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Old 28th September 2009, 12:04 AM   #151
AKSA is offline AKSA  Australia
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Hey, Iko,

You are a bit optimistic!! Once the schemat is agreed to, a proto ratsnest will be built by one of us, then a good listen, then I have to do a pcb design (2 weeks, and quite a bit of time figuring all the options people want since this is a group project), then another 10 days turnaround for the pcb from China, then layout revisions after a further pre-production version is built and tested, and then the final 100 board manufacture - do you get the picture? This stuff is like breeding elephants, it takes a long time, all the more because it's not a money maker, so all that effort is gratis.

We should have a pre-production version up and running with about four weeks, but no sooner.

Kean, I will ensure you receive a board, and I will try to get the difficult bits to you as well. You are something of a local hero in these parts, and we need to look after you!

Say, anyone here know if there are any 2 x 30mH common mode chokes rated to 150mA easily available in the market? A rectifier bridge followed by C (cmc choke) C would make the best power supply in my opinion.....

Cheers,

Hugh
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Last edited by AKSA; 28th September 2009 at 12:09 AM.
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Old 28th September 2009, 12:47 AM   #152
MJL21193 is offline MJL21193  Canada
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Quote:
Originally Posted by AKSA View Post
Say, anyone here know if there are any 2 x 30mH common mode chokes rated to 150mA easily available in the market? A rectifier bridge followed by C (cmc choke) C would make the best power supply in my opinion.....
Close, 28mH, 200mA
Cheap too.
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Old 28th September 2009, 02:07 AM   #153
iko is offline iko  Canada
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Quote:
Originally Posted by AKSA View Post
Hey, Iko,

You are a bit optimistic!! Once the schemat is agreed to, a proto ratsnest will be built by one of us, then a good listen, then I have to do a pcb design (2 weeks, and quite a bit of time figuring all the options people want since this is a group project), then another 10 days turnaround for the pcb from China, then layout revisions after a further pre-production version is built and tested, and then the final 100 board manufacture - do you get the picture? This stuff is like breeding elephants, it takes a long time, all the more because it's not a money maker, so all that effort is gratis.
Cheers,
Hugh
I wouldn't say optimistic, rather, practical. I normally build a point-to-point circuit if it's small like this, before going to a pcb design, to iron out the wrinkles on the real thing. I've had enough surprises when going from simulation to reality to be cautious.

And I totally get the picture that anything looking like a proper pcb is still a way off.

To be honest, I am surprised that you're doing the pcb design for free.
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Old 28th September 2009, 02:18 AM   #154
Bigun is offline Bigun  Canada
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Back to reality ... did some experiments on my TGM amplifier which tells me that the bias current in my sims is too 'hot'. The devices Hugh has selected for the output don't have metal tabs and we don't want to be worrying about heatsinks too much. Headphones aren't greedy, the worse case should be 23 Ohms.
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Old 28th September 2009, 02:59 AM   #155
keantoken is offline keantoken  United States
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Originally Posted by Bigun View Post
I think your suggestion for the bootstrap take-off point is the better approach. It might be worth playing with the relative values of the bootstrap divider resistors to see if we have them optimized, I haven't looked at that at all yet.
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1: Choose bias current value, with which we determine net bootstrap resistance.
2: Choose cap size.
3: Then, we can choose bootstrap resistor values based on that. Increasing the lower resistor will increase load rejection of the bootstrap, but the cap will have to work harder through the top resistor, which will decrease bass rejection... So really this is a question of what kind of bandwidth do we want?

This is my "bootstrap optimization" procedure...

So, what is our bias current and what size cap can we use... Bigger is better, generally...

- keantoken
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Last edited by keantoken; 28th September 2009 at 03:03 AM.
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Old 28th September 2009, 03:32 AM   #156
Bigun is offline Bigun  Canada
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Sounds like a reasonable plan.

I would suggest that resistors are easy to change, their footprint on the pcb doesn't depend on their value, just power dissipation. The caps are more critical because their footprints depend on voltage rating, capacitance value and possibly manufacturer. I suggest a cap of 220uF would be a good value.
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Old 28th September 2009, 04:00 AM   #157
keantoken is offline keantoken  United States
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We could use a POT with the wiper to the capacitor...

For different cap sizes, we could have a hole for one lead and a slot for the other so you can insert any size cap you want.

- keantoken
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Last edited by keantoken; 28th September 2009 at 04:07 AM.
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Old 28th September 2009, 05:07 AM   #158
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by keantoken View Post
For different cap sizes, we could have a hole for one lead and a slot for the other so you can insert any size cap you want.
A nice bit of lateral thinking !

- but commercial pcb vendors like drilling holes. However, if we leave space for a large cap, a smaller one can have it's legs bent out to fit.
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Old 28th September 2009, 05:07 AM   #159
AKSA is offline AKSA  Australia
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John,

Many thanks, that's a great choke, really impressive, cheap too!! Only 4R of series resistance, low losses!

Now, the biasing. How about three 4148 diodes to set the output stage bias at around 40mA?

Would that be OK?

And if we assume a max output of 12Vpp into 32R (that's 590mW) then the max current ever drawn by the load would be 6/32 = 188mA, and with 40mA bias the Class A zone would run out at 2.56Vpp, which is 25mW, plenty loud in fact if we assume 100dB/watt, around 70dBA sound pressure at the cans.

Since we are reading for 188mA max of output current, the transistors will need plenty of base drive. Assuming a beta of 150 for the 4793/1837 at 190mA (taken from the curves), we have a nominal base current demand of 1.25mA. The bias generator has no bias losses, unlike a Vbe multiplier, so 8 times this figure is reasonable for the VAS, and incorporates minimum delta I at the collector in all circumstances. This gives us 10mA collector current, which means bootstrap resistors, for a single rail supply of 24V, will be 470R and 680R (npv), as originally chosen. In fact with these values we shall have a VAS current of 9.5mA, which is close enough for government work......

Doing the pcb, IKO, will be a pleasure. I enjoy the work, it's relaxing.

Cheers,

Hugh
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Last edited by AKSA; 28th September 2009 at 05:09 AM.
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Old 28th September 2009, 06:05 AM   #160
keantoken is offline keantoken  United States
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Quote:
A nice bit of lateral thinking !

- but commercial pcb vendors like drilling holes. However, if we leave space for a large cap, a smaller one can have it's legs bent out to fit.
..Or we could just use one hole for one lead, and put a line of holes for the other lead. Same thing as the slot, really (Are you sure most PCB manufacturers don't do slots? It would be cooler).

Quote:
Now, the biasing. How about three 4148 diodes to set the output stage bias at around 40mA?

Would that be OK?

And if we assume a max output of 12Vpp into 32R (that's 590mW) then the max current ever drawn by the load would be 6/32 = 188mA, and with 40mA bias the Class A zone would run out at 2.56Vpp, which is 25mW, plenty loud in fact if we assume 100dB/watt, around 70dBA sound pressure at the cans.

Since we are reading for 188mA max of output current, the transistors will need plenty of base drive. Assuming a beta of 150 for the 4793/1837 at 190mA (taken from the curves), we have a nominal base current demand of 1.25mA. The bias generator has no bias losses, unlike a Vbe multiplier, so 8 times this figure is reasonable for the VAS, and incorporates minimum delta I at the collector in all circumstances. This gives us 10mA collector current, which means bootstrap resistors, for a single rail supply of 24V, will be 470R and 680R (npv), as originally chosen. In fact with these values we shall have a VAS current of 9.5mA, which is close enough for government work......
Changes have been made to my schematic.

So, possible features:

1: Switchable crossfeed.
2: "Low bias" mode for battery operation (?)

Aren't I missing some? Perhaps a "line out" for use as a preamp?

- keantoken
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