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-   -   Aleph P 1.7 - distortion (http://www.diyaudio.com/forums/pass-labs/208228-aleph-p-1-7-distortion.html)

Mfly 6th March 2012 08:31 AM

Aleph P 1.7 - distortion
 
I hope you wise people can give me some pointers on the following issue:

Me and a friend are finishing up our Aleph P 1.7 build. We used Kristian's boards and input selector / volume control from Dantimax.

As a source I'm using 2V out (balanced) from IVYIII (Buffalo II DAC). When using balanced signal and no input attenuation (selector switches), the sound is extremely distorted (high frequency). When using single ended inputs or attenuation on the input (switches all open (10k on the input)), the distortion disappears.

Any ideas on what could cause this distortion?

I read in some other post that this could be due to oscillation and this could be solved by changing the gate resistors on the 610's of the input from 221ohm to 1k?

Or could a too low input impedance on the Aleph P make the IVYIII go haywire?

Any other suggestions?

AudioSan 6th March 2012 11:24 AM

how is your gain set? by R36.

Mfly 6th March 2012 11:32 AM

Quote:

Originally Posted by AudioSan (Post 2935205)
how is your gain set? by R36.

Currently at 1k..

AudioSan 6th March 2012 11:41 AM

okei. try lowering your gain. im running the same setup my self. but i only have unbalanced source. all switches open and jumpers on R36.
and use your volum controll on the output.

AudioSan 6th March 2012 11:43 AM

1 Attachment(s)
here is mine:)

Mfly 6th March 2012 12:04 PM

Nice layout. Ours is still a bit of a spiderweb :-).

Maybe I wasn't totally clear in my inititial description:

-volume control is on the output
-when the switches on the input side are all open (so the 10k resistors in series with the input are in use (R5 & R6 in the Aleph service manual)), there is no distortion, even with balanced input
-when the switches 1&2 (switches 2&4 in Kristian's manual) are closed (input bypasses 10k resistor and goes directly to the input cap), then there is distortion (and quite a lot of it).

I was under the impression that with a normal input signal (consumer level, 2V), you shouldn't get any distortion, regardless of the setting of the switches at the input. I however do not know if this assumption is correct, regardless of the input source. Or that there's something with the layout of Kristian's board which makes them behave differently. Or something in the grounding. Or if this is dependent on the source.

I will have a look and see if increasing the gain resistor (decreasing the gain), results in less distortion.

Zen Mod 6th March 2012 12:15 PM

just leave that input attenuation in place and do not worry

AudioSan 6th March 2012 12:20 PM

Quote:

Originally Posted by Mfly (Post 2935249)
Nice layout. Ours is still a bit of a spiderweb :-).

Maybe I wasn't totally clear in my inititial description:

-volume control is on the output
-when the switches on the input side are all open (so the 10k resistors in series with the input are in use (R5 & R6 in the Aleph service manual)), there is no distortion, even with balanced input
-when the switches 1&2 (switches 2&4 in Kristian's manual) are closed (input bypasses 10k resistor and goes directly to the input cap), then there is distortion (and quite a lot of it).

I was under the impression that with a normal input signal (consumer level, 2V), you shouldn't get any distortion, regardless of the setting of the switches at the input. I however do not know if this assumption is correct, regardless of the input source. Or that there's something with the layout of Kristian's board which makes them behave differently. Or something in the grounding. Or if this is dependent on the source.

I will have a look and see if increasing the gain resistor (decreasing the gain), results in less distortion.

i just have the inputs switches open. use a 1-2Kohm stereopot insted of R36 if you need to adjust the gain.
yes, my amp is not finnished yet:) but it it works as a charm:)

kropf 6th March 2012 01:15 PM

Quote:

Originally Posted by Mfly (Post 2935249)

I was under the impression that with a normal input signal (consumer level, 2V), you shouldn't get any distortion, regardless of the setting of the switches at the input. I however do not know if this assumption is correct, regardless of the input source.

I will have a look and see if increasing the gain resistor (decreasing the gain), results in less distortion.

With a scope take a look at the output signal at the drain of Q18. You may find that the peak to peak voltage is very high and the preamp is essentially clipping.

I've never run my version of the P1.7 balanced so I don't know if there are any special considerations.

Jeremy

Mfly 6th March 2012 01:32 PM

Quote:

Originally Posted by kropf (Post 2935320)
With a scope


And now for a scope...

I indeed came to the conclusion that if I want a definitive answer on oscillation or clipping for that matter...I would need an oscilloscope, which I don't have. Maybe invest in a second hand.

Would something in the 20Mhz region suffice or do I really need 100Mhz or more for these kinds of analysis (including checking oscillation (also on opamps)?


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