Tweaks for NAD 3020

[planet10]

Originally posted by djk
Better parts, OK, but don't mess with the values.

Except the C for the output stage.

dave

[sam9]

"How can you give advice about a circuit you don't know what it even does?"

I've seen caps in series with the input before. Normally they are simply for AC coupling. But I thought I wrote the comment with enough uncertainty to express cautuion if need be.

A 1uF cap in series with the input will roll off the low frequencies well into the audio range. This demonstrated by somevsimple arithmetic, by a simple spice simulation and by listening. In equipment from that period, this (low value AC coupling caps) seems pretty common. I'm guessing the reason is that the primary signal source was a turntable. Although very good for its price an NAD 3020 was still a budget item, and likely to be teamed with a budget turntable. Thus rolling off lows like this supressed "rumble" , and as there was less LF content on the typical LP of the day than CDs or modern vinyl, little hard was done. I suspect it was also justified by the expectation that speakers that were likely to be teamed up with the NAD didn't go too deep as well.

I presume the posdter will be listening to CDs so the effect of the low values caps would have a greater impact on him than they would in 1980. In addition to the above, a 1uF electro at that point does more than just roll off the lows, it will add distortion as high up in the spectrum as 100Hz. This isn't just "capacitor lore", you can demonstate it with SPICE and it can be measured. Individual people vary as to whther they can hear it.

[cpemma]

Originally posted by planet10
There are a number of revisions of the 3020 so your unit may not exactly match the scematics you find... i have a pdf of the older units service manual.

dave

I'd be grateful for a copy. I've no firm idea of the build date, it was bought s/h in '82, the manual is dated 1980. It does have MJ29655 PNPs, not the 2N2955 shown on the schematic, I've not compared any deeper than I can see through the grille. My main thought is to replace the electrolytics, which must be past their sell-by date, though there's no serious problems with hum.

[planet10]

Originally posted by cpemma
I'd be grateful for a copy.

I'm uploading it now. Mail is on the way with the URL... anyone else want it, let me know as i can only afford the 10MB of disk it uses for a short priod.

dave

[djk]

"I've seen caps in series with the input before. Normally they are simply for AC coupling. But I thought I wrote the comment with enough uncertainty to express cautuion if need be."

Have you ever seen an active filter?

Do you know how to figure the frequency and Q?

Blindly messing with the RC time constants affects the PRAT of the amplifier.

"Except the C for the output stage."

There are limits to that too. How big do you think you can go before the rectifier will blow? Do you know how to calculate the % of ripple? What is the point of diminishing return? Considering that the voltage gainstages of the PA and the first current gain stage run off a fully regulated power supply, hows does that change things?

[jleaman]

Originally posted by planet10


I'm uploading it now. Mail is on the way with the URL... anyone else want it, let me know as i can only afford the 10MB of disk it uses for a short priod.

dave

Give me the link to the file. ill put it some where for every one..

J

[planet10]

Originally posted by djk
There are limits to that too. How big do you think you can go before the rectifier will blow? Do you know how to calculate the % of ripple? What is the point of diminishing return? Considering that the voltage gainstages of the PA and the first current gain stage run off a fully regulated power supply, hows does that change things?

There are, but before i knew better, in the late 70s, i stuck 2x 68,000 uF on one of these. That amp is still running today, with no need for repairs yet. And it did make a BIG difference.

dave

[phase_accurate]

Any suggestion why there was potential on the chassis?

How did you measure this ? Maybe it is just capacitive coupling between primary and secondary of the transformer and therefore harmless. But it could also be a dangerous insulation failure.
How does it behave when you turn the mains plug by 180 degrees ? How much current flows from the chassis to earth when you measure with a multimeter (use a series resistor to avoid fireworks or worse things !!!) ?

Regards

Charles

[NamWong]

I would like to have a copy of the service manual. Please let me know where I can download it.

Thanks.

[jleaman]

http://wingzero.ath.cx/jleaman/NAD_3...viceManual.pdf