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Commercial Gainclone kit- building instructions

Hi, I’d 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. I’m 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 PCB’s, 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,
 
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.
 
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 aren’t equal, when connected up? The Hotting up of the transformer, some current drop/leaching?
 
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
 
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.
 
Hi Tommy_o, I was after the the schematics to go with Peter's lm3875 board. I've worked it out by now thanks.

I've build the premium kit amp. In many ways it sounds amazing, very detailed and refined.

However I still have a few issues:

- Once in four of five power-ups I get a oscilating sound, "whu" sound- 1 sec silence- "whu" sound etc.., coming from both channels. I have separeted PSU boards and it contineus even when I turn my pre-amp off. It doesn't sound like a hum either, very strange. It goes away after 1 min max. Does anybody have a clue what might be going on?

- Secondly. I'm picking up very high frequency noise coming from the pre-amp. I don't think my pre-amp/dac is particularly bad, it's just that the GC is brutal. I'd like to trade in some gain against raising the noise floor by increasing the shunt resistor at the LM3875 input. Would that work? Also what would be a good high frequency filter, if anybody has got any suggestions.

Lastly I think a EMI filter is needed too in front of the transformer. Things like my doorbell are clearly audible through the amp.

Any other options suggestion to my issues above would be highly appreciated.
 
Right, I’m taking the following approach.
I’m going to change out R1 to a 1k resistor and am going to put a 470pf HF filter cap across the RCA’s, to start with

Also:
I’m still wondering if I should use a C-in, Peter suggests an optional 4.7uf or higher. I read elsewhere that C-in should be atleast between 5-10uf. As this cap will be inline I suspect it will have quite an influence the amps sound. What value/ size voltage wise do people recommend here? Also as there doesn’t seem to be a position other then in the place of R1, would I need to place these caps outside of the board?

The possible benefit of a 10uf at C1/2 is not clear to me, can someone explain what I stand to gain with adding these optional caps?

Thanks,
 
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The input cap is also referred to as the input DC blocking cap.
It does two quite separate jobs.
It blocks accidental DC entering the Power Amplifier.
It applies a high pass filter to the signal sent into the Power Amplifier Input.

The value of the input capacitor does NOT affect it's ability to block DC. However some types of capacitor pass more/less leakage than other types. For least leakage (=good) do not use an electrolytic, nor paper. Some of these can leak significantly.
Use a film type.

Thge value of the capacitor does affect the roll off frequency (or passband) of the filter passing the signal.
There is an enormous variation in user preferences and some technical requirements.
The F-3dB (= 1 / 2 Pi R C) is usually set lower than 20Hz. Some will set it lower than ½Hz.
I like 1Hz to 2Hz for that F-3dB, to ensure that 4Hz is within 1dB of flat. -1dB @ 4Hz is probably not audible, but this error adds to all the other errors both inside the Power Amplifier and before the Signal reaches the Power amplifier. You should listen to your own system and decide how low you need the filter frequency to be such that it is inaudible.

RF filtering.
The series resistor and the following shunting resistor to Signal Ground form a low pass filter that attenuates the high frequencies. Again there is enormous variation in user preference. Listen and decide what suits you.
Start with F-3dB @~50kHz and see if that is audible. You may have to go as high as 300kHz to make this RF filter inaudible for you and your system.
 
I've ordered a pair of AURICAP XO Series, 4.7uF Metalized Polypropylene Cap, to use as input caps, which I've read good things about. I've already tested with 2.2uf and that seemed to solve the Vdc problem.

Is there an online calculator or formula in which I can plug in the two resistor values to determen the RF cut-off frequency?
 
Thanks udaily, if you are so in the know, you could have just answered my question. But instead you chose to try and patronize me. That gives me the impression you are not a very pleasant person, prove me wrong. Please be assured that, I did "read" his comment very well, I Googled "rf filter calculator" and several other combinations and did not find what I was looking for, hence I asked the question. Don't believe me if that's your disposition, but don’t blame me for it.