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Old 20th January 2013, 01:53 PM   #21
DF96 is offline DF96  England
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Both AC and DC voltage are measured in V, mV etc. In a typical amp AC appears in two forms: power source and signals. The AC voltages specified at the test points will be so that you can trace a signal through and check that it is being amplified as it should, starting with 5mV at the input TP7.

The DC voltage at many of the test points should be small, as it arises only from opamp offsets. The exact value won't matter, provided it stays small (few mV or 10's of mV).

For other test points such as TP5 and 6 the DC value is what matters. These two measure the supply voltage to the opamps. The rails are +-16V because of the presence of the zeners. No zeners means +-42V, which would probably blow the opamps.
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Old 20th January 2013, 05:37 PM   #22
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Default Test Points

DF96

If I understand correctly these test points are not checked with the AC setting of my DVOM? I set to mV setting and check see what I get.

The values shown in the TP balloons are when I am running a signal, like an "test signal" or hooking the guitar up and plucking a note? And I assume that would involve using something that recognizes the signal being generated so as to trace or follow it through the circuit?



Quote:
Originally Posted by DF96 View Post
Both AC and DC voltage are measured in V, mV etc. In a typical amp AC appears in two forms: power source and signals. The AC voltages specified at the test points will be so that you can trace a signal through and check that it is being amplified as it should, starting with 5mV at the input TP7.

The DC voltage at many of the test points should be small, as it arises only from opamp offsets. The exact value won't matter, provided it stays small (few mV or 10's of mV).

For other test points such as TP5 and 6 the DC value is what matters. These two measure the supply voltage to the opamps. The rails are +-16V because of the presence of the zeners. No zeners means +-42V, which would probably blow the opamps.
I thought R144 and R145 were what drops the 42V to 16?
If TP5 and 6 are what drops the voltage that means its being dropped to 16 into the ground rail? I don't get it. Sorry, I'm a novice.

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Old 20th January 2013, 05:49 PM   #23
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the power amp starts at Q8-9, there is an opamp right before it that drives the input diff pair.

TP means "test point".

Those resistors 144/145, do not "step down" anything by themselves.
They form a "divider" with the two zener diodes.
The zener diodes set the voltage.
The resistors limit the current, and also prevent too much current from passing through
the zener diodes, so they don't get too hot and blow up.
There are other ways to use a zener to handle more power, but that's something to look up.

IF those resistors are getting really HOT, then something may be drawing too much current from
one or both rails. You can check the current draw by measuring the vdrop across each resistor
and putting into ohms law, find the amps.

The mV figures along the way in the amp refer (most likely) to the test condition spec'd on the schematic,
with an AC signal input at a spec'd freq. See note 6. on the schematic.

Ok?
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Last edited by bear; 20th January 2013 at 06:01 PM.
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Old 20th January 2013, 06:05 PM   #24
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Quote:
I thought R144 and R145 were what drops the 42V to 16?
The zeners regulate the voltage to the values shown at TP5 and TP6. R144 and R145 must drop whatever voltage exists over those test point values; in that respect your above statement is correct. The resistors also limit current through the zeners.
Quote:
If TP5 and 6 are what drops the voltage that means its being dropped to 16 into the ground rail? I don't get it.
Ground is not defined as the lowest voltage in a circuit.* This fact is especially appropriate for an audio amp, and reasons why should be easily determined from the article you linked.
I skimmed the article yesterday and will read it more thoroughly, but at this point I honestly couldn't characterize that last page as anything less than bizarre. But one bad apple blah blah blah

*mathematically speaking
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Last edited by sofaspud; 20th January 2013 at 06:07 PM.
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Old 20th January 2013, 06:42 PM   #25
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Bear, Sofaspud

Well thank you for explaining the relationship between R144.145 and the zeners .

Sofaspud, speaking of the last page of the article. Yeah, he got quite inventive with his analogies to say the least...


By the way, the name sofaspud is quite cute... lol
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Old 20th January 2013, 08:01 PM   #26
DF96 is offline DF96  England
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You can check AC signals with a DVM. Most have a 200mV AC range. It may be less accurate at very low or very high frequencies, but should be OK over the normal audio range.

To understand how zeners are used, first you need to understand potential dividers. To understand those, you first need to understand Ohm's Law. To understand Ohm's Law, you first need to understand voltage and current. Think of electronics as an upside-down pyramid, with things like Ohm's Law and a few other principles supporting a huge expanding structure above.
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Old 21st January 2013, 04:08 AM   #27
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While cold testing the diodes on my suggestion, he found that some diodes don't show the forward voltage on diode test and he ignored the suggestion to open one of the lead and check them, on another advice. And I stopped myself further.

Gajanan Phadte

Last edited by gmphadte; 21st January 2013 at 04:10 AM. Reason: sentence formation
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Old 21st January 2013, 05:16 AM   #28
qusp is offline qusp  Australia
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not sure if it was corrected, yes tubes are not complimentary devices, but solid state circuits can run on a single rail power supply as well, having a +0 -(bipolar) power supply is not something I would use to differentiate between tube amp and SS amps, its another issue altogether.
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Old 21st January 2013, 05:19 PM   #29
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While cold testing the diodes on my suggestion, he found that some diodes don't show the forward voltage on diode test and he ignored the suggestion to open one of the lead and check them, on another advice. And I stopped myself further.

Gajanan Phadte
That is not true. I have replacement diodes on order. Just waiting for them to arrive.
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Old 21st January 2013, 05:54 PM   #30
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Quote:
Originally Posted by DF96 View Post
To be honest, it reads like it was written by an opinionated teenager.
Hey, you! To criticize what other people do is very easy, but why don't you make one better that that???
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