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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Salas DCG3 preamp (line & headphone)
Salas DCG3 preamp (line & headphone)
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Old 7th November 2019, 08:27 PM   #4051
Antsav is offline Antsav
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Thank you, just wanted to be sure.
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Old 7th November 2019, 09:01 PM   #4052
Salas is online now Salas  Greece
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You are welcome.
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Old 8th November 2019, 07:52 AM   #4053
alexkosha is online now alexkosha  United States
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Salas DCG3 preamp (line & headphone)
Short question. My FFT is very clean with no top lid. I see some picks getting up about 10-15dB from 200 to 500Hz when lid starts to cover DCSTB, but only on left channel. Is any obvious reason witch causing that effect? It is quite impacting my total THD+Noise %.
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Old 8th November 2019, 09:47 AM   #4054
Salas is online now Salas  Greece
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The Tx fields bouncing off the lid I would think. Impact should be inaudible. Is the lid making continuity with the rest of the chassis?
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Old 8th November 2019, 02:51 PM   #4055
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Yes, eBay purchased aluminum-junk alloy lid does make electrical connection to Earth GND chassis. Strange that it is only left channel is susceptible for that reflection of Tx stray. If it is still below 105 dB in any way, does it make sense to try to use Mu-Metal sheets or some Fe trany covets to attenuate Tx flux? Or it is just unnecessary perfectionism to satisfy placebo?
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Old 8th November 2019, 06:52 PM   #4056
Salas is online now Salas  Greece
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Is the left channel nearer to the Txs in your build?

Yes it makes some sense to experiment with mu metal sheets for barrier or dressing.
Surely for technical satisfaction and maybe for a wee bit of more clarity? But then you would know.

Can you post the lid off & lid on now FFTs?
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Old 9th November 2019, 04:39 AM   #4057
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Quote:
Originally Posted by Salas View Post
Is the left channel nearer to the Txs in your build?

Yes it makes some sense to experiment with mu metal sheets for barrier or dressing.
Surely for technical satisfaction and maybe for a wee bit of more clarity? But then you would know.

Can you post the lid off & lid on now FFTs?
My left channel is located farther from Tx vs. right channel.
Please see test results attached.
I tried to be as detailed as possible there.
I took out my small Tx (witch dedicated to iSelect) for troubleshooting purposes and did not see any impact with or without it. So, it is not one that involved. I also used smaller aluminum cover to simulate partial lid and the effect starts to be obvious when I cover DCSTB from the top. Left or right enclosure sides coverage with that small lid are not effected left channel noise.
Attached Files
File Type: pdf Right Left lid.pdf (1.36 MB, 20 views)
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Old 9th November 2019, 04:42 AM   #4058
alexkosha is online now alexkosha  United States
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Here are my unit images. BTW, I forgot to mention that RMAA test results are very very good still and I see almost same values as audio card loop test.
Attached Images
File Type: jpg image0.jpg (1.02 MB, 109 views)
File Type: jpg IMG_2640.jpg (1.01 MB, 106 views)

Last edited by alexkosha; 9th November 2019 at 05:04 AM.
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Old 9th November 2019, 05:09 AM   #4059
Salas is online now Salas  Greece
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The general results are very good. Still... Don't push the audio interface to 0dBFS because it adds enough of its own THD. Keep it at -6dBFS. Also use 48kHz sampling because 192kHz bumps the interface's noise over 30kHz. Upscale the FFT points in ARTA to 65536 or even better to 131072 if the PC & interface are fast enough. So to see the noise floor better and any spikes better.

Response isn't naturally down not a bit at 20kHz. It must be a trait of the interface or RightMark software & hardware related or something with long high capacitance test cables and interface Zout. DCG3 on gen & scope goes -3dB at 2.9MHz without a pot and 800kHZ-1MHz for 20K pot at half level attenuation worst case point.

Has 107nS rise time for 1kHz square wave when without using a pot (i.e. at 50Ω gen source impedance straight in).

See about the cabling first. Especially the line out route take it close to the chassis floor firstly. Its closest to the lid now. Try shielded cable for the line and headphone output routes too if no better with moving as they snake around a rather long way.

To control what interference the lid sends to the left channel, experiments with Mu Metal sheets could help. Maybe a cover for the preamp board itself, or a barrier around it, or lining of the lid, or the selector & connectors are the pick ups to protect, you will see.
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Old 9th November 2019, 05:31 AM   #4060
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I'll re-run with 48kHz tomorrow. RMAA is not stable with Win 10 and it takes some effort to force it to run. Min dBFS is -3,5 and otherwise it will not start the test. I'll also go for that level during next test.
You're right and my current cables are not "something". Also I have several adapters from RCA to 6.35mm. All these all together probably adding some high pFs. My pot is 25k. Unfortunately, not Goldpoint and not Khozmo are making these with 20k values.

I'll experiment with Permalloy. I'll report soon.
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