Bottlehead Forum
General Category => Technical topics => Topic started by: sl-15 on October 12, 2013, 08:54:18 AM
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No sure if this is in the right category so please feel free to move it accordingly. I finally have a working Oscilloscope again. I would like to do some testing. One would be to figure out a channel imbalance that I have in my Seduction due to tube differences. I would like to feed a test tone in my Seduction and look at the output. What would be a good rms voltage/signal strength for that? Some of my MM carts have a 5mV output. Would that be too low to do testing?
Thanks for any suggestions.
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If you feed in 6.4mV peak to peak at 1k Hz it will be 4.5mV RMS. That is the suggested input for the Seduction.
But it has a much greater overload capability. So feed in 1k Hz and watch the output. When it visibly distorts cut it back by 10%.
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Thanks Grainger. How do you get to the RMS value? I thought for a sine wave the RMS is 0.3535 the peak to peak value. So following that the RMS for 6.4V PtP should be 2.2624V. Just checking if I am overlooking something. God idea too to start low and go up until it distorts.
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Root two times the Peak to peak value. At least I think so. Hard to remember that long back.
Nope, that is what I did. It is the peak to peak divided by root two.
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Feed in 4-6mV, not 4.6V.
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Paul,
I had it written 0.004.5V then decided to go to 4.5 and forgot the milli prefix. I corrected the post.
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Thanks Paul and Grainger for chiming in. Here is what I was doing. I am using a computer based Signal Generator, it comes with Pro Tools that I have and is running through an external sound card via RCA into the Seduction. One can choose the frequency, waveform and amplitude in dB. I checked the signal generator output voltage with my multimeter and started at a very low setting, somewhere around 5mV. The problem is that my Oscilloscope (Phillips PM3212, 25MHz) is not able to display a signal that is that low. It finally starts to register at a signal strength right around -40dB which is 26mV. I was feeding this 26mV 1kHz signal into the Seduction, ramping up the amplitude slowly until the sine wave would start to flatten out on top, which means its starting to distort. I thought it was a great way of telling the differences in tube gain for each tube. Anyways the signal that started to distort the tubes measured as follows: it starts to distort at -13dB input which measured 0.49V. At that input the Seduction output measured 32.8V. If I put the voltage difference into this online dB conversion calculator I get 30dB gain. I then started to check another pair of tubes and the results were different. So I guess every tube has its own gain character and the Oscilloscope was a great tool of making these differences visible.
Paul, do you think that this signal is too high for the Seduction?
And Grainger I still do not get your conversion from PtP to Rms. I found this online calculator and it is the same formula that I found in a book RMS is 0.3535 the peak to peak value. ?
Thanks
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The distortion occurs because the first stage is overdriving the second. So the limit is determined by the gain of the first stage times the input signal. If that value exceeds the grid bias voltage of the second stage it will clip. The mu of the first stage with a C4S installed is about 33. Sorry I don't have the second stage bias figure in my head, but it's just a couple volts. So half a volt input is way off the scale for a phono preamp that is designed to be putting out about half a volt with 5mV input.
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Okay, so that means that testing the Seduction with such a high input is just not the right way of doing it? I wonder how I would be able to make the Oscilloscope register such low signals but maybe its just not possible.
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The 26mV you started with for input level is probably ok. I was suggesting that the .49V you ended up measuring with is way too high. Just keep input signal down at the lowest level you can still get a reliable scope trace for.
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Put the scope on the output of the Seduction. The voltage level will be higher and the scope will show both channels at the same time.
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If he is trying to measure gain he's going to need to look at the signal level at both the input and the output.
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Yup. But he is having trouble reading it at that low level.
Stefan, Are you using a X10 probe or is it going into the scope through a set of jumpers? My scope goes down to 10mV per division.
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Grainger, I am feeding the Seduction output directly into the Scope input with RCA to BNC adapters. The 10x probe makes things even more difficult to read/register.
In any case I figured things out a little further.
I was able to get the Signal Generator output down to 6.8mV and get a reading on the Scope. The Scope is not precise enough to be able to see a difference between input and output but thats what the multimeter is for I think. I just wanted to get a visual of the signal and see right away which tube amplifies more/is louder. I am not so interested in the allover gain, but it is rather interesting to see the differences in the types of tubes used. eg. 6DJ8, 6922, 7308.
As an average value with all these different tube types at 6.8mV input (1kHz Sine Wave), I get about 0.5V output. I was not able to determine the gain of that voltage increase in dB yet. But maybe someone with crazy math skills can chime in.
I think at this point I am just playing around with my Scope, trying to learn stuff along the way.
What I have not figured out yet is why the output of the Seduction at 6.8mV input is pretty 'jittery' looking on the Scope and the Multimeter reading wanders up and down slowly as if a cap is slowly charging - getting saturated - and slowly discharging again. Not sure why the output is unstable but he input stays pretty steady at 6.7/6.8mV.
Anyways thanks for the tips.
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Stefan,
What is the lowest setting on your scope, Volts/Division? If it doesn't go to 10mV/Div. you won't see much on the scope. Also be certain that the "Cal" knob at the center of the V/Div. knob is fully clockwise. Otherwise you are attenuating the signal.
The reason I suggested 1k Hz is that it sees little RIAA EQ and is easy to trigger on. Low frequencies are a bear to get to trigger.
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The Scope goes down to 2mV/Div and the Cal knobs are in the right position. I think using such a huge amplification on the scope shows a lot of the noise thats on the 1 kHz signal too. So all I see is a fuzzy distorted waveform in the beggining, but once the signal strength goes up to about -50 dB it starts to smooth out on the scope. It might be that the signal is not that great or that it is simply too silent. Or my scope is not that great at these settings.
Next up will be the Foreplay and then the Sex monoblocks.
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At 2mV/Div. you should have in excess of 4 divisions of signal. You might try just feeding your source directly into the scope.
You could be picking up noise from unshielded connections too.
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I'm back from vacation, and slowly starting to look at old posts. Here's some math:
RMS is peak-ro-peak times 0.35, or peak-to-peak over (2 * root 2) which is also peak voltage times 0.7 or peak over (root 2). 0.49 volts peak to peak is thus 0.17v rms.
dB gain is 20 times the base-10 log of the voltage ratio. The 1kHz gain of Seduction is approximately 40dB, which means the output is 100 times the input.
100 times 0.17v rms is 17 volts rms, and Seduction can put out that much before distortion sets in.
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Thanks Paul for clearing up the math.
Grainger, I forgot to mention that I was feeding the signal directly into the scope in the scenario of my last post. I have to figure out a way how I can feed the signal into the Scope without the cable to see if the cable I am using is responsible for the noisy signal.