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Old 16th October 2009, 04:16 AM   #541
jcx is offline jcx  United States
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not sure where the circuit is exactly but an earlier schematic had a pot in the diff pair tail - if you swap order with the fixed R such that the fixed R is connected to the rail then the DC bootstrap CCS trick becomes an option with one more resistor from the output
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Old 16th October 2009, 04:22 AM   #542
AKSA is offline AKSA  Australia
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Thanks JCX,

I will investigate, but I believe we have a small amount of injected H2 with this mod, arising from the bootstrap supply node, and this is nice to have.....

Hugh
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Old 16th October 2009, 06:04 AM   #543
RKH is offline RKH  United States
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Hi Hugh and all,
Yes, I'm in. Just lurking and enjoying listening to the conversation. I like the one-board idea.

Cheers,
Ryan

PS I used to raise sheep as a lad... we only sheared them, never shared them...
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Old 16th October 2009, 07:40 AM   #544
Nico Ras is offline Nico Ras  South Africa
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So be it then, a single board.

Are going to use the wall wart as discussed or a PCB mounted transformer or a separate transformer wired to the board.

Quescient current is 40 mA per amp if we go 100 mA max then a 24VAC, 5VA transformer should do the job.

Gareth/Hugh if you can start sending me partnumbers and manufacturer or better still PDF data sheets of the major parts, I can start creating the parts library which takes most of the time since I draw these in autoCAD which is mechanically a far more accurate than any PCB program.
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Old 16th October 2009, 07:50 AM   #545
Nico Ras is offline Nico Ras  South Africa
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One more thing, choosing aheat sinks that is internationally easily available is paramount since it would probably have to be bolted to the PCB.

This amp can cost a pretty penny if all the components are imported. I think what I should do is post a PDF of the PCB and people can see the physical sizes of the componets and can place what they already have to see if they fit the footprint of those specified.

This will be double sided PCB since there will be a lot of tracks running across channels to get to the comon points such as power supply, input, output, control switches and pots.
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Old 16th October 2009, 07:55 AM   #546
Nico Ras is offline Nico Ras  South Africa
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Maybe it is worth considering that all power devices uses the base plate as heat sink, this will make it much cheaper and easier to mount.

The base plate could be 1.6mm Aluminium sheet and form part of the mechanical structure of the amp.
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Old 16th October 2009, 08:22 AM   #547
AKSA is offline AKSA  Australia
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Nico,

1. Power Supply. It seems that AC supplies are harder to find than the world mains smps DC ones. Maybe it's worth considering a relatively accessible 18Vdc 500mA supply, regulating it further to 15V with a LM317, then splitting the 15V single rail supply into two 7.5V rails using the virtual earth topology.
This would be inexpensive, and available off the shelf. I do not see a need for an output to exceed 12Vpp, so this would fit nicely. To me, the wallwart is the way to go; cheap, compact, easy to source, mains powered from 70-250Vac, removes AC induction from the HP amp, and reduces internal complexity.

2. Let's drop the chokes/CMCs. This will save much expense, and in light of the smps, ripple will be extremely, indeed vanishingly small anyway.

3. If we use a single board, mounting controls one side and power transistors the other, we are prescribing a specific case size which limits assembly from the DIY junk box. It makes wonderful sense to use the base of the enclosure for the hot devices, and the controls, input/output/power plugs can all be on one side (or pcb mount controls one side, flying leads for all the connections on the other). The C4793 and A1837 outputs I specify do NOT require wafer insulators, simplifying assembly. This approach means many different enclosure sizes could be utilised, as long as they are metallic. I favor diecast alloy boxes; they make outstanding heatsinks, and this saves on buying discrete heatsinks, which then have to convect their heat to the outside of the enclosure anyway. Inefficient, and undesirable.

4. The dimension of the enclosure will largely be dictated by the size of the board. I suggest we wait until you have some idea of the size during the layout process.

I will do my utmost to supply the information concerning plugs, couplers, devices, pots, etc this weekend.

Thanks again for your willingness to contribute so significantly. The layout is far and away the biggest part of this job (with documentation not far behind!).

Cheers,

Hugh
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Old 16th October 2009, 08:35 AM   #548
Nico Ras is offline Nico Ras  South Africa
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Thanks Hugh, Iam okay with this all.
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Old 16th October 2009, 08:58 AM   #549
Luke is offline Luke  New Zealand
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Nico, Im dissapointed that a south african doesnt know nz has the sheep. Aussies have a few but they are all thirsty
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Old 16th October 2009, 09:20 AM   #550
PaulBysouth is offline PaulBysouth  Australia
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Default smps concept

Hugh,
Recently I decided to test some of my Switch Mode Power Supply (SMPS) plugpacks:

- My 12volt router power supply had about 50mV or ripple at about 15kHz,
- A Cisco 48volt plugpack (from a Wireless Access Point) had about 5mV of ripple at about 150kHz.
- The smallest open frame SMPS from Jaycar specifies 100mV (rms) of ripple.

I tried simulating with LTspice a variety of power supply ideas to clean this ripple up. I finally decided that a Capacitance Multiplier was (just) better than either a simple series or shunt regulator.

As 24 volt SMPS seem to be universally available (my test is if Mouser has them, then anyone can get them), I suggest you go with as high a voltage as possible (ie 24 volts) as this gives you the most options on filtering the ripple and noise out. Attached is a concept diagram that has some simple input filtering, Nico's virtual earth circuit (but slightly modified), followed by capacitance multipliers (Using separate capacitance multipliers for each channel and rail).

If you are interested in a non inverting Baxandel style tone control circuit, I have a nice circuit from an old Crown preamp. It has gain, and is switch defeatable. Email me if interested and I'll send you a copy.

Paul Bysouth, October 2009
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