Benchmark ABH2. This is the new test equipment on the bench.

[DualTriode]

Hello All,

I have been eyeing this Benchmark ABH2 power amplifier for several months. This last week I went over the edge. The big brown truck delivered it yesterday. The APx1701 test amplifier is not going away just sliding over to make room.

The AHB2 is cooled without a fan which is a huge plus. The goal is to get rid of the instrument cooling fans that may be adding noise to my speaker measurements.

The plan is to share the literal ins and outs of all the connections, microphone placements and the like. There is an assortment of load resistors and heat sinks coming next week. Also still to be purchased are “sense” resistors to measure voltage drop and calculate driver impedance phase and Thiele Small Parameters (included in the APx500 software)

I have some accessory Speakon cables that need to be fabricated.

First off will be testing of the Benchmark ABH2 with load resistors, the idea here is to test the amplifier with an assortment of different types of load resistors. Perhaps there is a difference among the load resistors?

There are a couple of 500 Watt cheap elevator braking resistors of unknown thermal coefficient (ppm/degree C) that need to be tested with the amplifier. I am thinking that at 5 watts, the standard test around here to test the THD+N and SINAD, that I do not expect to see much heat caused distortion caused by the cheap 500 nominal Watt resistors. Any guesses about THD+N and SINAD?

Thanks DT

 

[DualTriode]

Hello All,

I had a couple of 500 watt 8R elevator resistors on the shelf and connected them up to the AHB2 and ran a SINAD test.

The SINAD was within a couple of red curly hairs of 105dB's. There is nothing too exciting going on here. Similar tests by @amirm with 4R test resistors was 8 or so db"s better. With what type of load resistors we do not know.

I am expecting some other test resistors later in the week from Mouser along with some SpeakON plugs. I expect the SpeakON plugs to improve SINAD things by about 3dB's.

After settling in on some reasonable test leads I will test drive some different types of speaker cable for noise and distortion. Then we can zoom in on the load resistors as well.

I have some 12AWG THHN stranded conductors, a roll of CAT5 either net cable and some silver plated Teflon conductors as well to test.

While are at we get out the Bode 100 and test all the above for impedance and phase.

Lots of fun DIY stuff to play with.

If anyone turns CAT5 cable with SpeakON plugs into a commercial product I claim 20%.

Thanks DT

 

[DualTriode]

Hello All,

The first ABH2 FFT.

The load was 4 33R (8R) wire wound 50 Watt Vishay RH 50 resistors in parallel for each channel. The cable was 14 AWG zip cord.

The SINAD was 113 dB for each channel.

As you can see in the FFT was within the Benchmark specification. Each channel is different than the other.

This is a base line; other cables and SpeakON connectors to follow later.

Thanks DT

Edit 8/11/21

About the Vishay RH 50 resistors:

Oops should have done this before but I did not. Wire wound resistors are inherently less noisy. The bulk of resistor distortion is due to thermal modulation. Thermal distortion is dominantly 3rd Harmonic. Thermal distortion is characterized by ppm/degree C. Thermal distortion is a function of ppm/C and an inverse function of thermal mass, thermal distortion increases with degrees C temperature change. These 33R resistors are 20 ppm/C and have a mass of 28g. A single 8 R resistor has a ppm/C of 80 or 4 times the thermal modulation distortion (+6dB) of an equivalent 20 ppm/C resistor. A single 8 R resistor has a thermal mass ¼ of the thermal mass of 4 33 R resistors in parallel for an additional (+6dB) increase in thermal modulation distortion.

Littler known fact, resistors in series and in parallel are used for ultra low THD+N in the internals of the APx555 analyzer.

Thanks DT

 

[DualTriode]

Hello All,

I have been trying a few different things without consistent results.

I have tried a couple or three types of speaker cable with a combination of SpeakON connections and without. So far the bare wire ends of 14 AWG zip cord/ speaker wire without SpeakON connections has performed the best by far. For some reason the SpeakON connectors cause a large 2nd Harmonic spike where there is none with the plain end zip cord to the binding post on the back of the AHB2 amplifier.

I was surprised by the current poor level of performance of the SpeakON plugged connections; I will sort out a few more things prior to posting plots and making claims

Out in the garage there is a hinged steel junction box, still packed from the move that I will use to shield the load resistors. The AHB2 amplifier is capable of low levels of noise and distortion so I suppose that a little extra effort is required beyond what is needed for the average amplifier.

See the attached Dash Board view and odd looking FFT with SpeakON induced 2nd Harmonic spike. Ideas about what is going on with the wavy looking left side of the plot?

Thanks DT

 

[bigjacko]

It is quite disappointed to see speakON does not perform well. Is there any other reason to use speakON if it does not have low distortion?

 

[JohnYang1997]

It is quite disappointed to see speakON does not perform well. Is there any other reason to use speakON if it does not have low distortion?

https://www.audiosciencereview.com/...asr-dummy-load-configuration.8026/post-196116

 

[DualTriode]

Thanks Guys,

The SpeakON connector has a good reputation and is easy to use.

It could just be me that did not get it right or the off brand item that I got at audioxpress.

I am going to keep after this. I just ordered the real, Neutrik, deal from Mouser and quad star cable as recommended by Benchmark.

The VCR discussed by @amirm in his Review of the AHB2 amplifier and in the link posted @JohnYang1977 is also interesting.

I was also reading Bruce Hoffer stuff about TCR also being very important to Ultra Low THD+N design.

I am away from the bench for a couple of days.

Thanks DT

 

[MaxBuck]

The fact that Benchmark uses star-quad configuration in its speaker cables gives me great comfort, since my own Blue Jeans cables use the same Canare 4S11 wire.

If I don't understand what I'm doing (and I don't), I can at least copy people who have definitively demonstrated that they do. Benchmark is my benchmark, even if I don't own any of their stuff. In fact, I may need to remedy that.

 

[bigjacko]

https://www.audiosciencereview.com/...asr-dummy-load-configuration.8026/post-196116

I had read through the whole thread, but I am not sure what you mean. Is the speakOn distortion coming from VCR or it is just not as good as bare wire as Amir said in the post?

 

[DualTriode]

Hello All,



There is a lot to unpack in the world of test load resistors. Much of this will come later; VCR, TCR, inductance and the like. I have the impression that much of this is misunderstood.

If you can find it, take a look at Bruce Hofer’s Audio Xpress article “Designing for Ultra-Low THD+N (Part 2)”. Included are quality insights about resistor caused distortion. found it.
https://hifisonix.com/wordpress/wp-content/uploads/2017/11/Designing_for_Ultra-Low_THD_N-1.pdf



I believe that the APx555 and Benchmark ABH2 are near ideal test instruments. These are on my bench.



First I want to zoom in on the connections and cable between the AHB2 and the load resistor(s).

My initial attempt with an off brand speakon and a 4 wire 14AWG length of SJOOW cord was a disaster.

30 plus dB’s of 2HD suddenly appeared in the FFT. See my posts above. Could be the off brand speakon, could be the cord, could be the screw connection between the cord and the off brand speakon or it could be something else.

Meanwhile I have Neutrik Brand name SpeakOn plugs and a roll of Canare 4S11 speaker cable on their way to the lab.

Breaking it down into manageable parts I want to nail down this cable stuff first.



Thanks DT



How about that for DIY?

 

[JohnYang1997]

I had read through the whole thread, but I am not sure what you mean. Is the speakOn distortion coming from VCR or it is just not as good as bare wire as Amir said in the post?

It's mainly the load. And speakon output is better than banana jack in AHB2 because speakon is directly soldered on the board where banana jack is connected to the board with twisted wire.
This is not an issue with 1et400a reference design because the jack is directly soldered on the board also the feedback and the ground sensing aredirectly taken at the speaker terminal.

So basically if there's some output impedance, VCR become very apparent. If you have extremely low output impedance and test setup is good enough, you may still have very low distortion with poor VCR loads.

 

[JohnYang1997]

Hello All,



There is a lot to unpack in the world of test load resistors. Much of this will come later; VCR, TCR, inductance and the like. I have the impression that much of this is misunderstood.

If you can find it, take a look at Bruce Hofer’s Audio Xpress article “Designing for Ultra-Low THD+N (Part 2)”. Included are quality insights about resistor caused distortion. found it.
https://hifisonix.com/wordpress/wp-content/uploads/2017/11/Designing_for_Ultra-Low_THD_N-1.pdf



I believe that the APx555 and Benchmark ABH2 are near ideal test instruments. These are on my bench.



First I want to zoom in on the connections and cable between the AHB2 and the load resistor(s).

My initial attempt with an off brand speakon and a 4 wire 14AWG length of SJOOW cord was a disaster.

30 plus dB’s of 2HD suddenly appeared in the FFT. See my posts above. Could be the off brand speakon, could be the cord, could be the screw connection between the cord and the off brand speakon or it could be something else.

Meanwhile I have Neutrik Brand name SpeakOn plugs and a roll of Canare 4S11 speaker cable on their way to the lab.

Breaking it down into manageable parts I want to nail down this cable stuff first.



Thanks DT



How about that for DIY?

The key to good measurements setup is to eliminate unwanted sources of distortion. For Poweramps splitting signal at speaker terminal is very important, every another inch of wire carrying current being measured, worse the performance you would get. And VCR comes to play when output impedance in the actual setup is not extremely low.
Is this practical not necessarily because you always have speaker cables connecting the amplifier and the speaker's. But it's critical part when measuring this low level of distortion from the amp itself.

 

[JohnYang1997]

For VCR to be at play there must be voltage division somewhere. Feedback network is basically a voltage divider. You wouldn't have issue with VCR if the circuit is unity gain.

For amplifier measurement, the output impedance and the load forms a voltage divider.

There are two aspects that you can improve to eliminate distortion.

One is to get low distortion load so no matter what division is added it's still no distortion.

Another is to reduce the output impedance in the setup. The amplifier itself has to have low enough output impedance and you have to measure as close to the output terminal(feedback point) as possible.

 

[DualTriode]

For VCR to be at play there must be voltage division somewhere. Feedback network is basically a voltage divider. You wouldn't have issue with VCR if the circuit is unity gain.

[....]

I am going to need some convincing of this.

The way I understand it is that distortion may occur as a result of a voltage drop across a resistor to the extent that specific resistor has a VCR. No other resistors or voltage divider are required.

Carbon comp resistors have a higher VCR than thick film resistors.

Thick film resistors have a higher VCR than thin film resistors.

Thin film resistors have comparable VCR to wire wound resistors.

Wire wound resistors have vanishing levels of VCR, still not to be neglected for ultra low THD+N design.

The largest factor that causes resistor distortion is AC heat modulation, measured in ppm/degree C. The distortion is directly related to the heat/temperature change in the resistor caused by the AC signal across the resistor. At DC the resistor temperature will reach equilibrium. At very high frequency the AC voltage changes so fast there is another sort of equilibrium. The temperature change is much smaller at very high frequencies. Across the audio band frequencies TCR is the dominate mechanism of distortion.

Thanks DT

 

[JohnYang1997]

I am going to need some convincing of this.

The way I understand it is that distortion may occur as a result of a voltage drop across a resistor to the extent that specific resistor has a VCR. No other resistors or voltage divider are required.

Carbon comp resistors have a higher VCR than thick film resistors.

Thick film resistors have a higher VCR than thin film resistors.

Thin film resistors have comparable VCR to wire wound resistors.

Wire wound resistors have vanishing levels of VCR, still not to be neglected for ultra low THD+N design.

The largest factor that causes resistor distortion is AC heat modulation, measured in ppm/degree C. The distortion is directly related to the heat/temperature change in the resistor caused by the AC signal across the resistor. At DC the resistor temperature will reach equilibrium. At very high frequency the AC voltage changes so fast there is another sort of equilibrium. The temperature change is much smaller at very high frequencies. Across the audio band frequencies TCR is the dominate mechanism of distortion.

Thanks DT

Mostly correct. But if there's no "voltage division" the voltage is then defined by certain active elements, voltage source.
I have tested this through and these based on my measurements.
Ignore carbon composite and thick film first.
Larger sized resistor usually is better. Through hole metal film resistors can be both cheap and good in VCR. 1206 0.1% is better than 0603 0.1%. 0603 0.1% is better than 1206 1%. Multiple resistor in series reduces distortion. Better TCR usually indicates better VCR. A better quality resistor usually is just better in almost every way.
For normal circuits 1206 0.1% and multiple in series is the most economical choice or you can use through hole part. Multiple 0603 0.1% in series for less area. For dummy load thick film can work. I use EBG uxp800 800W thick film.

I have not used wire wound. But it should be better than thin film and metal film. Metal foil seems to have some weird characteristics at hundreds of hertz. Also from Bruce Hofer

Apart from VCR, current induced magnetic coupling of wires/cables can also be an issue. Not as important for THD but crosstalk etc.

 

[DualTriode]

Back in Sparky's lab today.

Much improved results with the AHB2 and 8R test load.

Now with real SpeakON connector.

The Canare speaker cord has not arrived.

14AWG 4 conductor SJOOW power cord used as speaker cord today.

WAGO 221 thumb operated wire nuts used as connector between cord and test load.

Test load is 4 Vishay RN50 17R wire wound resistors in parallel.

EDIT: Sampled at load resistor.

See below
Note FFT is scaled in dBV. The goal is to show the noise floor in relation to 1 Volt, not to be relative to the input voltage.

Thanks DT

 

[DualTriode]

The plot below is the Frequency Response.

The test load is 4 vishay RH60 50 Watt resistors in parallel, remember that inductance in parallel adds like resistance in parallel.

There are no inductance worries about using low value wire wound Vishay RH5

0 resistors for test loads.

Thanks DT

 

[pma]

See the attached Dash Board view and odd looking FFT with SpeakON induced 2nd Harmonic spike. Ideas about what is going on with the wavy looking left side of the plot?

I measured similar wave shapes on some bipolar opamps, see

https://www.audiosciencereview.com/...pecially-of-the-lm4562-lme497x0-family.10687/

 

[DualTriode]

I measured similar wave shapes on some bipolar opamps, see

https://www.audiosciencereview.com/...pecially-of-the-lm4562-lme497x0-family.10687/

Yes, those wave shapes are odd.

I believe that the odd shape waves are out of the air.

I did the testing in the open on the bench. I recall seeing similar waves using a AP2522 years ago. Just for fun I tried waving my cell phone over the breadboard on the bench at the time. I did have my cell phone in my pocket the other day when I saw the "Wavy Gravy".

Things are improved with Name brand SpeakON's.

Next I am building some Canare Star Quad cable and name brand XLR test leads, then may be try the cell phone again.

Thanks DT

 

[DualTriode]

Hello All,

It looks like the point of diminishing returns.

The AHB2 gets rid of the power amplifier fan noise in Sparky’s lab.

The black rubber 14 AWG SJOOW cord works well and to my surprise the thumb operated WAGO 221 wire nuts work equally well.

The Benchmark power amplifier plus speaker conductors and test leads perform near the limits of the AP analyzer.

Now any distortion from the speakers being tested does not originate with the power amplifier or test leads.

Thanks DT