Amplifiers

[Tube Radio]

I have a Carver TFM-15 amp and a Scott LK-72 amp.

I used one then the other to drive a pair of Altec 811B horns.

the Scott sounds brighter on the horns, but does not integrate well with the Carver TFM-35 amp on the Midbass drivers.

The Carver TFM-15 doesn't sound as bright, but integrates much better with the TFM-35.

By integrating I mean how the midbass drivers and horns sound around the crossover frequency of 1.2 KHz.

There are a few possibilities here.

1. The different amp technologies.
2. In reference to #1 the Scott may have to be turned up louder to better integrate with the Carver amp at the crossover frequency.
3. The fact that tube and solid state reproduce different harmonics of the original signal.
4. The efficiency of the horns makes differences in amplifiers more noticeable.

Here's the output waveforms as measured using one channel of each amp connected to the Altec 811B horns. Loose connection with the alligator clip lead which is why the trace for the Carver amp looks like it does.

The Scott has a little hash on the positive peak of the output waveform. Not sure what causes that hash. I did have my xbox360 running along with my HDTV (all near the amp when testing) and cellphone charger (known source of RFI) so maybe one of those caused the problem since I was using unslielded leads to connect the amps to the scope.

 

[JoeESP9]

Unless the Scott has been restored to it's original specs the comparison is unfair. Once it's been brought to original specs my bet is that you'll find most of the gross differences are level related.

Please note, I'm not saying all properly working amplifiers sound the same. I am saying amplifier differences if any (with properly working amplifiers) are subtle and not grossly audible as you describe.

 

[Jon_Logan]

For a more even brightness of the traces, you might see if your scope has "chop" mode. You can use that instead of "alt".

 

[Tube Radio]

The scope traces was bright and I did use chop mode, but one of the leads was making intermittent contact which caused the top trace to look like it did and the camera was fast enough to catch it.

The Scott was fully restored about three years ago.

I applied the signal to the 6U8 pentode control grid through a 15K/15K balanced/unbalanced transformer to bypass the controls since they were not needed with the external active crossover driving the amp.

I adjusted gain until both waveforms were the same amplitude as shown on the scope. Output of both amps was flat from 1.2 KHz to 20 KHz and the amps remained at the exact same or very close to the same output levels.

In order to really tell what the differences are I'd need measurement software and a calibrated mic

 

[Jon_Logan]

Ah, got it. So that's a 2-channel scope but the "shutter" happened to be open while the top trace was being intermittent.

You can look at the wave shape on a scope amongst the 2 amplifiers, but for the differences in sound that you're hearing that's better measured with oscillators, level meters and/or a distortion measuring setup, IMO.

 

[Tube Radio]

Yup. I need to get proper measuring equipment to see what is really going on.

 

[JoeESP9]

A used HP 334A will cost around a kilo buck. If you do a lot of testing and measuring it comes in handy. It's not cheap even used, but worth the money

 

[Tube Radio]

I'll probably just get a calibrated mic and computer software.