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Old 18th November 2004, 03:13 AM   #1
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Default Dual Mono Gainclone Issues

I had visions of my first ever post in this chip amp forum as an open letter to BrianGT and to all the incredibly helpful and talented individuals who populate this board.

This was, unfortunately, not meant to be. I have a situation with my newly completed dual monoblock gainclone that is causing me some concern.

I finished the wiring on my dual monoblocks using the BrianGT PCBs (thanks Brian, couldn't have gotten this far without those) and his most basic schematic this evening. I fired them up with a 10 Ohm load across the speaker terminals and measured the DC offset at 75 mV on one block and 95 mV on the other. So far so good. Oh yeah, and the LED came on (thanks NUUK for the wiring schematic and the formula for calculating the resistor value).

My next step was to hook up a pair of disposable 8 Ohm drivers to make sure that nothing bad happened. I tested them with no input one block at a time. No problem. The LED stayed on for a little while after I switched off and there was a slight turn-off thump.

I next hooked up a source to see if the blocks actually amplified music. The source is a dirt cheap Lenox brand boom box that my son uses in his room. I tapped into this via the 1/8" headphone jack and one of those RatShack (Tm) adapers that allows you to hook up standard RCA cables.

By golly they amplify whatever's coming out of the headphone jack. I was pleasantly surprised that there was no smoke and that music was playing. There was a huge hiss which I attribute to the low quality of the boom box. You can hear it at zero volume through the boombox speakers. I'm sure that will go away when I hook up a better source (through an eventual pre-amp) but right now I have an issue that makes me reticent to hook up anything of value to these monoblocks.

The trouble started when I switched the source selector on the boom box from CD to radio and then to tape/off. The speakers went to maximum excursion and smoked.

I got out another pair of disposable midrange drivers and hooked them up to repeat the experiment. I turned everything off except the boom box which from which I removed the 1/8" adapter.

When I turned the blocks back on with the new speakers, all was as before and just fine. No noise, no smoke, no problem.

I hooked up the boom box again with the source still on tape/off and the drivers went to maximum excursion again. I turned the blocks off before the smoke came out and saved the drivers for further tests. I then selected CD and turned on the blocks and there was no problem even when the CD wasn't playing. I pushed play and voila! music.

When I stopped the CD however, the drivers went to maximum excursion and I had to hit the off switches pretty quickly.

The monoblocks will amplify the input with no problem if I turn them on while the CD player is not playing and then push play. I think most of the time I can push stop on the CD player with no problem, but this situation has me puzzled.

What causes speakers to go to maximum excursion when the source is changed or shut off?

Anybody have any idea?
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Old 18th November 2004, 03:21 AM   #2
Tim__x is offline Tim__x  Canada
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Wire a high value resistor, around a 100k, from the input to ground.
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Old 18th November 2004, 03:27 AM   #3
troystg is offline troystg  United States
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Hi cytokine-

Sounds like a DC grnd loop with boombox and amps.

Put a cap in the path for testing purposes to block the DC.
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Old 18th November 2004, 03:31 AM   #4
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Those boards already have 22k input shunt resistor. What you have to do, if you still insist on using that boombox, is to use input coupling caps that will prevent DC from a boombox geting to your speakers. A value between 2u - 10u should be fine.

And before connecting speakers, check for DC offset while switching the sources.

While 24VDC will not damage your speakers (if you dosconnect them quickly enough), while burn the test drivers?
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Old 18th November 2004, 04:41 AM   #5
Tim__x is offline Tim__x  Canada
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Whoops, I didn't know that. Ignore my earlier post.
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Old 18th November 2004, 01:34 PM   #6
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Default Re: Dual Mono Gainclone Issues

As minimal as the DC-offset of the source is, it will be amplified by as many times as the gain of the amp.
Many sources have some DC-offset, unfortunately, and it can be easily solved, of course.
But if you guys want to sleep well and be able to connect your amps to everything without worries, use input coupling caps.
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Old 18th November 2004, 03:06 PM   #7
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Default Re: Re: Dual Mono Gainclone Issues

Quote:
Originally posted by rabstg
Hi cytokine-

Sounds like a DC grnd loop with boombox and amps.

Put a cap in the path for testing purposes to block the DC.

Quote:
Originally posted by Peter Daniel
Those boards already have 22k input shunt resistor. What you have to do, if you still insist on using that boombox, is to use input coupling caps that will prevent DC from a boombox geting to your speakers. A value between 2u - 10u should be fine.

And before connecting speakers, check for DC offset while switching the sources.

While 24VDC will not damage your speakers (if you dosconnect them quickly enough), while burn the test drivers?

Quote:
Originally posted by carlosfm
As minimal as the DC-offset of the source is, it will be amplified by as many times as the gain of the amp.
Many sources have some DC-offset, unfortunately, and it can be easily solved, of course.
But if you guys want to sleep well and be able to connect your amps to everything without worries, use input coupling caps.

Everyone seems to agree that input coupling caps are the answer to my current problem. I'm glad that opinions do not seem to vary widely because I don't really understand the details of what I'm doing. I'm following detailed instructions very carefully.

As I understand it, I want a capacitor in series with the input signal from the RCA in to the PCB with value of between 2 and 20 uF. Is that correct so far?

If this is correct, what effect will that have on sound quality and to optimize this what kind of caps should I use?

Peter, you suggest that I check for DC offset before hooking up speakers while switching sources. Does this mean to hook up a 10 ohm resistor like I did to check DC offset initially and to measure while I'm switching with the boom box from say CD to radio?

I did this and the DC reading began to fluctuate wildly going from positive to negative in the hundreds of mV range. I expected it to stabilize at a much higher value but it did not. I wasn't sure what that means and I am still not sure in light of your comments. I'll do this again tonight after the kids are in bed and see what happens.

Is there any reason to not use input coupling caps? As carlosfm suggests, this will let me hook up my monoblocks to any source while keeping my speakers safe. Is that correct?

Why did music play and the speakers not burn up until I switched sources? Does the fact that there is signal coming through affect the DC offset? I measured with my multimeter with the source playing and I don't remember what the reading was but it didn't seem to be out of the "safe range" whatever that may be.

Finally, no I don't insist on using the boom box, but I didn't have a source with a volume control that I could use.

The boom box has a horrhendous hiss. Will I have better luck (quieter signal) if I get a headphone jack to RCA converter and run these off my Yamaha HT receiver headphone output? I'm going to build a Bottlehead Forplay but not until after Christmas (ho ho ho).

I thank you all most sincerely for replying and being so very helpful. I would not have begun this without such a great resource as this forum and I would not know what to do without your help.
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Old 18th November 2004, 04:46 PM   #8
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Default Re: Re: Re: Dual Mono Gainclone Issues

Quote:
Originally posted by cytokine



As I understand it, I want a capacitor in series with the input signal from the RCA in to the PCB with value of between 2 and 20 uF. Is that correct so far?


Woops. That should read between 2 and 10 uF.
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Old 18th November 2004, 09:42 PM   #9
troystg is offline troystg  United States
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Hi-

Buy a $5 50K Ohm pot from rip shack and use that as a passive volume from a better quality source until you can build what you want.

It will sound WAY better and not kill anything.

I actually saw a person use a pot in a medicine bottle for a pre.

He drilled the bottom and put his pot in with female leads hanging out of the opening and then screwed that down to a piece of wood. Worked like a charm.

But boy was it ugly... Almost as ugly as me.
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Old 19th November 2004, 01:59 PM   #10
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I actually already have a couple of 10K pots that I got with the rest of the stuff I bought to do this build. I decided that I wanted to leave them out since I have chosen to build a pre-amp later. I may go ahead and wire them in anyway without actually mounting them to the chassis as you suggest. I don't want anything but the LED on the front face of the chassis.

I got a couple of 10 uF caps from Rat Shack last night and wired them in line between the RCA in and the IN on the PCB. They effectively protect the test drivers from the switching and burning situation I had earlier. The drivers now just cave in or extend out for a little while and then return to their resting position.

I think I have other problems. I'm going to do an extensive search and start a new thread if I don't find what I'm looking for.
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