How do you guys suggest I find out how much gain my phono pre offers? Welborne labs - diyAudio
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Old 19th January 2006, 05:12 PM   #1
240z4u is offline 240z4u  United States
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Default How do you guys suggest I find out how much gain my phono pre offers? Welborne labs

I have an older, ie early 90s late 80s welborne labs phono preamp. I am trying to find a new cart but have no idea what kind of gain this thing offers. Is there any good way to test or find out?
I was trying to avoid bugging ron at welborne labs.

Evan
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Old 19th January 2006, 05:33 PM   #2
poobah is offline poobah  United States
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You should apply a sine wave at the input of about 50Hz.. above or below this and will either get tangled up by the rumble filter or the RIAA curve. Making a signal this small can be hard. Place a 5 or 10 K resitors in series with a 100 ohm resistor. Drive the series string with your signal generator. Use the voltage across the 100 Ohm resistor to drive the cartridge input. Adjust your generator (or the 5-10K) unitl you see about 0.5 VOLT peak to peak OUTPUT on your scope. Measure the signal generator for peak to peak as well. Then calculate the voltage at the input.

Gain (dB) = 20 * LOG [output (p-p) / input (p-p)]

If you don't have all these toys... maybe better to ask.

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Old 19th January 2006, 06:53 PM   #3
240z4u is offline 240z4u  United States
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What you mean its not written in plain english under the circut board or something? haha

Thanks poobah, I definitely do not have those fun toys to use. Maybe Ill ask first!!

Evan
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Old 22nd January 2006, 10:05 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I would recommend a switched attenuator (Tpad - 600r in & out) before the preamp.
You would need to research the building of this. Just a bank of resistors and 2pole 2way switches.

Then, you stand a chance of measuring the gain fairly accurately.

The idea is to set the input voltage to a value that you can measure. Adjust the attenuator to give a feed into your preamp that produces an output that exactly matches the input voltage. Read off the attenuation on the Tpad. This is your gain.
Because you are comparing the same voltage and the same frequency this method eliminates most of the errors that using a bare DMM would generate.

You can extend the usefullness of your circuit by using it to measure frequency response. Mine seems to work from 15Hz to 200kHz (+-0.05db). The lower limit is set by the sampling frequency of the DMM. I have not properly solved the low frequency measuremnts but by setting an oscilloscpe to no sweep I can estimate the lower end response down to 0.5Hz +-0.5db

Conclusion, you need some equipment. It will be easier to phone or Email the designer.
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Old 15th February 2006, 02:24 PM   #5
kevinkr is offline kevinkr  United States
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Yeah, I would ask Ron Welborne first, and incidentally phono pre-amplifier gains are generally specified at 1kHz. Pre-amplifiers designed specifically for MM/ high output MC cartridges will generally have gains in the range of 36 - 46dB. Ones designed for low output MC types may be 10 - 20dB higher than this.
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Old 16th February 2006, 12:24 AM   #6
240z4u is offline 240z4u  United States
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50db is the magic number, thanks ron welborne!

Evan
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