Project Polaris

I see on HF, there are people hungry for a new DAC.


+ 2
I was thinking the same thing the other day, because I need a DAC for my Polaris. Maybe we should let them know that there is interest in GarageDac.

Yes he does,

problem is that if you really want to make a GOOD and capable DAC that suits the Garage 1217 range this isn't going to be a bog standard one.
You know the simple single or dual chip USB powered thingies from $30 to $ 150.-
Everything other than that is difficult to make without programming tools etc. and will take a lot of prototypes.
I see no point in using a simple DAC chip and USB chip (or even combined) and putting it on a board.
I would want galvanic separation, good clocks, separated power supplies and capable DAC chips and capable of at least 32/368 + DSD.

I've spent the day hiding from kids in the house going nuts, since Lucy had some friends over.

So on with the DT150 and put on some Enthiogenic which is interesting electronic type music. (Like Shpongle, but perhaps not as great) The DT150 has loads of room for the ears and blots the world out nicely for me. I'm very used to the DT150 sound but on many other amps that I've used it on, it's been disappointing compared to what I get at work. On the whole, I've kind of avoided using it at home because of this.

The problem has been flabby bass, which I really don't get when using them to work with.

Surprise, surprise, the DT150 sound that I love is there with a Polaris. So it has to be a power thing I guess. The Polaris gets a grip with the bass so that it hits hard (as ever) but it stops really quickly, so you get no smearing or flabbiness. It's actually a good combo. At last, I feel that the DT150 is useful at home, so I have attached a normal quarter inch jack plug to them and they will sit at home for a while. The din plugs are off!!!

Bass levels are raised but it's a very fast bass which is lovely with electronic music. The DT150 never goes into harshness either but it does have an extraordinary clarity on the end of a bit of power. It's a smoother alternative to the DT770 and has a more tuneful bass once it's controlled properly.

I have no idea why this might be. Other than the amp stops and starts very quickly; especially in the bass and it has enough welly to kick it properly into action perhaps? I have it set to low output power and close to zero ohms output impedance. (At least I think it's on low power)

Of course, the others thing is that they are 'sit proof' and are built like a tank. Also, so ugly that no one in public nicks them!!

It's a good match and I've had a great afternoon without hearing nutty kids running around the house.

Both Ember and Polaris are similar Gordon. The Ember is a bit more 'tubey' sounding so possibly a bit more colourful or 'rounder'

I've used the DT150 as a workhorse for a very long time. They're a kind of standard piece of gear tbh. You occasionally see the DT100 with cameramen. Less so for audio studios, it's more common to see DT150s for sound. It is like an HD650 but would you believe, has a stronger bass clout and is a bit harder in the treble. Not harsh but just harder .... Not exactly sure how to describe the treble. It's not sibilant.

I like them actually, in spite of the bass ramp. That's its downfall on lesser amps, I think. The bass goes fat and flabby, which doesn't happen on the two Garage amps. It has a similarity with the Senn in terms of sound. Some don't like the pleather pads which are quite hard, but your ears are encased inside a chamber. It's a real pro headphone in terms of build and I'd say that it doesn't try to be spectacular in its sound. It does the job and is pretty honest with a rose bloom in the bass.

The Polaris and Ember seem to get more of a grip on it than my MF V8 so that the bass is very taught. They are very fast amps I think. Sounds stop dead with them, which is why I never understood someone saying that another amp sounded much faster. These really stop and start with lightening speed so there's a real clarity in attack and decay on most headphones with them. That fast speed of attack and decay brings with it a clarity that many amps don't get with the DT150 in particular which has a tendency for flabbiness if fed something not quite up to it.

Mind you, the Senn HD650 is no slouch on them both either. Very fast headphone that many describe as flabby. Not on the Polaris or Ember. Again, it may be the very same thing - power and speed of attack/decay.

If I stick the 650 on something with no real power, like the Ipods, it goes a bit thin and inconsequential tbh. Not flabby.

I'm not sure whether it's the sheer power and amount of headroom or the pace of the attack/decay that helps the Beyers become more transparent, which is how I perceive them at work.

Nothing fancy though, I'd say. Down to earth tank like headphone.

Best of all is that I was listening this afternoon with the tv on and two nutty kids shouting in the room and I was blissfully unaware with them on.

Mind you, I need to be careful ... You haven't got your own Ember yet and a headphone on top with it is going to take some explaining!!!!!

My Polaris finished its tour in Denmark.
It can either be sent home to me or to someone wanting to audition it.

I really love the sound of my Polaris, in fact so much that I start digging a little bit about amps architecture and I got curious about few things in the Polaris.
If it is possible Frans to tell us a bit more about the Polaris design and I have few questions about it.
What is the Polaris topology? Is it class AB?
Why have you choose OPA551? Are they in the input JFET stage?
Is it THD: > 0.045% high because of the JFET?
I'm sorry if that was mentioned somewhere or the questions are silly, but I don't know much about amp designs.
Thanks

The polaris topology differs from most other designs.
Most designs are overall feedback or use only local or no feedback.
Well... the so called non-feedback designs ALL use feedback otherwise they wouldn't work, have extremely high gains and distortion.
The non-feedback designs use forms of local feedback.

The Sunrise/Horizon amplifiers have no feedback at all.
Also no local feedback, to achieve this (single ended) class-A output stages are needed.
One could also opt for 'diamond' output stages which consume less power.

All the other G1217 amps have an class AB (power) opamp output stage.
The Starlight differs in that it has 4x a 'CMoy' in parallel (sort of what in the O2 is done but slightly different)
The OPA551 is chosen because it works well on voltages up to 60V AND can deliver 250mA of current.
I tinkered around with buffers and other circuits but none work as problem free and well as the OPA551.

The OPA552 which is faster cannot be used here as the OPA551 is stable in unity gain which is needed.

The less amplification an opamp has to make the less distortion there is in the higher frequencies and the lower the output resistance.
The OPA551 is thus stable, can provide a lot of power with good qualities in unity gain mode (1x amplification)

In all the G1217 amps (the ones with tubes) all the voltage gain is done with tubes which do not amplify nearly as much as semiconductors.
More than enough (a bit too much actually) for headphone amplifiers.

Jeremy liked to have an all solid state amplifier with tube like qualities.
Tubes are not that good in amplification as in 'faithful' amplification.
The output thus is NOT an exact (but amplified) copy of the input signal.
There is quite some distortion added which gets worse for larger output signals.
One would thing distortion = bad as most people think 'harsh' or 'deteriorated' and clipping alike sounds are associated with distortion.

Distortion simply means the signal differs.
Whether this sounds pleasant or nasty depends on the type of distortion, the amount, the character (whether it is constant with all amplitudes) and the harmonic spread.
If there are only 2nd and maybe third harmonics this is not that obvious as 'distortion' as when 4th, 5th, 6th etc harmonics are present.
The latter is the case with SS circuits that are poorly designed and when clipping is present.

The fun thing about tubes is that they add the pleasant kind of distortion (so lower order only) and do so in small quantities for smaller signals and becomes higher at increased signal levels.
Not as abrupt as SS, which remains 'low' untill clipping borders are reached and then turn nasty.
Tube distortion rises with amplitude as do increase the amount of harmonics.
This happens in nature as well so isn't found objectionable.
In fact the added (natural occuring as well) harmonics 'add' sounds that are related to the original signals.
Of course the dreaded IM distortion increases as well but obviously this isn't as bad ad most sites will like you to believe.

Anyway... the added harmonics (distortion in essence but pleasant sounding) add to the sound when the amplitude is high enough.
Fortunately low frequencies have the biggest amplitudes in music and treble the lowest.
So it is the same with added harmonics.
Bass has more harmonics and thus some 'energy' is added.
This is perceived as added 'warmth'.
The treble signals are relatively MUCH smaller and thus are reproduced more faithfully.
Lower freq. harmonics thus gain warmth, mids are smaller in amplitude and may just reach levels that may 'add' some coulour where treble passes untouched.
With SS when bass distorts the harmonics often reach up to 20kHz as well, fogging up the whole sound unless distortion is really low (like most SS amps really).

BUT even though SS amps are technically FAR superior (they have inaudible amounts of distortion if well designed) the 'added' harmonics of the tubes 'add' some warmth which is perceived as realism.

The 'worse' the tube the more tubey it sounds.
For this reason the G1217 amps have non feedback triodes as gain stage.
When pentodes would be used or 2 stages with overall feedback less 'tube goodness' (plain and un-needed distortion for signal purists) will be there.

For Polaris we wanted similar 'added harmonics' as tubes bring to the table.
J-FET's do this quite well, much better than MOSFET and transistors, in fact the latter are NOT that pleasant at all.
Some FETs are 'better' than others.
In the prototype I use old (and now obsolete) FETs which I had a bunch of.
Got it exactly similar to tubes in the proto and assumed these FETs would be available still.
They weren't so had to search for other FET's that are available.

FET's are usually not able to work on voltages above 30V so needed some trickery as well, hence the 'other than standard' design.
In the production model OTHER FETs are used and these have less and have a slightly different harmonics spread.

So the distortion numbers are solely coming from the tubes and FET in the Polaris.
As the OPA's only have to follow the signal their added harmonics are very low and are well below the audible threshold and 'masked' by the tube/FET distortion as well.
The distortion numbers thus don't say anything about sound quality as they are nice and welcome instead of detrimental.

So the Polaris has a 'hint' of tubeyness, the power and versatility of the Ember and thus a 'cleaner and leaner' sound than Ember but 'slightly warmer' than SS amps with similar output power.

The OPA is simply a voltage follower and power buffer in G1217 amps with NO gain.
In the Polaris it has 2x gain because the FET's don't reach the max voltage swing.