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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Audio Measurement and Testing => Topic started by: John Penkala on June 18, 2018, 02:29:40 pm

Title: Best way to do a transfer function of an amplifier output?
Post by: John Penkala on June 18, 2018, 02:29:40 pm
I'd like to do a transfer function of an amplifiers output to "see the processing". How are folks padding down the output of the amplifier to a level that a USB I/O can handle? Any help is appreciated.

JP
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Nathan Riddle on June 18, 2018, 02:46:15 pm
You can connect it directly, just don't exceed 24v or so (for an interface with line level inputs) :P

(I've done it, just be careful of the gain knob ;) )

Any *nearly voltage divider will work though.
http://www.epanorama.net/circuits/speaker_to_line.html/

If you really don't want to blow up your equipment then isolated transformer is best way to go.

If you want to run the amp (depends on max voltage) into clipping then you'd need higher value resistors and wattage.

good luck have fun!
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Ivan Beaver on June 23, 2018, 02:09:55 pm
I built a box that has a pad on the amplifier output, then that goes into an old Altec line level transformer.

The main reason for the transformer is to provide physical isolation between the amp and the interface input.

This way you can look at bridged outputs, or amps that have both output terminals floating.

Be sure to use a high quality transformer to avoid saturation.

The pad size will depend on the size of the amp being looked at.  But if you calculate it to get to around a -10dBu or so, the transformer should be fine.

Or you can simply turn the amp down.

It all depends on exactly what you are trying to measure and why.

BTW I use this method when setting up/matching transfer functions of amps with DSPs in them.
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: David Sturzenbecher on June 23, 2018, 03:43:09 pm
I have always used the VIbox from linearX in the past with great success. Unfortunately they are no longer available due to the passing of the companies owner.


Sent from my iPhone using Tapatalk Pro
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Hayden J. Nebus on July 18, 2018, 05:39:50 pm
I'd like to do a transfer function of an amplifiers output to "see the processing". How are folks padding down the output of the amplifier to a level that a USB I/O can handle? Any help is appreciated.

JP

Most DSP amps have some sort of handle to turn it down, just turn it down by about 30dB. Stim near -20dBfs/0dBu and you should be well above the noise floor still and near unity gain.
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Lyle Williams on August 01, 2018, 11:57:43 am
You really should have a dummy load that can handle full amp output (some great heatsunk monster) and have a voltage divider across that.

But the result will be very very flat.  If there is DSP it will be textbook-perfect EQ and resulting phase shift.
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Ivan Beaver on August 01, 2018, 07:01:33 pm
You really should have a dummy load that can handle full amp output (some great heatsunk monster) and have a voltage divider across that.

But the result will be very very flat.  If there is DSP it will be textbook-perfect EQ and resulting phase shift.
With solid state amps you don't need to put a load on it.
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Justin Segbers on August 13, 2018, 03:13:46 pm
I'd like to do a transfer function of an amplifiers output to "see the processing". How are folks padding down the output of the amplifier to a level that a USB I/O can handle? Any help is appreciated.

JP

Put a countryman Di with it selected to speaker input.  May not be perfectly flat? but I had no issues reverse engineering some l'acoustics presets to load them onto some crown amps for some extra rental flexibility.
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Chris Grimshaw on October 23, 2018, 05:35:29 pm
With solid state amps you don't need to put a load on it.

While true in theory, I think an interesting part of testing amplifiers is when you push them to the ragged edge with 2ohm (or worse) testing and see what they do.
Most amps are capable of low distortion, flat response, etc. The good ones will keep doing that when the rest have overheated and shut down.

I just wish I had a load bank capable of dissipating the multi-kilowatt power levels required.
My last load bank (some fairly big aluminium-clad resistors bolted to heatsinks) did okay until a big Crown amp got all the resistors to go open-circuit in less than 20 seconds.

Next time I'll probably just get some oil-filled electric radiators and put a few in parallel.

Chris
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Mark Wilkinson on October 23, 2018, 05:44:00 pm

I just wish I had a load bank capable of dissipating the multi-kilowatt power levels required.

Chris

Lowes https://www.lowes.com/pd/Utilitech-Water-Heater-Screw-Element/3693202#362342

16 ohms, $6 bucks.....I just stick em, however many needed for load desired, in a 5 gal paint pail full of water...
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Art Welter on October 23, 2018, 06:18:15 pm
Next time I'll probably just get some oil-filled electric radiators and put a few in parallel.

Chris,

Since I already had them, used to do that, but it took 5 heaters to do a 2 ohm load, since those 1500 watt 120V heaters were about 9.6 ohms. Dragging 5 heaters to the shop was too much work, and could only test one side at two ohms.
Then I found that wrapping baling wire around a brick worked quite well, and is even cheaper than Mark W's solution, but does have some heat safety issues  ;)
IIRC, using a  8"x 3" x 2.25" brick and 16 gauge steel baling wire at about 1/4" spacing came out to 4 ohms.

Art
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Mark Wilkinson on October 23, 2018, 08:02:04 pm

IIRC, using a  8"x 3" x 2.25" brick and 16 gauge steel baling wire at about 1/4" spacing came out to 4 ohms.

Art

 ;D

Did peeing on the bricks increase thermal capacity ? Lol
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Art Welter on October 24, 2018, 01:12:58 am
;D

Did peeing on the bricks increase thermal capacity ? Lol
Yes, but changes the impedance too much for repeatable and reliable measurements.
Stinks like hell, too.
Title: Re: Best way to do a transfer function of an amplifier output?
Post by: Frank Koenig on October 24, 2018, 11:50:22 am
I've mentioned this trick before but this sounds like a good place to mention it again. You can arrange the load resistor as a voltage divider so that you can hook a speaker to the tap and hear, at a pleasant level, what the amp sounds like when it's killing itself. Since the attenuation is high the source resistance seen by the speaker is low, and all is hunky-dory. -F