My First Build

[AndrewT]

An AC coupled amplifier using a single polarity supply and DC blocking cap in the output should have ZERO mVdc into the speaker/terminal.

What have you connected in error?

[benb]

Perhaps it's a leaky C7?

[AndrewT]

1uA across 6r5 results in 6u5V.
I can't measure that. It is essentially zero mVdc.

[benb]

I mean a REALLY leaky C7.

[Nurbit]

Originally Posted by AndrewT

An AC coupled amplifier using a single polarity supply and DC blocking cap in the output should have ZERO mVdc into the speaker/terminal.

I'm sorry, I really am quite thick, please excuse my ignorance but what do you mean by that?

I have the amp powered from an AC transformer but then it's rectified into DC before being fed into the amp.

.....or am I missing the point here?

On a positive note, when the amps are working, they sound absolutly brilliant. Even to the point that I've been going round listening to several other systems in my house and have found that they don't seem to sound quite as good.
I suppose the speakers make a difference but I'm only using some old ones I found

I've even started planning to case it up soon

[Nurbit]

I've done some googling.

Am I correct in thinking that it's AC coupling bit relates to the input source?
i.e we're inputting an analogue source so it needs to be AC.
If we were inputting a digital source, it would be DC coupled ????

Or have I missed the point again?

[danielwritesbac]

Originally Posted by Nurbit

I've done some googling.

Am I correct in thinking that it's AC coupling bit relates to the input source?
i.e we're inputting an analogue source so it needs to be AC.
If we were inputting a digital source, it would be DC coupled ????

Or have I missed the point again?

The MP3 player, CD and PC all have onboard DAC and are all analog at the headphone jack. However, an actually digital source fed into an audio amp sounds like a fax machine except louder and even worse. Please use the analog output jack, not the digital output jack.

DC is 0Hz, no signal on clean DC, although there may be plenty of current. Basically, you're doing AC coupling to avoid melting the speaker's voice coil. C7 is blocking the speaker fire, blocking DC offset. Apparently, C7 is a bit shorted and needs replaced.

That DC offset can happen from flopping about with all that duct tape or it can happen with voltage overrun (overcharge damage). Here's an example of a 63v 2200uF Nichicon: Nichicon PW Series 63V 2200UF Low lmpedance Capacitor

[john blackburn]

Hi Nurbit.

You may find this a little more understandable.

Capacitors

Your "perfect" audio signal is only AC but we all know things aren't always perfect. If any DC is passed into the amplifier input it will be amplified along with the signal. Once you push the DC into your speaker coils they will be held away from their normal resting point upsetting your sound and possibly burning the speakers.

You see the line on the linked page that says "They (capacitors) are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals"

So, if you pass your signal through a capacitor only the AC signal will pass through. The DC will be blocked preventing it from being amplified and keeping the magic smoke inside your speaker coils, always a good thing. If no capacitor (or other means of dc removal) is present the amp is considered to be DC coupled.

John

[Boscoe]

Try using a new cap with a higher voltage rating for C7 just to be sure?

[Nurbit]

AH-HA

I get it now Thanks Daniel and John.

I can't re-test the circuit at the minute because they're both getting measured up for their shiny new heatsinks.

I've got some different caps to try too but sadly none of them are new and I don't have an ESR meter to test them.

I will be using brand new parts for the next amp. I'm going to build a true gainclone next time using LM3875s and point to point soldering.... perhaps

Thanks for the help though guys, it's easy to get disheartened when learning this stuff, I appreciate all your answers