Is the EXAR XR-084 quad bi-fet opamp any good? - Page 3 - diyAudio
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Old 22nd March 2012, 01:06 AM   #21
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SMPS is more complicated than all 3 amp sections.
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Old 22nd March 2012, 02:57 AM   #22
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Originally Posted by cyclecamper View Post
Measure voltage across R14 with no signal?
Yes, without any signal.

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Interesting that that point CSA goes back to the power supply...near the "V0 trim" and "VTH Trim" pots. Is this protection circuitry or remote-sensing for the regulated supply?
The CSA is not part of the protection or regulated supply. It has to do with the Fan Speed.

The 4 transistors on the heatsinks and one hidden inside the power transformers sense temperature. When temperature increases, fan speed increases. CSA senses current (CS=Current Sense). When more current passes through the mosfets, the fan speed also increases.

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Looking at the device spec sheets, what's the critical "turn-on voltage" spec called? Is that "Gate-Source Cutoff Voltage"
min -0.15 max -1.45 V for 2SJ56
min 0.15 max 1.45 V for 2SK176
Yes, it's just a fancy name. The mosfets need a certain voltage at the gate to conduct (ie. switch ON). Below that voltage, the mosfets are OFF. For the 2SK176, it ranges from 0.15 to 1.45V. For simplicity, let's take the upper limit. Below 1.45V (measured from Gate to Source), the mosfet is OFF. To switch it ON, the Gate Source voltage must be above 1.45V.

I must add that in biasing the mosfets, you don't look at the this Gate Source voltage. You measure the voltage across R14. That will tell you the amount of current passing through the mosfets.
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Old 22nd March 2012, 03:30 AM   #23
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Should I adjust pot for 0 V DC at the speaker terminals after replacing big power supply caps?
If the caps are still good, there's no need to replace them.

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What does "VTH TRIM" likely stand for?
TH=THERMAL. It's part of the fan speed circuitry. There's no need to adjust that.

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SMPS is more complicated than all 3 amp sections.
The power supply is not Switch Mode. It is a Regulated Power Supply. Instead of using transistors for regulation, 4 SCR are used. By controlling their gates, the SCRs pass current when the voltage is at its peak. It has to do with conduction angle and phase, hence the term PHASE CONTROLLED REGULATION.
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Old 22nd March 2012, 08:49 PM   #24
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Thanks a million, this really really helps me start to understand the schematic and look deeper.

I meant, is the "0 V" pot for minimizing DC offset at the speaker outputs?

Yes, it's regulated, I assume by modulating the duty cycle during which the SCRs conduct. That's what a SMPS is, with several possible variations. The SCRs are on or off, rather than "ohmic losses" burning excess as heat, so it's a switch-mode regulator, and a SMPS is any poser supply with a switch-mode regulator. It's probably not tuned with fancy resonances to get more power from the inductive transformer at additional phase angles, and it doesn't have a fancy controller chip. but it sure seems to be a very basic SMPS.
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Old 22nd March 2012, 11:58 PM   #25
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I meant, is the "0 V" pot for minimizing DC offset at the speaker outputs?
The "VO" Trim Pot does not stand for Zero Volts. It is not for minimizing DC Offset. There are no provisions in the amp for DC Offset adjustment. "VO" Pot has to do with "VTO" (U1D output).

Looking at the top left corner of the first page, you'll see the 'VTO" and "VCA" inputs to U1A.

When the Compressor is switched to "ON", the compressor (LDR1) will activate when the amp clips. Compressor trigger is via 'VCA" input of U1A.

When the switch is "OFF", VTO is now connected to U1A. My guess is apart from the compressor activating when the amp clips (VCA input), it will also activate when the heatsinks or power transformer hits a certain temperature.

The action of VTO is different from VCA. The "VCA" compressor action is dependent on the amp clipping (ie momentory). VTO, in contrast, shuts down the input when the heatsink/transformer overheats. That means no signal can enter the amp until it cools down.
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Old 23rd March 2012, 01:29 AM   #26
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I am so grateful...now more of the schematic is starting to make sense.
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Old 23rd March 2012, 02:17 AM   #27
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Glad I could be of help.

The schematic appears complicated but if you strip away the features, the amp itself is not so daunting. Page 3 is the voltage gain circuit and Page 4 is the current section.

It's a shame they are no longer in production. They made quite an impression on me when I auditioned them in the 80s.
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