SB2: reduce analog out voltage by 10x?

[usualsuspects]

I would like to run SB2 analog out direct to my amp (Parasound HCA-750) - my amp manual says "0.775v input for full output". At 6v on the analog outs - I am afraid it will be way too high. Is there a simple analog resistor network I can build that will reduce the voltage by a factor of 10 to 0.6 volts? The input impedance on my amp is 33K ohms. I tend to listen at low volume and having the most range in the volume control is desired. My speakers are fairly efficient – 90db. Thanks!

[cbemoore]

I've copied my post from another thread:

You need to vote for this bug:

http://bugs.slimdevices.com/show_bug.cgi?id=1601

I REALLY want this functionality, but so far it looks like I'm the only person who's actually voted for it!

Running directly into a power amp (and using the digital volume control) gives a far cleaner signal path than connecting via a pre-amp. But the signal out of the SB2 is far too hot, and its impossible to turn the digital volume down low enough - 1 out of 40 is still really quite loud!

The DAC used in the SB2 already has a built-in attenuation capability, but its not used. If the Slim guys could add an option to make use of it, it would solve all my problems....

Chris

[mazurek]

Parts express sells line level attenuators, look at them if you are in the US

[Davey]

I'm not sure where you're getting the 6 volt figure from. The maximum output from the SB2 is approximately 1.85 volts RMS. I would try using a simple volume control between the SB2 and your amp. That should work fine.

Cheers,

Davey.

usualsuspects wrote:
> I would like to run SB2 analog out direct to my amp (Parasound HCA-750)
> - my amp manual says "0.775v input for full output". At 6v on the
> analog outs - I am afraid it will be way too high. Is there a simple
> analog resistor network I can build that will reduce the voltage by a
> factor of 10 to 0.6 volts? The input impedance on my amp is 33K ohms. I
> tend to listen at low volume and having the most range in the volume
> control is desired. My speakers are fairly efficient – 90db. Thanks!

Hi,

You don't actually need to reduce the output by that much.

The 0.775v figure is almost certainly a "standard" 0dBu level.

dBu measurement is referenced to 0.7746v (often rounded to 0.775v), i.e.:

dBu = 20log10(v/0.7746v)

where v is the rms voltage, 0.775v in this case (rounded from 0.7746v).

The 6v figure for the Squeezebox is a peak-to-peak figure.

Converting this to rms gives:

Vrms = (Vpp / 2) / sqrt(2)
= (6 / 2) / 1.414
= 2.12v

This corresponds to:

dBu = 20log10(2.12 / 0.7746)
= 8.75 dBu

So, to get an rms level of 0dBu you would need to reduce the output
voltage by 2.12 - 0.7746 = 1.35v rms

In actual fact, it is highly unlikely that your amplifier can't cope
with this signal level. So I would just plug it in and see how you go.

R.
--
http://robinbowes.com

If a man speaks in a forest,
and his wife's not there,
is he still wrong?

[usualsuspects]

Just wanted to say thanks to everyone for responding. I connected my SB2 directly to the Parasound amp, adjusted the gain pots on the amp a little, and it works great!

PS - the DAC in the SB2 is not bad at all. Not Classe Dac1 range (what I am used to) but not bad.

On 9/7/05, occam <occam.1v0utz (AT) no-mx (DOT) forums.slimdevices.com> wrote:
> A man walks into a doctor's office, and says to the doctor-
> "Doctor, when I do this with my arm, it really hurts."
> and proceeds to show his arm all the way up his....
> The Doctor looks at the contorted patient, ponders a moment, and says -
> "Well, have you considered not shoving up ALL the way?"
>
> Have you considered not shoving the volume up all the way?

Nice analogy

The problem is, there's nowhere near enough control with the low
volume ranges. On my Grado SR-80s driven directly from the headphone
jack, with some music, a volume level of 3 is a bit on the loud side
but 2 is too quiet. Sometimes I need to crank it up to 4 or 5 though.
If I could scale everything down (up?) by a factor of 5, it would be
much easier to find a happy medium.

- Jacob

[occam]

Dang Jacob..... I was editing that post and I see you copied the edited, more 'tasteful' version of the anecdote. I saw Usual's reply, while editing, mentioning that the Parasound had attenuators on its inputs which seemed to address the problem quite well. So I deleted my post. But you're right, inevitably, you loose resolution as you've only got the 40 set volume levels that are done in software. It would be nice if Sean, et al, would implement 256 levels of .5db attenuation. Barring that, if one doesn't want external attenuators, you could replace the input 4.7k resistors, R37 & R39, of the output opamps, with something close to 11.22k which would bring down the max to about .775Vrms.
http://www.seanadams.com/dac.pdf
This would still give those 39-40 steps up to the max desired output, which begs the question of whether X fixed steps is ever enough.
But I'm old and uncoordinated, and replacing smt resistors is a daunting task for me.

[cbemoore]

I still think Sean's solution (option 2 in bug 1601) is the best solution.

When I hook my SB2 direct to my power amp, volume 1/40 is fairly loud. Volume 2/40 is very loud. Volume 3/40 is too loud! And there's no way I can make it quiet enough for background music.

The best solution would be to add a a per-player option to attenuate the signal. That way, its up to the user whether they want to select the option or not.

And since the DAC has an inbuilt attenuation capability (which can be controlled by the firmware), I'm guessing this would be the obvious place to perform the attenuation.....

So if you're having problems with the analog output being too loud, make sure you vote for bug 1640!!

Chris

[JJZolx]

Qustion: _Why_ is the SB2 output level so high?