Development of a "reference" class D starting point

[classd4sure]

Hi,

Here's a thought I had. While we now know how to adjust for DC offset in this circuit..

Many posts ago I had suggested putting a resistor in each leg of the current mirror, which would allow the use of a trip pot to adjust for DC offset. Bruno said that didn't work, because after a second or two the resistors would heat up enough for the DC offset to be right back where it was.

So, I wonder if it would be possible to use a positive temp co thermistor in each leg of the current mirror, to balance the DC offset automatically.

It wouldn't be good at start up, but the idea is with say a 5 second warm up period, which is not exactly unheard of, DC offset would be auto balanced by the time the output stage is switched on.

What do you guys think?

[JohnW]

I use Amidon Iron powered toroidal cores “Type 2” material (red cores), losses are very low even at 1MHz, while distortion OPEN loop is just 0.0015% - 120W 8Ohms 500KHz, with no need to mess about with air-gaps. Distortion components are predominately ODD order due to change of permeability of the core across the audio cycle due to “non even air gaps” within the Iron Powered core.

For lower Open Loop distortion with Iron Powered cores “Gapping” reduces these distortion components even lower.

With Iron powder core’s at low flux densities, the flux tends to concentrate around the “easiest” paths (lowest reluctance) where the magnetic particles are in close proximity – Gapping concretes the “Flux” in a “linear” gap.

“As the flux density increases, the easier path’s are the first to saturate. Those portions of the magnetic particles that have saturated become non-magnetic, making their paths more difficult. Incremental flux increase shifts to adjacent paths, where the magnetic material has not yet saturated, but where the gap is somewhat wider.

This process continues – effectively widening the distributed gap with increasing flux density and as a result, the permeability (and inductance) is progressively reduced – the result is observed as rounding of the B-H curve.”

My thanks to Jaka Racman for providing the leads to this distortion mechanism.

For the DC drift issue, the simples way to resolve the problem is to use thermally coupled transistors for the Input pair & current mirror. Zetex provide dual packaged high voltage transistor – which are also VERY low noise.

Note: - the Zetex duals are dual die, so electrical parameters are not matched (although most likely from the same Wafer batch – if not Wafer) – but thermally they are very closely coupled.

John

[PMA]

You back in Prague, John?

[classd4sure]

Hi,

I just got some audio out of my prototype. Cleanest noise I've ever heard off of a plastic protoboard!

I can only listen to it for seconds at a time, zero heatsinks on the mosfets as of yet and they get hot fast~! I have no scope to work with though so I'll likely have to settle for that, but I can add some heatsinks.

John, that's excellent information, thank you! How does those type 2 cores you have differ from the average iron powder, if at all? As I do have a few IP cores here..

Thanks,
Chris

[subwo1]

Hello,
I just thought I would throw in a few words about the red/uncolored(clear) material #2 cores from Amidon. Those are the same ones as from CWS ByteMark. I was surprised to hear that they are useful in class D filters as I have observed them doing well as power supply output chokes too.

[JohnW]

Chris,

By comparision, the Pulse eng. cores designed for the Zetex Class D, are very poor with 0.1% OL 100W 8ohms. Also the Pulse cores suffer from large switching losses.

Other "unknowns" also seem to measure about the same i.e. 0.1% 100W

Todate, I havn't found a better core to the "Amidon" type II cores. I no longer source my type II cores from Amidon, but from CWS (Coil Winding services) Sanata Ana CA.

Note: - the results are open loop, so with post inductor FB, the THD will be lower.

John

[ssanmor]

Talking about output inductor, Micrometals cores also perform very well. I think their type 2 must be very similar to Amidon.

Btw, just for curiosity, does anyone know how does the LC audio Zap pulse perform in this arena? The output filter seems to be 2nd order, and the L has a RM ferrite core. What is the output ripple level, more or less, at typical supply voltages (say, +/-40V)? It is in the 1Vpp order, isn't it?
I have asked LCaudio ,but they seem to be always at holidays, so they don't answer.

Best regards

[JohnW]

TriPath originally used the Amidon Type II then recommended the MicroMetals version so I guess there the same.

The original version was designed by James Lau at Amidon for TriPath - James now owns and runs CWS - has manufacturing in China so very good prices.

Also noticed other Class D manufacturers using them on their Class D Eval. boards.

John

[classd4sure]

Hi John,

Thanks for the reply.

I was impressed by the open loop figures and if they perform as you say (I'm sure they do) I see no reason why not to use them over anything else.

I have more questions but I think I'll read up on them before I go on with what is likely "basics".

With that in mind you seem to also have an impressive source of information I was wondering if you wouldn't mind posting a link to it?

Tripath, I think, also switches alot faster than we're interested in here, so that's good.

I may be blind at this hour of the day, but I was just going through the Zetex site, they don't seem to list any noise ratings in their data sheets?

The only reference I've found there regarding low noise is that they seem to prefer their medium power transistors over small signals ones, but that wasn't in regards to their dual die models. While their dual die medium power seem to be for switching, yet have 150+ ns rise and fall times? Again, no noise figures on them at all. It's got me a bit confused.

Must sleep.

Thanks,

Chris

[JohnW]

Zetex don’t publish noise figures for their device’s, as it’s not there target market. Transistor noise is a function of Base resistance and is heavily dependent on operating current. That’s why low noise head amps use parallel small signal transistors or medium power devices. The lowest noise input stage I’ve built uses ZDT758 (PNP transistors have lower noise then there direct NPN counterparts); they have a noise figure of less then 0.25dB per device. My experience so far, is that all Zetex medium / high power devices have very low noise.

I’ve no direct working experience of Bruno’s UCD circuit topology, it’s my understanding (but not confirmed), that the UCD180 modules use BF821 for the input pairs, and BC847 for the current mirrors. The BF821 are not known for there switching speed or low noise performance. This would seem to suggest that the ZETEX medium power dual die devices should work well as input devices and possibly current mirrors.

I’m afraid my sources of information have been collated over many years, and from my own research. I believe that the information about “Rounding of the B-H characteristics” originally come from a Unitrode / Motorola paper titled “Magnetic Core Characteristics” – Jaka Racman originally gave me the link (once again thanks Jaka)….

After research that basically involved taking the standard Iron powered mixture used to manufacture the Type II cores, and “Rumbling” it for longer (1 week extra!) to reduce and smooth the particles / grain size, I can confirm that the smoother / smaller particle size iron powered cores performed much better, with open loop THD of 0.0006% 120W 8Ohms 500KHz compared to the control batch manufactured from the standard pre “extra rumbled mix” of 0.0015% under the same test conditions.

John