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I have had those leading-edge audio thumps since the beginning of my QMX (v27 FW) as well and wondered if it was going to be fixed but it doesn't even look as though its on the list to get fixed. To me it's a big problem and would welcome a software change rather than hardware fix. I will post a YouTube video demonstrating the symptom when I have a chance.

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Myron WV?H

Hi Myron,

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Further up this thread, Hans has indicated he’s aware of the issue and it is on his list. It has affected me too since the beginning of my QMX revision 1. ?The issue has been raised in the group from time to time, but not many users have complained of it. I think that has slowed down the process to find a resolution. Hans has indicated he’ll look into it once the SSB development is complete. In the interim, I intend to apply this cap fix.?
John

Hans knows the issue and the fix.? It is on his list and not forgotten.?

He removed most of the pulse shaping on the sidetone while addressing a different issue, which resulted in the 'click' part of this problem.? He just needs to restore that shaping.

The DC bias 'thump' issue is in hardware, and requires the larger bias capacitor we have described, or a Vin source that can't droop during tx.?

Stan KC7XE?

Here is the video of my popping QMX.

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Myron WV?H

Exactly what my 3 QMX and my QMX+ sound like! But as mentioned before, the popping sound gets better with increased sidetone volume and Hans will take care of it.?

Hi,
As discussed in previous threads, the likely cause of the low frequency thumps on the sidetone audio output is due to the voltage change on the headphone amplifier bias signal during transmit. The bias signal is derived from the 12V rail. Increasing the filter capacitor C430 seems to provide an improvement as shown by Matthew (M0MZB) and Stan (KC7XE).

In general, the effect is likely to appear worse if the QMX audio output is set to a low level and driving into a high impedance load such as an active speaker or PC sound card (see notes below).
73, Dave

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Assuming QMX is intended to drive headphones, with a nominal impedance of around 32?. With the 10uF capacitor on the QMX output (C420 & C421), this equates to a high pass cutoff frequency of 500Hz.

If the output is connected to an AUX input on an active speaker or PC sound card input, the input impedance is going to be in the k? range. Assuming a 1k? impedance, with the 10uF capacitor on the QMX output, this equates to a high pass cutoff frequency of 16Hz. In this case the low frequency thumps will not have as much attenuation and will be more noticeable.

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A few basic simulations to demonstrate this are shown below.

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Simulation with 32? load: 900mV dip on 12V rail during transmit, Bias filter C430 = 2.2uF (default)

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Simulation with 1k? load: 900mV dip on 12V rail during transmit, Bias filter C430 = 2.2uF (default)

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Simulation with 1k? load: 900mV dip on 12V rail during transmit, Bias filter C430 = 2.2uF//100uF

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