Aleph P and D1 Analog IV stage - diyAudio
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Old 13th January 2011, 12:37 AM   #1
ctong is offline ctong  United States
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Default Aleph P and D1 Analog IV stage

I have a Denon 2900 universal player modified with a D1 analog IV stage. I only have single-ended output, and only use the IV converter of the D1, not the source follower. I recently noticed that for high-level signals (3.2 V peak), the Aleph P generates a noise that sounds as if there is a bad solder joint. The noise is in both channels but is not identical (mono). This happens only when I drive the Aleph P directly from the Denon. If I put another unity gain device in between, the noise disappears.

I put a 10K series resister before the Aleph P input. That solves the problem. But I also noticed that for moderate level signals, the sound is more clean and transparent, especially for the treble. If I connect my Sennheiser HD600 headphone directly to the Denon, it is even more transparent. I am suspecting that the ultra sonic noise from the Denon is causing some kind of problem in the Aleph P, since the post-IV filter in the Denon is only first order. Does this make sense?
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Old 13th January 2011, 11:44 AM   #2
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The P 1.7 has zeners at the inputs, and they can sometimes make an upstream piece of equipment go crazy if there is no isolation. I had the exact same problem and put a 1K resistor on the input for isolation, total success.
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Old 13th January 2011, 02:19 PM   #3
ctong is offline ctong  United States
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Thanks for the interesting explanation. How do the zeners affect the upstream component?
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Old 13th January 2011, 04:15 PM   #4
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I don't know the exact mechanism involved but it is well documented. It caused my signal generator to go into high frequency oscillation. There have been posts by others on the subject.
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Old 14th January 2011, 01:10 AM   #5
ctong is offline ctong  United States
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I see. Would it be better to remove these zeners? For DIYers the protection by the zeners is not a bit deal.
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Old 14th January 2011, 11:03 AM   #6
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That´s exactly what i use.
D1 I/V stage and aleph P 1.7
No problems and no funny noises here.
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Old 14th January 2011, 11:07 AM   #7
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What capacitors did you use in the P 1.7 to cancel oscillation?
680 PF like in the service manual?
I don´t think that the zener causes any problems.

regards
Oliver
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Old 14th January 2011, 02:27 PM   #8
ctong is offline ctong  United States
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Do you have the complete D1 analog stage? Mine does not have the follower output part, just the IV part.

I do not use a capacitor. I use a 10K resister at the Aleph P input, just like the one in the manual for reducing the gain.
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Old 14th January 2011, 03:19 PM   #9
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Yes i have the complete analog stage, but not balanced.
The 680 pF capacitors are there to prevent oscillation, so if some noise appears
you might check them out first.
The 10k doesn´t help here because the high frequency oscillation is the problem.
The 10k resistor doesn´t reduce the bandwith.
You might here something like humming if oscillation appears.
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Old 14th January 2011, 03:33 PM   #10
ctong is offline ctong  United States
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May be the output follower acts as a buffer to prevent the problem?

I do not see any oscillation in the output. I am not really sure what is going on.
The 10k resister, however, does increase the input impedance for the gain stage, and therefore, acts as a RC (the input capacitance) filter to attenuate the high frequencies.
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