[ChewToy]

I was wondering what a good way to accurately test the gain of an amp is. I just completed a CMOY and its supposed to have a gain of 11. I used a 10K pot to control the volume, and the circuits spec is 11, but using a dmm is there a way to verify this? I've had odd results testing an audio signal through there (since its value is constantly changing voltage).

What kinda of range should i look for coming out of a MD (an old sharp MD-702) or PC Card (santacruz turtle beach) for voltage?

what about current?

should i match the impedences of the headphones with the amp (ie the source is 250ohm the headphones should be 250ohm to deliver max power?)

 

[guzzler]

use a DMM to measure the resistance of the feedback resistors...

formula for gain is:

(the one connected to the output ÷ the other one) + 1

or more conventionally (from META schematic...):

(R4/R3) + 1

edit: gain depends on what phones you've got... to be honest, i don't know why the CMoy and META both have gain 11 in their schematic which IMO is dangerously high... for low impedance phones such as Grado, a gain of 2-4; high impedance phones like Senn HD600s, a gain of 7 is often recommended but i only have a gain of 5 for mine! socket the feedback resistors so you can find the right gain for you!

g

 

[ChewToy]

Quote:

Originally posted by guzzler use a DMM to measure the resistance of the feedback resistors... formula for gain is: (the one connected to the output ÷ the other one) + 1 or more conventionally (from META schematic...): (R4/R3) + 1 edit: gain depends on what phones you've got... to be honest, i don't know why the CMoy and META both have gain 11 in their schematic which IMO is dangerously high... for low impedance phones such as Grado, a gain of 2-4; high impedance phones like Senn HD600s, a gain of 7 is often recommended but i only have a gain of 5 for mine! socket the feedback resistors so you can find the right gain for you! g

I'm actually more interested inverifying the gain of my circuit, not just calculating it. um... this was because i noticed that as i turned the 10k pot down (so the volume would go up) it eventually leveled off and got no louder after about 1/2 its full rotation. i'm using HP170's (32 or 40 ohm, which is also variable on each headphone because of the volume controls) right now, but i'll probably be getting the 250ohm dt 770's, so i guess i'll have to wait to then to properly test my CMOY.

edit: I'm kinda new to this type of audio, so you'll have to forgive me, i just joined like a week ago. and of course in that week i've already built a CMOY and plannign to build a meta. god this stuff is addictive

 

[guzzler]

ok, two question then.... what kind of pot have you used? (I know, that sounds stupid!) and what is your power supply??

the gain formula will tell you exactly if you know the values... if you really do want to measure it directly though, you'll need an AC supply (assuming you've got coupling caps on the input) of known voltage; feed that through the amp and measure the output voltage:

out/in = gain

edit: to correct the direction of the pot (assuming you want to that is!), swap the input and ground on the pot...

g

 

[ChewToy]

Quote:

Originally posted by guzzler ok, two question then.... what kind of pot have you used? (I know, that sounds stupid!) and what is your power supply?? the gain formula will tell you exactly if you know the values... if you really do want to measure it directly though, you'll need an AC supply (assuming you've got coupling caps on the input) of known voltage; feed that through the amp and measure the output voltage: out/in = gain edit: to correct the direction of the pot (assuming you want to that is!), swap the input and ground on the pot... g

Oh, i need an AC source, alright thanks! :-D the direction of the pot is fine, its just that after a certain point, the volume stops gaining (when its obviously not turned all the way up), i'm fairly sure its properly wired
The power supply is +4.5, -4.5 with virtual ground 0 (Chu Moy's article), and the POT is the one recommended by tangent (panasonic i think) 10k. thanks again guzzler

 

[PeterR]

Quote:

Originally posted by ChewToy Oh, i need an AC source, alright thanks!

That "AC supply" could be as simple as e.g. your soundcard or CD player playing a sine wave.

 

[needgoodphones]

I doubt that a gain problem would cause the pot to not increase the volume after a certain point. What type of pot are you using? It sounds to me like you may be using a linear pot instead of an exponential audio pot.

 

[guzzler]

thats what I thought originally, but he's using the Panasonic pot recommended on tangents site (presumably the EVJ) which is a dual log pot! unless a mistake has been made!

g

 

[stereth]

Quote:

Originally posted by guzzler unless a mistake has been made!

I wouldn't be surprised. Though I can't find the records right now, I purchased an EVJ with my last Digikey order. I'm sure I ordered a log pot, but they sent me a linear pot with a center dentent (and horrible tracking, btw). Turn it to half and check resistance between a wiper and a pole - if it's a linear pot, it will be 5K, if it's a log pot, it will be either 1-2K or 8-9K.

Anyway, for testing gain. You should be able to play a sine wave and measure the AC voltage with a DMM. You might have to play it into a load, a pair of headphones or a fixed resistor, but I'm not sure. Then test the same thing after the amp (with volume at max--bypass the pot if possible). I'm not sure if you'll have problems with the different input impedences or not.

 

[ChewToy]

Quote:

Originally posted by guzzler thats what I thought originally, but he's using the Panasonic pot recommended on tangents site (presumably the EVJ) which is a dual log pot! unless a mistake has been made! g

Yah its definitely a 10k EVJ, but i'll have to check about the possible screw up at digikey (if they accidently sent me a linear pot). Oh, and does anyone know where i can download a wav/mp3 file for a sine wave (also maybe a site for frequency sweeps to help burn in phone's and test frequency response)?

Thanks again guys

 

[tangent]

More likely you've just hooked it up wrong. Study the wiring diagram on the datasheet and compare it to the symbols on the schematic.

You can make your own sine waves (including sweeps, etc.) with a demo version of CoolEdit. It's easier to use the full version, and it's a great program, so consider registering it. Good stuff.