Analog Discovery 2 test results for various tube amps

[prww]

Thanks for the explanation. I have seen tube amps with a small trimmer capacitor across the feedback resistor. I wondered what you would trim to, or how to see "correct" trim. Makes sense now. Paul.

 

[BmWr75]

Thanks for the explanation. I have seen tube amps with a small trimmer capacitor across the feedback resistor. I wondered what you would trim to, or how to see "correct" trim. Makes sense now. Paul.

The AD2 allows you to run a continuous wave signal. If the amp under test had that feedback trim pot, it could be adjusted while watching the Square wave I imagine.

 

[ronenash]

The ringing is very normal and happens to some degree in any circuit, tube or solid state. Settling time should be short. On poor performing amps you will see this ringing lasting throughout the cycle.

 

[John Berard]

The AD2 allows you to run a continuous wave signal. If the amp under test had that feedback trim pot, it could be adjusted while watching the Square wave I imagine.

I am sure there are many factors relating to square wave over-shoot and ringing. I do know that the test I did on my EL34 did show feedback does have an effect.

 

[prww]

I would think that this square wave "ringing" should be minimized, but since audio is sine waves, how much does it really matter? Does a small amount of square wave ringing effect what you hear or measure in audio? Paul.

 

[restorer-john]

Square waves are incredibly illuminating as to the response, performance and stability of an amplifier. But, they don't tell you how an amplifier will actually 'sound'.

A 100Hz square-wave will show you (with a bit of math) what the low frequency -3dB point is.

A 10kHz square wave will show you a bunch more stuff, including stability, BW, etc.

 

[StepheK]

I would think that this square wave "ringing" should be minimized, but since audio is sine waves, how much does it really matter? Does a small amount of square wave ringing effect what you hear or measure in audio? Paul.

My experience from testing and then listening: Amps with a short duration overshoot on a square wave sound detailed and more open. When I have added caps or done other things to eliminate this in testing, the amp sounded dead/woody with a reduction in perceived detail. I quickly quit worrying about this, and actually worry when it's not there.

 

[ronenash]

My experience from testing and then listening: Amps with a short duration overshoot on a square wave sound detailed and more open. When I have added caps or done other things to eliminate this in testing, the amp sounded dead/woody with a reduction in perceived detail. I quickly quit worrying about this, and actually worry when it's not there.

This is 100% correct and matches my experience as well.

 

[BmWr75]

Can you post a loop-back of the analyzer (generator out to analogue in), so we know what we are dealing with here?

Have not found any way to do this using the AD2, BNC breakout board and Audio Analyzer Suite software.

 

[BmWr75]

Harmon Kardon Trio A224 integrated test results added to Post #12.

Some weirdness going on in this restored amp.

 

[gadget73]

Have not found any way to do this using the AD2, BNC breakout board and Audio Analyzer Suite software.

what are the actual connections for this? I'm guessing you can get output to an RCA plug to get into the amp, but whats the connection arrangement on the speaker side of things?

 

[John Berard]

what are the actual connections for this? I'm guessing you can get output to an RCA plug to get into the amp, but whats the connection arrangement on the speaker side of things?

Flywire connections:


The BNC adapter board and hookups:

 

[BmWr75]

what are the actual connections for this? I'm guessing you can get output to an RCA plug to get into the amp, but whats the connection arrangement on the speaker side of things?

The second picture above answer all your questions @gadget73 ?

 

[gadget73]

yup, BNC in, BNC out. Do you have a BNC to BNC cable? Connecting the input directly to the output with a cable should be all you need to run a loopback test to see the analyzer's internal performance.

 

[BmWr75]

yup, BNC in, BNC out. Do you have a BNC to BNC cable? Connecting the input directly to the output with a cable should be all you need to run a loopback test to see the analyzer's internal performance.

Don’t think I have a cable like that but will look. Never had any BNC cables before getting this AD2.

Edit: just ordered one.

 

[BmWr75]

Rogue Audio M120 monoblocks push/pull - power tests done at 10 watts

The specs for these amps say they are 120 watts/channel, but they are really about 60 watt/channel amps at reasonable distortion levels. Am going to have to remove the cover and see if the amps are set to Ultralinear or Triode mode. Am guessing Triode mode for now for they are only rated at 60 WPC in that mode.

EDIT: Nope, switches are set to the Ultralinear position, see next post for inside pics.

EDIT2: The AD2 unit is limited to 25V input. That is the issue causing the THD vs Power curve to indicate these 120 WPC amps are only putting out 60-70 WPC.

 

[BmWr75]

Inside pictures of Rogue Audio M120 monoblock amp. These amps weight 55 lb. each, big iron in them. Had to remove the KT-88s to get the cover off.

 

[Hajidub]

For those of us dipshits not experienced in graph reading (me included) I'm be curious to see the results in a spreadsheet format w/text, listed best to last (or vice versa).

 

[StepheK]

For those of us dipshits not experienced in graph reading (me included) I'm be curious to see the results in a spreadsheet format w/text, listed best to last (or vice versa).

There is no "best to last", just information on the THD vs power etc. It's not difficult to learn to read a 2D graph.

 

[JonL]

Harmon Kardon Trio A224 integrated (1959 vintage push/pull EL84)

Had to use the balance knob to get the Sine waves to match up. Ended up setting it to about 1:30 o'clock. Can't remember what EL84s are installed in this amp, but that might have something to do with the channel imbalance and other differences shown below. Also have no idea what is causing the weirdness on the THD vs. Power chart starting at about 3.5 watts, but it is coming from the Input Signal (green line). I've seen this strange behavior before testing other amps.

I have one of those amps, been sitting in the "someday I'll get to it" pile for probably close to 30 years. Bought it at a neighbor's tag sale, untested. Found one blown OPT, found a correct replacement that is simply gathering dust with the amp (and the quad of original Mullard EL84s). What did you think of the amp's sound? Maybe I can be motivated to get it working.