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Commercial Gainclone kit- building instructions

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#882
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.
 
#886
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. 🙂
 
#888
udailey said:
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
Click to expand...

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.
 
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#889
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
 
#890
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
 
#891
Well, The B1 wont color the sound. Thats the nice thing about it. Mine didnt anyway and I had a standard LM3886 amp. I dont remember the volume changing any different before/after B1 but it may have been easier for my amp to do its job. I suppose its the difference between sources.
I wonder if changing the first resistor that goes to ground from signal in your amp would help since it would dump more of the signal to ground and probably give you further rotation on your pot, but that would basically be the same as putting that resistor parallel to your shunt resistors I was talking about.
You could find a pot of similar value to that resistor and replace that resistor with the pot. Then turn the amp on and dial the pot. Of course a pot with legs A W and B would only use legs A and W for this. So with the amp on you would dial it down very slowly and you might find a situation you like.
I know I would do it on my amp, but would do it more out of curiosity. Not saying that I am sure this would solve your problem, but I know that putting a resistor parallel on my pot did help me with a limited pot rotation problem I had.
Uriah
 
#892
But surely, taking a 10k pot and adding a 15k series resistor, results in something VERY similar to a 25k pot, negating any SQ advantage to using a lower value pot you were suggesting in your first post. The fact is, for me, with a standard Audiosector LM3875 chip-amp, a 10k pot is not practical, and needs additional resistance for a line level signal. I will be testing a range of cheap carbon track pots in the near future to determine the most practical choice, and then replacing it with a stepped attenuator of the same value range.
 
#893
No, its a resistor that goes parallel to the shunt side of your pot. So it goes from wiper to ground on your pot. This will make the shunt side of your pot move up in resistance more slowly. So the only drawback is that impedance will not be as linear, but if you are dividing voltage by a pot you have the A, W and B legs. A to W is signal in to signal out. Its series. W to B is Signal out to Ground. At W the signal decides to go to amp or to ground. The lower the resistance then the more goes to ground. Lower relative to A to W. So if we were to divide AW by WB the result would be more signal going to ground earlier in the rotation of the pot with things evening out a little and then shooting up towards the end of rotation. Thats my experience with it but the information for how it works is here
Tack a log taper onto a digital potentiometer - 1/20/2000 - EDN
and towards the bottom of this page
Potentiometers (Beginners' Guide to Pots)
Uriah
 
#894
Hi Peter and the other here.

I dont know f here is the best place to ask for your suggestions in my problem.

I have bought 2 LM3875 premium kits a while a go. I havent got the time to finish them. No i spare some time and i put them together in 2 monoblocks.

As i asembled the first one and put it to work the output was loud enought but with a weird phenomenon , besides the hum (that i can avoid i think) i got after some second a cliping audible loud sound like mors signal besides the music.
even if i disconnet the input the noise insists.
I tried the other amp without the chasis and without cooling in the chip and there was no noise. But when i installed one the noise appeared. !!
Its loud and repetitive.
Any suggestions?

Nick
 
#895
tiglitosa said:
Hi Peter and the other here.

I don't know f here is the best place to ask for your suggestions in my problem.

I have bought 2 LM3875 premium kits a while a go. I haven't got the time to finish them. No i spare some time and i put them together in 2 monoblocks.

As i assembled the first one and put it to work the output was loud enough but with a weird phenomenon , besides the hum (that i can avoid i think) i got after some second a clipping audible loud sound like mors signal besides the music.
even if i disconnect the input the noise insists.
I tried the other amp without the chassis and without cooling in the chip and there was no noise. But when i installed one the noise appeared. !!
Its loud and repetitive.
Any suggestions?

Nick
Click to expand...

I have a suggestion Nick. It could be power supply noise of a kind that is almost impossible to remove with a regulated supply, emanating from these horrendous Ethernet communication cables that use the mains to send router information. Here in the UK, British Telecom have a wireless internet/TV/phone modem called the HomeHub, and it communicates to the TV box via the mains using Ethernet cables to electrical sockets. When I was using it, I got an awful noise like the one you describe in my hi-fi, and it was impossible to remove until I threw the electrical-socket Ethernet adapters away and simply used a single long Ethernet cable for the HomeHub to communicate with the digital TV box. BT is actually breaking the law in issuing these devices, by the way. There's a campaign to ban them. If you share your power supply, it could be somebody else using on of these things that puts junk data into the power supply. Just a thought.

These are the dastardly evil things:
http://www.amazon.co.uk/ComTrend-Power-Line-Ethernet-Adapter/dp/B001M06Y0M

Lucas
 
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#896
LucasAdamson said:
I have a suggestion Nick. It could be power supply noise of a kind that is almost impossible to remove with a regulated supply, emanating from these horrendous Ethernet communication cables that use the mains to send router information. Here in the UK, British Telecom have a wireless internet/TV/phone modem called the HomeHub, and it communicates to the TV box via the mains using Ethernet cables to electrical sockets. When I was using it, I got an awful noise like the one you describe in my hi-fi, and it was impossible to remove until I threw the electrical-socket Ethernet adapters away and simply used a single long Ethernet cable for the HomeHub to communicate with the digital TV box. BT is actually breaking the law in issuing these devices, by the way. There's a campaign to ban them. If you share your power supply, it could be somebody else using on of these things that puts junk data into the power supply. Just a thought.

These are the dastardly evil things:
ComTrend Power Line Ethernet Adapter: Amazon.co.uk: Electronics & Photo

Lucas
Click to expand...

Thanx Lukas.
Its a little strange i havent thought of it and it might not be the case since in my building only 2 have internet including me and the other one is not so relevant to get those powerline adaptors. Any way i will ask hom to be on the safe side.
The strange thing is that this happened when i attached the heatsink !! Withiut heatsink no noise !! With heatsing a signal machine gun in slow rate. 🙁
 
#900
Thanx Peter I will try it.

But if is the case of electrical interference why this was not happend when i havent installed the heatsink?

And if i unplug all the other devices , fridge, routers, etc should i still get the noise? I ask this to eliminate all the other possibilities.

Thx

Nick
 
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