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Old 16th October 2013, 07:52 AM   #1681
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Hi, Id like to use Peter Premium kit. I have a 400VA transformer (from old 6 channel Rotel power amp) with three 24-0-24 three wire windings. Im wonder if there are any objections against using the three sets of windings in parallel to connect to one rectifier PCB? Or would I do better by using two separate PCBs, one per channel, and discarding the third set of windings on my Transformer?
Second question,
I would like to build a balanced power amp out of the LM3875 Premium Kit (or a combination of) if possible, as my pre-amp offers balanced out puts. Are there instructions around, which show/explain how to do this?
Many thanks,
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Old 16th October 2013, 08:05 AM   #1682
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why don't use 3 windings in parallel if they have the same voltage?
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Old 16th October 2013, 08:29 AM   #1683
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Hi Yes, this is the question, i.e. if there are any objections against doing this?
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Old 16th October 2013, 09:19 AM   #1684
AndrewT is offline AndrewT  Scotland
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check that the windings really are the same voltage. They MUST have exactly the same number ot TURNS. One TURN error is NOT permitted for parallel operation.

You can check for zero Vac when the phase of the test winding is reversed with the phase of the reference winding. i.e. series connect one pair of ref and test windings in phase and you get double the voltage of the ref winding. Reverse one phase and you should get 0.0Vac if the turns exactly match. Repeat for the other test winding/s.
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Old 18th October 2013, 06:40 AM   #1685
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Hi AndrewT, I tested this with another transformer I had lying around. The wires weren't clearly marked so had to make some guesses. Firsts I had the phase the wrong way around and the transformer got very hot very quickly, but survived. Ooops. ;-)

What are the symptoms if the windings arent equal, when connected up? The Hotting up of the transformer, some current drop/leaching?
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Old 18th October 2013, 09:40 AM   #1686
AndrewT is offline AndrewT  Scotland
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Use a Mains Bulb Tester to power up EVERY project.

This will prevent you blowing the fuse that your have fitted to the transformer Primary circuit.

Even if you connect the primaries out of phase the bulb tester will protect you and your equipment.

DO IT !!!!!
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Old 19th October 2013, 01:21 PM   #1687
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Default LM3875 Integrated with Phono

Hello,

I'm new to the forum and this is my first post. There's a real wealth of information here, albeit some seem buried quite deep! Hope I've posted in an appropriate manner and not hijacked the current topic.

I'm anticipating my LM3875 Premium kit from Peter soon. This will be my second build (first was Transcendent OTL amp), so I have a bit of experience and hoping to learn even more with this kit. I've fired Peter a few questions via email, but thought this would be a better forum to avoid flooding his inbox

To the point: My goal is to build an stereo integrated amp using a single transformer. The amp would include a pot for gain control and a built-in RIAA phono stage. I have the phono board already, and 24V DC is suggested (although it can run off lower voltages). Current draw is reported to be about 15 mA.

I am trying to think ahead to how I could best power both the LM3875 amp boards AND RIAA board simultaneously using one transformer. The options I've thought of are:

1) 300 VA with 4 x 24V secondaries. Run one set of secondaries to one rectifier PCB to power LM3875, and a second set of secondaries to a second rectifier board to power the RIAA board. My concern is that each secondary will carry 1/2 the current; not sure if I will starve the amp boards.

2) 300 VA with 2 x 24V secondaries. Connect the secondary AC feed from the transformer to a common node (each wire separate of course), then branch off each coloured wire, one to rectifier PCB for amp, and one to second rectifier PCB for RIAA board.

3) 300 VA with 2 x 24V secondaries. Connect to a single rectifier board and connect DC outputs from board to common nodes, splitting off to amp boards and RIAA board.

4) Separate transformers, or simply use a wall wart with separate switch for the RIAA board (last resort for me).

Hope this makes sense. Any input as to the best option would be great. I'm hoping for the option that will be safest and quietest.

Thanks!!
Martin
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Old 22nd October 2013, 01:34 PM   #1688
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Thanks AndrewT, in future I will.

Does anybody have the schematics for the LM3875 AMP module?
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Old 25th October 2013, 01:21 AM   #1689
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Join Date: Oct 2013
Location: Great White North
Quote:
Originally Posted by AudioMartin View Post
Hello,

I'm new to the forum and this is my first post. There's a real wealth of information here, albeit some seem buried quite deep! Hope I've posted in an appropriate manner and not hijacked the current topic.

I'm anticipating my LM3875 Premium kit from Peter soon. This will be my second build (first was Transcendent OTL amp), so I have a bit of experience and hoping to learn even more with this kit. I've fired Peter a few questions via email, but thought this would be a better forum to avoid flooding his inbox

To the point: My goal is to build an stereo integrated amp using a single transformer. The amp would include a pot for gain control and a built-in RIAA phono stage. I have the phono board already, and 24V DC is suggested (although it can run off lower voltages). Current draw is reported to be about 15 mA.

I am trying to think ahead to how I could best power both the LM3875 amp boards AND RIAA board simultaneously using one transformer. The options I've thought of are:

1) 300 VA with 4 x 24V secondaries. Run one set of secondaries to one rectifier PCB to power LM3875, and a second set of secondaries to a second rectifier board to power the RIAA board. My concern is that each secondary will carry 1/2 the current; not sure if I will starve the amp boards.

2) 300 VA with 2 x 24V secondaries. Connect the secondary AC feed from the transformer to a common node (each wire separate of course), then branch off each coloured wire, one to rectifier PCB for amp, and one to second rectifier PCB for RIAA board.

3) 300 VA with 2 x 24V secondaries. Connect to a single rectifier board and connect DC outputs from board to common nodes, splitting off to amp boards and RIAA board.

4) Separate transformers, or simply use a wall wart with separate switch for the RIAA board (last resort for me).

Hope this makes sense. Any input as to the best option would be great. I'm hoping for the option that will be safest and quietest.

Thanks!!
Martin
Ok, so perhaps too complex of a post. Basically, I want to connect the two loads (amp boards and phono preamp board) in parallel using one transformer. Is there any particular advantage to connecting at the transformer secondaries and having a separate rectifier stage per load, or connecting in parallel after a single rectifier board, using star ground for all?

Thanks.
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Old 5th November 2013, 10:22 PM   #1690
tommy_o is offline tommy_o  United States
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Sorry, but you'd probably get better responses with posting that as a separate thread.
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