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Old 4th July 2012, 03:12 PM   #1
Nuuk is offline Nuuk  United Kingdom
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Default Higher voltage regulated GC

I've recently got back into playing with my GC's after some time away in Class D land.

I still love the SMPS LM3875 version although at only 12v it clips into my MS Pageant speakers on some tracks at high volume. That said it will drive my old Goodmans 201's well enough.

The Pedja Rogic regulated LM3875 sounds sublime running at +/-26 volts.

I was surprised at how much I liked the LM338 regulated LM3886. So I was thinking of building the snubberised version, and as I have a 30 volt traffo spare, I wondered about using that and then regulating the 42 volt output down to 39.

There are those who reckon the LM chips perform best at higher voltages and I wondered if anybody else had tried this.

Another observation I made in these latest auditions is that I prefer the Black Gate NXQ 47 uF caps on the input to almost any film cap. The exception being a Russion PIO that I only have in 0.1 uF. On the LM338 clone I actually preferred a 47 uF Panasonic ECA lytic to the 2.2 uF polypropylene that was there before. The moral is don't always assume that a film cap is the best option.

Anyway, it's fun to be tinkering around with the clones again.
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Old 4th July 2012, 03:16 PM   #2
SoIL4x4 is offline SoIL4x4  United States
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Welcome back! And thank you for all of the great information you have posted, it is a great resource.
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Old 4th July 2012, 03:56 PM   #3
djoffe is offline djoffe  United States
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My customers have been building the updatemydynaco retrofit kits for Dynaco Stereo 120's. It uses a LM3886 run from a regulated power supply at 72 volts.

Here are some links that you might find interesting:

http://www.updatemydynaco.com/docume...ualRev2.08.pdf

GT100 Power Amplifier

http://www.updatemydynaco.com/docume...entRev2p03.pdf

Last edited by djoffe; 4th July 2012 at 03:58 PM. Reason: add link for power supply design
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Old 4th July 2012, 05:27 PM   #4
Nuuk is offline Nuuk  United Kingdom
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Quote:
Originally Posted by SoIL4x4 View Post
Welcome back! And thank you for all of the great information you have posted, it is a great resource.
Thanks - nice to be back.
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Old 4th July 2012, 05:28 PM   #5
Nuuk is offline Nuuk  United Kingdom
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Quote:
Originally Posted by djoffe View Post
My customers have been building the updatemydynaco retrofit kits for Dynaco Stereo 120's. It uses a LM3886 run from a regulated power supply at 72 volts.

Here are some links that you might find interesting:

http://www.updatemydynaco.com/docume...ualRev2.08.pdf

GT100 Power Amplifier

http://www.updatemydynaco.com/docume...entRev2p03.pdf
Thanks for the links. That looks like a single rail supply so it would be the equivalent of running at +/-36 volts. But it looks to be an unregulated supply.
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Old 4th July 2012, 05:35 PM   #6
djoffe is offline djoffe  United States
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You're right...the single 72 volt supply is pretty much the equivalent of +/- 36 Volts. However, the supply is quite well regulated at 72 Volts.

I will readily admit that the schematic is a bit confusing, as the regulator is in the low side of the supply. This does have some advantages, allowing N-Channel pass transistor, plenty of gate drive for low overhead, and a direct connection of the drain (metal tab) to ground, which lowers the thermal resistance.

Update My Dynaco
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Old 4th July 2012, 05:58 PM   #7
Nuuk is offline Nuuk  United Kingdom
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Quote:
Originally Posted by djoffe View Post
You're right...the single 72 volt supply is pretty much the equivalent of +/- 36 Volts. However, the supply is quite well regulated at 72 Volts.

I will readily admit that the schematic is a bit confusing, as the regulator is in the low side of the supply. This does have some advantages, allowing N-Channel pass transistor, plenty of gate drive for low overhead, and a direct connection of the drain (metal tab) to ground, which lowers the thermal resistance.

Update My Dynaco
Nice concept djoffe! Given that the biggest outlay on a chip amp is the case and transformer, it's a budget way to a good amp. And green too with the recycled components.

I assume that the large cap on the PCB with the inductor coil around it is the output cap - how does that affect the sound or don't you notice it? It's what has always put me off a single rail GC.
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Old 4th July 2012, 06:18 PM   #8
djoffe is offline djoffe  United States
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I've never noticed (heard) any problems from the output cap, and it makes a nice coil form for the output inductor.

It's important that the output cap is big enough to not limit the low end. When you measure distortion, there can be a rise at the lowest frequencies and highest powers owing to some non-linearity in the output cap...But with a bit of careful design, it's not an issue.

On the plus side about the output cap, it saves speakers if the output stage fails stuck at the rail. This also saves the cost and potential distortion from an output relay and dc protection circuits.

Update My Dynaco
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Old 4th July 2012, 07:01 PM   #9
Puffin is offline Puffin  United Kingdom
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Hi Nick and yes a warm welcome back. Even though I built MY 3886 and 3875 amps a few years ago I still dip into the Dungeon for tips on various things. There was a time when it was a bit hit and miss as to whether you would get the page you wanted, but it seems fine now.

I built two LM3875 monoblocks using 30-0-30 trannys and the regulated supply on DD. I get about 42v per rail, so nearly on the limit of the chip, but it sounds great to my ears, and has been totally reliable. I recently re-built the amp modules in order to enlarge the heat sinks (they were really small initally - but then I was using 97dB horns), I can now use them with a wider range of speakers.

Rob.
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Old 4th July 2012, 08:36 PM   #10
Nuuk is offline Nuuk  United Kingdom
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Hi Rob. I had a few problems with free web hosting but I hope that I have found a good host now. And may be I can even add a bit to the Gainclone section.

I presume that you used the LM338 regulator for your clones? If so that is what I will be copying for an LM3886 amp. I am hoping to have six or seven different GC's all built to completion so that I can compare them all quickly. In the past it has been a case of of stripping down one version to get parts to build another. Not the ideal way to make accurate comparisons!
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