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Old 26th March 2009, 05:38 AM   #51
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Quote:
Originally posted by Lumba Ogir
I would remove C14. Nice box.
I've messed around with schematics so much that I should double check that you mean the lag compensation cap on the VAS ?

My plan is to try and reproduce the 'magic' of AKSA 25W for the first go around based on the TGM1 design (try the TGM2,3 later). That design used a lag capacitor as far as I know.

Here's a cleaner schematic of what I am doing. You will see that I am incorporating the power supply - I plan to use a single EI-style transformer on each channel:
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File Type: pdf mi-sugar schematic.pdf (27.8 KB, 270 views)
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Old 26th March 2009, 06:12 AM   #52
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Bigun,
Quote:
I've messed around with schematics so much that I should double check that you mean the lag compensation cap on the VAS ?
Yes I do, in post #48. I would remove C14 and keep C6.
Quote:
My plan is to try and reproduce the 'magic' of AKSA 25W for the first go around based on the TGM1 design (try the TGM2,3 later).
It`s going well, actually, you are about to make it better.
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Old 27th March 2009, 03:42 AM   #53
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Default Which one, A or B ?

I'd like some advice/opinions on power supply philosophy. I will have 5 channels in total for my HT system and I have set a budget for the transformer. I have sensitive speakers that really should do fine with a 25W/ch design. I have narrowed it down to two options and I'm not sure which is best:

A) a single Toroid, roughly 300VA with AC output fed to all of the channels in parallel which will each have their own rectifier/capacitors. I should get reasonable isolation between channels and it allows whichever channel 'needs' the most current to draw it from the main transformer. It gives me a bit more choice on voltage but for 25W I need only 18V AC. If I want more headroom I could use up to 24V AC.

B) a separate EI transformer for each channel, pcb mounted 56VA with 18V secondaries. Higher power levels and voltages for pc mount are not available (or practical) as far as I can see but this should be enough for a 25W channel. Total price is the same as for a single Toroid. I should get excellent channel isolation, building each channel as a complete 'mono-block'. The design would keep the high currents away from the input stages of the amplifier and the transformer will be oriented to 'point' it's magnetic leakage away from the board. I consider dual-bobbin EI transformers to be superior to Torroids for rejection of HF noise. I should point out that a large transformer for each channel is not justified in terms of cost and power required.
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Old 27th March 2009, 08:27 AM   #54
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my choice would be 15 or 18 VAC dual-bobbin EI transformers, 2 diode bridges per channel, 25V capacitors.
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Old 27th March 2009, 10:08 AM   #55
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the position of comp cap C6, (between VAS output and inverting input) is less damaging. Consider using polystyrene, even if higher value required and a TO-126 type with very low Cob and emitter resistor for T3.
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Old 27th March 2009, 06:10 PM   #56
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I have seen the use of two rectifiers before, but hadn't thought about the benefits. I assume it's as simple as halving the ripple noise because the voltage across each rectifier is only 18V AC instead of 36V AC. Digikey has only a few choices of slow recovery diodes and no full bridge rectifiers labelled as slow recovery (dual diodes are available only in common cathode if also slow recovery?) so I'll have to buy 8 devices per channel. I have an enquiry into another supplier that makes a full bridge using slow recovery diodes.

I understand that these diodes benefit from heat-sinking and the ones I have chosen do have metal tabs for this purpose. The good news is that I have found some abandoned heatsink material locally. Extruded aluminium, flat on one side with fins on the other, although the fins are not that long. There are 5 pieces of this stuff, each about 8 foot long and 6" wide. I took two home yesterday that I can cut up to suit the job.

It's more work than I thought going from schematic to pcb. I have to choose the components based on availability, performance and to some extent cost, in order to know their footprint and essentially do a mechanical & thermal design before I design the pcb. The electrical design is just one part of the challenge.
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Old 29th March 2009, 09:15 PM   #57
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Default pcb for Power Supply

So I've started on the pcb for the TGM. Each channel will have one integrated mono-block design on a pcb that's 3" x depth of chasis (about 10")

I've used wide traces to help with large peak current flows.
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File Type: gif tgm_psu.gif (31.0 KB, 636 views)
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Old 29th March 2009, 10:04 PM   #58
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two diode bridges provide higher ripple rejection. Have you really decided to stick with forty discrete fast recovery diodes?
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Old 31st March 2009, 05:04 AM   #59
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Default First Draft PCB (probably full of errors)...

I will keep the lag comp capacitor as an option on the pcb. I will try to design the pcb to reflect all the optional components up to version 3 of the layout but I can construct it as option 1 to begin with. The main difference is that the later version adds the LED based current source for the LTP and beefs up the LTP with supporting devices.

I don't see a problem using lots of diodes for the power supply, they're not so expensive.

This project is turning into a lot more work than I thought because I have to end up with something that accommodates 5 channels in my box. However, I think I've got a first draft on the layout. It's a single pcb per channel. A heatsink is bolted on the back-side of the board (not in direct contact of course) and adds some stiffness. The devices to be cooled are sandwiched between the heatsink and board. There will be over-sized holes in the pcb to allow me to bolt the devices to the heatsink.

The attached drawing shows preliminary pcb and side view of stack up. I don't claim 100% originality, it draws on those that have gone before and left traces of their designs on the web... to whom I am most grateful !
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File Type: gif tgm_ht2v1.gif (69.7 KB, 619 views)
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Old 31st March 2009, 05:31 AM   #60
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Bigun,
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I don't see a problem using lots of diodes for the power supply
You won`t regret it. Which type?
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