DC Offset Question

dsavitsk said:
Anyway, with a different source (my preamp this time), I am now back up to about 70mV.

With a source connected you will have the 22k input resistor to ground paralleled with the source's output resistor to ground.
So, depending on the source you connect to the amp you will have different DC-offset.
With an input coupling cap that wouldn't happen.
Your high(ish) DC-offset is also because the datasheet-recommended (and copied) resistor values are not a very wise choice.

dsavitsk said:
Is this a reasonable amount to run into speakers (B&W)?

A little high, but OK.
 
carlosfm said:
So, depending on the source you connect to the amp you will have different DC-offset.

The preamp (a modified version* of Nuuk's discrete buffer) has an cap (4.7uF) on the output and the pre shows no offset.

carlosfm said:
With an input coupling cap that wouldn't happen.
Your high(ish) DC-offset is also because the datasheet-recommended (and copied) resistor values are not a very wise choice.



A little high, but OK.

Where, and of what value, would an input coupling cap go -- just on the input bofore the boards?

Additionally, can you point me to a thread or a link with more appropriate resistor values?

* the modification was to add a 1uF cap and a 1K resistor on the input (after the pot). It seems that there is DC offset on the input, and when the volume is all the way down is shorts the circuit. These mods fixed it, however.

Thanks,

-d
 
dsavitsk said:
The connected source was off, and unplugged. I don't really understand your answer, so I am not sure if this matters.

Anyway, with a different source (my preamp this time), I am now back up to about 70mV. Is this a reasonable amount to run into speakers (B&W)?
Your problem is "input bias current" mainly. If you want to have an independent offset you must have a coupling capacitor or a DC servo. People thinks that a DC servo is pretty unneeded, it _is_ statisticly but some people will experience trouble with their LM3875 LM3886 designs because those IC's are not precision devices.

Unfortunately I haven't got the time to look up for material, nor make it myself but Carlos, Peter D, Brian etc are all experts in LM3875, LM3886 and offset trimming techniques and how you in theory should optimize the design. It's also pretty easy.

Besides trimming and inject current you should always strive to have the same DC resistance from both inputs, which is not the case when the amp is DC connected and you will use different signal sources.

Remember though, is this really a problem? When you have the unconnected, then 100 mV and a silent amp can't be a problem?
 
peranders said:
This is normal. You have a current flowing into the inputs. Take this current and let it go through the input resistor. This will create a voltage.

Datasheet says 0.2-1 uA. Take this current through 22 kohms => 22 mV times your gain which is 30 => 660 mV!

You seems to have around the lower limit in the current.

660mV -- consider this -- 1 watt into a nominal 8 ohms requires 2.63 VRMS -- most of our listening is going to be at low levels...

hmmmm, I think you mean to say that you have a pair of bias currents flowing FROM the opamp (LM3XXX) inputs through the source resistor causing a POTENTIAL between ground and the Opamp. The difference between the POTENTIALs on the + and - inputs is multiplied by the gain of the amplifier resulting in a DC OFFSET voltage expressed at the output.

precisely matching the resistors is a first step toward reducing DC OFFSET.
 
peranders said:
Remember though, is this really a problem? When you have the unconnected, then 100 mV and a silent amp can't be a problem?

Well, even connected it is still 80mV. Really, the problem is that the amp hums, quietly at first, but the longer it is on, the louder it hums, and within 30-40 minutes is really unacceptable. Perhaps this is not DC offset causing the problem, but then what is it?


jackinnj said:
precisely matching the resistors is a first step toward reducing DC OFFSET.

Resistors were precisely matched (they are mostly PRPs with a Kiwame for the feedback.) What do I look at next?
 
jackinnj said:


Could be heat --

It seems to happen even if there is no of little signal, that is, at low volume. Also, the chips are mounted to the chassis (with arctic silver) which is pretty thick aluminum. Touching the chassis there is essentially zero heat.


janneman said:
Your hum points to a mechanically humming transformer. Could that be the cause? mech noise from the transformer rather than hum from the speakers?

If it is a transformer, it is transferring the noise to the speakers as that is where the hum comes from. The amp box itself is very quiet.

I am installing some AC noise filtering this week (one of those filtering IECs) as the AC is very nasty here. I'll post again w/r/t/ whether that helps or not.

-d
 
I'm not convinvced by the other comments......gradual increase in o/p hum level is a trouble indicator....Generally SS amps should give o/p offset of around 20mV speaker connectoed or not..after quiescent current has settled down... the o/p impedance is already very low........100mV offset is high and will gradually cause LS coil offset due to flowing DC current. If amp hums from o/p there is something amiss in the electronics.....basically the ripple rejection in the early amp stages is pretty good. But it could be unstable and going into HF oscillation....
You shouldn't hear anything on the o/p's.

Are the supply rails holding up ?

richj
 
richwalters said:
I'm not convinvced by the other comments......gradual increase in o/p hum level is a trouble indicator....Generally SS amps should give o/p offset of around 20mV speaker connectoed or not..after quiescent current has settled down... the o/p impedance is already very low........100mV offset is high and will gradually cause LS coil offset due to flowing DC current. If amp hums from o/p there is something amiss in the electronics.....basically the ripple rejection in the early amp stages is pretty good. But it could be unstable and going into HF oscillation....
You shouldn't hear anything on the o/p's.

Are the supply rails holding up ?

richj

richj,

I follow you. What if the bias current gradually runs away to very high values, that could increase the hum gradually. The telltale would be a steadily increasing temp of the heatsinks, right?

Jan Didden
 
Yup your'e on the right track......the output stage class A current drivers get the juice from raw +/- supply rails....in a properly working amp these transistors will run hot drawing roughly 50mA but should stabilise. On cheaper amps a chain of diodes is often used (neg temp cooef) in contact with the heatsink to offset runaway. Other complicated circuits exist.

The other cue is to thoroughly examine the input diff end components. A failed tranny would give you complete failure so something is "fuzzy", a wrong value resistor or cap wrong w. round.

With DC-DC stage SS amp.......which is behaving weird, DON't put your best speakers on it.

The other poss is RF instability.......my simplest trick is a small portable radio tuned to the low end MW band waved over the amp and listen. YOu will get some buzz from the rectifier end but a circuit that is oscillating will give obvious results.

richj
 
richwalters said:

The other poss is RF instability.......my simplest trick is a small portable radio tuned to the low end MW band waved over the amp and listen. YOu will get some buzz from the rectifier end but a circuit that is oscillating will give obvious results.

richj

hmmmm, or bzzzzzzzzzz -- coincidentally this is the technique mentioned in an application note by Analog Devices (a long time ago).

that RFI from the rectifier arises from the diode's capacitance and the inductance of the transformer secondary...