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Old 24th September 2005, 12:49 AM   #31
Stabist is offline Stabist  Slovenia
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Quote:
Originally posted by chip_mk



Hi Stabist

My point is that in most cases DC offset can be adjusted by slight trim of R11 (+/- 20%). Of course, matching of Q1 and Q2 should be as good as possible. I don't recommend simple replacement of R11 with trim pot since accidental turn to 0 ohm can damage the amp. Instead you can use serial combination of 180E resistor + trim pot of 100E.

However, since you have provided measured values (24.8mA via Q3 and 12.8mA via Q1) I was able to calculate that instead messing with trim pot you can achieve decent correction simply by putting 6k8 resistor in parallel to the existing R11 (which is 220E).

BR

Chip

Gee ... I guess it's really time to match a lot of my own 9610 and forget about those "matched" ones ...

After adding 6k8 in parallel - I've only improved DC offset for around 5mV - but it is still somewhere around 40mV ...

First I'll check also the right channel and see what the numbers there will say ...

After that - I'll see how to continue ...
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Old 24th September 2005, 01:30 AM   #32
jleaman is offline jleaman  Belgium
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Quote:
Originally posted by Stabist



Gee ... I guess it's really time to match a lot of my own 9610 and forget about those "matched" ones ...

After adding 6k8 in parallel - I've only improved DC offset for around 5mV - but it is still somewhere around 40mV ...

First I'll check also the right channel and see what the numbers there will say ...

After that - I'll see how to continue ...

Id have to say if yor fet's are not mached this is going to keep happening
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Old 24th September 2005, 03:19 AM   #33
Stabist is offline Stabist  Slovenia
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Yep - time to match them on my own ...

Right channel even worse - aroun 50mV DC offset ...

All other numbers pretty much same for both channels - maybe just couple mV difference between channels ...

BTW - one Q about AC current ratio - if voltage drop on both source "power" resistors is same - is maybe that also an indicator of 50% ratio - and as "upper" resisitor goes to higher voltage than the other one - that means the ratio goes from 50 --> 100% or not??
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Old 24th September 2005, 03:22 AM   #34
Stabist is offline Stabist  Slovenia
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Oh btw - power fet were purchased in local store - 4pcs - no selection etc - and guess what!! 3 of them have VGS 4,60V and the fourth one 4,61 ... hehehe ...
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Old 24th September 2005, 03:47 AM   #35
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This is a lot of excitement over an offset voltage well within
spec. I suggest you kick back with a nice glass of wine and
enjoy the result of your labors.



Tell you what - I'll bless it over the Internet.

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Old 24th September 2005, 04:01 AM   #36
jleaman is offline jleaman  Belgium
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Quote:
Originally posted by Nelson Pass
This is a lot of excitement over an offset voltage well within
spec. I suggest you kick back with a nice glass of wine and
enjoy the result of your labors.



Tell you what - I'll bless it over the Internet.


Good Job Nelson
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Old 24th September 2005, 04:25 AM   #37
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Old 24th September 2005, 12:19 PM   #38
Stabist is offline Stabist  Slovenia
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Quote:
Originally posted by Nelson Pass
This is a lot of excitement over an offset voltage well within
spec. I suggest you kick back with a nice glass of wine and
enjoy the result of your labors.



Tell you what - I'll bless it over the Internet.


Hi Nelson,

You're more than right - it's in my nature to do all the time

Now I must complete AlephL and than join them into same chassis and so get a nice integrated amp which will , I'm sure of that

The thing that matters most is that the has now entered and another Aleph is born

Btw - is my conclusion about measuring AC current ratio via voltages on source resistors correct?
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Old 24th September 2005, 05:35 PM   #39
Stabist is offline Stabist  Slovenia
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uff, time out ...

Aleph L almost completed But I needed some timeout ...
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Old 24th September 2005, 08:35 PM   #40
chip_mk is offline chip_mk  Macedonia
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Quote:
Originally posted by Stabist




Btw - is my conclusion about measuring AC current ratio via voltages on source resistors correct?

Hi Stabist

Identical voltage on Source resistors indicate that the output MOSFETs are well matched and are biased by equal currents. Since they are devices of the same type, the same manufacturer, probably the same batch, you can be sure they will exhibit the same AC (dynamic) performance.

Now I would like to correct what I've wrote in one of my previous postings. Adding a source resistor to some of the fets of the input pair in order to correct DC offset does not introduce asymmetry in the transfer curve of the input stage, at least not at lower frequencies when the impedance of the constant current source (Q3) is very high and ignorable. You said the current through Q2 is about 12mA, so adding a source resistor 3E3 to Q2 will reduce DC offset for about 40mV without worsening the current distribution. However, source resistors do decrease open loop gain of the amplifier and the loss would be more than 1/3 if you add the mentioned resistor. So matching is still the best way to go.

In mid '80s my neighbour used to own early Stabi platter with Grace tone arm and Supex pickup. I was about to buy Stabi in '87 but finilay decided I could not afford it. Anyway, my LP collection was/is pretty limited.

Lep(?) Pozdrav

Chip
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