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Chip Amps Amplifiers based on integrated circuits

My_Ref Fremen Edition - Build thread and tutorial
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Old 13th August 2019, 11:54 AM   #4161
giacintopapa is offline giacintopapa  Italy
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I would like to submit these two measures to you. We have seen that at the output of a transformer preamp, as expected, there is no trace of DC, it is blocked by the transformers.

I wanted to see how much DC was present at the output of my dac. As always I don't know if I used a correct method ...

Dac turned on without signal. On the left channel I measure about 1.0 mV, on the right channel I measure about -0.9 mV.
I didn't expect to find a negative current on a channel.
Would anyone know how to explain these measures (if they are correct and have a meaning)?

Thank you all
Ciao

giacinto
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File Type: jpg IMG_20190813_131507.jpg (287.5 KB, 113 views)
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Old 13th August 2019, 01:20 PM   #4162
Joseph K is offline Joseph K  Hungary
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Yes it is absolutely normal, even more, your result is very good. Your dac is a correctly designed/ executed product.
Output offset can be any polarity, it's normal. Think of it that your dac is producing an AC output, with 0V DC being the center value. The actual momentary magnitude is either above or below in the time; like between +2,8V / -2,8V peak maximum, in case of an average dac output level.

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Old 13th August 2019, 04:20 PM   #4163
giacintopapa is offline giacintopapa  Italy
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Grazie, George, e ciao

Giacinto
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Old 13th August 2019, 04:36 PM   #4164
Panelhead is offline Panelhead
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First, George it is so good to see you back!!!!
The dac measurement is fine. It tells me the dac is dc coupled, a good thing. Also, hook up the TVC and measure again. The dc voltage offset should be even lower when loaded by the dc resistance of the input side of the TVC.
My transformer is on the input of My_Ref. Been running some REW sweeps. Not happy with results. The transformer is raising the noise floor.
Going to play with routing and transformer position to see if there are easy fixes.
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Last edited by Panelhead; 13th August 2019 at 04:37 PM. Reason: Comma
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Old 13th August 2019, 06:27 PM   #4165
Panelhead is offline Panelhead
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Default With plots

One saving grace is the high efficieny speakers, very low power at normal listening.
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File Type: jpg 15:15B 26.7 mW..jpg (98.6 KB, 100 views)
File Type: jpg 15:15B, Fremen, REW, 270 mW, 6 ohm.jpg (92.3 KB, 86 views)
File Type: jpg 15:15B 2.67 W.jpg (110.6 KB, 86 views)
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Old 13th August 2019, 08:33 PM   #4166
lehmanhill is offline lehmanhill  United States
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Am I assuming correctly that the curve on the left is the Fremen only and the curve on the right is the Cinemag + Fremen? If so, a very curious result.

Is there any chance that the input ground to the transformer is common with the output ground? I'm really just guessing in the dark.

Jac
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Old 13th August 2019, 10:57 PM   #4167
Joseph K is offline Joseph K  Hungary
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I see saturation in the last plot.. somebody is saturating. Is it possible that You overdrive the transformer? Or the input ADC, given that the signal is -1,6dB, with respect to the -14dB in the first.. -4dB in the second.. when you get close to the Zone, saturation sets in all at once ..
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Old 13th August 2019, 11:21 PM   #4168
lehmanhill is offline lehmanhill  United States
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George,

Very good point, as usual. The transformer is good for +20 dBu, but the graphs are at -14, -4, and +1.7 dbFS. I believe that is decibels relative to full scale, so depending on what is full scale, the source or something else is likely clipping or saturating.

Jac
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Old 14th August 2019, 02:58 AM   #4169
Panelhead is offline Panelhead
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Default All graphs are using a Cinemag 15/15B line input

Just showing the overload and saturation.The low level is normal listening output. Speakers are three way horns with a sub.
The chain here was REW signal generator, Focusrite Clarett 4Pre output into a Fremen. The measured voltage across the 6 ohm power resistor is fed into an ADC on the Clarett and looped into REW.
All three graphs use the same power resistor and input transformer. High enough input really gets it ringing.
A different signal generator is next, an Akitka 79.00 generator. I hope to install Cinemag CM10/600 4:1 stepdown transformers soon and retest.
Had a pair of Lundahl transformers and configured them for 4:1 step down. Saw a lot of ringing and removed. Guess all ring if driven at a high enough level.
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Old 14th August 2019, 01:16 PM   #4170
lehmanhill is offline lehmanhill  United States
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Interesting. As usual, I learn something new every day.

We are wandering off topic, but please report your experience. I guess a simple RC post transformer snubber wouldn't fix that ringing.

Jac
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