Commercial Gainclone kit- building instructions

[nipper575]

That looks fantastic, very tidy. I also received my classic kit and I couldn't wait to get started, first time I have attempted a project like this.

Couldn't be happier with the product, still have a long way to go though (need a toroidal and case along with everything else to get it connected). Can't wait to get it done.

[Lightman]

Originally Posted by koogar

The chassis was a scratch built by me from aluminium with a temp Corian front panel before i get a 10mm aluminium plate CNC machined.

How did you drill the Corian? Were you able to do it with a conventional hand drill and steel bits, or did it require diamond tipped bits and a drill press, or some other method?

I've never worked with Corian, but I once made a pair of custom speaker stands with granite bases. It took me a couple of hours using diamond bits to drill a total of eight holes.

(Awesome looking GainClone case, by the way.)

[koogar]

Hi Lightman

thats the beauty of Corian you can basicly use normal powertools such as routers and drill bits because its Man made from resin. you can thermo form it heat it sand it bend it drill ,cut it all with normal tools when i cut it on the table saw theres no melting what so ever my friend makes kitchen worktops from it he often glues offcuts together to form sheets and you cant see any joints.you can see here i just used a collar on the router and a guide i made using some thin ply and a hole saw to make the volume knob resess.

i only intended this as proof of layout before i find someone to CNC a piece of 10mm aluminium plate which will serve as additional heat transfer when bolted to the front .

a great tip is to use Front Panel Designer its free i made a 2d layout and you can print it out to scale stick it on the panel and you have all the drill holes exact ready for the drill press.









Thanks k00gar

[stelakis1]

Hello I've just finished my kit, I used the dual mono configuration with a 20V 150VA center tapped transformer on each channel
It's simply amazing! The kit quality and the sound! Thank you very much Peter!

And here are some photos.

Since the transformer is center tapped I'm using only one bridge per supply.



No pot was used since I'm going to use a preamp for volume control. Red plexyglass was used for front and back plate (it was the only one that I had available)









Both together over the preamp (sorry for the blur photo)



And now a question, actually two...
The first is, does anyone have a schematic showing the single bridge configuration with a center tapped transformer? I try to understand how the connections are, but I am a big newb...
And second is, does anyone have an idea about a cheap transformer cover?

[Peter Daniel]

Bride schematic with CT will look like this: http://www.tortran.com/drawings/dual..._rectifier.gif

[udailey]

I think Antek, Inc, the transformer supplier now has toroid caps.
Lets see....
Yep, here they are. Not cheap though.
Antek - Products - List View[]=3
Uriah

[stelakis1]

Originally Posted by Peter Daniel

Bride schematic with CT will look like this: http://www.tortran.com/drawings/dual..._rectifier.gif

Thank you very much Peter!

Again thank you for this amazing design!

Originally Posted by udailey

I think Antek, Inc, the transformer supplier now has toroid caps.
Lets see....
Yep, here they are. Not cheap though.
Antek - Products - List View[]=3
Uriah

Thank you very much my friend, I have already checked Antek, but as you said they are not cheap, at least for what I want to do.
I have some ideas in mind (a cd cake box cap for example, painted). I have the whole summer in front of me to decide.

[nipper575]

I fired up my gainclone for the first time last night, for the most part I think it is working correctly but I have some questions.

The DC offset of each output is -80mv for the left, and -40mv for the right with the volume maxed (25K pot). Is that just the luck of the draw with the chips, or should they have similar DC offset?

I'm happy to say I don't have any hum after following the star ground advice at the start of this thread. I do however have a buzzing noise that seems to be amplified with volume through both channels. It's not loud enough to be heard over any music, but you can hear it if you place your ear within ~30cm of the speaker. This is with no source connected, I'm guessing this has something to do with my input wiring?

My last question (hopefully) is that when I touch my potentiometer, it makes my speakers pop slightly. If I connect my voltmeter between the pot shaft and my starground, it reads between 0 - .10mV. I'm guessing that's not normal.

If anyone can shed some light on those questions, or point me in the right direction, I would be very appreciative.

Other than that, I have to say that I am very impressed with the sound that this amp produces, I have enjoyed building it greatly.

[AndrewT]

the position of the volume control should not alter the output offset.
DC coupling of the input will allow variations in source resistance to change the output offset. DC coupling of the input will also allow a fault producing DC on the signal to pass right through to the amp output.

What schematic have you used?

[nipper575]

Hi Andrew

I'm afraid I am still very much a newbie to DIY audio, so I will try my best to answer your questions. You might need to spell things out for me if I miss the point. So you are saying that my input is DC coupled and it shouldn't be?

I used Peter's LM3875 classic kit with a 300VA 25V transformer. I followed his instructions at the beginning of this thread to connect the components.

Originally Posted by Peter Daniel

The first thing we do before connecting any speakers to the amp is measuring the DC offset. This can be done directly at the binding posts with no load connected. If the potentiometer is installed, the offset will vary depending on volume setting. This particular amp (right channel) measures 32mV offset when volume is completely down (input shunted to ground).

Originally Posted by Peter Daniel

and 70mV when volume is at the maximum (15K combined input impedance of 50k pot and 22k input shunt resistor).

I'll just take this time to mention that I removed the pot from the input and just joined the wires and the buzz is gone. I don't know if that points to the pot being of poor quality or if it has something to do with how I grounded the pot to the PCB (to the SG as per the instructions in this thread).

After I removed the pot, my DC offset between the posts climbed to -111mV on the left channel and -68mV on the right (roughly -30mV difference for each channel).