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DIY Tube Preamp causing Power Amp Protect Mode
DIY Tube Preamp causing Power Amp Protect Mode
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Old 23rd August 2019, 06:27 PM   #31
leadbelly is offline leadbelly  Canada
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Quote:
Originally Posted by jean-paul View Post
It is that "if" One will definitely learn after the first set of burned woofers (by accident, by a partner or child). A muting circuit costs way less than a set of woofers, adds convenience and removes a risk.
Your position is the "if", not mine. I troubleshoot to actual observed symptoms.
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Old 23rd August 2019, 06:30 PM   #32
jean-paul is offline jean-paul  Netherlands
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We can get caught up in semantics but a muting circuit prevents damage at a very low cost and protects valuable devices. The constant need to focus on the "right" power up and power down sequence is not convenient.

Your troubleshooting to actual observed symptoms should tell you the same. I repaired many amplifiers and speakers because of the lack of protection circuits.

Besides that, who is worried about 10$ extra cost that takes away risk and has no negative side effects? It differs with various devices but experience tells that exactly the device that has no bad power up or power down phenomenons is the one that fails one day taking away the woofers.

I aks myself why there alway seem to be the need to explain simple good practice items like muting circuits, gate stoppers, input filters etc? Any mass produced quality device has those.
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Last edited by jean-paul; 23rd August 2019 at 06:37 PM.
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Old 23rd August 2019, 06:37 PM   #33
leadbelly is offline leadbelly  Canada
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I'm not against a muting circuit, but if that's all the OP does it simply won't fix the problem he's having. It shut down after 30 min.
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Old 23rd August 2019, 06:39 PM   #34
jean-paul is offline jean-paul  Netherlands
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True but my gut feeling tells me that when that problem is solved (probably oscillation) the other described phenomena are waiting around the corner. Adding the suggested input RC filering might be of some help in solving the issues.

It would help tremendously if OP replies to posts.
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Last edited by jean-paul; 23rd August 2019 at 06:43 PM.
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Old 23rd August 2019, 06:51 PM   #35
joshvito is offline joshvito  United States
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I ordered parts yesterday to add
  • input filtering caps
  • check for oscillation
  • add screen stopper resistors

I will report back once I have ability to modify and/or test the circuit.
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Old 26th August 2019, 11:55 AM   #36
DF96 is offline DF96  England
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Have you considered the suggestion I made about mains voltage variations getting though the PSU and the preamp? There is a frequency band around 0.5Hz which is not attenuated much by either of them. To solve this problem you would need a bigger cap in the PSU or a smaller cap in the preamp, so the passbands no longer overlap.
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Old 26th August 2019, 01:55 PM   #37
joshvito is offline joshvito  United States
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How much bigger of a cap in the power supply? Which one to replace, the 40uF? or the 120uF? Are you talking about lowering the 4.7uF output caps in the preamp? To what value? 2.2uF? The stereo is on a dedicated circuit, plugged into a Furman (EMI,RFI,Surge).
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Old 27th August 2019, 03:24 PM   #38
DF96 is offline DF96  England
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Dedicated circuit, mains filter etc. will do nothing to protect you from mains voltage variations. If your mains goes from 110V to 115V then the supply rail voltage will increase and some of this increase will appear at the amp output.

The PSU cap to make larger is the 1uF feeding the valve grid. Alternatively (or in addition) reduce the 4.7uF output cap. The aim is to ensure that there is no overlap between the passbands of the PSU low pass filter and the amp high pass filter.

You could do a check. Attach a meter to the supply rail and see if the DC voltage varies by much. You may need to watch it for a few minutes.
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Old 28th August 2019, 01:50 AM   #39
joshvito is offline joshvito  United States
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The supply voltage does vary by 1-2 vDC on the auto range meter. I have a 2.2 uF cap I can add in parallel with the 1uF for the PSU low pass filter. Still waiting on some parts to check for oscillation.
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Old 28th August 2019, 08:20 AM   #40
Mark Tillotson is offline Mark Tillotson
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If the output network was 1uF and 47k load, the subsonic cutoff would be about 4Hz, not 0.03Hz, at the expense of slight gain reduction.

Adding a bidirectional TVS diode across the output would tame the voltage excursions on turning on and off to levels that won't blow up a front-end. Without this the existing output stage can source upto about 50mA or so at upto 220V, not semiconductor safe.
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