Gainclone advice?

Hi, I'm Mark, another FNG building a gainclone. I'm looking for advice in a couple areas. I REALLY don't want to toast my speakers - ever. They are 4 ohm speakers, Klipsch Heresy II's. My surround receiver does a lousy job with music and the 4 ohm load. So I looked at used McIntosh and can't afford them. Wandering the Internet looking for amplifiers led me here.

I bought the chipamp LM3886 dual mono kit (two power supply boards, and two amplifier boards). I also bought two speaker protectors from Ampslab. I bought ONE 18V, 250VA toroid from Parts Express - Avel #Y236651. Here are my questions.

The speaker protection circuits run on 24V. I figure I should have about 25V rails. So I was thinking about running one amplifier and one speaker protector per power supply. Is this a good idea, or should I run both amplifiers on one power supply and both speaker protectors on the other? Or, should I get a recifier and give the speaker protectors their own supply?

What about the feedback capacitor? I built the boards in standard configuration, without the feedback cap. Good? Bad? It could be changed... I understand it reduces DC offset, but is not really necessary. How do I measure DC offset (without a scope), and why is it good to minimize it?

Any help is appreciated. I am not an electronics guy and I know it. (I won't blow myself up though)

Mark
 
Be Very carefull of connecting your newly built amplifier to your speakers.

I blew one of my ones the other day due to the +35V wire touching the output. Accidents are very easy to make.

Finish your amp first and test all basic function on cheap old speaker.

For dc on output testing.

Short the input to ground , Get an 4 ohm load on output (old speaker) and measure with a normal multimeter.

Tang
 
I'd recommend that you acquire either:
A Pioneer B20 and a big box or
A Tang W4654SE and a small box
(or similar)

The lack of responses to your post indicates that the option of the speaker protector circuits is a puzzle, and nobody wants to be wrong on a recommend because of the expense of your speakers.

Therefore, I will only risk a recommend if you can arrange to drop the cost of any consequences down to an inexpensive speaker on your test bench!!!

Sorry to be obnoxious there, but let's get you unstuck and on with the project. ;)

I think that simplifying should be first.

First, its easiest to build standard stereo layout, and then expand it later, if desired. Let's start with your one transformer and one power supply.

Build the power supply first, and test it.
The cables to feed left and right amps will be the same length per left and right (applies to stereo layouts).
Any grounds, once met, will never meet again (no ground loops).

Use gel flux with these boards. Otherwise the gold terminals are like nonstick teflon. ;)

Let's examine your application and give a few normal speaker protection options.

First, the DC protection capacitor included in that kit is (rant deleted), but if you use a cap at Ci, it gets you additional speaker protection and usually a higher-resolution bass (absence of DC).
Elna Tonerex 45uF 100v is, in my opinion, a nicer alternative than the one in the kit. Others may differ on this opinion.
However, one of our esteemed moderators has also mentioned Elna at position Ci (NFB, gain loop), but his Elna is orange. ;)
In my own personal testing, almost any 100v "made for audio" cap is workable at Ci, regardless of price.
These are surprisingly inexpensive, and if you also find them copesetic, then they will provide nice additional protection.

Second, it is also necessary to prevent DC from entering the amplifier through its input. Has this already been done by your preamplifier? Or not?
Will you always use this amplifier with just one safe preamp, or will someone someday connect an Mp3, computer, or TV?
Then do consider an input filter--usually an input filter cap.
Those are not ever price based performance, but they are sometimes price based variety to support a vast array of personal preferences.

With both of these caps in use (per amp), then your speaker protection *could be* simple fuses.

It seems that the majority of chip amp accidents could have been prevented by:
1
A light bulb (in series with) cord tester for power supply testing.
2
Avoid putting DC into the amplifier signal-in
3
A DC voltmeter (to measure DC at speaker output).
Here is where you find out if a cap at Ci (NFB, gain loop), is paramount or if you have the option of incurring a small risk (if no DC without cap) to get a sonic difference by leaving that cap off.
I'll say that leaving the Ci cap off is inherently irresponsible, but that the risk for doing so depends on the DC voltmeter speaker output reading.

With our low cost test speaker in use, you can build the amplifier normally, test it, and then. . . after it is working to your satisfaction. . . later increase the complexities, if desired.

Yet, in addition to all of the above personal determinations (to match your own application) there is one step left before increasing complexities. . .

You need to A-B your amplifier in comparison to another amplifier. Now, you need to undo any changes that you have made that may cause masking. Its possible to voice an amplifier for personal tastes, without resorting to masking. The most common approach for voicing that kit is to study the difference between the two 100uF caps for power aboard the amplifiers versus the two 1000uF caps in use aboard the AudioSector kit. There are several seemly options to accomodate personal tastes in audio--without masking. Just do a search here at diyaudio.com and you'll see ever so many options. ;)

Finally, its time to explore adding optional components.
Check DC output just once more before connecting to expensive speakers.
Have fun! ;)
 
Oh, I will DEFINITELY test the power supply, then the amp, and then connect one of my garage speakers before I even think about hooking anything else up to it.

If you think it's smart to use the Ci feedback capacitor, I will do exactly that. The inexperienced need to listen and learn, so that's what I'll do!

The input will, for now, be a pre-out from a Nakamichi surround reciever. This is the one that deals poorly with the 4 ohm load. Eventually, it will be replaced with something else, but the receiver will always do the signal switching and preamp duties. If the gainclone works well with the 4 ohm Klipsch's, then it will stay there even when I replace the receiver.

Mark
 
craigg4c said:


Very good. You will notice a huge difference when the dog suddenly unplugs the input with everything playing at full volume -- with the capacitor in place, your speaker cones will not go flying across the room and break Grandma's heirloom vase -- a very serious form of distortion.

PLEASE pass the word to other novices, decky: DO NOT build gainclones without Ci!


Ok, I saw this in another, older thread. I'll move the resistor and add C1. Consider it done.
 
Ok, I got C1 connected and R3 moved to the other hole. Good thing I left the leads on the resistor a little long, just in case.

I'll see what I think of the sound, and I just might try the carbon resistor swap that Daniel suggests. I used the C1 supplied with the kit for now.

I will run one amp per power supply with my garage speakers, THEN I will connect the speaker protectors and see if there is any audible difference with them. If not, the Klipschs will get hooked up.

Of course, I will run the tests as I go for correct voltage out of the power supply, etc.

I got a black anodized case from ParMetal. Pretty nice. Now it's time to assemble this stuff.

Please explain how to measure DC offset. Do I connect a load to the speaker terminals?

Mark
 
The most verifiable position for a resistor swap (from metal to carbon) is just one resistor, your R1, at position Rb, known as the input in-series resistor.

At this location, a lesser conductor may or may not provide additional source isolation. Any benefit seen does depend considerably on the source and very little on the resistor itself.

I haven't mentioned "sounds of" resistors in some time, because it seems to cause mass panic. ;) However, if you're going to try it, then see what it does for you at Rb.

Its simply more important to voice that particular kit with power cap options, so you may change the 100uF to 1000uF before exploring endless resistors. ;)

My method of DC offset measure would not be appropriate to mention at this forum; however, if you do a search for DC offset, then you'll find popular measuring methods discussed many times.

An additional concern is DC at the input, so you either need to measure each source used, always use the amp with a hifi preamp (that contains this protection), or add optional components Cin (standard input filter caps). Use of input filter caps requires an additional load for the source to see. Most people use a potentiometer for this, but a plain resistor works just as well. The value (from signal to ground) can be as little as 1 megaohm to as much as 20k.
 
Well, I figured out the transformer-to-power supply wiring after searching this forum. That all appears to be in order. I don't get a steady DC reading on the output, but I read on the Gainclone FAQ that you need a load connected to measure the voltage.

Enough for tonight. Tomorrow (ok, later today...) I'll clean up the wiring. Need to get a heat sink before I can connect the amplifier boards.