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Old 6th January 2013, 01:29 PM   #121
dddac is offline dddac  Germany
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I just checked on my board. Did an FFT and found 177kHz "peak" of -86dBV at 177kHz directly after the 8 Volt regulator. After the LC filter it is gone in overall noise.
When a larger capacitor (I used 2,2uF which was at hand..) was applied directly at the output of the regulator the 177kHz also disapears in the noise band. The effective RMS Value was round 1mV. This is all quite harmless. What values did you measure?
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Old 6th January 2013, 09:04 PM   #122
clivem is offline clivem  United Kingdom
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Originally Posted by dddac View Post
mmm, need to check (measure) this again. I believe I tried and this messed up the DC bias of the other DAC output.
so, if you ground the not used output, what DC voltage did you get on the used one? did you measure this? Will check myself as well
Erm, no I didn't measure the DC offset at the time. I've gone off at a bit of a tangent since then. With the neg grounded, I'm now using a simple CCS'd 6C45 tube gain stage with a ~5.5R, (5.8R in parallel with the original 133R), I/V resistor from the positive out. (Gain of 46, Ia 18mA, biased @ 0.7V with a diode, 2.3V RMS out @ 0dbfs). But I did measure the bias on the DAC positive outs as being 110.5mV with that configuration, (both channels), with the negs grounded, which is what would be expected with 20mA bias @ digital zero.
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Old 8th January 2013, 02:10 PM   #123
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Quote:
Originally Posted by dddac View Post
I just checked on my board. Did an FFT and found 177kHz "peak" of -86dBV at 177kHz directly after the 8 Volt regulator. After the LC filter it is gone in overall noise.
When a larger capacitor (I used 2,2uF which was at hand..) was applied directly at the output of the regulator the 177kHz also disapears in the noise band. The effective RMS Value was round 1mV. This is all quite harmless. What values did you measure?
Not the same situation indeed. One LF80 was very bugging: oscillations around 195 KHz but 500 mV signal. After inductance the voltage was 7.8 V instead of 8V. As I mentionned earlier, more capacitance solved the problem.
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Old 9th January 2013, 12:18 AM   #124
2A3SET is offline 2A3SET  Canada
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Default Ground reference

How about raise the ground reference and getting rid of the output capacitor?
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Old 9th January 2013, 03:31 AM   #125
dddac is offline dddac  Germany
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but than you would have this in the signal return path? what would be the gain?
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Old 9th January 2013, 09:26 AM   #126
clivem is offline clivem  United Kingdom
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I'm reminded of the, "Why do you want to para-feed and put a capacitor directly in the signal path?", argument.... Whilst conveniently ignoring, that in a traditional series feed tube output stage transformer topology, there is typically a large, undesirable, power supply electrolytic in the signal path.
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Old 9th January 2013, 09:34 AM   #127
clivem is offline clivem  United Kingdom
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Originally Posted by crazyfrog View Post
One LF80 was very bugging: oscillations around 195 KHz but 500 mV signal.
Ouch! I checked mine after reading that. A teeny bit worse than Doede measured before the filter, but nothing for me to worry about.
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Old 11th January 2013, 04:32 PM   #128
clivem is offline clivem  United Kingdom
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Default My latest lash.....

After reading kevinkr's Tube And Transformer I/V Converter For Differential Current DAC thread, I remembered that I have a spare pair of Sowter 9545 I/V transformers.

Sticking with the 6C45 tube output stage that I had been using with a ~5R I/V resistor from the + out, a quick back of an envelope calculation and I soldered 4x 1R2 I/V resistors to the DDAC board. I have the secondaries of the 9545's in parallel, so ratio is 2:5. Output is ~2.2V RMS for 0dbfs signal.

Not had a scope anywhere near it yet, but it sure does sound good!
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Old 13th January 2013, 05:55 AM   #129
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clivem ... is the big black cylinder situated next to the big black box a cap? The function of the black box is ... ? Thanks. Project looks good.
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Old 13th January 2013, 11:48 AM   #130
clivem is offline clivem  United Kingdom
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clivem ... is the big black cylinder situated next to the big black box a cap? The function of the black box is ... ? Thanks. Project looks good.
I think you must mean the 100uF/400V Obbligato PSU capacitor. It's a film PS cap. No "nasty" electrolytic caps used here!

The black box.... Not 100% sure what you mean. Directly behind that cap is the HT regulator for the tube output stage. It has a fairly large black heatsink strapped to the cascode transistor. Right at the back/left of the picture, there is a Hammond 370DAX PS transformer (260V-0-260V & 0-6.3V), and next to that is a Hammond 193C choke (20H / 100mA). HT is rectified by the EZ81 tube, and followed with 1uF / 20H / 100uF Pi filter before the regulator.
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