Hifi Preamps for your Record Player

By: skrodahl | November 26, 2017

Check out this post for test setup and limitations.

ITU-R IMD is one way to measure intermodulation distortion. I am feeding the circuit with two test tones at -6 dBV, which will sum to 0 dBV. The test tones at 19 kHz and 20 kHz will produce a modulated signal at 1 kHz. The difference between the test tone and the modulated signal is the ITU-R IMD.

The ITU-R IMD for Burson V6 Classic is -118.1 dB / 0.000124%.

Burson V6 Classic - ITU-R IMD at 1.36 dB gain
Burson V6 Classic - ITU-R IMD at 1.36 dB gain

How does it fare in comparison to the audio analyzer itself and, equally interesting, our OPA2134 reference IC op amp?

Well, the audio analyzer can measure down to -122.2 dB / 0.000078%, so the Burson measurements are well within the analyzer's capabilities.

QA401 - ITU-R IMD
QA401 - ITU-R IMD

The Burson V6 Classic loses out to the OPA2134 once again. The OPA2134 measures -121.0 dB / 0.000089%.

OPA2134 - ITU-R IMD, 1.36 dB gain
OPA2134 - ITU-R IMD, 1.36 dB gain

Category: Measurements 

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By: skrodahl | November 25, 2017

Following up on the THD measurement of the Burson V6 Classic (which also shows the test setup and limitations), here's its frequency response.

These measurements are showing up now because the QA401 is not linear. The measurements had to be compensated for that.

The frequency response is measured with an output of 0 dBV / 1V rms and a gain of 1.36 dB / 1.17X.

The Burson V6 Classic's gain is 0.1 dB lower than expected. It performs great otherwise, showing -0.2/-0.1 dB all the way from 10 Hz to 80 kHz. This is as high as I can go with 192 kHz sampling frequency. The graph uses a relative scale since it would otherwise disappear under the 0 dB horizontal line.

Burson V6 Classic - Frequency response at 1.36 dB gain
Burson V6 Classic - Frequency response at 1.36 dB gain

But how high does it go? Time to whip out the Rigol DG1022 function generator and the Rigol DS1054Z oscilloscope. ~1.5 MHz is the answer (with the noise starting at about -20 dB below the signal). Go beyond that and the amplification still holds for a while, but the signal is no longer a sinus.

The signal is down -3 dB at approx. 3.7 MHz. By then it's a pure saw tooth signal.

By: skrodahl | November 23, 2017

Background

To see if the Burson V6 operational amplifiers are viable in the Muffsy Phono Preamp, I went ahead and bought the dual version of both the V6 Classic and V6 Vivid (*). They don't come cheap, about 20x the price of an OPA2134. Getting two of these instead of 40 OPAs for my kit stock was quite a difficult decision...

The Burson V6 Classic and Vivid (dual op amps)
The Burson V6 Classic and Vivid (dual op amps)

The op amps from Burson were delivered with cable ties. I have not been able to figure out why... ;)

I have both feet firmly planted in the objective side of things, so I decided to get the Burson's to do some measurements. I did listen to them in the gain stage of my headphone amp, comparing them with both NJM2068 and LM4562. They sounded pretty damn good (as in more or less indistinguishable from the others) in that scenario (both 2.5 and 6.5X gain), so I really don't feel fooled or anything. This seems to be a proper product.

The Burson V6 Classic in NwAvGuy's Objective2 headphone amp
The Burson V6 Classic in NwAvGuy's Objective2 headphone amp. A tower of DIL sockets were required to make the op amp fit.

JFET inputs are best suited for moving magnet cartridges. The Bursons do have JFET inputs. I would like my op amps to be able to run on +/-15 volts. The Bursons handle up to +/- 16.5 volts. The basics are covered, let's see what they're capable of.

(*) Yes, I did buy them myself. They were not given to me by Burson.

Preparations

In my quest to measure the Burson V6 discrete operational amplifiers, I've established the measurement baseline and used the OPA2134 as reference.

I'll start with the Burson V6 Classic, and I'll have to visit the Burson V6 Vivid later. The V6 Vivid that I received appears to be defective. Burson kindly agreed to send me a new one.

I have shown that I can measure THD down to -118.7 dB / 0.00012%, Here's how the Burson V6 Classic fares.

The op amp is placed in a test rig with 1.36 dB gain. There is no filtering circuitry in the test rig. The output signal is -10.33 dBV, which is the level at which my audio analyzer performs the best.

Op amp test rig
Op amp test rig
Op amp test rig - Schematic
Op amp test rig - Schematic

NOTE: I have reduced the input impedance from 33k to 511 ohms, and updated the measurements. A man's gotta do something with those Tesla TR161 precision resistors...

The plastic around the Bursons gets comfortably warm (not hot, but they would be perfect for warming my hands now that it's winter), and they need a minute or so to warm up before the measurements stabilize. This is taken into account when performing the measurement below.

THD Measurements

Burson V6 Classic - THD
Burson V6 Classic - THD

As it turns out, the V6 Classic is within the measurement limits of my QA401 with about 1.5 dB to spare. The THD shows up at -117.1 dB / 0.00014 %. The noise below 1 kHz shows up in this graph as well, showing that the Burson doesn't have the same noise rejection as the OPA2134.

While the OPA2134 performed beyond the abilities of the audio analyzer, the Burson didn't. This shows that the V6 Classic can't compete with the OPA2134 when it comes to THD at low gain. Still, this is pretty respectable.


Based on the THD alone, they should be virtually indistinguishable from most "normal" op amps in the gain stage of a DAC, headphone amp, CD player and similar line level equipment. I won't be testing their output capabilities, but they're probably not suited for output directly to a headphone (and neither are more than 99% of the IC op amps).

By: skrodahl | November 23, 2017

Measurements on their own are no good unless they can be seen in relation to something. Having established the measurements baseline, I want to get an idea on how the preferred OPA2134 performs.

The conditions are the same as they were with the baseline setup, and the op amp is placed in a test rig without any filter circuitry. The gain is 1.36 dB.

The OPA2134 shows a THD of -118.6 dB / 0.00012%, which is the same as I got on the loopback test. Notice that a lot of the noise below 1 kHz has vanished. This must be because of the OPA2134's noise rejection.

This is obviously beyond what the QA401 can measure, and Texas Instruments claims that the OPA2134 has a THD of 0.00008%. It does show what the audio analyzer is capable of, which is very useful for the next measurements.

OPA2134 - THD Measurement
OPA2134 - THD Measurement

By: skrodahl | November 23, 2017

I've been asked about how the Burson V6 discrete operational amplifiers would perform in the Muffsy Phono Preamp quite a few times. They don't seem to have many published measurements, so I thought I'd give it a go.

To perform the measurements, I have a QuantAsylum QA401 audio analyzer. This will obviously not be measurements of datasheet quality, but I still hope I can find out if these op amps will work for the low signal levels from an MM cartridge. They have JFET inputs and work with up to +/- 16.5V power, which are two of the basic requirements.

QA401
The QuantAsylum QA401 Audio Analyzer

To get the very best results from the QA401, I followed the tips in this blog article. Using the R- output and the R+ input gives the best measurements. I tested signal levels to find what gave the cleanest input signal and the very best THD measurements, which was -10.33 dBV.

A loopback test on the QA401 shows that I get a THD of -118.7 dB / 0.00012%. Operational amplifiers have great noise rejection, so I might get somewhat better measurement results. It shouldn't be by much though, this is basically as good as it gets.

The noise graphs at the bottom of this post shows that using a BNC cable (I tried several of them of varying brand and length) adds about 0.5 dB noise. I can't do anything about that...

QA401 - Loopback
QA401 - Loopback

For reference, here's the noise floor of the QA401, inputs shorted with BNC cables (hires image):

QA401 - Noise floor, inputs shorted with BNC cable
QA401 - Noise floor, inputs shorted with BNC cable

And finally the QA401's noise floor with the inputs shorted with BNC terminators (hires image):

QA401 - Noise floor, inputs shorted with BNC terminators
QA401 - Noise floor, inputs shorted with BNC terminators
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