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problems with vinyl rips, strange waveforms..., is it a problem with tonearm or pre-amp?
ekg
post Dec 15 2012, 00:44
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So I recently got myself gear for ripping vinyl. It consists of the following:

1. Technics SL-1200MK-2 (used with upgraded RCA and internal ground)
1a. Ortofon Archiv cartridge and needle (both brand new)
2. TC-750LC (a brand new phono pre-amp)
3. Indigo Echo IOx (PCMCIA soundcard for my laptop, new)

I hooked my gear in the following manner:
Technics SL-1200MK2 (internal ground) -> TC-750LC (knob never goes higher than 8.5) -> Echo Indigo IOx (3.5 mm jack)

The interesting thing that happens on ALL my vinyl rips regardless of the level set on the TC-750LC is that the amplitude of the peaks is smaller than that of the valleys by a few db. Here's what a waveform looks like after post-processing.



THIS IS NOT DC-OFFSET

I have tried changing the output of my pre-amp with peak levels ranging from -1db to -8db. I also tried setting it lower than that and the waveform still comes out the same. The soundcard utility/mixer stayed at 100% the whole time. I chose not to adjust it since I can adjust the input via the pre-amp). I could try reducing it as well but I don't see the point when the line-in is obviously not clipping.

This is independent of my soundcard as I tried it on my internal card and had the same problem.

So basically this leave me with it either being the turntable or the phono pre-amp.

Today I tried recording without the phono-preamp and normalized the output to see if it's a problem with the way the tonearm was grounded. The result of this experiment is the following:



The left channel is louder than the right but that's not a major problem for me now

At first glance it looks OK. But looking at it more closely it's hard to say if it's any better as certain parts still appear to have the same problem but it looks obfuscated by noise or whatever else might have gone on in between.

There was something interesting I noticed about the preamp. When I disconnect the right channel from the input but keep the left one in I hear noise. The noise goes away when I touch the right channel of the rca cable (which is unplugged). Doing this the other way, keeping the right one in and pulling the left one out results in no noise. The noise is on both left and right output.

This is leading me to either one of the possibilities: my tonearm isn't grounded properly (it's an internal ground) OR the pre-amp is bust.

Any help on this matter would be most appreciated.

This post has been edited by ekg: Dec 15 2012, 00:45
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DVDdoug
post Dec 15 2012, 01:16
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QUOTE
...amplitude of the peaks is smaller than that of the valleys by a few db.
The waveform doesn't look bad to me? You didn't say there's anything wrong with the sound...

QUOTE
Here's what a waveform looks like after post-processing.
What kind of processing?

QUOTE
Today I tried recording without the phono-preamp and normalized the output to see if it's a problem with the way the tonearm was grounded...

At first glance it looks OK. But looking at it more closely it's hard to say if it's any better as certain parts still appear to have the same problem but it looks obfuscated by noise or whatever else might have gone on in between.
With no RIAA equalization?

QUOTE
There was something interesting I noticed about the preamp. When I disconnect the right channel from the input but keep the left one in I hear noise. The noise goes away when I touch the right channel of the rca cable (which is unplugged). Doing this the other way, keeping the right one in and pulling the left one out results in no noise. The noise is on both left and right output.
Like a hum or buzz? That sounds like the left cable has an open ground, or one of the channels in the tonearm/turntable is ungrounded. That could be intentional... With only one ground path, there is less chance of a ground loop.

QUOTE
The left channel is louder than the right but that's not a major problem for me now
That could just be the stereo recording. If it's on all records, your cartridge may have sligntly more output from one side... That kind of thing happens with analog. wink.gif Or, it could be the preamp... Stereo volume controls don't always track perfectly...

This post has been edited by DVDdoug: Dec 15 2012, 01:16
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ekg
post Dec 15 2012, 02:11
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-The post-processing was noise reduction and volume increase (the peak was originally -8db). The problem is that ALL of my vinyls come out with flat tops as shown above. I doubt they are supposed to be like this.

-Second waveform is with no RIAA equalization (no pre-amp just direct to soundcard then normalized).

-The noise when I unplug the right channel sounds like a low frequency saw wave.

This post has been edited by ekg: Dec 15 2012, 02:12
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ekg
post Dec 15 2012, 02:24
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Here are some waveforms of raw audio with the pre-amp (no post processing). The peak for the top part is about -6 db. The difference in the levels of peaks and valleys will increase when gain is added.
This type of dynamic does not look correct for a vinyl.




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