I recently purchased a i84 Tube Amp from Boris at Vista Audio. The amp is very well made and sounds excellent. I recently also built a John Lindsay-Hood designed Class A power amp, uses 4 transistors per channel and produces 10 watts into an 8 ohm load at 0.1% distortion. I first did a distortion test and frequency plot on the i84 at work using a Textronix scope, a Racal 2955 test set for distortion and audio voltmeter and a B&K signal generator for audio source.
The results for the i84 are shown below. One channel only was tested.
Left channel 8 ohm resistive load
% distortion Vrms out
0.1 0.72
0.2 1.84
0.3 2.44
0.4 2.725
0.5 3.275
0.6 3.575
0.7 4.050
0.8 4.5
0.9 4.8
1.0 5.3
1.1 5.625
1.2 6.15
1.3 6.4
1.4 6.6
1.6 7.6
1.7 7.9 - onset of clipping
1.8 8.1
1.9 8.25
2.1 8.4
2.2 8.5
2.3 8.55
2.4 8.6
2.5 8.65 - 9.3 watts
Frequency response amp fed via 27k resistor Vin = 3.325 at generator
Frequency Vrms Out
20k 4.175
15k 4.225
12.5k 4.275
10k 4.325
7.5k 4.375
5.0k 4.475
2.5k 4.525
1.0k 4.525
500hz 4.525
250hz 4.525
150hz 4.525
100hz 4.5
80hz 4.5
60hz 4.5
50hz 4.5
40hz 4.5
30hz 4.5
20hz 4.3
excellent bass response
Squarewave response 1 cycle ring before settling down at 1khz
First, I tried the i84 amp using it connected straight to a Toshiba SD-2109 dvd player. Listening/watching Symphonic Yes dvd, Steve Howe's guitar solo was very clean and pleasant, good definition. Next was Starship Trooper. The bass on this track was very respectable, although it could have done with some bass eq uplift. I first tried a pair of speakers with a 6" woofer. Bass was tight but not overly loud. I then switched to a speaker set with a 12" woofer, similar good response, although not much improvement in bass volume, again a little eq would bring it right out. I suspect the amp drops off a little in the lower bass as is to expected. I'm very impressed overall and happy with the amp. My listening room is quite large with a high vaulted ceiling, the amp provided lots of volume, in fact I had to turn her down after a while. Most people would laugh at 10w/channel, but then they don't understand the relationship between sound pressure and power in watts.
I then fed the amp from the Toshiba through a graphic equalizer. I put on Emerson, Lake & Palmer's 1996 DVD of their live concert in the Royal Albert Hall. With no eq the sound was very good, I added a little bass eq and a little boost at 1khz and 2.4khz. The music just shone right out. Carl Palmers drum kit was excellent esp. the hihat ensemble he has. Keith Emersons keyboards sounded out so clear I was hearing subtleties in sound that I had not heard on the DVD before. Brought tears to my ears. And plenty of volume again.....
One set of speakers are home made (2 way with a 12 bass unit) The other pair is a 2 way unit by Quest with a 6" bass driver that I bought to use with my electric piano. Not sure of the sensitivity of either unit really, but the amp drives either pair very well. I listened to Peter Gabriel's "Secret World Tour" dvd last night, I'm still hearing new things on listening with the tube amp.
My friend gave me some Technics SB-G700 loudspeakers just recently, must be 25 years old. Four way units with a 13 inch bass. They are highly sensitive, lots of volume at 5 watts. The 5" paper midrange was far to prominent, I put a 3 ohm resistor to reduce the output from the unit. I replaced the bass drivers with 6" drivers from the Quest speakers I have, the sound is incredible, bass very tight. They have a cone tweeter and a metal tweeter, both have a single volume control on them.
I set myself up to do and A/B comparision of the i84 and the Class A transitor Amp. First I set the equalizer for the i84 at flat. I then fed signals at 60hz, 150, 400, 1k, 2.4k, 6k and 15k to each amp and read the loudness at each point with a microphone and db meter. I set the other equalizer on the Class A amp to the same volume setting at each equalizer point to be the same as the i84.
I went back to symphonic Yes DVD. Jon Anderson has a very unique voice. On the i84 his voice was very smooth, on the Class A a little more edgy. But both amps sounded excellent. I then put on ELP's Lucky Man, again Gregg Lake's voice was just a little more mellow on the i84. Each amp has well defined bass, the i84 a little more hollow, and possibly a little better defined. I then tried Sarah Brightman's La Luna CD. On "a whiter shade of pale" Brightmans voice was excellent on both amps. I then tried an opera DVD, The Gala Concert From Dresden, 1999. Angela Cheorghiu's voice was strong and powerful. In the soprano range there was no discernable difference in the two amps.
The i84 has a sweeter sound at lower listening volumes than the Class A, when cranked up they both do music justice. I believe the difference in sound is so minimal at higher volumes that both can be said to be excellent amps. It seems to my ears that the Class A ouput is slightly compressed compared to the i84, possibly due to higher negative feedback used on the transistor amp.
Its been an enjoyable and interesting day of testing/listening for me. I had built the Lindsay-Hood amp years ago (30 to be rough) and was very impressed then with the sound compared to what the industry was kicking out as transistor amps back then.
All said the i84 is my favourite of the two, but not by much..
Regards John..
