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Dark LED passive filter-I/V stage for NOS DACs

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#162
Vunce said:
Your power amp should be the last component turned on and the first turned off. You won’t have a problem if this sequence is followed. Never turn upstream equipment on/off while the power amp is on.
Click to expand...

+1.
Vunce said:
That said, I don’t remember my Dark putting out that kind of offset at turn-on, maybe Abrax could chime in.
Click to expand...

David is quite correct, there is a large offset until the output cap fully charges. And the time constant is fairly long because of the size of the cap. I use a large value because I'm normally driving a trafo with it.

To reduce the time it takes for the offset to decay the cap value (330uF, C8 & C9) can be reduced - if you're not driving a trafo then 15uF likely is sufficient and will give a 20X reduction in settling time. The resistors at the output (R43 & R44) could also be reduced in value to have the settling time reduced further.
 
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#164
abraxalito said:
+1.


David is quite correct, there is a large offset until the output cap fully charges. And the time constant is fairly long because of the size of the cap. I use a large value because I'm normally driving a trafo with it.

To reduce the time it takes for the offset to decay the cap value (330uF, C8 & C9) can be reduced - if you're not driving a trafo then 15uF likely is sufficient and will give a 20X reduction in settling time. The resistors at the output (R43 & R44) could also be reduced in value to have the settling time reduced further
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Ok great. I'll drop in some smaller value caps and remember to fire up the power amp last 🙂
 
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#166
#168
Steve I think you need this link

M

Post in thread 'DAC AD1862: Almost THT, I2S input, NOS, R-2R'

Protection against DC offset voltage on DAC or Preamp output

It is definitely not needed for DACs like AD1862, AD1865, maybe PCM63? Because they have zero DC offset even if the signal source is not turned on.
TDA1541A is another animal and when the signal source is turned off, then a DC offset appears on the output. It has some internal biasing and most manufacturers used audio capacitor in the signal line to prevent this DC offset on the output. I am not a fan of coupling capacitor because it colors the sound 😏 I assume you won't use it either, so the correct power-on...
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#169
My PMs don't seem to be getting through so I thought I'd post this here.

abraxalito said:
Hi Dan,

Hoping you're getting listening satisfaction from your Dark LED I/V stage. Unfortunately we made an error in building your kit and six of the resistors we fitted were of the
wrong kind - we fitted thick-film ones where the design originally specified thin-film for lower noise and better stability. The difference in SQ is fairly small but we figure is
worth having so we'd like to ship you replacement resistors if you're able to replace them yourself. If that's too inconvenient for you then we would like to ship you a
replacement baseboard (excluding the filter board) with the correct thin-film resistors fitted. Would either of those solutions work for you?

Please accept our apologies for this inconvenience,

Best wishes
Richard
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Can we continue this discussion?

Regards,
Dan
 
#174
Ahoy, Richard!
I have found some time to assemble and test the Dark LED I/V and some of the DUT measured voltages are a bit below those in Step 1 of the soldering guide in Post #1. I hope you can comment on whether these are 'okay' or if there are further tests that may be necessary.
The test rig is: 1) a board-mounted 22V/25VA Talema toroid, feeding 2) a VRDN regulator set at +/-18.00 V, which feeds the Dark LED.
Voltages at the test points are:
DUT (V) Expected (V)
TP1 7.20 7.8
TP2 7.20 7.8
TP3 0.62 0.64
TP4 -2.53 -2.3
TP5 15.23 15.2 - 15.3
TP6 7.17 7.9
TP7 7.17 7.9
TP8 8.33 9.1
TP9 8.44 9.1

Test points 1, 2, 6, 7, 8, 9 are a bit low whilst TP4 is a bit high.
Any thoughts or notes you have would be appreciated.
 
#175
Hi Keith - those voltages were taken with the shorting links on the 20 way connectors in place of the plug-in filter? In which case I'd expect no difference between TP1 and TP7 but you have 30mV which seems odd. Yet both channels are consistent. With your meter on ohms range and no power applied, try measuring resistance between TP1 and TP7, also between TP2 and TP6.

In general though they're within the range of component tolerances except for TP9 which looks out of whack with TP8 (the other channel). Please measure the voltages across R35 and R37 (1k5s next to the PXT transistors) to maybe give some insight into that variation. TP8 would normally be just two diode drops (so ~1.2V) higher than TP6, likewise TP9 the same amount higher than TP7.
 
#176
Hi Richard, thanks for your quick reply.
First, let me apologize for a typo - TP9 above should read 8.34V. Much better.
Yes, the shorting links are in, on top of the 2x10 sockets with the shorted links near the inductors.
I left the I/V on for a few hours to let everything come to thermal equilibrium, and retested:
TP1 7.12V
TP2 7.12V
TP3 0.618V
TP4 -2.596V
TP5 15.09V
TP6 7.12V
TP7 7.12V
TP8 8.27V
TP9 8.28V
These seem much better than previous, so perhaps something in the test apparatus just needed to settle down.
On the other items, the resistance from TP1 to TP7, as well as TP2 to TP6 are the same at about 0.5 Ohm, after adjusting for the resistance of my DMM probes and cables. The voltage drop on R35 is 0.642V, and 0.644V on R37, so I think these are okay.
Thanks again for your help.
 
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