Commercial Gainclone kit- building instructions

[puma2]

Peter Daniel, one of your LM3875-kits got a new home this weekend in my fathers old Pioneer SX-400L chassi.

http://img197.imageshack.us/img197/2888/dsc00172gu.jpg

It required a little bit of modification though, but hey, the sound is great considering the use of some 20 year old equipment.

Thanks for this thread. Greetings from Sweden.

[GregH2]

Hi Peter,

Thanks for the link.

I'm interested to know if there is any particular reason (either scientific or subjective) for your choice of 0.75W ratings for 22k resistors? For example, are larger power resistors more stable or less inductive or something? Or do you just think they sound better?

I would have thought a 0.25W rating would have been enough for this purpose (though I'm pretty ignorant to this stuff), and I'm just interested to hear your thoughts. I would like to design my own gainclone circuit one day and am trying to learn everything I can.

Thanks,

Greg.

[Peter Daniel]

It's not the actual wattage of resistors that I'm after but I like that particular brand of resistors in that application (Caddock MK132). If you check my regular kits you will notice that I'm using 0.5W resistors there; 0.25W rating will work fine as well.

[GregH2]

Thanks Peter.

One more question, judging by your photos, when you use an external PSU, you are not grounding the chassis of your amplifier (only the PSU chassis). Is that correct?

Thanks,

Greg.

[Peter Daniel]

I connect PS enclosure to Earth ground. Additionally, you can run a separate ground wire from PS enclosure to amp enclosure.

This way, everything is properly grounded.

Amplifier's output ground connects to chassis through CL60 thermistor.

[puma2]

I have a little problem. Today after playing for approximately an hour my left speaker started to sound really bad.

After disconnecting the speakers I measured the DC offset, the left channel had DC ratings at -22,7 V, while the right channel was as normal.

The voltage from the rectifier is 25 V.

Now my plan is to remove the amp board from the chassis and check everything again. I would really appreciate it if someone has an idea of what could be wrong.

[udailey]

Has anyone found that really low impedance on their pot is way way better than 25k or more?
I love impedance around 8k. The sound is so much more alive.
Uriah

[LucasAdamson]

Originally Posted by udailey

Has anyone found that really low impedance on their pot is way way better than 25k or more?
I love impedance around 8k. The sound is so much more alive.
Uriah

I use a 10k stepped attenuator on my lm3875, and I find that with a line level input, I have to keep in on the bottom two or three steps of the 22 available. Above that it begins to distort.

When my Pass Labs B1 buffer is built, however, I hope that these problems will be over. I plan on using a 25k or 47k pot with the buffer, so that it brings it within a more practical range.

It sounds good with the 10k attenuator, but I have nothing to compare it to, and can't imagine why a small pot would sound any better than a larger one, as any given volume will require exactly the same degree of attenuation with any device. The only difference is the range of resistance on offer.

[udailey]

With higher impedance I get a more muffled sound. The lower the impedance the livelier the sound. The pot along with your cables capacitance creates a filter so we can lose part of the music depending on that. I built a pot with LDRs and had made it so i could change impedance at will, during music, but not change volume much. The change is definitely there. Better with lower. Then I built the DCB1 buffer and used that same pot on it and since its a buffer I could get away with wild impedances without sending my amp oscillating away. I ran it from 400k down to 600R and liked the range in 600R-4.7k best. Makes you want to move! So crisp and clear.
I think if you use that 10k attenuator but try a little fix it might do well for you. Try getting a few different values of resistors. Like 500R, 1kR, 2k, 5k, 10k. Take each one individually and put it parallel to your shunt resistors you will find a value that allows you much more rotation on the pot so you can bring the volume up in smaller steps. When you say distort I am not sure if the sound just gets bad or if you mean that it gets loud really fast and sounds poor when its that loud. If its to loud to fast that fix might do it.
Just a suggestion that I have tried and it worked for me to get more range out of my pot having an amp with lots of gain and super sensitive speakers.
Uriah

[LucasAdamson]

Hi Uriah,

The gain is too high when feeding a line level signal into my gainclone, owing to too high a voltage. These amps aren't line level input, nor even close, so attenuation is definitely in order, even for top volume level I find, or else they will distort. The buffer I'm making right now should help, but I'm not exactly sure how, to be honest. It won't amplify the signal, but what I really need is to reduce the voltage from line level to something closer to the amp's ideal input.

Lucas