Offset nulling - kind request of advice

[beppe61]

Dear Sirs,

while any amp has a bias trimmer it seems to me that a lot of amps do not have an offset nulling trimmer
Am I wrong ?
In general, how much offset at the speakers terminals is usually acceptable ?
And what can I do if the offset is higher and there is no nulling trimmer ?

Thanks and regards,

beppe

 

[gmphadte]

ETI 480...

For this amp, an output offset of more than 50mV is considered as fault. At least that is what the site (don't remember the website) on the net says.

If the amp is newly constructed, the offset will be high if the LTP pair is not matched.

The symasym needs three pairs of matched transistors and offset will be low if followed.

Gajanan Phadte

 

[beppe61]

Re: ETI 480...

For this amp, an output offset of more than 50mV is considered as fault
At least that is what the site (don't remember the website) on the net says.
If the amp is newly constructed, the offset will be high if the LTP pair is not matched.
The symasym needs three pairs of matched transistors and offset will be low if followed.
Gajanan Phadte

Thank you very much for your kind and valuable reply.
Is there a "normal" value for an acceptable offset ?
Does a such value exist ?
Can you confirm me the fact that an offset nulling trimmer is rarely seen inside an amp ?
Excuse me, just one question.
What the LTP pair is ?

Kind regards,

beppe

 

[richie00boy]

Upto 100mV offset is not a problem. This range is easily within the scope of a well designed amp, which is why such amps never feature a need to adjust the offset.

 

[beppe61]

Originally posted by richie00boy

Upto 100mV offset is not a problem.
This range is easily within the scope of a well designed amp, which is why such amps never feature a need to adjust the offset

Thanks very much for the very helpful information.
Now that I think more about it I have never seen an amp with a offset nulling trimmer.

Reading it otherwise an amp with an offset > 100mV should have some problems, like a bad component ?
Am I wrong ?

Thanks and regards,

beppe

 

[richie00boy]

Yes. Maybe a burned resistor or transistor causing improper performance.

 

[beppe61]

Originally posted by richie00boy [/i]
Yes.
Maybe a burned resistor or transistor causing improper performance

Thank you very much again. Very helpful to me indeed.
Clearly if offset were a real problem a trimmer for nulling it could be found somewhere.

Kind regards,

beppe

 

[gmphadte]

In a typical amp, a Long Tailed Pair abbreviated as LTP will be used. It can be made from BJTs or FETs. There will be two transistors in the simpler and most designs. These two control the total power amp. Any deviation in their parameters viz. Vbe and Hfe, will lead to improper error correction which leads to offset at the output in a functional power amp.

The ideal offset value is no doubt 'Zero' volts, but even if u do not match the LTP transistors, u will end up in offset of less than 10mV. One of my symasym with no matching of any transistors shows 6mV offset.

Attachment shows the LTP in Symasym.

Gajanan phadte

 

[beppe61]

Originally posted by gmphadte

In a typical amp, a Long Tailed Pair abbreviated as LTP will be used. It can be made from BJTs or FETs.
There will be two transistors in the simpler and most designs. These two control the total power amp.
Any deviation in their parameters viz. Vbe and Hfe, will lead to improper error correction which leads to offset at the output in a functional power amp.
The ideal offset value is no doubt 'Zero' volts, but even if u do not match the LTP transistors, u will end up in offset of less than 10mV.
One of my symasym with no matching of any transistors shows 6mV offset.
Attachment shows the LTP in Symasym.
Gajanan phadte

Thank you so much Sir for the very kind and valuable explanation.
Moreover I have learnt that a high off-set (>100mV) means is an index of something wrong inside the amp.
But a friend of mine is modding two identical mono power amps.
In one offset is 15mV in the other 35mV
Not to worry ?

Thanks again and kind regards,

beppe

 

[boholm]

FYI:

There is a third option: A DC servo, which is a circuit that holds the output at 0,000 volts - or close ;-). This can be made also with or without a trimmer.

 

[boholm]

But a friend of mine is modding two identical mono power amps.
In one offset is 15mV in the other 35mV
Not to worry ?

Not to worry! If the offsets do not change too much, but is fairly constant, there is no cause for alarm.

Do not be alarmed if you look at the voltmeter and see it change. If it changes a little bit up and then a little bit down - and keeps being around 15 / 35 mV, there are no problems.

But if one rises and rises and rises *or* falls and falls and falls (below zero volts and into minus ?? mV) then there is a problem.

 

[richie00boy]

Yes those differences are fine, it's just down to component tolerances.

 

[beppe61]

Originally posted by boholm

FYI: There is a third option: A DC servo, which is a circuit that holds the output at 0,000 volts - or close ;-).
This can be made also with or without a trimmer.

Thank a lot Sir.
Now that you mention it, I have an amp with that arrangement.

Not to worry! If the offsets do not change too much, but is fairly constant, there is no cause for alarm.
Do not be alarmed if you look at the voltmeter and see it change. If it changes a little bit up and then a little bit down - and keeps being around 15 / 35 mV, there are no problems.
But if one rises and rises and rises *or* falls and falls and falls (below zero volts and into minus ?? mV) then there is a problem.

I will check about this soon.
Thanks again for the very helpful advice.

Kind regards,

beppe

 

[beppe61]

Originally posted by richie00boy [/i]
Yes those differences are fine, it's just down to component tolerances

Thanks again.
I assume of course that the offset must be measured without any load (without speakers) with a mVmeter.
Kind regards,

beppe