Squeezebox Output Impedance?

JDUBS · « **on:** 1 Dec 2005, 02:55 am »

Hey All

I've done a few searches and can't find this anywhere. Can someone tell me what the output impedance is for the SB3?

Just trying to figure out if its a good candidate for an autoformer-based passive. It will be paired with a Clari-T (120k ohm input impedance....should be plenty high enough).

Thank you very much!

-Jim

JoshK · « **Reply #1 on:** 1 Dec 2005, 02:59 am »

I don't know what the output impedance is but typically transformers trade voltage for current. If you bypass the opamp in the output of the SB then it is most definitely not a candidate, but if you leave the opamp alone, I am not sure what the output voltage is. IIRC Davey measured it to be around 1.8Vrms?? If so, then autoformer should be fine I'd think.

Val · « **Reply #2 on:** 1 Dec 2005, 01:45 pm »

It is 220 Ohms, I understand, so you'll be OK in any reasonable case.

Davey · « **Reply #3 on:** 1 Dec 2005, 02:01 pm »

Yes, it is 220 ohms (set by R42 and R44) in the SB2. I don't believe this has changed for the SB3.

http://www.seanadams.com/dac.pdf

As Josh mentioned, the output voltage is about 1.86 VRMS for 0dbFS recorded program material.

Cheers,

Davey.

JDUBS · « **Reply #4 on:** 2 Dec 2005, 03:41 am »

Thanks guys...I appreciate it. I guess I should have mentioned that this is a modded unit (i.e. bypassed opamp).

So, this won't work with a transformer based passive? Vinnie mentions that the DAC can drive a load as low as 5kohms.

-Jim

Davey · « **Reply #5 on:** 2 Dec 2005, 03:05 pm »

Well, if you have a modded unit then all the information we gave you is meaningless. You should post this query in the Red Wine forum.

Davey.

mgalusha · « **Reply #6 on:** 3 Dec 2005, 12:42 am »

I just measured the Zout of my SB2 at 1KHz and it was 286 ohms. I didn't check it at other frequencies, so this obviously isn't the last word. Unloaded output voltage was 1.046 with a 0dB 1KHz tone at full volume.

My SB2 has the OpAmp removed and is using a Sonicap 3.3uF bypassed with a Sonicap Platinum .01uF. Bolder uses the Sonicaps and I believe Red Wine uses the Auricaps. Not sure how much Zout would change with the Auricaps.

I installed some 1:2 step up transformers in a friends amp which lowered the Zin on his amp from 39K to 9K750 and the modded SB doesn't seem to have any trouble driving it. Can't say how it would work with a transformer based attenuator. Should be OK as long as the Zin of the transformer stays above 3K.

Hope this helps.

Mike

JDUBS · « **Reply #7 on:** 3 Dec 2005, 01:10 am »

Thanks guys!

What I think I'll probably end up doing is just getting the passive pre made up with one input / one output and use it with my turntable only (200 ohms from the phono pre) into my (soon-to-be-ordered) Clari-T. I'll then drive the Clari-T directly via the SB3.

My system will be bi-amped, so I will need to add dual outputs to the Clari-T...but other than that...I think I should be all set.

Will be kind of a pain to have to swap interconnects but the signal won't get much purer, I think!

-Jim

mgalusha · « **Reply #8 on:** 3 Dec 2005, 01:54 am »

Quote from: JDUBS

What I think I'll probably end up doing is just getting the passive pre made up with one input / one output and use it with my turntable only (200 ohms from the phono pre) into my (soon-to-be-ordered) Clari-T. I'll then drive the Clari-T directly via the SB3.
....
Will be kind of a pain to have to swap interconnects but the signal won't get much purer, I think!

This is similar to what I'm doing. I have the TT and it's electronics ready to go and the SB feeding the amps directly. If I want to listen to Vinyl I have to swap cables but it's a small price to pay for the quality of sound obtained when driving the amps directly. The SB is good enough that I don't reach for the Vinyl as much as I used to. Still, the I love the glow of the vacuum tubes and sound of a good record that sometimes I just have to fire it up for a few albums.

Vinnie R. · « **Reply #9 on:** 3 Dec 2005, 02:19 am »

Quote from: mgalusha

I just measured the Zout of my SB2 at 1KHz and it was 286 ohms. I didn't check it at other frequencies, so this obviously isn't the last word. Unloaded output voltage was 1.046 with a 0dB 1KHz tone at full volume.

My SB2 has the OpAmp removed .

Hi Mike,

Thanks for posting your findings.

The Burr Brown DAC used in the SB2/SB3 (PCM1748E, and KE) has an internal output amp stage that is rated to drive as low as a 5k load. This is pretty much a built-in opamp, but without the 2nd pole LPF and limited voltage output (no +/- 15V rails here). It sure drives my DACT 10k stepped attenuator very well!

Regards,