Heed Obelisk SI

[drummerman]

 

[drummerman]

Leaving the possibility aside that all amplifers may, under certain conditions, sound the same and with the Heed Obelisk SI being current flavor of WhatHifi for amplifiers around £1500 I had a quick look at some internal pictures. Looks interesting and well made, with a smallish but adequate (for the rating) torroid and supply caps and a relatively neat layout of the circuit board. A couple of output devices per channel are par of the course. All this can arguably be had for much, much less but the build quality really looks nice. Kind of follows Cyrus and Naim with a similar modular approach but the formers size. I like the look. There seems to be a couple or three different versions, all with the suffix SI; One with ribbon connectors from input board to main circuit board and motorised (Alps blue) volume control to main board. - The other one has a shaft driven input selector to mainboard and ribbon to main volume control. Then there's a version with selector switch coupled to small board which in turn is connected to the mainboard and finally there seems to be one where the motorised volume control (still an Alps blue) is shaft driven to the main board. I think they've covered every eventuality ... go figure.

I always like it when a different brand makes inrows into the 'establishment' and of all the available options at that price and there are a few, its probably one of the more interesting ones but it does seem to get a fair amount more expensive once adding separate power supply etc. and it will still be a low powered amplifier.

Its all relative I guess and if it sounds good, so what. After all, there are valve amplifiers with even less power that cost more. On the other hand, you could get something like AVI actives, which (each) have a beefier and still neatly engineered amplifier in their cabinets. So in essence you get a good speaker thrown in for free and double the number of amplifier (plus many extra watts) so to speak. Diversity ... like it.

Would be interesting to hear from anybody having bought the Heed ...

regards

 

[oldric_naubhoff]

Its all relative I guess (..). On the other hand, you could get something like AVI actives, which (each) have a beefier and still neatly engineered amplifier in their cabinets. So in essence you get a good speaker thrown in for free and double the number of amplifier (plus many extra watts) so to speak.

not quite so. the ADM woofer amps's rating hoovers closer to 75 W into 8 Ohm load (I always thought that for purposes of comparability you should give the rating into resistive load of 8 Ohms and what AVI does they give theoretical power output of the transistors without load into DC resistance of the woofer. none of those conditions will ever be met in real life. transistors will sag under load. even more so when closing to boundaries of their SOA. likewise a dynamic driver will never present a load equal to their DC resistance value due to inherent inductive nature). so as you can see you don't get too much more power. which seems quite right anyway because there's no way you can get 250W from a couple of output devices - like in ADM. 70W sounds much more credible and ties in nicely with experimentation (I saw a movie on youtube where a guy was proving that 250W rating for ADMs is a nonsense).

another thing to consider is biasing of the output stage. you don't know how things look like in Heed. and neither do I. however, just to give you an idea, my smallish amp outputs 75Wpc and first 7 watts (I believe, maybe slightly more) is outputted in class A (as everybody knows class A is less susceptible to harmonic distortion and there's no issue with crossover distortion of class B output stage which is good). but trust me, there's no way this thing would fit into ADM's enclosure. the power supply is simply too big. and you need a decently sized power supply for those more dynamic moments in music.

 

[drummerman]

I remember the argument over quoted power of the ADM's at AVI forum a while ago well. I believe, if I remember right, the consensus at the time was that it was around 150w/ch continuous with peaks reaching or exceeding the quoted 250w, there was talk about 500w. into what load and for how short a burst, I dont know. Still, you do get two reasonably sized power amplifiers with torroids and capacitance which seem to back up AVI's claim of substantial peak power. - They use the back plate as heat sinks. - The Obelisk on the other hand really has not just a very small heat sink but also a relatively small supply. Then again, they rate it at only 40W which seems entirely reasonable.

This thread was less about AVI than the Heed. However, I thought it was a valid comparison on price alone and does show both the material value and engineering prowess AVI have put into their product/s.

Whether you like one or the other is up to the buyer.

As I've mentioned elsewhere, there are valid reasons for both low and high power amplifiers (as is there for actives). The camps will probably find it very difficult to agree but I am trying here not to judge products by topology.

regards

 

[jcbrum]

transistors will sag under load.

This is not accurate. Transistors don't 'sag under load', - power supplies do that.

Transistors under load have a rising junction temperature proportional to the current flowing. The junction temperature is cooled by the transistor case which may be connected to a heat sink.

It is a common design principle to arrange the power supply voltage to sag under excessive load so as to prevent the junction temperature from exceeding safe limits, and thereby prevent the transistor from destruction if an unsafe drive level is applied.

General purpose amplifiers, such as might be used to drive passive loudspeakers, with unknown crossover impedance characteristics, require rather different design parameters to amplifiers specifically designed for use in an active speaker design. Damping factor alone might be just one such consideration.

BTW, AVI active speaker amps don't use a couple of simple transistors as output devices. They are designed around very special compound devices, with very high gain, capable of very high power, and with great efficiency (low heat). I assure you there is more inside the casing than might seem apparent at first sight, and that the speakers will go very loud indeed with no shortage of power. AVI's stated power handling statistics are representative of this fact, and give a good guide to the 'real world' performance. Ask any, of the now many, experienced AVI users for confirmation of this.

It's worth bearing in mind that music isn't an 'RMS' continous signal, and for true fidelity very high power is required for true signal handling if clipping and associated distortion is to be avoided. An 'RMS' test, or rating, is unlikely to be sufficient to represent musical performance under real conditions.

JC

 

[drummerman]

Interesting. What is the VA rating of the torroid/s of these actives?

regards

 

[jcbrum]

I assume you mean the mains transformer, but power supplies, and amplifiers, can employ toroids in other duties too.

It varies according to the model, and in the ADM designs you have two of them anyway, compared to most separates amplifiers which only have one, which therefore needs to be a least twice as big.

In a modern amplifier power supply, the mains transformer is simply used to charge up the reservoir capacitors which are the real souce of power to the amplifier. Charging occurs at a comparatively constant level, with the big fluctuating current demands being supplied from the reservoir according to the music waveform (signal). It's not unusual for a demand of hundreds of amps to be drawn from the reservoir for short periods.

In some amplifier power supply designs there is no mains transformer at all, toroid or otherwise, since the mains supply is fed directly to a diode ring. This is usually the case where a powerful local oscillator is used to provide an internal supply at a much higher frequency than 50Hz.

JC

 

[jcbrum]

p.s. The last time I had much to do with military aircraft (about 40 years ago ) it was common to find a 'mains' supply rail running at 400Hz rather than the domestic household norm of 50Hz. This had the effect of making the power supplies about a quarter of the linear size of domestic stuff, and consequently about one eighth of the weight.

 

[drummerman]

Knowledgeable as usual and interesting to follow.

Still, just out of interest I would be intrigued to know what the supply of the ADM's consists of ...

It looks like a 200 - 250VA or thereabouts trafo and a bank of 6800vu caps reservoir? (per channel) Are these purely for the power stage or is a pair used for other duties? What is the consumption at the mains for the ADM's?

I would assume that the commonly applied factor of a minimum of approx 2.5 for transformer rating vs output rms is used even in actives no? If so that would roughly agree with that previously mentioned conversation over at AVI forum some time ago where a figure of roughly 150w rms was assumed.

Claimed peak power (real world power to use your words) would therefore also be plausible.

regards and wishing you a lovely Xmas

 

[jcbrum]

There are at least three physical sizes of mains transformers used in AVI active speakers, according to the model and production year.

The physically smallest one has the highest duty cycle, but is the quietest. Bigger transformers seem to hum more, even though their duty requirement is lower. All have proved reliable in service.

I would assume that the commonly applied factor of a minimum of approx 2.5 for transformer rating vs output rms is used even in actives no?

Many actives use transformerless amplifiers these days, as do computer power supplies.

As far as AVI ADM speakers are concerned, I doubt that Martin took much notice of this, and almost certainly drew up his own specification for the ADM mains transformers. I do know that they are produced and wound for AVI, rather than a commonly available 'off-the-shelf' item.

I don't have the spec sheet for them, but Martin might tell you if you asked him.

JC

 

[jcbrum]

This is an ADM amplifier plate, as used in the latest model.

 

[jcbrum]

Here is an ADM amplifier plate used in an earlier model.

 

[jcbrum]

There have been at least six incarnations of AVI ADM Loudspeakers so far, with more in development, and at least three sub-woofers,which all have variations in design.

Martin is developing a new 500 watt amp at the moment, which is at the experimental prototype stage.

JC

 

[drummerman]

JC, thanks for the pictures.

That looks impressive and if anything, I perhaps under-estimated the trafo size by a few VA.

It also puts into stark contrast just how much hardware and engineering you get for the price of a mid-range integrated, completely leaving sound preferences aside as not to start another ... ). Especially as AVI is a tiny operation, outsourcing excluded.

regards