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25 watts Class A or 70 watts Class A/B ?

I understand these days there are many good examples of both Class A and Class A/B amps. To the point that a well designed Class A/B can beat a similar pedigree Class A amp. However my question is in particular related to these two amps:

1. First Watt F5 (Pass Labs)
2. Naim NAP 300

The speaker to be driven is a Tannoy dual concentric Turnberry SE. It is a 93 db sensitive 8 ohm load and in general considered to go well with both high powered and low powered amps.

F5 is a highly purist class A 25 watts design with a robust power supply. It is a push-pull design so it can generally drive difficult loads better than typical single ended first watt models (F3, F4 etc). I have heard the F5 on an Altec horn system and I loved its pure and direct sound.

Naim amps in general has always appealed to me, again because of their very direct presentation. Music has a certain excitement and bounce through them. The NAP 300 is one of their top models and I am sure it will be a very refined amp.

However I will only buy one. Both these amps must be having few fundamental differences in the way they present music considering they are coming from very different designers. Even the topology is different here.

The first question here is, can a 25 watts F5 drive the Tannoys well ? My room size is 200 sqft and I listen to music at reasonably medium to loud levels. I dont play very loud though. I listen to all kinds of music from Mozart to Metallica.

Qualitatively, sound per sound, how do these two amps compare ?
pani

Showing 1 response by almarg

almarg's avatar
almarg
9,670 posts
 
While I have no doubt that in many setups the F5 would provide great sound (within its power range), the facts that it has a very wide specified 3db bandwidth of 1 MHz, a high input impedance of 100K, and uses feedback, raise some caution flags in my mind.

You've probably already seen it, but just to be sure I'll quote this paragraph from the manual:
A caveat is in order here – this is a very wide band amplifier with a high input impedance. In order to prevent the output voltage from bleeding back to the input at very high frequencies (thus making a fine power oscillator), keep the input and output cables separate, and don’t externally connect the speaker ground to the input ground. Good ground shielding on the input cables is important, and caution is called for in using Litz and other specially low inductance / high capacitance cables. I have not seen a specific example of a problem, but historically it is to be expected when an amplifier’s bandwidth exceeds 200 KHz. If the amp makes funny noises, runs extra hot, or blows fuses, this might be an indicator of such an issue.
Also, I would be particularly cautious if the F5 would have to be placed close to a component containing significant amounts of digital circuitry, or if there is reason to suspect that you may be in a high RFI area (e.g., nearby radio stations), or if there is reason to suspect that your AC power is particularly noisy (e.g., nearby industrial plants).

Also, keeping speaker cables as short as possible would seem to be desirable, both to minimize capacitance and to minimize RFI pickup that may be injected into the amp's feedback loop.

Not sure how critical any of this is, but these kinds of things strike me as being potential sonic issues (or worse) with any very wide band high input impedance amp that uses feedback.

Regards,
-- Al
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