Gainclone-chip amp?

mmadden28

heiney9 wrote: »

Lost my motivation and have no extra time at the moment so I got a great deal on this:

http://www.audiogon.com/cgi-bin/cls.pl?ampstran&1253218507&/Gainclone-LM3875-Stereo-Amp

I couldn't make it cheaper than I bought this for. I like the layout and he used Caddock and Riken resistors and Black Gate caps.

I know I sort of whimped out by not building it myself, but there are still room for improvements on this one.

H9

Sweet-nice deal. Looks like a nice build--and yes would've cost you more to build that exact config yourself. Well if nothing else it will serve as a model for when you're ready.

megasat16

heiney9 wrote: »

Lost my motivation and have no extra time at the moment so I got a great deal on this:

http://www.audiogon.com/cgi-bin/cls.pl?ampstran&1253218507&/Gainclone-LM3875-Stereo-Amp

I couldn't make it cheaper than I bought this for. I like the layout and he used Caddock and Riken resistors and Black Gate caps.

I know I sort of whimped out by not building it myself, but there are still room for improvements on this one.

H9

Congrats on a nice gainclone amp. I hope you'll enjoy it. I agree it'll be more expensive to make one instead of buying one.

You have plenty of time to build another one when you feel like it. I may be buying some parts for the gainclone since I have the transformers for it already.

heiney9

The gainclone is sounding pretty good at this point. Very beefy with the Black Gates and large power supply. It has a surprising amount of bass for such a small PS section (<10uF total smoothing PS caps). It's not harsh, or hashy at all, but it is somewhat analytical. I'd call it a little on the "steely" side w/o being fatiguing. There is good delineation of instruments and voices.

I'm using it with the Dared Sl2000A tube pre-amp and to be fair I'm doing a fair amount of tube rolling so some of the characteristics I'm sure are influenced by the tube choice. I will say with the tube pre, it's not as mellow as one would think. But I need to spend more time with both pieces as they are both new to my office rig.

I do have an issue with the input on the amp being too high. The original Audio Sector GC was designed to be used with a 50K stepped attenuator. I need to figure out which resistor is the input resistor and then come up with a suitable swap to lower the input gain to be able to use it with a pre-amp. GV27 you out there? Need some opinions on what value to use and which *exact* resistor needs to be swapped.

H9

me_myself_I

heiny9,

If you want the "exact" components to change, you'll need to supply a schematic. Do you have a schematic for this? There are a lot of ways of assembling a chip amp amplifier and more than one way to reduce the gain.

heiney9

me_myself_I wrote: »

heiny9,

If you want the "exact" components to change, you'll need to supply a schematic. Do you have a schematic for this? There are a lot of ways of assembling a chip amp amplifier and more than one way to reduce the gain.

Page 2 of this document shows the REV B board and page 3 shows the schematic. I believe the input resistor is 220 ohms. It also has a feedback resistor soldered to the underside of the pcb. I assume (but have no idea) that it can stay the same, and by adding a higher value resistor to the input will allow the amp to have lower overall gain and I will have more control with the pre-amp I'm using.

Right now I can barely turn the vol knob and it becomes very loud very quickly.

http://www.audiosector.com/nigc_kit-users_guide.pdf

Face

That would go perfect with my passive preamp.

heiney9

Face wrote: »

That would go perfect with my passive preamp.

Yes it would............as I used in my main rig with the GFP750 in passive mode, sounded pretty good. Not really on par with the Aleph, but better than it really ought to be.

I've got my eye on some small tube mono's for the office rig......so who knows you may be mating it with your passive pre before you know it

H9

Systems

It.s not surprising you have too much gain with the tube pre,they have sufficient gain to work great with passives.You can do one of two things,add a pot at the input or reduce the overall gain by increasing R3 (680 ohm) to something like 1.2k.

me_myself_I

GV#27 wrote: »

It.s not surprising you have too much gain with the tube pre,they have sufficient gain to work great with passives.You can do one of two things,add a pot at the input or reduce the overall gain by increasing R3 (680 ohm) to something like 1.2k.

Like GV#27 mentioned, the gain is set by the feedback resistors 22k and 680 ohms.

Changing the 680ohm resistor to 1.2k will lower the voltage gain from 30.5dB to around 25.7dB. Not an insignificant change, but I really think there is more in the range of 10dB too much gain for heiney9's system. 20dB of gain is typical for a lot of power amplifiers.

The voltage gain is basically Av=(1+Rf/Ri) where Rf is the 22k resistor and Ri is the 680 ohm resistor. Increasing the feedback reduces the gain and will probably improve the distortion characteristics. Voltage gain in dB units is Av(db)=20*log(Av).

So either increasing Ri, or decreasing Rf will have the same effect. There are some effects from the absolute values of the feedback components, but that is for another thread.

GV#27s 1.2k reduces the gain to 25.7dB. If you want to reduce it even further say to 20dB, increase the resistor to 2.4k. 20dB will give you full power at probably somewhere around 1V input.

Adding a pad in front of the amplifier is probably the easiest and crudest method of them all, but it will raise the relative noise floor of the system. You may or may not notice it.

heiney9

Thanks GV27 that the exact info I was looking for!

Thank also to Me-myself_I for a nice explanation

Face, I won't be making any modifications until I'm sure I'm keeping it for awhile. You are first in line if I decide to move it on down the road.

H9