play with.

The headphone socket is a Neutrik and mates nicely with 1/4 inch headphone jacks.

The back panel is as simple as you can get with AC IEC connector and a pair of RCA jacks:

I am not one for detail as you can tell from my posts but come on: why is "designed" not capitalized?

Measurements
I started with my unity gain test of 2 volt in and 2 volt out...

This is a review and detailed measurements of the Oppo HA-1 DAC and headphone amplifier. It is on kind loan from a local member who literally flew to our nearby airport to deliver it to me! The HA-1 is discontinued but it cost USD $1,199 when it was being sold by Oppo. I just searched on ebay and was surprised that it goes for $1,200 to $1,700!!! Will this be a top find as the Oppo UDP-205 UHD player was? We will find out.

The HA-1 is a hefty unit with a beautiful, high resolution graphical display:

I wish all DACs/headphone amps above $1,000 came with such a display.

In some respect though, beauty is skin deep as the display is very sluggish to keep up with volume control changes. Worst yet, even though the volume control appears to be digital, it is not. My son has had an HP-1 for a few years and...

This is a review and detailed measurements of the Class D Audio (the company) CDA-250C Power (speaker) Amplifier. It is on kind loan from a member. The CDA-250C costs USD $495 from the company direct which barely sneaks it into affordable class.

There is not a whole lot to look at on the face of the unit:


As you see, the unit is made in USA. I suspect the case is generic Chinese though given the DIY look and feel it has.

The power button is oddly in the right side of the unit.

The back panel sports very large speaker jacks which is nice:



Unfortunately there are no balanced inputs even though the holes seem to be made for them.

Another miss is lack of any safety or regulatory certification.

In testing, the CDA-250C ran completely cool. I could not feel slightest rise in temperature of the case whether I touched the top, left or right, which is really good.

Measurements
Let's start with our...

This is a review and detailed measurements of the Essence HDACC II-4K HDMI and USB DAC. It is on kind loan from a member. It costs USD $699 from company's website although it is on sale currently for $599.

For years, I have been looking and asking DAC manufacturers to include HDMI input in their DACs. As many of you know, HDMI is the standard delivery format for digital video. Without HDMI input, you cannot listen to the advanced, multi-channel audio formats on such platforms as Blu-ray Disc and its UHD version. When there is a HDMI input on some DAC, it is for PCM only and no compressed format is offered. Essence solves fills this much needed gap with their DACs with HDMI input. Both Dolby True-HD and DTS Master Audio HD are decoded internally allowing you to feed the native bitstream to it.

It includes all this functionality plus balanced output and analog input plus headphone out in a very small and attractive enclosure:

As the...

This article is to expand upon earlier discussions about reflections and their potential for harm in a digital transmission system as applied to the audio world. I will be qualitative as much as possible, but numbers are bound to creep in. This is also not meant to be a rigorous analysis, more a hand-waving explanation to help folk see what is happening in their system.

First consider a digital driver (source) and load (receiver) connected by a transmission line (cable). The driver and cable are matched with impedance Zo, and the load has mismatched impedance Zx as shown in the figure below. At the top, a single pulse is launched from the source toward the load. The pulse, now a forward signal, travels down the line to the receiver. Even at nearly the speed of light it takes a little time to get there.

As the...