Tube Phono Preamplifier Measurement Methods

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Re: Tube Phono Preamplifier Measurement Methods

Permanent link to this post Posted by Spaced » Sat Jul 15, 2017 1:48 am

What I meant by no major grounding issues is that there's no audible hum for me during normal playback. I've used this in normal listening before attempting to make measurements. Obviously there are some power supply issues that need to be sorted out.

Yes I agree the first question should be how to I calibrate / create a baseline or reference point.

I'd like to remove the vinyl from the equation and use an inverse RIAA so that results are more uniform.

I don't require laboratory grade readings I'm just trying to do the best with what I have available to me.

I thought my initial question was how to do just that?

Flynwill's response came closest to providing an actual course of action.
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Re: Tube Phono Preamplifier Measurement Methods

Permanent link to this post Posted by flynwill » Sat Jul 15, 2017 4:15 am

kozikowski wrote:And nobody's addressed the vinyl yet. Do you have a CBS Test Record? I still have mine here somewhere.

Koz


MIne is in a box somewhere (and it's not CBS, but some other version... HiFi magazine I think)

Steve is right I did neglect to address the calibration issue. I would probably start by calibrating the output of the Behringer, and then use it's output to calibrate the input. I'd probably just use my relatively new Oscilloscope as for this sort of work 5% accuracy is probably ok. A good DVM can probably be used to get a calibration starting point as well. But as Steve point out you have to be a bit clever, very meticulous, and willing to accept that your measurements will probably not have laboratory accuracy.
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Re: Tube Phono Preamplifier Measurement Methods

Permanent link to this post Posted by kozikowski » Sat Jul 15, 2017 3:00 pm

CBS Laboratories. They were heroically obsessive. They claimed a calibrated, constant-velocity lathe and they published what the rainbow looked like when you held the vinyl disk up to the light. Some engineer died happy.

@Spaced
60Hz is rough to hear, that's an organ pedal, and 120 and 180 are very low level. I'm not shocked you missed the sound. That's why you apply instruments and tools to find under-the-radar problems like that.

Do post back when you find it.

Koz
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Re: Tube Phono Preamplifier Measurement Methods

Permanent link to this post Posted by Spaced » Sun Jul 16, 2017 2:03 am

Can you explain this statement: I would probably start by calibrating the output of the Behringer, and then use it's output to calibrate the input.

Or, can you provide a link or example of this being done?

I appreciate all of your willingness to help, but I still do not understand in the slightest how to apply anything of what you are saying in a practical use case. I'm not asking for a 10-page walkthrough I understand this is a forum but if I am to learn I need to know where to look, and for what, etc. Any links to useful information would be appreciated.

How does one "calibrate" the input and output?

Using a CBS record, what recorded volume should I be using?

What amount of attenuation is correct? Or, if there is no "correct" what should I be looking for?

If it is variable, how would I know when I have reached the right amount of attenuation in my use case?

I understand there is no basic on-size-fits-all solution, but also without any general reference points / approximations of things to look for I am adrift at sea.
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Re: Tube Phono Preamplifier Measurement Methods

Permanent link to this post Posted by Spaced » Sun Jul 16, 2017 2:05 am

"The correct way would be to install a 47k ohm resistor across the inputs because that is the nominal impedance that all turntable cartridges (or at least moving coil ones) are supposed to have. Leaving the inputs unconnected will usually lead to larger measured noise levels as the sensitive inputs pickup EM noise (like the obvious AC hum in your spectragrams). Shorting the inputs out will lead much lower (and equally wrong) noise measurements. Leaving the turntable connected will have two possible issues: First many turntables include a relay that shorts out the cartridge leads (mutes them) until the stylus is safely in the record groove (leading to the "too low" measurement). Second many cartridges are quite sensitive to EM radiation and will often pickup hum from nearby sources (leading you to improperly blame the preamp for the noise.)."


Ok. That's easy enough. Thank you for that.
Last edited by kozikowski on Sun Jul 16, 2017 2:37 am, edited 1 time in total.
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Re: Tube Phono Preamplifier Measurement Methods

Permanent link to this post Posted by flynwill » Sun Jul 16, 2017 4:33 am

Spaced wrote:Can you explain this statement: I would probably start by calibrating the output of the Behringer, and then use it's output to calibrate the input.


Yes what you need is to establish the relationship between actual signal levels in volts and the digital values that you will likely use for the actual measurements.

I would use Audacitys generate function to create a sine-wave of known amplitude play it out the UCA-202 and measure the actual output with whatever instrument is available. You might measure the P-P amplitude with an oscilloscope (hopefully a reasonably calibrated one) or the RMS (or pseudo-RMS) amplitude with an DVM. If the former I'd probably use a 1 kHz sinewave, for the latter I would use a 60Hz sinewave unless I knew for sure the DVM was rated to accurately measure a higher frequency. For this test I would set the software playback gain controls their maximum setting so you have an easily repeatable setup there.

I would probably do the measurement twice, once driving only the measuring instrument and again with 1K resistive load in order to also measure the output impedance of the UCA.

Next step is to connect the output of the UCA-202 back to it's input and to play the same sinewave while recording in Audacity. Again set the gain controls to the maximum, and this will give you the relationship between volts on the input and the digital values.

Once you have a calibrated input and output you have a basis to actually measure the amplifier. Did your reverse-RIAA filter come with a specification as to it's response? My inclination would be to build a passive attenuator (or possibly attenuators) from 1% metal-film resistors to get the level out of UCA-202 down to Phono cartridge levels.

You sound like you know some EE, how good are you at impedance calculations?
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