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Old 31st October 2012, 04:33 PM   #1
AndrewF is offline AndrewF  Australia
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Default Goldline Phono Tube Pre Amp PCB - Analog Metric

Pre amp noob and starting at the bottom

My power supply is giving 460V rather than the required 400V. I will change out the transformer tomorrow. Some ripple too, guess a better selection of caps will help.

What I am curious about is the slewing of the signals. These pics show ~100Hz sq and tri waves passing thru the circuit. The output is not really what I expected. Could somebody suggest what might be the problem?
The input signals are 200mV p-p.

Thanks

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Old 31st October 2012, 05:24 PM   #2
kevinkr is offline kevinkr  United States
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Are you applying the test signals through an inverse RIAA equalizer first? The RIAA EQ network will seriously distort square and triangle waves due to differing amplitude response and network phase shift with frequency - the inverse equalizer will "predistort"** the input wave form and when passed through the complementary response in your phono pre-amplifier should emerge as a relatively decent looking triangle or square wave. Also 200mV is a very large signal at the input of a phono stage, try 2 - 20mV instead.

**Admittedly not the best choice of words since the viewed waveform distortion is due to differences in phase and amplitude over frequency and are reversible when the proper EQ is applied
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Old 31st October 2012, 09:02 PM   #3
DF96 is offline DF96  England
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200mV at 100Hz into a phone preamp is likely to cause overload and possibly slew-rate limiting, which may be asymmetric. 200mV at 100Hz is roughly equivalent to 1V at 1kHz, so maybe 46dB overload.
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Old 1st November 2012, 06:03 AM   #4
AndrewF is offline AndrewF  Australia
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Thanks for your replies, I did not consider this aspect of the RIAA network, was only thinking about gain and should have known the 200mV test signal was way too hot.

Replaced the transformer, now runs at 400V, still a slight 50Hz hum to chase down otherwise it works as well as could be expected....
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