Eddie Current Studio B technical measurements

Eddie Current Studio B technical measurements

Standard Prologue
If you are unfamiliar with audio measurements please use a search engine with the query:
"audio measurements" or "audio measurement handbook"
Look for publications by Richard C. Cabot and also by Bob Metzler, both from Audio Precision. There are other useful publications as well. These will provide basic knowledge.
Interpretation of the following measurements is beyond the scope of technical measurements posts.

The data presented were collected as follows:

1. PrismSound dScope III, picoscope 5243B, Keysight 34465A, Cal Test CT2593-2 balanced probe (if used)
2. Balanced XLR cables Belden 1800F with Neutrik 110R AES connectors (if used)
3. Single Ended cables Audioblast HQ-1 1.5 ft with Rean RCA connectors
4. 32 and 300 ohm loads used for measurements
5. dScope analyzer sample rate 48 KHz unless otherwise noted
6. 0dBu level used for testing unless otherwise noted
7. Amplifier input to output gain set to achieve 0 dB unless otherwise noted
8. Audioquest Forest and Schiit Pyst USB cables used with measurement equipment
9. Vaunix Lab Brick USB hub
10. Shielded 14AWG and 16AWG power cables
11. Genalex Gold Lion 300B tubes used
12. Balanced 4-pin XLR output used

Measurements made in accordance with AES17:2015

Sensitivity data for two headphones to keep in mind while viewing these measurements:
HD 650 impedance 300R, sensitivity 98 dB/mW
HE-500 impedance 38R, sensitivity 89 dB/mW

SPL levels for above headphones for reference:
0 dBu 300R 2.00 mW - 101 dBSPL @ 98dB/mW
0 dBu 30R 20.00 mW - 102 dBSPL @ 89dB/mW

All testing performed at 0 dBu unless otherwise noted.
This level is consistent with listening to headphones (referenced above) at 90 dBSPL average with peaks to 100 dBSPL, if the music has 10 dB Peak to Avg ratio. That is LOUD for long listening sessions.

Studio B
Measurements commenced after 1 hour of warm-up.
Measurements were performed over a period of several days.

Index
Post 1 - measurement setup description, highlights
Post 2 - 300 ohm load SE input Balanced output
Post 3 - 32 ohm load SE input Balanced output
Post 4 - Frequency Response and Distortion vs Frequency sweeps
Post 5 - reserved for corrections or additional data

Setup picture:


Listening evaluation picture:


Output Impedance: 1.7 ohm

Notable highlights:
Due to some interesting attributes of this amp I will provide additional commentary and interpretation from which I would normally refrain.

Let's get this out of the way first.
Generator level set to 0 dBu, Amp set for 0 dB gain with 300R load

THD+N: 0.161% Left channel
THD+N: 0.132% Right channel

Those who worship THD+N should stop at this point and dismiss this amp. The remainder will be a waste of time. However if you consider there is more to an amp than THD+N please continue. This very special sounding amp offers an opportunity to demonstrate how chasing low distortion numbers out of context is the very definition of foolishness. It is noted that THD+N measures rather high on this amp. Those bothered by such high levels and regard measurements as some sort of diety should bail out here and not waste time with what follows.


The above complex display contains considerable information. The most informative are the FFT spectrums but many want to see numbers, of which there are plenty on this display. The following series will break the complexity into smaller increments and explain what is represented.

Beginning with THD+N we note what appears to be a rather high number of 0.16% THD+N which is an oversimplification. THD+N was one of the first audio measurements and historically easy to measure. Think back 60 years and imagine how to make a measurement without computers. THD+N is simply the insertion of a signal to a component, removing that stimulus with a notch filter and measuring what remains. The remainder comprises everything including mains hum and harmonics, harmonic distortion, residual noise, inharmonic distortion, etc.

So let's break the THD+N down into components.


Expanding scope a bit we see THD+N, THD and 2nd harmonic distortion are almost the same value. This is the first clue that THD+N and THD are 2nd harmonic dominant. I believe 2nd harmonic distortion is part of the magic that provides an enriched presentation of the music. Make no mistake, this is a coloration, but in my opinion a very pleasant one.


Now look at 3rd harmonic distortion. Note this value is 50x lower than 2nd harmonic distortion. Again, my opinion, I believe 2nd and 3rd harmonic distortion ratio comprise the voicing of a component. So this smaller amount of 3rd harmonic distortion adds just a hint of sparkle to the 2nd harmonic enriched sound.


4th and 5th harmonic distortion are 550x lower than the 2nd harmonic, an insignificant level. Hum and residual noise likewise are 200x lower than the second harmonic, also insignificant.


This is a measurement I developed, 4+HD+N, with stimulus, 2nd and 3rd harmonic removed. What remains is a simplification of 4th, 5th and higher harmonics, mains noise, residual noise, in-harmonic noise. This is what I consider the crap factor. A low number here is desirable. Studio B has 0.00126% crap factor in the left channel which is approximately -98 dBu, well below audibility.


Putting the foregoing into perspective the harmonic distortion depicted in the FFT is downward sloping as shown by the Blue line on the Right channel, highly desirable for those who appreciate what 2nd harmonic distortion offers, of which I include myself for many recreational listening situations; not all though.


Finally, adding another horizontal Blue line on the Right channel we note mains noise and other residual noise are all at or below -100 dBu, quite inaudible. Consider the following:

-100 dBu 300R 98 dB/mW HD800 will produce 1 dBSPL
-100 dBu 55R 104 dB/mW Focal Clear will produce 14 dBSPL
-100 dBu 27R 106 dB/mW ER4P-T will produce 19.5 dBSPL

gain linearity

Exceptional gain linearity down to -105 dBu, remarkable for a SET amp.

Bottom line, this amp represents an accumulation of experience and learning over decades which Craig has applied to create a SET amplifier with very special presentation of sound, a 2nd harmonic heaven for SET magic. The transformer design in conjunction with the selected tube characteristics, circuit topology and associated component selection have all been optimized. Performance measured is achieved with very little negative feedback, 1.75 dB.

Well done Craig!

 

300 ohm load SE input Bal output

Studio B A04 THD+N THD nth-HD FFT 300 ohm load SE input Bal output


Studio B 50 + 7000 Hz 300 ohm load SE input Bal output - Left Channel


Studio B Gain Linearity 300 ohm load SE input Bal output - Left Channel


Studio B THD+N vs Frequency 300 ohm load SE input Bal output - Left Channel


Studio B Residual Noise 300 ohm load SE input Bal output - Left Channel


Studio B Square Wave 2000mV 5uS/div SE input Bal output - Left Channel

Bandwidth estimation: BW = 0.35 / RT
Where RT = 10 to 90% Rise Time
0.35 / 3.493 uS = 100.2 KHz

Complete 300 ohm load SE input Bal output analysis report pdf attached

 

32 ohm load SE input Bal output

Studio B A04 THD+N THD nth-HD FFT 32 ohm load SE input Bal output


Studio B 50 + 7000 Hz 32 ohm load SE input Bal output - Left Channel


Studio B Gain Linearity 32 ohm load SE input Bal output - Left Channel


Studio B THD+N vs Frequency 32 ohm load SE input Bal output - Left Channel


Studio B Residual Noise 32 ohm load SE input Bal output - Left Channel


Studio B Square Wave 2000mV 5uS/div SE input Bal output - Left Channel

Bandwidth estimation: BW = 0.35 / RT
Where RT = 10 to 90% Rise Time
0.35 / 3.741 uS = 93.56 KHz

Complete 32 ohm load SE input Bal output analysis report pdf attached

 

Frequency Response and Distortion vs Frequency sweeps

Frequency Response:

Studio B Gold Lion frequency response 5Hz - 96KHz 300R 0dBu


Studio B Gold Lion frequency response 5Hz - 96KHz 32R 0dBu

In the graphs above Studio B is +1 dB at 10 Hz and -1 dB at 80 KHz.
Observe the interesting +2 dB bump at 5 Hz.

note: with a 192 KHz sample rate frequency response measurements are impacted -1 dB by the analyzer anti-alias filter beginning at approximately 85 KHz and -3 dB at 90 KHz. Compared with the 20 Hz square wave bandwidth estimates Studio B is likely hitting between 95 and 100 KHz with the Genelex Gold Lion tubes.


Distortion vs Frequency sweeps:

Studio B Gold Lion THD+N, D2 D3 4+HD+N vs freq 0dBu 300R


Studio B Gold Lion THD+N, D2 D3 4+HD+N vs freq 0dBu 32R


Legend
THD+N is the blue trace
D2 - 2nd harmonic distortion is green
D3 - 3rd harmonic distortion is yellow
4+HD+N (everything else) is red

D2 and THD+N are identical from 60 Hz to 10 KHz
Of interest note how D3 rises with sweep from 7 KHz to 20 Hz. It becomes the dominant distortion below 32 Hz.
Also note how 4+HD+N is higher than D3 above 950 Hz yet 35 dB below D2. Below 950 Hz 4+HD+N rises but always remains significantly below D3. This indicates the non-musical distortions are likely never audible as they are always masked by either D2 or D3.

Consider that these last two graphs convey far more information about distortion than the THD+N number in the original post. Single numbers are cornerstones for simplistic one dimensional thinkers. Those capable of grasping multidimensional information will gain far more insight from graphs such as these last two, which I believe better describe an aspect for the characteristic sound of an amp.

 

reserved for corrections or additional data

 

Studio B specifications from Eddie Current website:

SPECIFICATIONS
• Custom output transformers, rare earth magnetic core with tertiary feedback standard of 1.75 dB (as with Studio).
• Minimal point-to-point wiring.
• Balanced headphone drive.
• Two input selector switch, two pairs RCA unbalanced inputs.
• Speaker kill switch.
• 100K Alps Blue Velvet volume control.
• One pair Speak-On speaker outputs. 4, and 8 ohm.
• 4-foot detachable umbilical cord.
• Vacuum tube rectified passive LC power supply.
• Will drive 4,8 ohm speakers to 4 watts. Input sensitivity 1.5V for full power.
• Power bandwidth -1dB 10Hz-100kHz.
• Recommended headphone load 8 ohms or higher.
• Power into 32 ohms 2W.
• Power into 100 ohms 800mW
• Power into 300 ohms 400mW.
• Typical headphone frequency response–balanced or SE: -1dB 10Hz-100kHz.
• Balanced line output 11 volts into 600 ohms -1dB 10Hz-100kHz.

The percent distortion at which the powers were achieved for the various loads is not specified. Given this type of tube amplifier, it is reasonable to assume 10% distortion for the specified power levels.

Using the numbers from the above specification produces the following graph:


Voltage is Vrms.
P = Vrms^2 / R
Vrms = sqrt(P*R)

dScope measurements verifying the specifications.

common measurement conditions
Studio B amplifier
Gold Lion power tubes
GE 5670 JAN driver tube
SE input
Bal output

distortion vs level 1000Hz 300R load 5 dB amplifier gain

Be careful reading this graph. The x-axis depicts signal generator level input to the amplifier. For the 300R loads Studio B was adjusted to have 5 dB of gain to facilitate reaching the necessary output levels for specification verification. Thus it is important to add 5 dB mentally to x-axis levels indicated when observing distortion levels.

Observe 10% THD+N at +23 dBu (18+5) which is 10.95 Vrms
This computes to 400mW power at 300R load. Specification VERIFIED.

distortion vs level 50Hz 300R load 5 dB amplifier gain

Again remember to add 5 dB to x-axis levels when reading this graph

Observe 10% THD+N at +23 dBu (18+5) which is 10.95 Vrms
This computes to 400mW power at 300R load. Specification VERIFIED.

distortion vs level 1000Hz 32R load 0 dB amplifier gain

The amp is adjusted for unity gain for 32R load measurements
Max signal generator input is +20dBu. At this level observe 8% THD+N. A slight extrapolation by an additional 0.28 dBu the level will still be below 10% THD+N. 20.28 dBu with a 32R load is 2W. Specification VERIFIED.

distortion vs level 50Hz 32R load 0 dB amplifier gain

Max signal generator input is +20dBu. At this level observe 8% THD+N. A slight extrapolation by an additional 0.28 dBu the level will still be below 10% THD+N. 20.28 dBu with a 32R load is 2W. Specification VERIFIED.

HE-6 and Susvara planar headphones need only 0.5W at their respective nominal impedances to reach a blistering 110 dB SPL at 1000 Hz.

Most other headphones require considerably less power to reach such levels.

Assuming 100 dB SPL average with peaks to 110 dB SPL will allow the listener to go deaf quickly. Most normal listeners are likely to keep the levels down, around 75 to 85 dB SPL, if they wish to enjoy music, and their ability to hear it, for many years.

 

@atomicbob It's not clear what you mean by adjusting amp gain. What is the amplifier gain, and what input voltage is needed to reach max power into 300 Ohm?

 

Measurement diagram


dScope Signal generator sine amplitude is adjusted from +20 dBu to -30 dBu in 2 dB steps.
dScope Signal analyzer measures distortion and plots distortion vs signal generator level at each step.
To achieve an amplifier output of greater than +20 dBu it is necessary to adjust the Studio B volume knob
such that 0 dBu input results in +5 dBu output into 300R load.

For 32R load distortion measurements the Studio B volume knob is adjusted for
0 dBu input resulting in 0 dBu output, also known as unity gain.

 

Thanks! So you adjusted amp volume.

I'm asking because my amp has gain of ~3.35x . That means to reach maximum output voltage of 11V, we need 3.3V input. For this I assume we need a preamp. That is not how I expect a headphone amp to work. So I'm curious what gain you've got, with max volume.

 

First, most headphones don't need that much power to reach ear damaging levels.
I've posted this in many places, but once again:



HE-6 and Susvara only require 50 mW to reach 100 dB SPL at 1 KHz. That is approximately 1.6 ~1.75 Vrms.
Want them to make 110 dB SPL, then they need 500mW which is approximately 5.48 Vrms.
Most DAC converters produce 2Vrms at 0 dBFS. To achieve the 110 dB SPL a gain of 2.74 is required.
A preamp is not necessary.

Now look at a typical dynamic headphone application and it can be observed that most listeners will be attenuating the
DAC output, ie turning the volume know DOWN.


A preamp is not necessary unless using a turntable.

 

Thanks, that much I know. I was asking in principle. Is the max power specification correct, if you cannot achieve it with just a DAC? Shouldn't max power be the value that you can get with 2 VRMS input?

Or maybe your amp has higher gain, and mine is defective. That's what I'm wondering. I'm getting surprisingly underwhelming dynamics from my amp, and was investigating exhaustively to the extent of my knowledge, whether there's something wrong with it.

 

Gain on SET tube amps has a strong correlation to the gain of individual tubes in the signal path.

My amp has less gain than what you appear to have measured on yours. I'd bet there is nothing wrong with your amp. It may be your preference doesn't align with the way the tube compliment on your amp renders the music. Or maybe Studio B isn't a good fit with your preferences.

The following measurements establish gain of my particular Studio B with the specific tubes used as noted below:

common measurement conditions
Studio B amplifier
Gold Lion power tubes
GE 5670 JAN driver tube
SE input
Bal output
316R load (General Radio wattmeter, closest to 300R)
Volume control set to maximum

Studio B 1 KHz sine input 2Vrms output 5.25Vrms 316R load 80mW at 0.9% THD+N


Studio B 1 KHz sine input 4Vrms output 9.7Vrms 316R load 300mW at 5% THD+N


Studio B 1 KHz sine input 5.3Vrms output 11.5Vrms 316R load 400mW at 10% THD+N


Using data from the above measurements produces the following graph:

From the above measurements it would appear my Studio B with tubes listed has a gain between 2.17 ~ 2.63 depending on the operating point and distortion tolerated.


I dug out the General Radio Wattmeter for the following measurements made simultaneously with those from the oscilloscope shown above.

Studio B 1 KHz sine input 2Vrms output 5.25Vrms 316R load 80mW at 0.9% THD+N


Studio B 1 KHz sine input 4Vrms output 9.7Vrms 316R load 300mW at 5% THD+N


Studio B 1 KHz sine input 5.3Vrms output 11.5Vrms 316R load 400mW at 10% THD+N

 

With the previous data and examining the Distortion FFT from the oscilloscope measurements a question one might ask is why anyone would need power above 100mW for most headphones and suffer the consequences of increased distortion.

:Studio B 1 KHz sine input 2Vrms output 5.25Vrms 316R load 80mW at 0.9% THD+N

This is a favorable distortion profile with dominant 2nd harmonic and subsequent harmonics descending in level.

Studio B 1 KHz sine input 4Vrms output 9.7Vrms 316R load 300mW at 5% THD+N

At this level the rise in distortion may not be worth the extra power achieved.

Studio B 1 KHz sine input 5.3Vrms output 11.5Vrms 316R load 400mW at 10% THD+N

I wouldn't, but someone else may find this appealing.

Now consider the first example with approximately 5.24 Vrms or 16.6 dBu

Those sound levels are plenty loud enough for the 300R headphones.
Sennheiser headphones are in the 117 dB SPL territory.
Drop TH-X00 is at 124 dB SPL.
Ear damaging levels.

Now consider using +23 dBu or 10.95 Vrms with 10% distortion levels

Bloody Hell! Is that loud enough?!!!
Probably smoke both the headphones and the ears wearing them.
Recommendation: Don't do it.

 

A very brief comparison of how Studio B and Mjolnir 3 behave as they begin to hit voltage saturation.

common measurement conditions
Studio B amplifier
Gold Lion power tubes
GE 5670 JAN driver tube
SE input
Bal output
316R load (General Radio wattmeter, closest to 300R)
Volume control set to maximum

Mjolnir 3 amplifier
High gain
NFB off
SE input
SE output
300R load
Volume control set to max

1 KHz sine input to each amp, level adjusted to achieve approximately the same output saturation.

Studio B saturation at 9.7 Vrms

Observe the negative peak flattening.

Mjolnir 3 saturation at 10 Vrms

Observe the positive peak flattening.

Other than the polarity of the peak saturating and amplifier fixed gain, there is a strong similarity between the saturation behavior of these two amplifiers.


Studio B distortion profile at 9.7 Vrms

Observe the harmonics in a generally descending pattern to 40 KHz
Studio B has a 40 KHz amplifier driving the tube filaments.

Mjolnir 3 distortion profile at 10 Vrms

Observe the harmonics in a generally descending pattern and at almost the same levels as Studio B.

Again, a very strong similarity of distortion behavior at beginning of saturation between the two amplifiers.
Something to consider when A/B comparing these two amp in listening evaluation.

 

What are the considerations when setting the bias level? Does relative higher or lower bias impact FR or distortion, AMIRnads, etc?

 

Mainly heat. Lower bias tends to be higher distortion and/or lower power. Higher bias = more heat = bigger heavier heatsinks = more $$$.