Oddity with NAD 3020A

[GlidingDutchman]

TIME OUT!

I am now totally confused by something a bit odd.

This morning, fiddling with the "scrap" 3020B pcb I have here I noticed that I missed something on my 3020A's board. (follow this thread for full explination: The "Volksamp" - NAD 3020)

I had noticed that the previous owner's DIY efforts did not only remove the fuse holder section but also severed a supply line to a small network connected to the output of the pre-amp section. This supply line feeds a very odd network that in turn feed the FET's [Q509 & Q510] (I think they act as a buffer stage) on the pre-amp output.

When I restored the connection the amp behaved slightly different but not in a good way. When powering on there is now a violent "zonk-buzzzz-hummmzzzz" that stays a while until it fades away.

The oddity is that the amp functioned just fine without this connection.

See attachment. Can anybody explain what this circuit does?

TIA

[analog_sa]

Looks like a mute circuit at the output. Cannot be sure without seeing more of the circuit but disconnecting it will probably improve sound a bit. Why it doesn't work is anyone's guess.

[GlidingDutchman]

A_SA... well, the circuit is basic. The thick black line coming from the left side that goes into the FET is the signal from the line-stage...

[analog_sa]

The question is where does the gate go?

[analog_sa]

Found the circuit. It is indeed a mute/delay switch. Short-circuiting will deliver a slightly better sound which is probably what the previous owner did.

[GlidingDutchman]

Originally posted by analog_sa
Found the circuit. It is indeed a mute/delay switch. Short-circuiting will deliver a slightly better sound which is probably what the previous owner did.

But now I want to pose the following question: What does it "mute/delay"? There is a terrable sound when the unit is switched on and the "plop!" sound remains when the amp is turned off.

What do you mean by "short-circuiting"? The previous owner didnt disconnect it... he probably never saw that he has cut it off. Should I connect the AC feed line to ground?

[jaycee]

Originally posted by GlidingDutchman


But now I want to pose the following question: What does it "mute/delay"? There is a terrable sound when the unit is switched on and the "plop!" sound remains when the amp is turned off.

It shorts the preamp output to ground, which mutes the sound. The nasty noise is probably coming from the preamp circuit which takes some time to stabilise on power up. The circuit would've muted for about a second on power up.

Originally posted by GlidingDutchman
Should I connect the AC feed line to ground?

NO - if you do that, you will blow up the transformer. If it's been removed, leave it as it is.

A better solution would be to add a DC Protect circuit between the output of the amp and the speaker terminals, which has a power on delay. It doesn't look like theres enough room in the case for that though.

[analog_sa]

It doesn't appear to short the output to ground which would have been a better solution. The "switch" is in series with the signal.

[analog_sa]

Originally posted by GlidingDutchman


The previous owner didnt disconnect it... he probably never saw that he has cut it off.

Difficult to say who did what and when. It appears that the mute circuit was working properly before the "repair", so something is not right.

[GlidingDutchman]

Originally posted by analog_sa
Difficult to say who did what and when. It appears that the mute circuit was working properly before the "repair", so something is not right.

My suspicion as well.