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Simple way to measure preamp output voltage/amp input V...

knockbill

knockbill

Lunatic Member
I searched this, and the common answer is "you don't need to" so that's no help! I assume I need to use a tone signal, not sure what frequency, so it isn't constantly changing...
I have a Ballentine AC volt meter which should measure very small ranges,, DMMs have larger AC ranges, at least the ones I have!!!...
I'm looking to see primarily, what the V out is of these 50C clones... But then I realized, I have a SS phono preamp that will cause certain tube amps to oscillate(motorboat )... If the level pots are low enough, it can work OK, but not at a volume level I want...
May be a stupid question,, but most commercially made amps/preamps list the input output Voltage levels, so it must be relevant... How does one know the output V of scratch built amp/preamp without measuring it tho?
Thanks...
 
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For basic testing I use a 1kHz input signal. If you build a preamp, you should know the type of circuit used, the characteristics of the tubes and the input signal voltage. It's a matter of calculations based on those factors. Without knowing the details, I can't be more specific. If, say, the design offers 4x amplification, you can bring a 500mV signal up to 2V, a 1v signal up to 4V, etc. A sweep tone can later be used to test frequency response and output at other frequencies. Hope this helps.
 
Specifically, you’ll need a 1kHz test tone recorded at 0dB - and a source at which to play it into the preamp. Output should be monitored with a distortion detector (like an SMD DD-1) and output voltage measures with a true RMS volt DMM.

The amp is a tad more complex ...
 
Unless someone comes along that can, and wants to, analyze the circuit to determine the approximate gain based in the parts, it will be faster to measure it.
 
Thanks for replying,,, seems this is more complicated then I thought... The preamps are fisher 50C clones, but the schematics don't give a target output V, other than distortion percentage at given Voltages, maybe that's the info needed to calculate...
Guess I need to understand theory more to understand the answers...
 
If you want to reduce worry about volt meter accuracy, try feeding it some 60 cps. A small variac, and a filament transformer will do quite well.
cheers,
Douglas
 
Then, on the other hand, if they are clones the specs of the 50C should be pretty close.

What I just looked at said the 50C max gain is 22.5dB on the high level inputs. The specs also listed 0.4% distortion @ 15V output.

But, not too many amps require more than 2.5V input, and many require only 1.5V give or take for full rated output. So, with 22.5dB gain possible you need only about 190mV for 2.5V out or about 113mV input for 1.5V output.
 
knockbill said:
Thanks for replying,,, seems this is more complicated then I thought... The preamps are fisher 50C clones, but the schematics don't give a target output V, other than distortion percentage at given Voltages, maybe that's the info needed to calculate...
Guess I need to understand theory more to understand the answers...
Click to expand...

What is your concern about the output voltage?

The ouput voltage varies relative to the input signal strength and the volume control position. There isn't really a "target" output, per se.
 
Variac and small Tx seems like a doable input source... I used the 50Cs as an example, just to see if the output Voltages match on both chassis, as a way of comparing them, maybe not the best way to accomplish that, tho? I can balance the outputs with the level pots, but one seems to drive an amp to distort before the other...

I also mentioned the SS phono preamp, as I have one tube amp that will motorboat when its used... The tube phono preamps (RCA clones) are fine with it...

I figured the output V of the preamps has something to do with distorting/motorboating, and something could be done to either the amp or preamp ckts to adjust it... I may not be approaching this right, or just way over my head here, tho!!!!
I appreciate the replies...
 
Could it be an impedance mismatch that John's actually asking about? Something to think about. I know that FISHER tube amps and receivers generally don't like low impedance Solid State sources like the later R2R and cassette decks. and vice versa. Which is why none of my newer R2R's(akai GX-3700 and Akai GX-4000D's) don't sound good with my 400's and 800c's and consoles. However my TEAC 4000GS and GSL's sound fine. And none of my cassette decks sound decent on my Tube gear. However, hook them up to anything Solid State, and they sound great. So it might be an impedance mismatch he' asking about. The Phono preamp the OP's using might be over-driving the amplifier or near shorting the preamp line sections (impedance mismatch) and causing the "motorboating".

Talk to Dave Gillespie about this. He's very very good on this issue with the early FISHER stuff. If your pre-amps are a close or identical clone to the 50c, and you're going into a Tube unit, you shouldn't have a problem with impedance matching.

I think the voltage question is; Does the tube preamp output pretty much match the input voltage necessary to drive the amp @ full volume. For instance if the amp requires 1v to drive the output @ full volume, does the preamp output enough to match this? You'll also need to know the Amp's MAX voltage to determine wattage at the speaker connections.
 
whoaru99 said:
Then, on the other hand, if they are clones the specs of the 50C should be pretty close.

What I just looked at said the 50C max gain is 22.5dB on the high level inputs. The specs also listed 0.4% distortion @ 15V output.

But, not too many amps require more than 2.5V input, and many require only 1.5V give or take for full rated output. So, with 22.5dB gain possible you need only about 190mV for 2.5V out or about 113mV input for 1.5V output.
Click to expand...

I've been pondering this info,,, seems if an amp requires 1.5V out, and its fed 2.5V, that's going push it to distort? If so, and I can adjust the input with the level pots, that's all that necessary? Or in that case of the SS pre, the amp need to be adjusted to need more gain, so it won't distort at low input levels? However, the SS preamp works as well with other amps same as the 50Cs,, or WA-P2s...
I think I'm asking questions I can't understand the answers to!!
 
Larry,, one of my problems is... Most, OK all, of the tube amps I used with the 50C clones seem to operate the same with them... I built them to known schems, more or less and its only the 811SE amp that motorboats with the SS phono pre...(haven't tried it with the 50cs yet)...
That's why I thought measuring output V of the pres, and input V of the amps was a good thing to know.... but I didn't realize it would be a complicated process,,, and with homemade preamps/amps, I don't know if they are getting the proper V without measuring them...
 
It doesn't put out 1.5V or 2.5V (or whatever) all the time. It starts from 0, with the volume all the way down, and increases the more you rotate the volume control.

For a lot of normal listening a preamp isn't even boosting the signal going into the amplifier, it's attenuating it.
 
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whoaru99 said:
It doesn't put out 1.5V or 2.5V (or whatever) all the time. It varies from 0, with the volume all the way down, and increases as you turn up the volume control.
Click to expand...

The level pot limits input Voltage available thru the amp , and VC also, correct? Level set too high seems to allow amp to distort earlier...
 
The level controls on the back can do a couple of things.

They could set the operating range of the volume control, e.g. if there is only a small usable range of volume control before it's too loud, you could reduce the level setting on the back to get more range of the volume.

Or, you could adjust them to normalize the volume of the different sources.

Or you could do both.

And, yes, if you have a strong source, it might be necessary to reduce the level setting if it is overloading the input.

Bear in mind neither the back panel level controls nor the volume control are limiters, per se. They work together to change the ratio of output to input (net gain).

In effect, you could consider the back panel level adjustments as input volume controls, and the front panel volume control as the master volume.
 
OK,,, That's why I originally asked about measuring Voltage,,, thinking is, V readings would allow comparing the chassis' with each other... May be a better way to compare them, tho... I can adjust the level pots to allow the preamps
to distort earlier or later..., which relates to input V,,, correct?
 
What was said earlier about not needing to measure or calculate the voltage it is fairly true.

I'd think you'd turn the back panel level all the way down for the input you are using. Then, turn the front panel volume to maybe 3/5th or 2/3. Start the source playing with your typical music then increase the rear panel level control until it's as loud as you'd normally ever want, or just a bit more. From that point you use the front panel volume to adjust to your desired listening level.

These are separate mono units, yes? If so, then set the volume controls to the same positions and adjust the back panel levels as above, considering side to side balance.
 
That's basically what I do,,, altho I have the VC pots set at max, and turn the rear level pots til they distort, and back down.... Yes, two mono units, I can easily adjust the rear level pots to balance the L-R output...
I'll fire them up again tomorrow, and take better notes... Just figured measuring output Voltage was the way to see if they are both operating at the same level...
Thanks for the input....
 
Not really an issue, just though it would be a good barometer of matching outputs, level pot, accuracy, etc... with VC and level pots on the pre's and VC pots on most amps... I can balance the speaker output,,,, trying to gauge how well the rebuild is going...
And to take it a step further,, I'm curious why the SS phono amp motorboats on one of the tube amps.... However maybe reading output V may not be the proper way to accomplish this...
 
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