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Asymmetric Waveform with Valve Phono Preamp

bar72

bar72

Active Member
Hi all. So I decided to move from solid state to a valve amp. I record a lot of vinyl to computer so I can save on wear and tear but have noticed that my waveforms are off centre. Waveform looks like it's leaning to positive polarity on left channel and negative on right.

Checked for ground loop in my system and if I check waveform in Izotope, there is zero DC Offset present so I'm assuming it's the phono preamp / Valves [EH 6922's].

I've read elswhere this can be caused by Valve Overdrive but I have no idea what this means. Can anyone help with this?

I have ordered a pair of PC88-Tesla's as they have a 7 volt heater as I also read that some amps can run at an excess of 6.3 volt, causing premature failure? Just unsure if this is what is meant by valve overdrive?

Code: 
https://www.watfordvalves.com/product_detail.asp?id=1144

Any help, thanks in advance.

Valve_Amp.jpg
 
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What happens if you switch right and left between turntable and riaa amp ? Does the asymmetry change side ?
 
Hi ,

What preamp are you using , and if you have a schematic that will also help .
The two waveforms look mirror imaged , could you record off of a test record or another source , a 1khz sine wave and or a saw tooth wave .

Also what phono cartridge are you using , it could be as simple as the a design fault in the cartridge or the way the cartridge is set up .

Over drive means that the grid is driven too hard causing clipping , in the output signal
 
@peterh Thank You for reply. I'll get channels swapped.
@battradio Thank You for reply. Parks Audio Budgie preamp, I'll get setup and capture my HiFi New test LP. Using an Audio Technica AT150MLx which ran perfect on my Project phono Box SE. Don't seem to be suffering from clipping, just an off centre waveform.
 
@peterh yes, polarity reverses when swapping leads. as in, negative polarity on left channel (was positive) and positive polarity on right channel (was negative)? thanks

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http://www.diytube.com/phpBB2/viewtopic.php?f=18&t=4755
http://www.diytube.com/budgie/phonoschem.pdf
http://www.diytube.com/phpBB2/viewforum.php?f=26
http://www.diytube.com/phpBB2/viewtopic.php?f=26&t=6325

Which Budgie is it the early one or the arduino controled one .

Tube amplifiers slew much faster than most consumer SS amps , so the tube preamp could show up flaws that a SS might not .

Did you build the Budgie yourself or buy it assembled , and just a little surprised to see one in your location ..
 
http://www.waltjung.org/PDFs/A_High_Accuracy_Inverse_RIAA_Network.pdf

If it is in the preamp , you might need a reverse RIAA network and a signal generator to troubleshoot it , if you have the Arduino Budgie it could be clock noise getting into the audio .

http://www.diytube.com/arduino/budgiepremanual.pdf

http://www.diytube.com/budgie/phonoschem.pdf

http://www.audio-technica.com/cms/resource_library/files/3fb05aa3e1096e5a/at150mlx_review.pdf

What loading capacitance are you using on the cartridge per channel , the factory recommends 100 to 200 PF per channel
 
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@battradio test LP doesn't have a sine wave on it.

I imported it from USA to UK and amp was pre built by Shannon. Only 6 weeks old.

Loading set at 47k on phono pre, van damme phono cable from cartridge to pre with seperate earth wire [rega rb300] so should be within Audio Technica's tolerance levels for cart.

With TT spinning but arm at rest, level meters float around -72dB but this is normal noise floor for the pre, so I believe.
 
No. Tried playing with that. set at 1.0 [Rega arm] and VTF [1.4g]. Strangely enough, the new Telsa PCC88's [7 volt heater] turned up, rolled them in and waveform is not perfect but a lot closer to centre?

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What he said. If flipping the leads at the cartridge shifted the asymmetrical output, the cartridge is what causes it.

frankly I wouldn't worry too much about it, but I've been often accused of not worrying enough about enough things :)
 
wasn't blaming the amp, possibly the valves, but waveforms are perfectly level if I hook up old SS Project Phono Box 2. Anyways, put whole lot on ebay, got TT and Phono Pre sold within 1hr so time to start over ;
 
bar72 said:
wasn't blaming the amp, possibly the valves, but waveforms are perfectly level if I hook up old SS Project Phono Box 2. Anyways, put whole lot on ebay, got TT and Phono Pre sold within 1hr so time to start over ;
Click to expand...
A solid state riaa amp _might_ saturate for surprisingly small peaks, this will manifest itself in a perfect maximum
output level. riaa amps built on tubes won't have that saturation effect .

I once measures 500mV peaks from a shure M75E pickup during dust clicks. ( using a 100mhz tec memory scope)
Guess what a ss amp optimized for 5mV signal does when they come ?
 
thank you for all the replies guys. whatever was happening, I'm heading back to ss, valve's not for me.
 
if you wish to eliminate the preamp as the culprit, get an inverse RIAA (couple parts if
you build it yourself), and a source for sine waves - phones/laptops with sine wave generator app.
and then record something (Audacity) and look at the waveforms.

having recorded lots of LPs back in cassette days, I didn't have many LPs that
didn't require recording volume changes on one channel or the other. Lots of
the differences were label dependent.

you'll have this issue over the course of all your LPs - tube/ss RIAA processing not-with-standing.
 
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