Budget passive preamp with stepped attenuator?

[Deleted member 58865]

Is there any? I just want volume control for RCA cables without channel imbalance.

 

[JSmith]

Review and Measurements of Schiit Sys Passive Pre-amp/Switcher

This is a measurement and review of the Schiit Sys switcher and passive pre-amp. It retails for just $49 plus shipping which is reasonable. The unit is on loan from a member so not new. I will do a teardown later but for now, there are just two inputs, one output, and a volume control in...

www.audiosciencereview.com

I just want volume control for RCA cables without channel imbalance.

Not perfect... but very cheap;

PAC LC-1 Passive Volume Control Review

This is the review and detailed measurements of the PAC LC-1 passive attenuator. It was kindly purchased new by a member and costs just $9.50 including shipping from Amazon. I must say for such a cheap device, the RCA cables feely nice and supple. And volume control itself is smooth. Inside...

www.audiosciencereview.com

JSmith

 

[Deleted member 58865]

I found

FV3 SMD Dact 21 Stepped attenuator 50K Passive Pre-amplifier preamp HIFI audio | eBay

Each step use 1% SMD High Quality resistors. Default send input impedance is 50K.

www.ebay.com

I don't know how good it is, but it comes with stepped attenuator. With stepped attenuator, you don't have to think about channel imbalance. It's taken care of.

 

[Deleted member 58865]

As far as I know, all potentiometers degrade over time, and nearly all of them have noticeable channel imbalance.

Stepped attenuators are the way of the past, the present, and the future.

 

[Blumlein 88]

I'd strongly suggest you make the steps no more than 1 db. That is so close to continuously variable there is no need for smaller. 1.25 db might almost be good enough, but just make it 1 db per step and you'll be happy. 1.5 db is nearly enough, but is not quite small enough.

 

[sonder]

I have no idea on the stepped attenuator side, but in looking in to the same subject there are the nobsounds on the budget side, and this https://luminousaudio.com/products/axiom-ii-passive-preamp on the more expensive side.

 

[Chr1]

I use one of these and am very happy with it...

Tisbury Audio Passive Pre

Available on Ebay.

 

[Deleted member 58865]

What is the right input impedance for a passive preamp? 10K or 50K or something else?

The output impedance of my Topping D10s is 20 ohm.

Update: https://www.diyaudio.com/community/...tenuator-for-passive-amp.221728/#post-3206048 says 10k input impedance for modern gears.

 

[mdsimon2]

If you are worried about channel imbalance why not just get a high dynamic range DAC with digital volume control? What amp are you using?

Michael

 

[RayDunzl]

I built my bargain basement passive attenuator with two pots - left and right.

Since it sat in the middle between the two speakers, it was simple to adjust by ear.

 

[Deleted member 58865]

a high dynamic range DAC with digital volume control

I don't know how expensive it would be. Topping D10s has 112dB of dynamic range. So, with the DAC volume currently at -16dB, I get 96dB of dynamic range.

With the DAC volume at -16dB, application volume at 100% is loud enough to disturb me and people around me. I keep application volume at 25~70%.

It seems 121dB is the best dynamic range you can get from DAC.

My AIYIMA A07 amplifier doesn't have 96dB of dynamic range because the AC/DC power adapter is not clean enough. If I buy a better power supply, I may get more dynamic range from the amp.

Really, AIYIMA should step up its game and put stepped attenuators in their amplifiers. I would buy AIYIMA A07 with a stepped attenuator instead of potentiometer as long as the gain structure of the stepped attenuator is correct.

 

[Deleted member 58865]

I just saw this comment on

However, reducing DAC volume still reduces DAC's dynamic range.

 

[Deleted member 58865]

Can a passive preamp raise noise floor? It seems all phono preamps reviewed on this forum have SINAD below 91.

 

[solderdude]

However, reducing DAC volume still reduces DAC's dynamic range.

Reducing the volume = reducing the dynamic range.
Reason is simple. The max amplitude is lower so lower volume signals will fall below the hearing threshold limiting the 'dynamic range'.
Yes, electrically the dynamic range may still be the same high level but what is below the hearing threshold you simple cannot hear.

The dynamic range of your hearing is about 70dB at any 'average' level so music at 120dB level means you won't be hearing any sounds at 50dB SPL at that moment (you can later on when the music is silent).
This means hiss will become audible (when high enough) when there is silence (or music playing at low levels)
When listening to music at 80dB SPL you won't be hearing things at 10dB SPL at the same time.


Yes, a passive amp can raise the noise floor (very slightly) depending on the input stage of the amp.
Selecting a lower resistance is always the best. (10k or lower)

 

[Deleted member 58865]

https://www.diyaudio.com/archive/blogs/rjm/1364-rant-why-passive-preamps-totally-stupid.html says passive preamps can induce a lot of noise.

I am thinking about adding

SMD Dact type 21 Stepped attenuator 10K Passive Pre-amplifier preamp HIFI audio | eBay

Each step use 1% SMD High Quality resistors.

www.ebay.com

between Topping D10s and AIYIMA A07. Topping D10s has output impedance of 20 ohm. I don't know the input impedance of AIYIMA A07.

 

[Deleted member 58865]

Selecting a lower resistance is always the best. (10k or lower)

Why?

 

[Deleted member 58865]

The dynamic range of your hearing is about 70dB at any 'average' level so music at 120dB level means you won't be hearing any sounds at 50dB SPL at that moment (you can later on when the music is silent).

You mean reducing DAC volume is just as good as setting it at 0dB and reducing the volume on stepped attenuator?

Topping D10s has SINAD of 112dB. If the total digital attenuation from application volume and DAC volume is -42dB, then the dynamic range would be 70dB.
-42dB of digital attenuation is probably going to be more than enough. I don't know whether my ear's dynamic range is larger than 70dB at quieter volume levels.

I often listen to video and music at low volume levels in order to protect my hearing and to not disturb people around me.

 

[Blumlein 88]

Why?

It lowers output impedance which lessens the effects of the cable on frequency response. It also lowers thermal noise which might be amplified further down the signal path. Lower resistance has less thermal noise than higher resistance.

 

[mdsimon2]

I don't know how expensive it would be. Topping D10s has 112dB of dynamic range. So, with the DAC volume currently at -16dB, I get 96dB of dynamic range.

With the DAC volume at -16dB, application volume at 100% is loud enough to disturb me and people around me. I keep application volume at 25~70%.

It seems 121dB is the best dynamic range you can get from DAC.

My AIYIMA A07 amplifier doesn't have 96dB of dynamic range because the AC/DC power adapter is not clean enough. If I buy a better power supply, I may get more dynamic range from the amp.

Really, AIYIMA should step up its game and put stepped attenuators in their amplifiers. I would buy AIYIMA A07 with a stepped attenuator instead of potentiometer as long as the gain structure of the stepped attenuator is correct.

You are simplifying this too much. You need to look at residual noise at the amplifier output as a function of volume control position considering residual noise and gain from the amplifier as well as residual noise inherent in your source.

If you don't have issues with audible hiss currently, adding an analog volume control is not going to help you.

Based on PMA's measurements -> https://www.audiosciencereview.com/...s-power-supply-measurements.19507/post-642669 A07 residul noise is 10^(-75.2/20) = 174 uV residual noise from the A07 with an improved power supply. For reference this is a 5 W in to 4 ohm dynamic range of 20 x log[sqrt(5 x 4) x 10^6 / 174] = 88 dB.

Actual system noise performance will depend on the volume control position of the A07, lower gain = lower amplification of upstream noise but this comes with a channel imbalance penalty. For this analysis let's assume 29 dB gain like the ASR review.

Residual noise from D10S = 2 x 10^(-112/20) = 5 uV
Amplified residual noise from D10S = 5 x 10^(29/20) = 142 uV
Total residual noise from amplified DAC output and amplifier = sqrt (142^2 + 174^2) = 225 uV

225 uV residual noise is pretty good and is likely not audible depending on speaker sensitivity, hearing and listening position.

Now let's say you typically listen at -20 dB, that will reduce your output signal but noise remains constant with digital volume control.

Voltage at DAC output at -20 dB = 2 x 10^(-20/20) = 0.2 V
Voltage at amplifier output at -20 dB = 0.2 x 10^(29/20) = 5.6 V
Dynamic range at amplifier output at -20 dB = 20 x log(5.6 x 10^6 / 225) = 88 dB

Now let's look at the case of perfect analog volume control. In this case both upstream signal and noise are attenuated.

Attenuated residual noise from DAC output at -20 dB (analog) = 5 x 10^(-20/20) = 0.5 uV
Amplified residual noise from DAC output at -20 dB (analog) = 0.5 x 10^(29/20) = 14 uV
Total residual noise at -20 dB (analog) = sqrt (14^2 + 174^2) = 175 uV

Now noise is dominated by your amplifier where as before it was roughly equal parts DAC and amplifier.

Dynamic range at amplifier output at -20 dB (analog) = 20 x log(5.6 x 10^6 / 175) = 90 dB

You've gained 2 dB dynamic range at -20 dB volume position with a perfect analog volume control. Without a lower noise / lower gain amplifier you will not be gaining much from using a higher dynamic range DAC or an analog volume control.

Michael

 

[Deleted member 58865]

Anyway, noise floor is different from dynamic range.