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Hi,
I've finally got around to building my Buffalo II DAC and have a problem with the lock LED - it did light up when I was building the DAC but now I've added the valve output buffer it doesn't, though that could be unrelated. Looking through the forum there appears to be a number of issues that can cause a lock problem so please bear with me as I'm a DAC newby. I hope it's something simple/silly and the more experienced can point me in the right direction.
When I first built the Placid (2.0.1) all the LEDs came on, 2 on the Placid and 4 on the BII. I set the voltage and current as per the Placid User Manual, and these haven't sagged/changed:
output voltage ~5.5V
voltage across R8 0.325V
voltage across R3 62.5mV
resistance of VR1 105R
So far so good. Then I built the output buffer which is a 5687 Broskie Cathode Follower and the voltages test fine on this. I've connected the DAC and buffer this morning and now the lock LED doesn't light. (It might not have lit before I connected the buffer, I don't know for sure.) So I've disconnected the buffer and rechecked the Placid voltages which are unchanged but the lock LED still doesn't light. I hope I haven't damaged anything.
I'm not sure if I've perhaps earthed the DAC incorrectly? I connected the DAC output earths to the RCA out jack earth tags, which are connected to the buffer star earth. This is lifted from ground by a 20R resistor.
I don't know what to try next, any thoughts?
Thanks,
Simon.
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Have you tested the voltages with the output connected and the DAC operational?
What is your input, and how is the SPDIF switch set?
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With the buffer attached and DAC on (but no input connected) the output voltage had risen a little this morning to 5.65V, which I've reduced to 5.45V. This value doesn't change whether the output or input are connected or not. Input is a Squeezebox 3 connected via an ordinary interconnect as I don't have any 75R to hand. Interestingly when I plugged the SB3 in just now the Mute LED lit momentarily then went out and the Lock LED flashed twice then stayed on. The only thing that has changed is the reduced output voltage. But I do now have music! So what does the Mute LED not lighting mean? Is this what should happen? I have noticed that at the DAC outputs I have +/-1.78V DC offset between ground and out, on both channels. Is this correct - an earth lift of some kind? BTW thanks for writing the Integration Guide, I found it really helpful. 
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No, this is not what should happen. You may either have a malfunctioning LED or perhaps you accidently reversed it? Whatever it is, the DAC is operational now and that is what is most important. Enjoy!
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simon wrote:
I have noticed that at the DAC outputs I have +/-1.78V DC offset between ground and out, on both channels. Is this correct - an earth lift of some kind?
Yes, that's by chip design and correct. You might have to ac couple if your tube stage doesn't like the that sort of voltage.
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LeonvB wrote:No, this is not what should happen. You may either have a malfunctioning LED or perhaps you accidently reversed it? Possibly, but it does light momentarily, and when I was testing it all four came on. LeonvB wrote:Whatever it is, the DAC is operational now and that is what is most important. Enjoy! Sure is, thanks for the help. :-) It's still in "quarantine" in my test rig upstairs so hasn't had a serious listen yet but the signs are good! I have a healthy respect for things that look okay but are a problem waiting to happen so it would be good to understand what's causing it, and indeed what the Mute LED not lighting means.
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avr300 wrote:Yes, that's by chip design and correct. You might have to ac couple if your tube stage doesn't like the that sort of voltage. Thanks for confirming that avr300. The buffer seems okay but I'll have a think about it a little.
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I had a good listen last night which was most enjoyable :-). How long does the Buffalo II generally take to run in? Or to put it another way how long should I leave it before I start tweaking the buffer?
I still haven't resolved the LED issue. Without the spdif plugged in the mute is on and the lock is off, plugging the spdif in reverses this. Any thoughts what this might mean?
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It means the mute is off and the DAC is locked onto the SPDIF signal.
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Thanks Russ, so this is normal then? Sorry for the newby questions, solid state stuff is a bit of a mystery to me I'm afraid.
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Just thought I'd update this as the Buffalo II has been running just fine since. I've had a bit of a play with the valve buffer for those interested. avr300 was spot on - thanks for the head-up. The buffer is a Broskie Cathode Follower - see page 6 here: Broskie Cathode Follower PDFIt's basically the second half of John Broskie's Unbalancer, so no gain which suits me fine. The direct coupling on the in- input was messing up the biasing of the lower triode so I added another cap, like in+. I've got about 4uF as coupling cap at the output which brings the f-3 to around 2Hz. The power supply is very simple - 1N4007 bridge followed by 30H 100uF 30H 100uF; it was a "get me going" power supply. There are a number of things I'd like to try, including shunt reg, but at the moment I have a 470nF Russian K40Y bypassing the second 100uF and I'm inclined to leave it as I'm surprised how much it's improved things - the DAC sounds absolutely stunning. I'm using a Squeezebox Duet and with this DAC and buffer this is the best digital I've ever had at home. Very nice indeed.
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