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View Full Version : squeezebox attenuators to a power amp - help



cat2
2008-03-25, 15:13
Hi I am getting an old Cyrus Q Power with the specifications below. My concern is of plugging it straight into the line out of the Squeezebox 3(soon to be a Duet) I found the wiki page http://wiki.slimdevices.com/index.cgi?ConnectToPowerAmp which states that the squeezebox3 will be 6 Vpp/2.1 Vrms but I am lost on how this applies to the amp specified below. I assume I need attenuators and ideally would like to have it sorted before I plug it in.

Does anyone understand this stuff and can tell me what I need, my current attempt at how it works is (first time I have used the log button on the calculator :-)) ...

SQ3 2.1v (max volume)
Cyrus Q 380mV (max)
so far looks like things are going to blow up,

dB = 20 * log ( .380 / 2.1 ) = -14.85

This will allow the full volume on the SQ3 to be the max the amp can handle. I assume a attenuator something like http://www.amazon.com/Harrison-Labs-Line-Level-Attenuator/dp/B0006N41B0/ref=sr_1_1?ie=UTF8&s=electronics&qid=1206482411&sr=1-1 but I will need 15 would be what I am looking for? Can I just daisy chain them together, i.e plug a 3db into a 12db to get 15db ?


Cyrus Q Power
Specifications

Continuous Power - 50W/CH (both driven into 8 Ohms) 70W/CH (both driven into 4 Ohms)
Frequency Response - -3 dB at 1Hz and 80kHz
Current - 30 Amps peak to peak
Sensitivity - (RCA input) 380mV (for 50W stereo into 8 Ohms)
Input impedance - 10Kohm (RCA)
Output voltage - 380mV (Chain out)
S/N Ratio - 110dBA

My speakers

M31
Specification
Enclosure type 2-way reflex loaded
Frequency Response (+/- 3dB) 65Hz-20kHz
Sensitivity (SPL/M @ 2.83V) 88dB
Cabinet Volume 7l
Impedance 8 Ohms compatible
Recommended Amplifier 25-75W
Dimensions (mm) 225 x 170 x 280
Included Accessory n/a
Optional Accessory1 Stancette
Optional Accessory2 n/a

cat2
2008-03-25, 17:33
Just found out the Duet uses 4.8vpp. After some research (google) I found the fomula Vpp*0.7071=2*Vrms

So assuming I worked it all out correctly in the first post.

(4.8 * 0.7071)/2 = 1.697 vrms

dB = 20 * log ( .380 / 1.697 ) = -12.998 dB

I would really appreciate somebody telling me that this is all correct.

Note the info for the Duet is at http://www.slimdevices.com/pi_duet.html under Analog RCA outputs.

zanash
2008-03-26, 01:14
there are a number of issues ...if you run the sb3 to a power amp and use the on board vol pot .....the quality will be diminished....as it only gives you all the bits at max volume ! [unless I'm mistaken]

I'd suggest getting a small passive volume control tvc or stepped attenuator type to fit between the sb3 and power amp ...that would allow the sb3 to run at full quality [I mean volume !] and the passive sets the listening volume.

in practices you set the passive with the sb3 at full volume to your rrquired listening level ...you then use the sb3 volume control to regulate the sound level ...normally between mute 1/2 vol and full vol if my experience is the same as others

radish
2008-03-26, 05:54
there are a number of issues ...if you run the sb3 to a power amp and use the on board vol pot .....the quality will be diminished....as it only gives you all the bits at max volume ! [unless I'm mistaken]


Not true. The SB pads the 16-bit output to 24-bit before the volume control, thus you have 8 bits of headroom before anything is lost. Even then, you have to decide whether the bit loss is more damaging to the sound than running the analog signal through a pot. I'm not convinced it is, but that's one for your ears to decide.

Regardless, the idea of running through some kind of attenuator (fixed should be less impactful on the sound I would think) before the amp is a good one - if only to protect the amp/speakers in case of a problem. The TP was designed with this in mind which is why it has an inbuilt physical attenuation function.

sebage
2008-03-26, 07:51
I recently went throught the same thought process. My SB3's output was too high an input for the power amp to handle and I wanted to have an attenuator in the signal path for safteys sake.

I also did experience the 'white noise of death' once or twice (a ghost the in squeezebox machine that I haven't since experienced in over 18 months) which I didn't want to damage my equipment.

I ended up getting a goldpoint attenuator. It works well for me. I had it matched for the amp I am using (10K stepped attenuator) by the guy who builds them.

cat2
2008-03-26, 23:20
I found the link http://www.russandrews.com/lookup/1/region/UK/currency/GBP/customer_id/PAA2683038708165UPHFYYBZUXDYNIML/product-Inline-Attenuators-4800.htm which had the attenuators I wanted, but after a bit of googling I found the circut for the attenuators made some 16db they work fine it is all plugged in and going (took an hour and that included the 30 minute walk to the shop and back to get the parts - 6 resistors and a 4 RCA plugs and sockets). I am not sure why the ones on the link above are 200 times more expensive then mine, I could have even stretched out and put them in better RCA plugs/sockets, this will have increased the price by about $3.

Thanks for the replies nothing, is smoking so I assume the above was all right.

zanash
2008-03-27, 01:22
I suspect that were saying the same thing .......the volume pot on the sb3 reduces the level of information as the sound level decreases ...regardless of what happens before the vol pot.....as compared with its full output.

the simplest solution is a set of in line attenuators ...you can make them for the cost of two rca plugs and two rca sockets plus 4 suitable resistors of sufficient quality ie dale vishay as a minimum.

cat2
2008-03-27, 03:11
1. I have made the recommended attenuators, as they work I will maybe look at getting quality components and getting more accurate resistors.

I used the website at http://radiomagonline.com/notebook/passive_attenuators/ to work out the numbers

2. The volume control on the SB3 as Radish said is digital and it drops bits from the signal (i.e information) so the higher the volume the better, I vaguely remember reading 80% been no loss but any lower and you start throwing stuff away, to complicated for me to work out exactly, not sure how the 8 bits Radish mentions connects to the volume control.

3. The Cyrus Q is cool, it will go from standby to on when the SB3 starts sending a signal, and will go back to standby 5 minutes after the SB3 is finished - will be good for an alarm.

-------------------------
a) SB3 -> (analog) Cyrus Q - > Mission M31
b) SB3 (replacing with a duet) - > (Digital Coaxial) Rotel RSX 972 - > Monitor Audio S6

chill
2008-03-27, 05:58
Cat

I realise you're sorted now, but you might consider a variable attenuator like this: http://www.vaneijndhoven.net/jos/switchr/design.html

It achieves a variable attenuation by using a series of relays to put different high quality resistor values in the path. I am considering one of these to go between my SB3 and a Hypex dual mono UcD180HG power amp.

The attenuator has an IR remote capability (it can also act as a source selector - ie a 'passive preamp'), so I plan to set the SB3 volume to maximum, disable the volume control on the SB3 (already done via a custom.map file), and teach the attenuator to respond to the volume buttons on the SB3 remote. Volume control would be completely in the analogue domain, so none of those precious bits would be lost, no pots are used (only high quality fixed resitors), and I will still have the convenience of volume control on the SB3 remote.

Of course, you might need to change the resistor values to achieve the required attenuation at maximum volume, or just leave your home-brew attenuators in the path.

Edit: Jos also has a version of his preamp that has a gain stage (http://www.vaneijndhoven.net/jos/relaixed/index.html), so in fact I plan to use this, rather than just his passive attenuator, as an all-purpose preamp.

Edit #2: It's worth pointing out that the PCB and all parts for Jos' passive attenuator come to only 20 more than the Russ Andrews in line attenuators, AND you get the fun of building it yourself.

radish
2008-03-27, 06:47
2. The volume control on the SB3 as Radish said is digital and it drops bits from the signal (i.e information) so the higher the volume the better, I vaguely remember reading 80% been no loss but any lower and you start throwing stuff away, to complicated for me to work out exactly, not sure how the 8 bits Radish mentions connects to the volume control.

The data coming from your files is (usually) 16bit, the data going into the DAC inside the SB is 24bit, the additional 8 bits are zero padding added at the LSB end. When you lower the volume below 100% it starts losing data at the LSB end, but the first 8bits are not from the original signal, they're the padding, and so you can reduce the volume very significantly before there is any loss of actual signal data at all. There's plenty of discussion of this in the audiophile forum if you care (or dare!) to go in there :)

My other point was that the attenuators you add after the DAC are not transparent either, they will affect the sound quality (and the SNR) to some degree. The question (which is a personal choice) is would you rather have the quality loss from the digital volume control or the quality loss from the attenuator. Only listening to your specific setup will tell you which is better. I use fixed output from my TP into my main amp, but all my other SBs are using the digital volume control.