# Oscilloscope for Testing Amp Output



## bigdwiz (Oct 7, 2010)

I'm on the search for a suitable oscilloscope to assist in testing the output of my old school amps. My dad used to repair TV's and had one which we recently tested and it no longer works (it used tubes). It does not have to be portable, although this would be nice! I don't want to break the bank with the purchase and if anyone has suggestions for obtaining max power w/o an oscilloscope, I'd be interested to hear. I've seen some vids on YouTube showing ways to test max power by using different frequencies and listening audibly for distortion, but I'd like a little more scientific approach.

Maybe some EE here can school me on what to look for? Thanks for any insight!

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## SoundJunkie (Dec 3, 2008)

Dereck,

Does the one you have just need new tubes? If that's the case I have some sources, that I used in the past, for tubes that I use for my home gear. Both NOS and new.


Sent from my DROIDX using Tapatalk


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## bigdwiz (Oct 7, 2010)

SoundJunkie said:


> Does the one you have just need new tubes? If that's the case I have some sources, that I used in the past, for tubes that I use for my home gear. Both NOS and new.
> 
> 
> Sent from my DROIDX using Tapatalk


It appears the tubes are warming up and working. The horizontal sync doesn't work and all we see is one dot in the center of the screen.

On another note, check your email here shortly...I may have found a gem!

-D


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## SoundJunkie (Dec 3, 2008)

If by chance you have the schematics and model number of the O-scope send it to me. My Uncle is a genius of an EE up in Jersey and I can ask his opinion.

Later

E


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## bigdwiz (Oct 7, 2010)

I'll have to check on the model, pretty sure I don't have the schematic for it.


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## bigdwiz (Oct 7, 2010)

cajunner said:


> if your oscope is an old tube unit it's possible that it's loaded with a couple of tubes that are worth more on epay than a used working oscope of newer vintage.
> 
> check out the tubes, you're looking for RCA and whatever Mullard made, and re-branded
> 
> some of these tubes go for upwards of 50 bucks each, and you can get a nice oscope for under a 100, a decent Tektronix or HP.


I will check those tubes for sure! Any specific models of Tektronix or HP I should look for? I know Fluke makes one with built in RMS meter, but a little out of my range (actually a LOT)!


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## Thunderplains (Sep 6, 2009)

I got this a little while back..

DSO Nano v2 [TOL131B2P] - $89.00 : Seeed Studio Bazaar, Boost ideas, extend the reach

I t is the Nano v2 pocket scope.. it is perfect for what I need (testing system, setting gains and levels and some small home stuff) It is only a single channel and you need to upgrade the probe to a 10x probe to handle the voltage on an amp, but all that for less than $100 ($10 probe thru ebay) If you need more, they are introducing a quad channel model in March. It is also open source based which means you can mess around with the way it looks.. Very cool little scope for the $$..


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## bigdwiz (Oct 7, 2010)

Thunderplains said:


> I got this a little while back..
> 
> DSO Nano v2 [TOL131B2P] - $89.00 : Seeed Studio Bazaar, Boost ideas, extend the reach
> 
> I t is the Nano v2 pocket scope.. it is perfect for what I need (testing system, setting gains and levels and some small home stuff) It is only a single channel and you need to upgrade the probe to a 10x probe to handle the voltage on an amp, but all that for less than $100 ($10 probe thru ebay) If you need more, they are introducing a quad channel model in March. It is also open source based which means you can mess around with the way it looks.. Very cool little scope for the $$..


Thanks for the recommendation. Not exactly the same, but similar one here?

Pocket-Sized Digital Oscilloscope (2.8" TFT LCD Module) - eBay (item 200475224453 end time Feb-16-11 06:56:47 PST)

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## Thunderplains (Sep 6, 2009)

bigdwiz said:


> Thanks for the recommendation. Not exactly the same, but similar one here?
> 
> Pocket-Sized Digital Oscilloscope (2.8" TFT LCD Module) - eBay (item 200475224453 end time Feb-16-11 06:56:47 PST)
> 
> --


Previous model.


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## Thunderplains (Sep 6, 2009)

cajunner said:


> bench models, keep an eye out, Tektronix is high reputation, I don't have any real experience with oscopes but I'm going by online reputations
> 
> depending on how much "juice" you find in those tubes, you could pick up a cheap chinese scope brand new, anything made today is like 2 meg or higher on a standalone, I don't know about using USB scopes, never tried 'em but they are usually pretty low on specs. Not much out there that doesn't qualify for audio checks, even the old tube clunkers
> 
> ...


I agree with cajunner here.. A good bench model is hard to beat. If you can spend a few $$, one of the better ones out there for a good price is the Rigol DS1052E:
Amazon.com: Rigol DS1052E 50MHz DSO: Home Improvement
Comes with all the bells and whistles and is a very accurate scope, far beyond what I was going to ever use it for (Thus why I picked up a cheap chinese made one that works very well) and like he said, if you look, you can get a Tek 475 in real good shape for about $200 or so on ebay. But again, if you can spend a few $$ more, I would go with the Rigol.. You will find nothing but positive reviews on that model. You get alot of bang for the buck.


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## bigdwiz (Oct 7, 2010)

Thanks for the replies guys. My main use of the o'scope would be for determining clipping, so I'm not looking to invest in a real fancy model. The portable one recommended is nice in that it won't take up too much space on my bench....need more space for the gear!

Good idea on attending the ham fest, I'll check locally on CL as well. Thanks for all of the info!

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## sqshoestring (Jun 19, 2007)

I think the one I have is a Hitachi 20mhz 2ch and it works fine. The one thing I wish it had is some of them show the voltage on the screen this one you have to count the lines. It was way way less than 100 on ebay iirc not far over 50, but if you get one like that make sure they prove it will read a signal. If it will not lock onto a wave its junk. The probes were 25 or something for a whole set with clips and all, new on ebay.


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## bigdwiz (Oct 7, 2010)

Hey guys, I just came across this Velleman HPS10 portable o'scope. It has a built-in RMS volt meter and measures RMS and Max power output. Unless someone puts up a red light as to why I shouldn't get one, I'm gonna get one and try it out. 

See it here:

Velleman HPS10 PERSONAL SCOPE 10MS/s - eBay (item 230569818946 end time Mar-04-11 10:37:37 PST)

-D

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## EriCCirE (Apr 14, 2010)

I am also about to pull the trigger on a scope. The Velleman HPS10 looks like a fun toy, but will I be able to do everything I want to on it?

I am specifically getting a scope to diagnose and fix a faulty channel on my amp. 3 out of the 4 channels work, and I think it would be a fun project to get her back up and running 100%

$119.95 Here

$109.95 Here

Interesting review

Looks like the HPS10SE is the better route: The HPS10SE is a special edition that﻿ comes with an hard case and﻿ has a backlit screen.

Youtube Review

Quick troll on google shows 139.95 as best price for the HPS10SE


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## DBfan187 (Feb 26, 2010)

Velleman HPS10, I own 2 of them.


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## bigdwiz (Oct 7, 2010)

DBfan187 said:


> Velleman HPS10, I own 2 of them.


Thanks, I ordered the Velleman HPS50 and as soon as I get my dummy loads built (as Fred Sanford would say, "You big dummy!" ), I'll start testing my amp output testing.

I also looked at the HPS10 and that one looked great as well, but the HPS50 allows direct connection to the PC and has a built-in rechargeable battery

Also, I ordered my HPS50 from kitsusa, they had the best price BY FAR, but shipping was high at $30. The unit was $199 when I bought it about a week ago. The HPS10SE was around $160 at kitsusa, try Velleman HPS10SE  for the HPS10SE, they are $149.

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## DBfan187 (Feb 26, 2010)

bought both of mine on ebay for $130shipped each


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## bigdwiz (Oct 7, 2010)

DBfan187 said:


> bought both of mine on ebay for $130shipped each


Great price! The SE models are about $25 more and come with a hard case and back-lit LCD. I couldn't find the HPS50 on eBay for under $300...

It sound like you approve of the HPS10? Can I ask what you use it for? My main purpose is measuring output power, but may have other uses for it as well. 

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## DBfan187 (Feb 26, 2010)

Honestly the 1st one I bought I left it in my jeep over winter and the cold took it's toll on the LCD screen, poor scope. R.I.P.  So I bought another.

I just used them to level match with really. lol


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## bigdwiz (Oct 7, 2010)

Just an update to those who are interested....

I finally got a chance to test the *Velleman HPS50* today. It was pretty simple to setup, even for the o'scope novice like myself.

I don't have my dummy load resistor bank yet, but I do have a 6.5" Realistic Dual Cone car speaker I was willing to donate to the cause. I measured it's resistance to be 4.2 ohms.

I used a 100Hz test tone for my tests and calibrated the HPS50 per the instructions. First, ensure the switch on the probe is set to 10x, also check the display to ensure 10x. Then use the tip of the probe to touch the calibration point on the side of the scope. Lastly, view the waveform, and adjust the small screw on the probe until the wave form has even square waves. (I was wondering what the small, plastic screwdriver was for in the box, now I know!)

You have a few choices for measuring power output. I initially tested Peak watts, then decided RMS was a better test. In the HPS50, it is displayed as *W^4Ω*. You can also set the ohms (Ω) to 2, 4, 8, 16 or 32. Since my speaker measured 4Ω, I set the meter to "*W^4Ω*" for RMS wattage at 4 ohms. I'll eventually put together a demo video of the unit, with setting up the unit for power output being the main objective.

I already had a Rockford Fosgate Punch 4020DSM on my test bench, so I decided it would be a good first test. The "old school" amp is rated at 20 watts x 4 @ 4 ohms. My test signal was a 100Hz tone and my power source was my Audio Authority Model 978 100A 13.8V power supply.

Here are my results:

*76.65 watts RMS per channel* (only 1 of the 4 channels driven) *@ 4ohms, 100Hz @ 13.8v*. Not bad for an amp rated at 20 watts per channel! The main difference to note is I can't measure THD, so Rockford's measurement (although still way underrated) was tested at 0.1% or less THD while mine could be up to 10%. They also test at either 1000Hz or 20-20kHz, my test was limited to 100Hz. Once I get my resistors, I'll be able to test at various frequencies.

I also had a Rockford Fosgate Punch 30HD close by and knew it would be a good test. The amp is rated at 15 watts x 2 @ 4 ohms. I measured *24.75 watts RMS* (from 1 channel) *@ 4ohms, 100Hz @ 13.8v*. 

The HPS50 will connect to a PC via USB and capture real-time data as well. I did not test this, but will in the near future.

So far, the *Velleman HPS50* is a fun tool! 


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## ChrisB (Jul 3, 2008)

I like my Velleman HPS40:









EDIT - I also use these:


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## bigdwiz (Oct 7, 2010)

ChrisB said:


> EDIT - I also use these:


Are those from Parts Express or did you build them? How much wattage for each?

I'm building (4) 200w dummy loads at 8ohms, so I can do 400w at 4ohms or 800w at 2ohms. I won't be able to test a few of my amps bridged at 4ohms, but can do single channels at 2ohms.

I'm essentially building (2) of these in one chassis:

http://terrydownsmusic.com/technotes/dummyload/dummy_load.htm

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## ChrisB (Jul 3, 2008)

bigdwiz said:


> Are those from Parts Express or did you build them? How much wattage for each?
> 
> I'm building (4) 200w dummy loads at 8ohms, so I can do 400w at 4ohms or 800w at 2ohms. I won't be able to test a few of my amps bridged at 4ohms, but can do single channels at 2ohms.
> 
> ...


I have four of these: 4 Ohm 200W Non-Inductive Dummy Load Resistor

They are 4 ohm, 200 watt, non-inductive dummy loads. I've hit a single 4 ohm one with short blasts of nearly 500 watts RMS, and it took it like a champ. I wouldn't recommend going over their rated power for long term testing though.


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## bigdwiz (Oct 7, 2010)

I looked at the PE dummy loads and thought they were priced a little high...that is until I added up the cost of 50w non-inductive resistors and an aluminum case to hold them all...I guess I saved about $40, but will have to do some soldering and mounting of the resistors. Not "plug and play" like the PE models.

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## bigdwiz (Oct 7, 2010)

Just another quick update to those interested in the Velleman HPS50....I have been unable (so far) to get the software on my Windows 7-32bit machine to see the HPS50 for capturing the output. The driver installs fine, but I can't get the Velleman software to see the device. You choose it by COM port. I believe this is a Win 7 permissions issue and will work to find a resolution.

Despite the Win 7 issues, the capture software works fine with a Windows XP SP3 computer (I tested 2 different laptops with Win XP and they both worked without an issue). I knew there was a reason I decided to create a dual-boot system on one of my laptops! I have not tested within an XP Virtual Environment, but will soon. :computer:

I'll create a video demo of the HPS50 soon...


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## sqshoestring (Jun 19, 2007)

I used those big ceramic resistors, managed to mount them in an old amp case. Then used part of another junk amp to mount a PC case fan on top of them. They are 4" long or so, I forget if they are 50 or 100w. Can't find a photo of it but was an ugly lanzar amp anyway. Still have not tested with it much, bought some 8ga and have not hooked up the other 30A PS I have.


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## bigdwiz (Oct 7, 2010)

Another update....I got my resistor bank (AKA "Fred Sanford" - You Big Dummy!) assembled today. See the pic and don't laugh too much at my soldering (I'm also out of ring terminals - DOH!). The unit has (4) separate 200W loads at 8ohms for running dual 400W 4ohm loads or a single 800W load at 2 or 8ohms (depending on wiring, parallel or series). See the pic below:











The resistors can handle up to 3x their rated wattage for short bursts. I benched my 1996'ish RF Punch 200.2 today and measured *173.4W @ 4ohms per channel* or *519.8W @ 4ohms mono*. Both ratings were taken using a 100Hz test tone and 13.8V from my Audio Authority 978 power supply. I had the Velleman HPS50 set to Watts RMS mode using the 4ohms setting. I also did the measurements manually by using the AC Volts RMS mode and calculated wattage...no need as the unit does this by itself!

Can't wait to test the amps I displayed in my "Old School Cheater Amps" video...(see below). I'll put up some results over the next week or so.







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## sqshoestring (Jun 19, 2007)

That looks nice, this is going to be fun!


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## bigdwiz (Oct 7, 2010)

This may be a little OT for this thread, but I thought it would be fun to post some of my results here. If you want to read more about my test bench setup, see my posting on my website oldschoolstereo.com for more details.


Here is a spreadsheet of my measurements from the "old school cheater amp" shootout:

(The shocker  was the Punch 40i DSM. Wow, rated at 20x2 and put out nearly 100x2 at 4ohms? Impressive! Now that's what I call getting what you paid for!!)


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## ChrisB (Jul 3, 2008)

Were both channels loaded down when you tested the Orion HCCA 225? I ask because I own FIVE of them, and the highest I ever got out of one was 57 watts per channel just prior to visual clipping with both channels driven into a 4 ohm stereo load. For the test tone, I generally use 50 or 68 Hz, so that may be the difference between your results and my results. 

If you really want to have some fun, you should measure power output between 20 and 40 Hz.


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## bigdwiz (Oct 7, 2010)

ChrisB said:


> Were both channels loaded down when you tested the Orion HCCA 225? I ask because I own FIVE of them, and the highest I ever got out of one was 57 watts per channel just prior to visual clipping with both channels driven into a 4 ohm stereo load. For the test tone, I generally use 50 or 68 Hz, so that may be the difference between your results and my results.
> 
> If you really want to have some fun, you should measure power output between 20 and 40 Hz.


Hey man, yes, both channels were used for the 4ohm test, but only one channel for the 2ohm test. Once I build another resistor bank, I'll be able to do 2 channels @ 2ohms or 1 channel bridged at 1ohm. I was going to add another test frequency just to see if there was much difference. I'll try 40 or 50Hz tonight and post the results. This 225HCCA is the "digital reference" series. I also have a "competition amp/x-over" series I'll test for comparison. And just to be clear, I'm testing RMS Power output, not just Max output.


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