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Budget passive preamp with stepped attenuator?

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Joe Smith

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Yeah. was testing my A07 yesterday and there's only noticeable channel imbalance at the very very lowest positions for the volume knob, using the 32v power supply. At the 8 o'clock position channel balance is fine. I think yes using a 24v/5a power brick would help if the main use case would be to play pretty softly.

I do enjoy having the digital volume control (and remote) on the SMSL AO100, it's perfect for my dining room setup paired with KEF Coda speakers where it is mostly used very softly for background during dinner or games... Hopefully more inexpensive digital volume controls will get implemented...
 
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However, I can't help but notice that after more than 10 full pages of discussion and thorough coverage of every single aspect multiple times, from rather amazing forum members sharing over and above, you've just went to the option you found yourself on ebay 8 minutes after posting about 3 days ago.
I almost always find my first instincts correct. My first instincts know what's right for me.

However, I also learned
  • Regular DC jacks do not have ground. To ground something through power supply requires 3-prong AC inlet or DC jacks that have 3 or more pins.
  • Medical power supplies minimize leaked AC current which may contain mains hum pointed out as the culprit for lowered SINAD of AIYIMA A07.
  • XLR minimizes noise by having a separate wire for ground. XLR is superior to RCA.
  • To minimize noise, I want XLR cables and direct connection to power supply's ground.
I think my audio knowledge is quite sufficient for now.
 
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Zek

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XLR is superior to RCA.
You probably mean balanced and unbalanced connection. Balanced connection helps if there is a problem with ground loop noise. If that's not a problem, an unbalanced connection is OK.
 
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I think clean medical power supplies can increase SINAD of audio devices by producing less high-frequency noises and allowing much less leakage current(mains hum).

My knowledge is that you should use a monitor that has a 3-prong AC inlet instead of a regular DC jack which has no ground. My computer monitor injects a lot of high-frequency noise into the entire computer system because its noise is grounded by HDMI cable. A 3-prong AC inlet behind a monitor would ground that garbage through an AC cable.

To minimize the entire system's noise,
  • If possible, use clean medical power supplies for audio devices.
  • Use a computer monitor that is powered through a 3-prong AC inlet. I'm going to buy one when I have more money.
    • Fortunately, a desktop computer is already powered through a 3-prong AC inlet.
  • Active PFC(power factor correction) tends to inject a lot of noise into the house wires. Use Passive PFC power supplies if possible. A little bit of inefficiency allowed by passive PFC can eliminate a lot of noise. If a power supply doesn't brag about active PFC, then it probably uses passive PFC.
 
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Grumpish

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Not really budget, but I use a Khozmo passive with separate L/R stepped volume controls. Order direct, you can specify what options you want, and what impedance you want, plus you are dealing with a real person at the other end.

https://khozmo.com/
 

Blumlein 88

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I've found with many pots, if you measure the total resistance in each channel and trim the low one by adding a small resistor to ground it usually tightens channel to channel balance though it will exhibit the most problems near minimum level.

What you want long term are conductive plastic pots. Like the RK271 series which btw, are available in a range of values. You might notice if you look up the specs even the best of pots show divergence as you reach -40db and lower. I've found good ones are usually better than spec, but I rarely use that level of attenuation for music.

Never used them, but TkD make some with an unusual design. Sort of a stepped resistor, pot combo for different than usual tapers. These aren't budget being something like $125 each stereo pair. Not knowing about them I did something similar once. I put resistors I could switch in and out in the center tap. This way I could have a nice gentle tapering for different volume levels. Then again I tried all sorts of things for a few years. Once I put together a 7 resistor matrix to ground I stopped all the other efforts.


1680396438792.png


Won't be answering any questions in this thread about it just here for some interesting ideas if any one cares.
 
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I've found with many pots, if you measure the total resistance in each channel and trim the low one by adding a small resistor to ground it usually tightens channel to channel balance though it will exhibit the most problems near minimum level.
There is no way I would work that much to fix a volume pot at low levels. I would just insert a stepped attenuator between DAC and amp. Find an attenuation step that I like. Fine-tune the system gain with DAC. Adjust application volumes. Done.

A cheap budget 21-step stepped attenuator can go a long way.
 

solderdude

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I think clean medical power supplies can increase SINAD of audio devices by producing less high-frequency noises and allowing much less leakage current(mains hum).

It could lower 'leakage related hum' problems with RCA cable connected devices with poorly designed ones or very crappy (read high resistance) return wires/screen or suboptimal PCB or wiring layout.
That does not change the SINAD of audio devices though.
It won't change self-noise nor change distortion at 1kHz (near) full signal or relative to 2V/4V.

Besides there is a difference between audible limits and measurable limits. As long as it stays below audible levels but increases in measured levels (because of external injected noise/hum) it still is irrelevant to music enjoyment.


re: potmeter vs stepped.

Also, what hasn't been discussed but is a technical advantage of stepped/relay is that a potmeter is never truly log type (also RK27). They are all semi-log (the affordable ones).
Basically the resistance tracks are 2 or 3 different (linear) ones merged together at some point giving a log-type attenuation.
Stepped and relay ones can easily be made truly log type (dB/step)
 
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It could lower 'leakage related hum' problems with RCA cable connected devices with poorly designed ones or very crappy (read high resistance) return wires/screen or suboptimal PCB or wiring layout.
That does not change the SINAD of audio devices though.
Reducing voltage from 32 to 24 allows me to turn up the volume pot a bit more. Going from 32volt to 24volt results in ~3dB attenuation.
So, I am going to buy a 24V power supply anyway to make the volume pot more useable for a stepped attenuator between amp and DAC.
Why do I not buy a cleaner one that will lower the noise floor by a few decibels? That's a good idea. Catch two rabbits with one stone.
 
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sonder

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Here's a couple of photos of the internals of two devices:

1) The NS-05P, this one has an ALPS 07 20k in it, linear pot, works great for XLR->XLR, XLR->RCA, RCA->RCA, and RCA->XLR is possible but degrades the signal, as you'd expect.

1680447984920.png


2) This is the HiFi 50k passive from nobsound/douk audio, it has a LOG 2x50k RK27 in it, and better CMC contacts.
1680448119579.png

1680448131552.png


General feedback, I prefer the experience of the RK27 for RCA to RCA, it's simple, well built, and the log volume control gives much finer grained control and is a nicer more tactile experience to use. However the 50k is definitely wrong here, it's like a muffler has been applied to the audio, and noise is amplified, if this had a 10k in it I'd /probably/ recommend it after trying, but certainly my the 20k, or dac control or stock pot on the pa3s perform considerably better from an audio perspective.

My takeaway is that I certainly prefer finer steps, or log, after being given the ability to fine tune volume to such an extent so easily, I couldn't go back to the stock pots on my current amps.
 
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solderdude

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Reducing voltage from 32 to 24 allows me to turn up the volume pot a bit more. Going from 32volt to 24volt results in ~3dB attenuation.
So, I am going to buy a 24V power supply anyway to make the volume pot more useable for a stepped attenuator between amp and DAC.
Why do I not buy a cleaner one that will lower the noise floor by a few decibels? That's a good idea. Catch two rabbits with one stone.
Is the gain of the Ayima powersupply voltage dependent ?
 
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Is the gain of the Ayima powersupply voltage dependent ?
I don't know. But, I read that lower voltage results in quieter volumes. Maybe, the same gain results in less amplification with less voltage.

According to TPA3255 measurements, reducing voltage from 32 to 24 halves the output power at 0.001% THD+N.
 
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Okay, going from 32volt to 24volt doesn't provide much attenuation. However, a medical power supply can still lower the noise floor by a few decibels.
We can see quite low THD=0.0014% and SINAD=86.74 dB and very good suppression of mains hum multiples.
SINAD of 86.74dB was achieved with regulated linear PSU at 30v. I think a cleaner power supply can definitely lower the noise floor.
 

JeffGB

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The gain of the Aiyima A07 is not dependent on the operating voltage. The operating voltage will affect the maximum power the amplifier can produce but the gain won't change.
 
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The gain of the Aiyima A07 is not dependent on the operating voltage. The operating voltage will affect the maximum power the amplifier can produce but the gain won't change.
If I reduce the operating voltage, will I hear quieter sounds at the same volume pot position?
 

JeffGB

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If I reduce the operating voltage, will I hear quieter sounds at the same volume pot position?
If you reduce the operating voltage the maximum power will be reduced and everything else, except longevity (it should last longer running at a lower voltage), will remain the same. That is if you are using a psu with equivalent quality to the stock supply. If you use a higher quality psu, with lower output impedance and noise, you will enjoy those benefits.

I have changed the psu for my Topping PA5 from the stock 38v one to the Aiyima 32v 5a one and the performance is the same except it has slightly less maximum power. It does run cooler though and that is what I hope will extend it's rather sketchy reliability.
 
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Do you mean the sounds would not be quieter at 9'o clock or 12'o clock with a lower voltage?
 

JeffGB

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Do you mean the sounds would not be quieter at 9'o clock or 12'o clock with a lower voltage?
That's correct. The gain of the amplier is determined by the amount of negative feedback and the gain of the individual stages, not the power supply voltage. If the power supply is better, it may lower the noise but not the signal.
 

solderdude

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If I reduce the operating voltage, will I hear quieter sounds at the same volume pot position?
No... hence my (silly) question.

A lower power supply voltage only results in less max. output voltage (power).
It does not change gain, noise nor anything else but max. output power.
Also a lower noise power supply will not reduce noise on the output of the amp by much (unless the amp has a really poor PSRR, not tested for).
The only advantage a medical grade PS could offer is less leakage current which would result in less 'tingling' when touching the amp when not connected to a (grounded) audio chain and it could result in less 'unwanted noises/hum in an RCA connection. I believe the Topping D10 is often mentioned when groundloops are present.
If that's the case then use an USB groundloop breaker.
 
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