How to determine if an amp is class A/B or class D?

Justinus

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Oct 10, 2005
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I've got a mono amplifier from a major brand that is advertised as being a class AB amp, however it sure doesn't look like it.

I found an IRS2092S class D digital amplifier chip on the board of the amp, in addition to a suspicious lack of heatsinking (if it were truly class AB). It looks like the output stage is a pair of FQPF13N50C mosfets, a MUR1620CTR rectifier, a MUR1620CT rectifier, and a pair of IRFB38N20D mosfets.

Is the presence of the IRS2092S class D amp chip proof it's not class AB as advertised?

I don't know enough about circuits to really do much analysis beyond identifying components and maybe some extremely rudimentary analysis of the PCB.
 

Justinus

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Oct 10, 2005
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What make & model is the amplifier?
It's the amplifier built into a Velodyne DB12 subwoofer, advertised by Velodyne as having a 350W RMS class AB amp.

The ad video from the product page:

image (1).png

The amp:

IMG_20210821_093833.jpg

The IRS2092S amp chip:

IMG_20210821_130158.jpg

Funnily enough the main board looks like a slight variation of this generic subwoofer amp that several brands seem to sell on AliExpress. All of the main components appear to be the same/in the same places on the main board with some aux board variations and some components differences (caps and stuff).

Screenshot_20210824-064407.jpg
 
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Justinus

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It's all a bit disappointing because the Deep Blue line was supposed to be Velodyne's comeback to home theater, but it seems it was really just Velodyne's exercise in how to cut as many corners possible to build the cheapest subwoofer they could to sell for a massive brand markup. This sub retails for $1200 and has the thinnest unbraced cabinet I've seen, nasty finish issues with glue coming out every corner of the vinyl, a wimpy magnet and disappointing driver in general, and whatever this probably not a class AB amplifier is that I apparently could buy on AliExpress.

Problem is, cutting corners in quality is one thing but outright false advertising with regards to the amplifier is something I'd like to confirm before I start complaining.
 

Muadib

Lifer
May 30, 2000
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You should look for the sub in avsforum in the subwoofer area. If what you say is true, it’s sure to be mentioned there.

 

mindless1

Diamond Member
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Besides the class D driver IC, having the output go through a significant inductor (LC filter on ouput) is a pretty strong clue it's class D, but IMO it doesn't matter much for a sub, and will save on the power bill.
 

Justinus

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Besides the class D driver IC, having the output go through a significant inductor (LC filter on ouput) is a pretty strong clue it's class D, but IMO it doesn't matter much for a sub, and will save on the power bill.
In the current market the top sound quality and "most musical and accurate" subs are majority class AB. It's a big selling point for someone looking for more than boom boom. Look at the majority of Rythmik F series (excepting a minority couple class D variants) and Rel's T series, both exceptionally regarded for sound quality.

Selling a subwoofer for $1200 that is, upon critical inspection, more cheaply made in every possible way and worse performing than options literally less than half the price is extremely poor form on it's own. Add the outright false advertising and it's a shitshow on Velodyne's part.
 

sdifox

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That looks like class D to me.

I had this Carver AV-505 which is 5*80W, (2*100w in stereo mode) @8 Ohm and each channel had more heat sink than this 300w (albeit 4 Ohm) amp board. Class D is efficient so they don't need massive heat sinks.

How much heat sink is there on the outside side of the amp plate?

The right hand side (back half) of the amp you can see the heat sink fins for the opamps.

a0tayccetmzfcfv8bvsb.jpg


Remembered it is in my basement, so about 7.5" long heat sink.
IMG_20210825_111634.jpg

And let's call it 4.5" tall
IMG_20210825_111920.jpg
 
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Justinus

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That looks like class D to me.

I had this Carver AV-505 which is 5*80W @8 ohms and each channel had more heat sink than this 300w (albeit 4Ohm) amp board. Class D is efficient so they don't need massive heat sinks.

How much heat sink is there on the outside side of the amp plate?

The right hand side (back half) of the amp you can see the heat sink fins for the opamps.

View attachment 49292
There is no heatsinking on the outside of the amp plate. It's just the thin ~3mm aluminum backplate and that's it.
 

Muadib

Lifer
May 30, 2000
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Velodyne Acoustics is now owned by a German company, Audio Reference. Sadly, I think you are SOL.
 

mindless1

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Class A/B is also used on some of the cheapest junk out there, and IMO (especially at that price point) high end shouldn't use a switchmode power supply subcircuit either, instead a toroidal transformer based linear circuit, except that this too, matters a lot less for the frequencies a sub handles, and considering that this is (probably?) a sealed rather than ported enclosure, both the PSU and amp circuit it powers, are adding to the heat produced vs lifespan if it were a toroidal linear and A/B amp instead of both being switchmode as they are.

At the same time, the switchmode PSU circuit is going to be pushing its output filter caps harder than a linear PSU would, heating them up far more, and they are likely to be one of the first failure points.

It is unlikely that in a blind ABX test, anyone could consistently pick which amp type was used if driving the same driver in the same cabinet.

I agree it is poor form to sell it for what they do, but such is often the case in boutique audio where things cost more than triple what the construction plus normal profit margin would dictate. The really curious part is getting the advertising wrong when Velodyne themselves have (a patent? Or just fancy marketing words?) for their Energy Recovery something something class D circuit, so they could still load on the BS in the marketing either way.

Far more concerning to me would be the quality of the driver and housing. I'd never pay that much for a sub but if I did, it would be returned immediately when I saw those issues.
 
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sdifox

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Sep 30, 2005
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Class A/B is also used on some of the cheapest junk out there, and IMO (especially at that price point) high end shouldn't use a switchmode power supply subcircuit either, instead a toroidal transformer based linear circuit, except that this too, matters a lot less for the frequencies a sub handles, and considering that this is (probably?) a sealed rather than ported enclosure, both the PSU and amp circuit it powers, are adding to the heat produced vs lifespan.

At the same time, the switchmode PSU circuit is going to be pushing its output filter caps harder than a linear PSU would, heating them up far more, and they are likely to be one of the first failure points.

It is unlikely that in a blind ABX test, anyone could consistently pick which amp type was used if driving the same driver in the same cabinet.

I agree it is poor form to sell it for what they do, but such is often the case in boutique audio where things cost more than triple what the construction plus normal profit margin would dictate. The really curious part is getting the advertising wrong when Velodyne themselves have (a patent? Or just fancy marketing words?) for their Energy Recovery something something class D circuit, so they could still load on the BS in the marketing either way.

Far more concerning to me would be the quality of the driver and housing. I'd never pay that much for a sub but if I did, it would be returned immediately when I saw those issues.
1200 for a 12" sub is a ripoff

this one says it is class D

 
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Justinus

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1200 for a 12" sub is a ripoff

this one says it is class D

Someone in marketing messed up, every product listing I've seen and their own marketing video embedded on the official product page says class A/B. I even found an extremely long brown nosing magazine article about these subs that specified they were class A/B instead of Velodyne's class D tech.
 

sdifox

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Repairs can be DIY if you have a little (enough) background and are ready to roll up your sleeves and provide good intel to a crowd in a DIY repair oriented forum, if the faulty component isn't obvious enough already. I can pretty much do this online if the situation is right, like, all ducks in a row, someone able to take good high-res pics, measure things with a multimeter in a methodical way, not manage to electrocute themselves while doing so, etc, ;)

You just have to break it down into the two subsystems and recognize what voltages and polarities should be where for power and signal.
Pretty sure he was talking about oppo bluray player and not amp.
 
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