Volume Adjustment Thump Troubleshooting

[Hypnoglitz]

I'm experiencing a low frequency persistent thumping from my speakers and a significant intermittent thump from my subwoofer when adjusting the volume to my preamplifier.

It began a few days ago when I changed out the output coupling capacitors to my dac. I did so in hopes of improving the clarity to my system, I changed the value from a 2.0uf to a 3.9uf. I changed the capacitance value due to the recommendation of a vendor, as my premaplifier has an input impedance of 10k, and it was recommended that a higher value capacitor may be better for my system. I found similar information using the V-cap calculator, as it indicated an optimal capacitor would be much higher than the stock value. I settled with the 3.9 value due to space constraints.

Upon changing the capacitors, I immediately heard the thumping in the system. Once the volume was changed, the sound was very good. However, the volume adjustment noise bothered me so I investigated my connectors. This led to what I thought was a discovery, as I noted the left rca output of the dac was creating a noise through my speakers when disconnecting and connecting. I followed up by purchasing a new set of rca jacks, I soldered them in tonight, and I'm still experiencing the thumping when adjusting the volume.

Curious what I should be looking at for a fix? Is this a problem with my preamplifier's volume control? Or is it the dac? Or something else entirely, perhaps?

Any thoughts would be greatly appreciated.

 

[ubergeeknz]

It's most likely the capacitors, since it started when you changed them. Did you use the same type of capacitor as the ones which were installed? Why did you change them in the first place?

 

[Douglas Blake]

I'm experiencing a low frequency persistent thumping from my speakers and a significant intermittent thump from my subwoofer when adjusting the volume to my preamplifier.

This started when you changed those capacitors.
If you have a voltmeter check for DC on the outputs of your DAC.

It's possible you have one or both in backwards. Generally for an output you wan the negative pole of the capacitor on the connector side, not the electronics side.

 

[Mark Tillotson]

Some sort of motor boating is happening or an unintended relaxation-oscillator has been created.


What kinds of capacitor are involved? What is the DC voltage across the capacitor? More details should provide clues.

 

[Hypnoglitz]

I changed the capacitors to improve clarity, my system was sounding a touch dark. The capacitors I put in are Jantzen Superior 3.9uf, 800vdc. From what I understand, the capacitors are not polarized. I did orient them both in the same direction, per the label writing. I removed the capacitors and checked the capacitance, not sure how to check the DC voltage...is that done in circuit?

 

[Mark Tillotson]

And the ones you replaced were what?
DC voltage is measured in circuit.

Those monster film caps claim to be low inductance, yet they are huge wound types so they will have complicated impedance v frequency curve simply due to the length of the foils.

Very large caps can pick up interference or feedback due to the large stray-capacitance of the outside of the thing.

Its a shame there isn't a proper datasheet for the caps, their self-resonant frequency, ESL and ESR are all important to know, and the polarity isn't given (large caps outer foil should be connected to ground if possible, or the lowest impedance side of the circuit if not.)

 

[Douglas Blake]

I changed the capacitors to improve clarity, my system was sounding a touch dark. The capacitors I put in are Jantzen Superior 3.9uf, 800vdc. From what I understand, the capacitors are not polarized. I did orient them both in the same direction, per the label writing. I removed the capacitors and checked the capacitance, not sure how to check the DC voltage...is that done in circuit?

I looked up those caps... they're not for PCB circuitry. Most likely they're for crossovers or industrial use.

I would strongly suggest you put back the original caps and see if your problem goes away.

 

[Hypnoglitz]

Interesting thoughts. I'm likely going to backfill with the old ones. They were proprietary, made by the dac manufacturer. Regardless, I'm still scratching my head, as these dacs, made by MHDTLabs, have hundreds of online pages of modding enthusiasts doing the exact same thing but with satisfactory results. I can't help but feel the change exposed an existing weakness but don't have proof of such a thought.

 

[ubergeeknz]

It seems that you have DC somewhere and the level is changing when you adjust the volume.

Apart from swapping out those caps, did you make any other change? You haven't accidentally bridged anything with solder or something?

I'm also not sure I understand why changing a 2u output coupling cap to 3u9 could fix a "dark" sound. It seems like it would only improve deep bass response.

 

[Douglas Blake]

Interesting thoughts. I'm likely going to backfill with the old ones. They were proprietary, made by the dac manufacturer. Regardless, I'm still scratching my head, as these dacs, made by MHDTLabs, have hundreds of online pages of modding enthusiasts doing the exact same thing but with satisfactory results. I can't help but feel the change exposed an existing weakness but don't have proof of such a thought.

One possibility is that your setup has some unique property that exposed the fly in the pudding. Of course, with the low levels of skill prevalent on the net, it's entirely possible others have the problem too and don't even know it.

 

[Hypnoglitz]

Ubergeeknz- The higher capacitance was a recommendation from a vendor and I found supporting information on another site....so I figured I would go with the higher value, the space was there. I went with Jantzen as I read a handful of testimonies that seemed in par with what I was seeking and their price when on sale seemed okay.

Douglas Blake- I get your drift, completely.

A shot in the dark but I read a similar experience by another listener that found a fix by replacing a signal relay. My device has a NAIS TX2-5V ATX 209 low signal relay just before the rca jacks, might this be the culprit? Admittedly, I have no idea what this component actually does.

 

[sgrossklass]

Relay is probably for output muting.

Could you show a picture of the DAC PCB in original form? What model DAC is this anyway?

2 µF seems like an odd value for an output coupling cap in a DAC. Usually I would expect dozens if not hundreds of µF, electrolytic.

All of this smells quite esoteric to me. I don't like esoteric.

 

[Hypnoglitz]

The dac is a MHDT Stockholm 2. This link MHDT Labs Stockholm 2 : A Review | Headphone Reviews and Discussion - Head-Fi.org shows the internals well. The capacitors that I changed were the large yellow ones in the upper right section of the frame.

 

[rayma]

There must be enough DC leakage in the new capacitors that the volume control's wiper is affected.
The specific uF value has nothing to do with this, although larger uF capacitors of the same voltage
will have more leakage. The relay just shorts the output during warm up.

 

[Hypnoglitz]

Swapped out the dac with an alternate. I could still hear the noise but it was much quieter. I also swapped out the preamplifier with an alternate passive preamplifier, no noise, and I really cranked it.

So maybe the trouble is with the preamplifier? My head is spinning from this.

 

[ubergeeknz]

Did you check for DC at the DAC output? There should be none (after initial settling time)

 

[Douglas Blake]

Ubergeeknz- The higher capacitance was a recommendation from a vendor and I found supporting information on another site....so I figured I would go with the higher value, the space was there. I went with Jantzen as I read a handful of testimonies that seemed in par with what I was seeking and their price when on sale seemed okay.

I can understand going up a standard value or two and maybe one step in the voltage ratings, with the same type of capacitor. Beyond that, I would quickly suspect my advisors were the same people who suggested charging iPhones in a microwave oven.

 

[Hypnoglitz]

Ubergeeknz- Forgive my ignorance but how exactly is the dc chceked? Ive checked the offset on my first watt clone but the rca jacks have binding posts. On my dac they're pcb jacks....just not sure how to measure them.

 

[Hypnoglitz]

Douglas- I get now that the vendor was perhaps out for more money. Lesson learned.

 

[ubergeeknz]

Ubergeeknz- Forgive my ignorance but how exactly is the dc chceked? Ive checked the offset on my first watt clone but the rca jacks have binding posts. On my dac they're pcb jacks....just not sure how to measure them.

Measure between the inside contact of the RCA socket and the outside (ground)