RJM Gainclone Help

[sorenj07]

Here's the schematic that I'm referring to:

http://www.geocities.com/rjm003.geo/...gc_lm3875a.gif

The power transformer I have is a Hammond 182P30. It has two 115V primaries and two 30V secondaries. RJM's site suggests a rail supply of 16VAC before rectification, which I could get by putting the primaries in series. My filter caps are 2200uF 50V 105C types, so they could withstand 32VAC rectified... I'm using one LM3875T per channel.

This is my question. Should I feed 16VAC or 32VAC into the PSU as RJM lists it here? ( http://www.geocities.com/rjm003.geo/...ages/gc_ps.gif ). My rectifiers are flat GBU types good for 8A and 200V. My volume control is a generic 50K dual log pot.

Thanks!

edit: this amp will be for 8-ohm loads only!

[jaudio]

I would use 16volt ac. 32volts after rectification(near 45volts) is too much voltage,it will kill the chip.

[sigurd]

hello,
I use a trafo with 2 secudaries of 16 volt. I use 4 rectifiers ULTRA FAST SINTERGLASS DIODE FE5D from VISHAY Semiconductors on www.vishay.com .Very important, likes it to me to acheive bright high tones!!! I have experience with many other kind of bridges.
So I have 2 zero's from the trafo, one go and one return and the two others from the diodes (in bridge) are the positive and the negative. I think it's important to use 1000 microfarad , not 2200. The go and return from the zero point must be connected together on the earthpoint on the LM3575. Earthing is very critical. Make it like the schema that you have. I use two trafo, one for the right and one for the left channel to avoid groundloops! The grounds come together at the outputs in most CD players! So they must not come twice together in the amplifiergrounds! If you have other questions, let me know. Succes!!! My email : sigurd21@email.com

[sorenj07]

OK, I'll go with the dual 16V secondaries. I'm using four bridge rectifiers to have a complete dual-mono setup, splitting off the secondaries and having separate V+, GND and V- rails for left and right channels. Each rail gets a 2200uF 50V capacitor, instead of 1000uF because I'd like to have a bit more headroom.

[sigurd]

Why four bridges? Two bridges are still good. The purpose is ( I think ) that the power, the amperes come direct from the trafo, not from the capacitors! The capacitors are for eating the ripple and lost them in heat. Also I have the experience to remove the plastic from the capacitors!!! So when the power comes from the trafo it's important to use good and ultrafast rectifiers, who I mentionned before : the sinterglass diodes 4 each channel. Also I have experience with cooling the chip! I use not the very large and big heatsinks!!! Not good for the 'open sound'!! I use a plate of red copper, 7 cm on 11 cm thickness about 1 mm. I use 0,9 mm thickness and lay it down horizontally so it can cool.
When the voltage is over 25 volt the chip can't powerd a 4 Ohm
loudspeaker. Higher voltage is for 8 Ohms loudspeaker and above.
Grounding is very important!!! See it like there must flow enormous amperes. I will send you next week a picture how I make my ground!!!!!!
When you will use a 2200 micro capacitor the sound is not the same as with 1000 micro. I experienced also with 500 micro, also not good. 1000 seems best for me. I maked it hardwired, not pcb.

[sigurd]

For the chip, you can look on National Semiconductors.
www.national.com and have a look on LM3875. You will have all the specifications. Succes.

[sorenj07]

Cool, thanks . My experience is with building tube circuitry so I'll probably point-to-point it, mounting the chips to the aluminum chassis, and trimming the unused pins. I'll probably use a small C-bar to press the chips to the chassis, and this could also act as a heatsink. I have some Arctic Silver 5 left over from my CPU heatsink that should work well.

Why skin the capacitors? Is there a sonic benefit to having the metal cans exposed?

I use four bridge rectifiers to minimize crosstalk between channels. The only thing that the two channels share now is the transformer - if they draw current from the same capacitors, more crosstalk is possible. It's probably not a big deal but the rectifiers are only ~$1 each anyway...

I will use a thick 12-gauge wire for each channel's ground buss, and also for the PSU. Star point, of course. I'd love to see how you implement the Gainclone, and will probably post pics of my amp once it's done.

[sigurd]

hello soren,

I experienced a great difference in sound when I removed the plastic from the capacitors. It's much better without plastic.
I will send you next week a few pictures of mine gainclone, my computer yet can't send it, I will work on this next weekend.
I wonder to see your gainclone. Succes!( My knowlegde of englisch is not so good,....)

[sigurd]

Hi Sorenj07, here some pictures of my gainclone. First it was some experimental box, now I use it for definite. I will build a very proper one in the future, but this sound great. Voltage 16V alternate so is proximus 22,7 volt +/-.

[sigurd]

Hi, sorenj07,
Can I have your email address? I will send you the pictures of my gainclone. sigurd21@gmail.com
Here on this site is'nt possible, too many Megabites...
Greatz.