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DIY 250W/4ohm amplifier based on "blameless" topology, and measurements

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pma

pma

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So the amp has been in operation about 2 months and it remains my main amplifier. I would like to share some construction details.

P1040402.JPG

In the middle is a delayed start board. It is absolutely necessary to have it because of the inrush current of the two 300W toroidal transformers. On left and right are PSU rectifier/filter boards.

P1040398.JPG

On left and right are output speaker protection boards with 90A MOSFET SSR circuits and opto isolation.

P1040399.JPG

Main amplifier board of one channel.
 

restorer-john

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Thank you for sharing your build.

I think it's the first time I have seen a bridge rectifier made in India. A sign of things to come.

1620717403305.png
 
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pma

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Thank you for sharing your build.

I think it's the first time I have seen a bridge rectifier made in India. A sign of things to come.

Vishay, manufactured in India. About 10 years in operation in the same box, only with a different amplifier module (CFA). As you know, the "old world" has almost resigned to SS parts production.
 

restorer-john

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peufeu

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Nice build!

The LF distortion comes from highly nonlinear ferrite choke 18mH. This is intentional and simulates the real speaker LF nonlinearity. Finite DF then makes rise of LF distortion

That's a good argument for bi-wiring actually: nonlinear woofer current * wire impedance creates distortion voltage at the speaker, which will be played back by the whole speaker, so if it's a 3 way the midrange driver will output this distortion voltage... With separate wires, distortion due to nonlinear woofer current will only be seen by the woofer.
 
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pma

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Several new measurements

A250W4R_1kHz_4R_thdampl.jpg


A250W4R_1kHz_8R_thdampl.jpg



and sine burst 50Vp/4ohm, 312.5W/4ohm
A250_burst_1k.png
 
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pma

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Following the current "bridging" discussion in the LA5 thread I have to say that not every power amplifier is a good candidate of bridging. This one is the one that should not be bridged. The main reason is that it is a dual-mono design and when the power grounds of both amplifiers are connected at the output and signal grounds at the input, noise due to induced mains spuriae is rising high.

Below some results of THD and THD+N measurements into 4ohm and 2.4ohm load in a "normal" scenario, and also for 4ohm in a bridged mode. Though we get about as much as 580W/4ohm, the elevated noise is unacceptable, to me.

A250W_2-4R_SE-bridged_thd.png


A250W_2-4R_SE-bridged_thdn.png
 

nn_in

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DIY 250W/4ohm power amplifier based on “blameless” topology

Hello all, the thread posted here by @sabristol
https://www.audiosciencereview.com/forum/index.php?threads/luxman-l-85v-integrated-amplifier.20657/

inspired me to build a new DIY amplifier functional sample. The circuit posted in the link above is called “Luxman L-85” but in fact the topology is rather the Douglas Self's “Blameless Amplifier” discussed in his book Audio Power Amplifier Design Handbook on also on his website
http://www.douglas-self.com/ampins/dipa/dipa.htm
http://www.douglas-self.com/ampins/dipa/dpafig33.gif

The original Luxman PB-1037 main amplifier circuit is more different with 2 differential stages instead of one, no use of EF VAS buffer etc.

@sabristol has made very good job in implementing improvements by Douglas Self (EF buffer before VAS etc.) and his circuit posted was a temptation to me to get even more power and less distortion from that circuit. The main change I made was to use 2 pairs of the output devices, to get more output current and power and less dependence on load impedance. I was thinking about my favorite and robust MJL21194/93 first but then decided to go for MJL3281/1302 pairs, which have even better linearity at high currents and are faster, though only very slightly weaker in SOA.

This is the complete schematics of the amplifier that I built

View attachment 119117

It was built into my prototype case, which determined the size of the PCB and also components placement and drilling. The case is 19” 4U, dimensions 450 x 415 x 180 mm. It has big side heatsinks and can accommodate two 300VA toroidal transformers, that are needed for the dual-mono 2x250W amplifier concept with long-term full-power capacity.

This is the amplifier PCB mounted on the heatsink
View attachment 119118


and this is the amplifier board in the prototype 19” 4U case (the bottom board. The top board is a CFA amp - it will be replaced soon)
View attachment 119119

There are two transformers, two rectifier-filter boards, two amplifier boards and two DC protection SSR boards inside the case. The metal case is grounded (connected to PE) but the signal grounds of the left and right channels are connected to the case through the Rth//C components, to prevent usual serious ground-loop hum issues. The design is dual-mono and the signal grounds of the left and right channels are not directly interconnected.

Two MJL3281/1302 output pairs make 250W/4ohm power possible with respect to SOA (Safe Operating Area) of the transistors. It is possible to use speaker/complex load that does not fall below 4 ohm in its impedance/frequency plot. The worst case simulation with the load that well reflects the woofer impedance shows that the SOA I/V trajectory of one output device is just at the edge.

This is the impedance response used in the simulation
View attachment 119120

and this is the SOA simulation for 1 power transistor, with dummy load impedance schematics
View attachment 119121

Interestingly enough the amp may drive purely resistive load of 2 ohm up to full output swing and still stay inside allowed SOA boundaries. It only tells that pure resistive loads are inadequate for both simulation and testing and do not reflect real-world speaker load.

Another interesting points of the schematics are the Q16 emitter follower (beta enhancer) that greatly reduces VAS distortion and increases open loop gain and all the current sources that improve PSR (ripple rejection).

Functional sample parameters

  • input impedance ….. 70 kohm
  • frequency range ….. 2Hz – 88kHz/-3dB
  • full-power bandwidth ….. 20Hz – 20kHz
  • output noise voltage A weighted ..... -84dBV(A)
  • output power ….. 2x250W/4ohm for THD < 0.1% at 1kHz
  • S/N at full power ..... 114dB(A)
  • harmonic distortion ….. THD < 0.007% at 200W/4ohm/1kHz (see graphs)
  • rise time of step response ….. 4us
  • gain ….. 34.4dB
  • dimensions ….. 450 x 415 x 180 mm
  • weight ….. 30 kg approx.
  • construction ….. dual-mono with 2 toroidal PSU transformers 300VA each

Measurements

Response to 10kHz square wave

View attachment 119123

Sine 20kHz at full power into 4ohm load
View attachment 119124


Distortion measurements - as mentioned in post #25, the soundcard originally used had H2 distortion similar as this amplifier so there happened a distortion cancellation in some measurements. These measurements have already been replaced by valid ones.

THD vs. output power into 4ohm load at 1kHz BW40kHz
View attachment 122356

THD vs. output power into 4ohm at 5kHz BW40kHz
View attachment 122357


THD 1kHz spectrum at 25W/4ohm/1kHz
View attachment 122219


CCIF IMD 19+20kHz at 56Vp-p/4ohm
View attachment 122220

View attachment 122221


Conclusion

The amp looks promising. Just one channel has been built at the moment. I will make a second board and then make some listening tests as well.
Thank you for sharing this design.For 100w/4ohm version is it ok to use one pair and scale down the VA or does it need a redesign
 
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pma

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Thank you for sharing this design.For 100w/4ohm version is it ok to use one pair and scale down the VA or does it need a redesign
For 100W/4ohm, it will be OK with just one pair of output devices. Power supply voltage 2 x 40V to 2 x 45V.
 

nn_in

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For 100W/4ohm, it will be OK with just one pair of output devices. Power supply voltage 2 x 40V to 2 x 45V.
Thank You. If possible any gerber files that you can share.This is only for personal diy build only .
 
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Measurement of THD+N vs. output power into 4ohm load, at 50Hz, 1kHz and 6kHz. Measuring BW = 22Hz - 22kHz. Distortion plots shown both in % and dBr.

A250W4R_THDN_50-1k-6k.png


A250W4R_THDN_50-1k-6k dBr.png
 

DonH56

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Two questions:

1. dBr relative to the power level of the sweep (sanity check)? E.g. at 10 W, -80 dB is with respect to 10 W? (That is what I always assumed, but a recent paper did it differently, confusing me a bit.)

2. What does your tool include in THD+N; is it really just THD+N, or really SINAD (everything but the fundamental tone)? There are so many tools that mix the two, curious.

Regardless, impressive performance.

Thanks,
Don
 
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pma

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Two questions:

1. dBr relative to the power level of the sweep (sanity check)? E.g. at 10 W, -80 dB is with respect to 10 W? (That is what I always assumed, but a recent paper did it differently, confusing me a bit.)

2. What does your tool include in THD+N; is it really just THD+N, or really SINAD (everything but the fundamental tone)? There are so many tools that mix the two, curious.

Regardless, impressive performance.

Thanks,
Don

Hi Don, thank you!

Re 1) dBr relative is related to actual power level at the point of measurement, yes at 10W (X-axis) it is with respect to 10W/4ohm output power. THD calculated from H2 - H9, with 22kHz bandwidth.

Re 2) THD+N is defined in REW by John Mulcahy @JohnPM as :
The THD+N figure is calculated from the ratio of the input power minus the fundamental power to the total input power.

Measurements are done in REW V5.20 using stepped sine (not sweep). More and definitions here:


Although much, much slower than a log sweep the stepped sine measurement can measure low distortion
levels much more accurately than a sweep, particularly at high frequencies and for higher harmonics.
Stepped sine distortion measurements show distortion components up to the ninth harmonic, THD and the
noise floor, in the same way as the sweep-derived results, but also include THD+N (total harmonic distortion
plus noise and non-harmonic distortion) and N (noise and non-harmonic distortion) alone. Note that the noise
floor plot shows the spectral content of the noise measured with no signal playing. The 'Noise' in the N and
THD+N shows the summed level of all non-harmonic distortions and noise across the frequency span for
each test frequency. It consequently sits much higher than the noise floor plot. For stepped level
measurements the X axis can be dB SPL, dBFS, dBu, dBV, dBW, V or W showing either the generator or
input level.
 
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An interesting comparison of 31-tone multitone distortion of A250W4R amp with my NC252MP. Though the NC252MP has better results in THD and THD+N vs. power measurements, the same cannot be said about the multitone. Yes NC252MP has lower noise, however NC252MP gain is 26dB and A250W4R gain is 34.4dB (conservative value of the days gone when we had signal sources with lower output level).

log X-axis
A250WxNC252MP_multi_log.png


lin X-axis
A250WxNC252MP_multi_lin.png


The class D amp here has cleaner low end, but the class AB amp has cleaner high end, and they both have almost same maximum power, so it is apples-to-apples comparison.
 

DonH56

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@pma : First, thank you for the explanations. It looks like REW uses SINAD for THD+N, which is what I think most analyzers do.

Second, in the log plot above, the signal (bin) width is smaller (narrower) for the NC252MP, judging by the skirts especially at low frequency. I do not think it will impact the results, but wonder if the FFT length is slightly different for the two runs?

Interesting that the NC252MP "wins" at lower frequencies, but spurs rise above the A250W4R at higher frequencies, and there is quite a forest of spurs uncorrelated to the input tones. Seems class-D'ish.
 
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I made a similar power-on, power-off test as @amirm has been doing recently. Speaker and speaker wires are connected. There is no audible "pop".

Power_on.png


Power_off.png
 
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Based on a discussion in another thread, I am attaching the result of THD and THD+N vs. power measurement at 20kHz/4ohm with 90kHz measuring bandwidth.

A250W4R_BW90kHz_thdnlevel_20kHz.png


A250W4R_BW90kHz_101W4R_20kHz.png
 
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We have been discussing in another thread the definition of measuring bandwidth for power amplifier distortion measurements, with conformance of practices used for decades and also with standards like IEC 60268-3.
The key is that distortion measurements with bandwidth of some 20kHz loose any technical reason and point above about 4kHz, because only few harmonic components are then taken into account which absolutely cannot reflect non-linearity of the amplifier. For decades, the common practice was to use at least 80kHz measuring BW. This has changed after the introduction of class D amplifiers, that were unable to compete under standard conditions of distortion measurements.

As a comparison, below please see THD+N vs. power measurements, into 4 ohm load, of Hypex NC252MP amplifier and of my A250W4R class AB amplifier (which has just average parameters for the class), measured at frequencies from 1kHz to 20kHz, with 90kHz measuring bandwidth. I think the plots speak for themselves. With the higher measuring BW, the class D amplifier cannot compete to an average class AB amplifier.

A250W4R_vs_NC252MP_THDNlevel_BW90kHz.png
 
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