Right tool to measure preamp outputs?

[Coytee]

I've got a little meter, a "GMT-12A" made by GB Instruments.

My goal is to take my Peach & measure it's outputs. connect that to the amps and measure THOSE at the outputs.

The meter has:

1. DC V, 10-500 and "max 500V ac/dc"

2. DC mA 0.5-250

3. AC V 10-500

4. Ohms x 1K

My question is, will this work to measure the output of a preamp? Isn't that measured in AC volts? yet this only goes to 10 on AC.

If this will work, which one do I use

??

[OldGuy]

Interesting. The meter you have is really not suited for your project. Do you have a tone source for a test signal? I gather you do not want to do any frequency response testing, but what is that you want to set up or acheive? The instruments to check out the Peach and other equipment would probably cost several 100 times the value of the meter mentioned.

Norm

[Coytee]

My end goal is to get the signal strentgh as balanced as I can. Why?

Because I'll have gain controls on my Peach, gain controls on my 30's and 2 sets of external, 1 internal gain control on my active crossover.

TOO many variables for this dummy so I want to try to measure things so they're as close as I can make them.

I think I was told I need to measure millilvolts at the preamp?

[efzauner]

2 issues here:

1:If you could measure, what would you adjust the output voltages to? This is a real practical question!

2:Choice of equipment. For measuring voltages there are several factors in the choice of a voltmeter:

1: range: Does the voltmeter have a low enough range to measure the preamp output of under a volt, and high enough to measure the 10-50 volt output of a Power amp?

2: Will it measure RMS and at what frequency? Most simple voltmeters will rectify the AC, and then convert the resultant DC peak to RMS by dividing by sqroot of 2. This assumes that the voltage is a sine wave, AND that the circuitry has the frequency response required. Most cheap voltmeters will read 60Hz sine OK, but may not measure 1KHz or 10Khz well. You could do all your tests at 60Hz, or invest in a better quality voltmeter. More expensive voltmeters also measure "True RMS" and will correctly measure the real RMS voltage of a severely distorted signal. But this is not your case. I just went to the Radio Shack web site and the key specs are sadly lacking .

3: The last spec is what the input resisance of the voltmeter is: How much does it load the circuit. This is normally given in ohms/volt. Cheap Analog meters may have 10K-100K while electronic meters will have 1Megohm or more. For measuring preamps and power amps, this is usually not critical since these have low output resistance.

So, where does this leave you:

Make a CD with a 60Hz tone, full amplitude. You can find lots of places on the web to get pc sine generators that will also make .wav files to burn cds. Then get a voltmeter that has a wide enough range of say 500mv to 100volts or more to measure all the outputs you need. Most modern HiFi equipment have very flat frequency response. You could burn a CD with higher frequency tones and test the response of your voltmeter!

[Coytee]

Surely it doesn't have to be this complicated? (can I call you Surely? [6])

To answer your question "1:If you could measure, what would you adjust the output voltages to?"

I have two amplifiers. One with a FIXED input sensativity and a pair with variable input sensitivity.

I'll be measuring the "throughput" (wrong word?) using the 2102 which has the fixed input sensavitiy and once I can get some reading on the output side of that amp, I'll try to adjust the two MC-30's to match it as close as I can. Just that simple.

Then I won't have to try to rely on my ears since I keep going back/forth when I try that method.

[Al Klappenberger]

Richard,

I think what you have run into here is the reason a lot of attempts a bi-amping fail. I think what I would do is play some recording that you are used to and know the volume control adjustment you normaly use with it. Set all the level controls on everything down to zip. Slowly start moving them all up equally until you hear the levels you expect. After you have that, move each on up higher, one at a time untill you are sure it's very loud with no distortion. That will tell you nothing is clipping. After you do that, turn the recording off and listen for hum or hiss. If you hear either you can use the individual level controls to determine what units is casuing the hiss. Turn down the OUTPUT of the unit causing the noise and output of the unit ahead of it up. That should get you in the ball park.

Al k.

[Al Klappenberger]

Richard,

If you'r interested, here's the instrument you actually could use to measure the levels:

http://cgi.ebay.com/HP-3400A-TRUE-AC-VOLTMETER-10-HZ-TO-10-MHZ_W0QQitemZ270021670827QQihZ017QQcategoryZ25411QQrdZ1QQcmdZViewItem

AL K.

[Coytee]

cool, thanks for finding/sharing that.

I'm game to bid on it, any idea what those things are worth?

IE, I'm not concerned enough to pay a couple hundred for it but if I could get it for $50?

Or is that value an insult to it's true worth?

[Al Klappenberger]

Richard,

The price tag on instruments like that depends on their condition and state of calibration. As usual on eBay, you are buying a pig in a poke! Look for any of the HP400_ sereis or newer. I used to have an HP400D and it was a fine instrument. I replace with an HP334A distortion analyzer which has a voltemeter in it. The one I posted is a newer model and probably in better shape. I'll look over what is up for bids and see if I can identify one that would be a good one.

AL K.

[Al Klappenberger]

Richard,

This one looks good:

http://cgi.ebay.com/Hewlett-Packard-HP-400E-AC-Voltmeter-EXCELLENT_W0QQitemZ270023438084QQihZ017QQcategoryZ25411QQrdZ1QQcmdZViewItem

AL K.

[Coytee]

I've got a bid on this one

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ih=017&item=270021670827&rd=1&sspagename=STRK%3AMEBI%3AIT&rd=1

I bid on another one and stopped bidding at $50 as $50 seems to be a high price. I DID talk to the company selling it (the one I bid $50 on) and got their tech guy. Seemed VERY nice and helpful. Was going to even make me a probe for it. Anyways, I didn't want to keep chasing it pricewise so I moved to the one I posted above.

It ends later on today, guess we'll see.

[BEC]

Richard,

You could also consider one of these.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220022686315

I have no idea of the condition of this one, but the meter was well accepted by industry when new.

Bob Crites

[Al Klappenberger]

Richard,

I see you won the HP meter!

There are a few thing you could do with it besides just checking levels into and out of you equipment. If you invested in a calibrated mike you could connect it directly to the input of the meter. If you get a CD of pink noise bands you could actually use it to adjust the frequency response of your room using the equalizer function of your EV digital crossover unit. The meter also has an output on the back that brings any input the deflects the meter on the scale to a fixed level, usually about 150 mV or so. It can be use that way to amplify the mike output.

Al k

[Coytee]

Yes, I evidently won the item. Not heard from the guy on how to pay him nor what shipping might be.

As for the other items you mention, I'll have to tinker with this thing and get a feel for it. You bring up some interesting ideas although at this point, they're way over my head as far as how to impliment them.

[]

[Al Klappenberger]

Richard,

Actually, it's pretty simple. A calibrated mike will come with data telling you what voltage comes out of it with a specific SPL at the mike. Connect the mike right to the input of the meter. Using the dB scale you can figure the exact SPL at any frequency. Put the mike 1 meter in from of the speaker and 2.8V into it and the meter will tell you the sensitivity. With your speaker, it will be very loud, so you just drop the 2.8V down by 10 dB and add 10 dB to the meter reading. Of course you will use the same meter to set the voltage going to the speaker. Don't use a stedy tone though. 1/3 octave pink noise bands is a must. 1/3 ocave "warble tones" work pretty well too. Just dont try to use a steady tone.

Here's a pink noise CD:

http://cgi.ebay.com/Pink-Noise-CD-for-Radio-Shack-SPL-Meter_W0QQitemZ190024819978QQihZ009QQcategoryZ3278QQrdZ1QQcmdZViewItem

I use an Old Colony Mitey-Mike II for this myself. It's frequency response is dead flat from 20 Hz to about 15 KHz. It's only down about 1 dB at 20Khz.

Here's the mike:

http://www.audioxpress.com/bksprods/kits/kd-4.htm

Al K.

[efzauner]

The HP ac voltmeter is way overkill if all you want to do is adjust levels. All you need is an simple voltmeter that measures AC at the voltage range you want. Most PCs have audio outputs that are dead flat and with free sw can generate sine waves, and you can also burn CDs. Use this flat output to test your voltmeter vs frequency.

I just tested my el cheapo 30 year old analog multimeter. The 10V ac range is flat to over 10Khz.