Variable Bushmaster

[pumpkineater23]

Apologies in advance if this question is answered as part of another Beresford thread.

I was hoping to upgrade my already brilliant 7520 to the Bushmaster while continuing using the DAC as a pre with my Quad606. Is it in the pipeline? I remember there was some talk of it at some stage.

[Labarum]

I too have an interest in that - to drive a Quad power amp.

[seoirse2002]

+1

[Krisbee]

After a recent exchange of PMs with Mr.B I got the distinct impression a "variable bushmaster" is not on the cards. But of course Stan can answer for himself.

[StanleyB]

I have loads of projects on the go, but only time will tell which ones turn out to be right for final production.
The production version of the variable output BM is on hold for now whilst I try to get the CaimanII from Alpha stage to Beta stage. Things are not helped by the fact that in many cases I know more than the manufacturers' support engineers about the technical aspect of some components that I am working with, which makes it a nightmare to get answers from them on stuff that their own parts are capable of!

[Labarum]

What can you tell us about the Caiman II, Stan?

[StanleyB]

Absolutely nothing at this stage.

[DSJR]

What I still think we need is a good quality analogue line buffer with low output impedance with precision volume control (a couple of properly power-supplied op-amps are fine for this IMO), with a DAC with digital inputs and a few analogue inputs too. Possibly an A_D option as well giving backwards and forwards compatibility. Surely this could be combined in a decent enough case and not to sell for £1250 as the new pretty naim one does!

[StanleyB]

It would still cost a fair amount. £500 would not be far off. Whilst I have looked at the idea, the market is far more limited compared to just a digital input only DAC.

[Richiebuoy]

I too would be interested in a Bushmaster / pre........

[StanleyB]

Someone on an Italian forum asked about the possibility of building a noise filter like the sbooster into the DAC since power supply noise would make a big impact on any variable output stage. The Bushmaster already has a noise filter built in as part of the VirtualDC circuit I came up with, but it's a work in progress design wise. I can understand the cost considerations of potential customers when taking into account the cost of additional upgrades. So I added some additional components in the filter circuit in order to improve the noise performance of the incoming DC supply just before it enters the voltage regulators.
I took some screen shots of the display on my scope. I have decided to share some of them here so that the technically interested can keep up to date with the progress of the project.
Sorry for the less than sharp results. They are just quick snaps with a camera phone.

Additional info: measurements made under dynamic load conditions. That's to say that the pictures were taken whilst the DAC was playing music. Static load conditions are nicer to look at and make a powerline reading look more impressive, but they don't give an accurate picture of what really happens when the music starts.

http://www.homehifi.co.uk/aos/smpsu.jpg
Standard SMPS power supply. Picture taken at the + terminal of the DC socket inside a standard Bushmaster.

http://www.homehifi.co.uk/aos/sbooster.jpg
Standard SMPS with sBooster and inbuilt noise filter. Picture taken just before whereethe filtered supply enters the voltage regulators.


http://www.homehifi.co.uk/aos/vdcfilter.jpg
Standard SMPS with improved VirtualDC circuit and noise filter. Picture taken just before where the filtered supply enters the voltage regulators.

I have also posted a picture of the output noise from the ultra low noise regulators in the variable output DAC. The picture was taken at a higher magnification compared to the previous pics.

http://www.homehifi.co.uk/aos/lowreg.jpg
Ultra low noise regs output from VBM

[Mark Grant]

Two of your pictures have the same file name which means they are the same picture posted twice in error even though the description of what it shows is different for each picture :)

[StanleyB]

Two of your pictures have the same file name which means they are the same picture posted twice in error even though the description of what it shows is different for each picture :)
:doh: Cheers for noticing that. I had the standard SMPS picture wrong. Now corrected.

[Stubies]

Stanley

Wow - the improvement is stunning.

Would there be a chance to see this new filter in one of your next DACs?

Perhaps the Bushmaster Mk 2?

I am waiting for your re-stock, need a new one, exactly with variable pre-amp Modification.

Stu

PS that filter circuit may be patentable - the result is certainly unusually good - maybe the rights could be licensable?

[StanleyB]

Stanley

Wow - the improvement is stunning.

Would there be a chance to see this new filter in one of your next DACs?

[]

PS that filter circuit may be patentable - the result is certainly unusually good - maybe the rights could be licensable?
The filter circuit is built into the design for practical and technical reason. I tried it as a separate external circuit, but it didn't perform anywhere as good compared to the inbuilt version.

Anyone with a bit of patience and knowledge of SPICE and filter designs can come up with a solution like mine. But most audio designers are not familiar with RF filter designs, which is however the critical area in a digital circuit. By experimenting with different designs I found out that to effectively filter the noise from a SMPS you need a balanced filter circuit. Most power supply filters are unbalanced which dates back from the era when linear power supplies were first fitted with a filter circuit. But a single rail SMPS doesn't have a true power supply earth. What it has is a negative and positive terminal. So filtering just the positive terminal with respect to "ground" still leaves the negative terminal as a source for noise. I have now added a filter circuit in both the positive and negative power feed of my own design. And by making it a balanced filter any common noise spike in the two power lines is added together out of phase, which then cancels out each other. Any remaining noise is then taken care of by the regulator circuits.
Mind you, it took me a while to figure that out.

[NRG]

Stunning. Great work Stan!

[ZebuTheOxen]

Very cool. The question is: Does it sound better?

[StanleyB]

Very cool. The question is: Does it sound better?
I haven't got that far in my testing yet:).

[pumpkineater23]

I haven't got that far in my testing yet:).

Exciting stuff. I'll be following the progress with great interest.

Pete