johnheath

How to hook up and use a scope for measuring gain and more.
« on: December 14, 2016, 08:44:11 AM »
Hi all...

I have recently bought a scope and a function generator to be able to measyre gain and do some analyzis of my mojos and for trobleshooting.

Anyhow... after Reading about it in numerous articles on the net I am still confused and uncertain on how to get it right. Being a total "scope virgin" I would like some advise on how to hook it up with the generator and how to read the figures.

Ok, I understand that this is a huge question but I will narrow it down to my level to get things started :)


If I have a tube preamp and want to measure the overall gain I have read that I send a 1kHz tone to the input... connect the ch1 probe to the same spot and connect the ch2 probe to the output and measure the ratio between the two.

Then I should have number for calculating the gain (dBu???)... but I also have read that when calculating dBm I need the Resistance. But I do not know if it is the DCR of the input transformer or what?

And... If I have a balanced input and output... Is it enough to measure on one leg of the secondary or do I have to measure both?


As you understand there are a some questions here for me to understand :)

I can do the math but as for now my Readings are pretty strange I guess since the gain of the preamp is lower than the signal sent from the generator and would result in a gain of 1 -2 dBu and there must be much more?

Help is very much appreciated.

Best regards

/John
Nothing is impossible - It just takes some more time


johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #1 on: December 14, 2016, 09:00:31 AM »
Some more info...

Probes are set to equal 1x or 10x

I have hooked up the preamp and scope as described above.

I send a 1kHz sine wave, 2V p-p which is clearly shown on the scope.

Reading on the secondary output is 7,4V p-p

Difference would be 5,4Vp-p

Input transformer is a Sowter 4935, 1:7, 200:10k ohm, terminated with a 68k resistor
Output transformer is a Carnhill connected 9600:600ohm, 4:1, no termination

Best regards

/John

Nothing is impossible - It just takes some more time

ruffrecords

Re: How to hook up and use a scope for measuring gain and more.
« Reply #2 on: December 14, 2016, 09:44:01 AM »
Given the transformers you mention, a 2V pp input signal is probably much too large. Both the input transformer and the amplifier are probably clipping. Is this a mic pre? If it is then you need to feed in a signal of about 20mV.

Cheers

Ian

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #3 on: December 14, 2016, 10:00:21 AM »
Thanks Ian

Yes, a micpre

I will check it with a 20mV, but if it is as you say that it is clipping... would that show on the scope... and if so how?

Best regards

/John
Nothing is impossible - It just takes some more time

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #4 on: December 14, 2016, 10:05:44 AM »
Ok - checked with a 20mV, 1kHz, sine and the ratio is still the same.

But I checked the input transformer mentioned above... the signal is strongly attenuated on the secondary and since it is a step up transformer I guess that the signal should be amplified... no?


best regards

/John
Nothing is impossible - It just takes some more time

ruffrecords

Re: How to hook up and use a scope for measuring gain and more.
« Reply #5 on: December 14, 2016, 10:14:13 AM »
Ok - checked with a 20mV, 1kHz, sine and the ratio is still the same.

But I checked the input transformer mentioned above... the signal is strongly attenuated on the secondary and since it is a step up transformer I guess that the signal should be amplified... no?


best regards

/John

Which suggests maybe the transformer is wired the wrong way round?

Cheers

IAn

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #6 on: December 14, 2016, 10:21:30 AM »
My thought too but I am sitting and looking at it now and XLR input is wired to pink and orange leads (4935F) and Brown is wired to grid at first preamp tube...

Best

/John
Nothing is impossible - It just takes some more time

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #7 on: December 14, 2016, 12:24:25 PM »
Disconnected the input transformer and checked with voltages and 20mV in gave 140mV out which is correct with transformer ratio of 1:7. So the transformer was correctly wired.

So... ?


Best regards

/John
Nothing is impossible - It just takes some more time

Andy Peters

Re: How to hook up and use a scope for measuring gain and more.
« Reply #8 on: December 14, 2016, 01:27:02 PM »
If I have a tube preamp and want to measure the overall gain I have read that I send a 1kHz tone to the input... connect the ch1 probe to the same spot and connect the ch2 probe to the output and measure the ratio between the two.

Then I should have number for calculating the gain (dBu???)... but I also have read that when calculating dBm I need the Resistance. But I do not know if it is the DCR of the input transformer or what?

Gain is unitless. If the output is 10 V when you feed in 1 V, then the gain is simply 10. Gain is a ratio of output to input. Gain of 1 is unity. A gain between 0 and 1 is attenuation (output less than input). A negative gain implies a polarity reversal.

Now, it is common to report gain in decibels, or dB, which is logarithmic. dB without a suffix (dBm, dBu, whatever) is unitless. Remember that gain is a ratio, and so too is gain in decibels.  The formula to calculate gain in decibels is simple.

Gain (dB) = 20 log (Vout / Vin)

This is independent of impedance, termination, etc.

What is the difference between gain in dB and a number like +4 dBu? The latter isn't a gain, it's another way of expressing a voltage.  Hidden in that expression, though, is that the number is "voltage with respect to some reference." And the reference is given as 0 dBu = 0.7746 volts rms and I suppose you can look up the derivation of that constant if it's interesting to you, and also the difference between dBu and dBm. The point, though, is that you never refer to gain in dBm or dBu or whatever, only dB.

So in the case of the thing you are trying to measure:

Put one 'scope probe on XLR Pin 2 of the output (and put the probe ground on the chassis), put the other probe on XLR Pin 2 of the input, measure the peak voltage of both, and do the math above, and that's your gain.

-a
"On the Internet, nobody can hear you mix a band"

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #9 on: December 14, 2016, 02:18:48 PM »
Thank you Andy

I have read about dB in different articles and I have also found those Formulas for calculating dB, dBu and dBm and some other... so far so good. Maybe I don't understand my calculator or I might be slow :/ but I give you the figures here and it would be interesting to see what you get out of it? :)

V in = 20mV pp
V out = 56mV pp

Best regards

/John
Nothing is impossible - It just takes some more time


mattamatta

Re: How to hook up and use a scope for measuring gain and more.
« Reply #10 on: December 14, 2016, 03:05:42 PM »
Maybe I don't understand my calculator or I might be slow :/ but I give you the figures here and it would be interesting to see what you get out of it? :)

V in = 20mV pp
V out = 56mV pp


I think your calculator is working, but it's the numbers that are off and causing you to doubt it.

56mV/20mV = Gain of 2.8
20*log(2.8) = Gain of 8.9dB

I'd say either something's wrong with the mic preamp or wrong with the measurement if that's the max gain.  I imagine you're expecting much higher numbers.

If you swap the probe channels (same scope settings) do you get the same numbers on in and out?  That would point to whether it's a scope/probe setting difference causing confusion.


Edit#2:

Some usesfull ballpark figures to keep in mind:

20dB = 10
40dB = 100
60dB = 1000

Every 20dB is x10, and that makes sense because by applying an additional 20dB gain, you're applying an aditional x10 gain.  That's one of the reasons that makes dB useful to work in as the logarithmic dB can be added, whereas the gain of 10 must be multiplied.  For instance,  if you had a signal with an absolute level of +4dBu and amplified it by a gain of 10dB, you'd now have a signal with a level of +14dBu (which is still "an additional 14dB relative to 0dBu=.7746V.  Also important to keep in mind always that 0dB is gain of unity, and thus 0dBsomething = your reference, whatever that may be)
« Last Edit: December 14, 2016, 03:13:55 PM by mattamatta »

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #11 on: December 14, 2016, 03:09:18 PM »
Thank you sir

Actually the preamp is working just fine... good gain and all (I've had for two years now)

I can't go near max gain in recording or live performance so I just don't understand what I am facing at the moment. I will swap the probes and see if there something different... I'll get back =)

Best regards

/John
Nothing is impossible - It just takes some more time

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #12 on: December 14, 2016, 03:18:52 PM »
No, no difference there.

I find it strange since the input transformer itself provide a gain of 7... hmm??

Got to get this working
Nothing is impossible - It just takes some more time

moamps

Re: How to hook up and use a scope for measuring gain and more.
« Reply #13 on: December 14, 2016, 03:34:16 PM »
Have you connected pins 3 of the input and output XLRs to signal common (ground) during the measurement?

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #14 on: December 14, 2016, 03:37:14 PM »
Yes... both in and out
Nothing is impossible - It just takes some more time

moamps

Re: How to hook up and use a scope for measuring gain and more.
« Reply #15 on: December 14, 2016, 03:48:21 PM »
Did you check the probes on test pin on the scope? Do they measure equally?

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #16 on: December 14, 2016, 03:55:53 PM »
Yes, I tested both on the test Point and it was no difference... I also tested with different settings on the generator and they are equal.


Maybe you could explain to me: When I alter the frequency the voltage also alters... as far as I can see the gain is Rising together with the frequency... this is at the output.

The gain of the generator remains the same.

At 1kHz there I have my "low" gain but if I rise the frequency towards 2kHz gain is way higher... is this normal?


Best regards

/John
Nothing is impossible - It just takes some more time

moamps

Re: How to hook up and use a scope for measuring gain and more.
« Reply #17 on: December 14, 2016, 04:02:42 PM »
You didn't connect second wire of the input and output transformer to the ground (pin 3 to pin 1 to the ground on XLRs).

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #18 on: December 14, 2016, 04:08:51 PM »
Hmm... I will try at once and get back to you :)
Nothing is impossible - It just takes some more time

johnheath

Re: How to hook up and use a scope for measuring gain and more.
« Reply #19 on: December 14, 2016, 04:16:53 PM »
A million thank you sir

I owe you one... just the kind of info I needed. The scope is showing the gain I was expecting (roughly based on calculations)

Again a great thank you

Best regards

/John
Nothing is impossible - It just takes some more time