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Old 12th August 2003, 09:50 PM   #1
LaZarus is offline LaZarus  Netherlands
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Default Quad Clipping

The Opamp of my Quad 405 starts clipping when I turn up the volume to a normal listening level . A drop of the suply voltage is measurable with my cheap multimeter.

So I already figured out what the problem is, I just don't know the solution. Maybe someone is kind enough to help me out with this one.

Attached is the schematic of the PSU of the Quad.

Main PSU capacitors are replaced by some 10000uF/105V units.

When I raise the voltage of the PSU (by selecting a 220V input) the problem becomes less audible.

The supply voltage of the opamp swings around the +- 16.8 V

I think i need to change the value of the resistor and the value of the zener. Maybe someone can help me out with the correct values, or tell me how to calculate them. Or maybe someone has seen this problem before and knows a different solution.

Regards
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Old 12th August 2003, 10:35 PM   #2
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Assuming that your AD-something op amp draws substantially more current than the original LM301/TL071, it may be a good idea to replace the zeners with simple emitter follower series regulators, as advocated by Bernd Ludwig on several web sites. See, for example, the modifications part of http://www.audiocircuit.com/9121-sol...9121CMQUA.htm.
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Old 12th August 2003, 10:57 PM   #3
LaZarus is offline LaZarus  Netherlands
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Sorry for the mistake, it's an OPA134 that I placed there.

I would like to avoid to place the voltage regulators, which were proposed by Bernd Ludwig.

Mvg,
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Old 13th August 2003, 05:49 AM   #4
sajti is online now sajti  Hungary
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Quote:
Originally posted by LaZarus
Sorry for the mistake, it's an OPA134 that I placed there.

I would like to avoid to place the voltage regulators, which were proposed by Bernd Ludwig.

Mvg,

In this case You can reduce the 3k3 resistors to 2k2. Than test Your amp. If the clipping still exist, go to 1k8, than test...... But all the time check the temperature of the zeners, or change them to 1.3W type (I don't know BZX84C)

Sajti
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Old 13th August 2003, 07:53 AM   #5
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There is something funny going on. The OPA134's quiescent supply current is specified as 5mA maximum, which should be low enough when there is
(50V-16V)/3.3kohm=10.303030...mA flowing through the resistors. I'd only expect the switch on and switch off bumps to be worse than with the original op-amp; for this reason, I'd recommend using 1.5kohm, 1W resistors and 15V, 1.3W zeners.

Does the distortion get less when you add decoupling capacitors (10nF to 100nF) between the op amp's power supply pins and ground (in parallel with the zeners, but placed close to the op-amp)? Maybe the impedance of the zeners is too high for an OPA134, causing instability.

Did you perhaps substantially reduce the values of the feedback resistors of the op-amp? That might explain the increased supply current with signal. Did you remember to remove the compensation capacitor for the LM301A? I've no idea what that capacitor would do to an OPA134, but probably nothing good.
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Old 13th August 2003, 07:58 AM   #6
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One other thing, are you sure that one pin of each zener is grounded, like you've drawn in your schematic? Because of the funny output signal level limiting scheme, that is not the case in the original QUAD405. The QUAD405-2 does have one side of each zener grounded.
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Old 13th August 2003, 05:35 PM   #7
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mmmk get ready to laugh.......

What's a Zener?

What's an OPamp do?

And what are PSU caps? I've never heard the term before.....






Jeff
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Old 13th August 2003, 08:49 PM   #8
Asen is offline Asen  Bulgaria
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LaZarus

I don't know why you want to avoid Bernd Ludwig's regulators. Maybe you've got a good reason for that. I assure you that this mod works great, and there are no sonic deteriorations.

Regards
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