agedhorse

For 400Hz power supplies, noise can enter through the power supply, through the inductive effects of then magnetic field of the transformer or from the effects due to ground loops. Sometimes, bridging topologies can be beneficial because IF you can get the noise to be common mode, much of that can be rejected because the noise falls in a band where the CMMR of most op-amps (including power op-amps) is the highest. The parent company of Genz Benz (one of the companies I designed for) was part of Kaman Aerospace (K-Max high lift helicopters and other aerospace subassemblies) so there were a lot of exposure to resources that revolved around electrical noise control in 400Hz environments.

The fan on the Subway amps (and other amps that I have designed in the past) are quiet by design. It's not something that can really be retrofitted AND still have acceptable performance under hot ambient conditions, As an example, it's not uncommon to reach 100 degrees F in my area so I typically design to somewhere around 105 degrees F as the high ambient temperature condition. At these temperatures, I doubt anybody would want to either play bass or listen to bass or a band or much of anything IME.

For small amps it's almost impossible to beat the performance of the TDA-2050 without spending a LOT more, but it looks like these are all going by the wayside in favor of integrated class D IC based amps. Look ma, no heatsink! There were a lot of cool tricks that could be done with the TDA series too, even paralleling a couple or 3 to drive a 2 ohm load! Paralleling took a good bit of effort to insure stability, and there was also the bridged parallel set-up that could get might impressive power into a 4 ohm load (close to 100 watts IIRC). All of that's now water under the bridge.

It should be noted that different manufacturers might have different definitions of presence also, so what may be obvious on one amp bay not follow intuitively on all amps that label a control or switch "presence".

Tha bass guitar in the lowest octave produces less than 50% fundamental, the rest of the sound is harmonic series of the fundamental. Position of the pickup in relation to the bridge changes the percentages of the various harmonics (and the percentage of the fundamental in the total), as does the type of string, their material, the position of the strings with respect to the pole pieces (or magnetic fields), it goes on and on. A bass that just played fundamentals would sound very much like a sine wave generator (which is what it would be).

Be careful swapping fans to be sure the characteristics match what the designer intended. the is ESPECIALLY true on amps that have variable speed fan control circuitry. These circuits and the fans must track for the cooling to be the same.

This wouldn't be SOAR protection (in the classic or traditional sense of the approach), SOAR doesn't track temperature, the limits need to be calculated and designed around the design thermal limit. There are some unique thermal limiting circuits, a particularly effective (but very intrusive) one is a combination of SOAR with the addition of thermal feedback that operates on almost a cycle by cycle basis at low frequencies developed by National Semiconductor called SPiKe, (Self Peak Instantaneous Temperature Ke ), the Ke is an abbreviation for a thermal element in the equation algorithm. It was used on some integrated amplifier IC's. It is a way of decreasing the current limit as temperature of the die increases. It's purely a protection mechanism, it induces nasty current clipping into a reactive load. There are other approaches too, some of which feed back thermal information that lowers the threshold of a more conventional limiter circuit, and some that (in the case of an SMPS) feed back thermal information that will reduce the main supply rail voltages. The limiter approach is pretty common with class D amps, especially those using DSP because it can all be integrated in software. Another approach that many lateral MOSFET amps use is the natural behavior of the device itself, where as the current through the device (and the temperature of the device) increases, Vgs also increases which reduces the rated power that's possible. This is a VERY complicated subject with a LOT of highly technical details and math involved to make it work well. The simpler it appears, the more difficult it actually is in practice.

I was just pointing out WHY fan cooling often becomes the better choice if the size of the amp has anything to do with your choice for an amp.

The problem with passive cooling as the power levels increase is that the size of the amp will grow to accommodate the space needed for passive cooling compared with fan assisted cooling. I have designed amps with both types of cooling and I know firsthand the challenges involved.

Agreed, fans don't have to be loud to be effective.

Some players find the need to use an amp for multiple applications (including practicing alone).

There is always the chance that the AC mains could fail too, resulting in the amp stopping at a gig. Putting things like this in their proper perspective helps identify real problems versus perceived problems IME.

Virtually no difference IME.

As long as the noise from the fan is REASONABLE, but Paul, you know my feelings on what real world reasonable are. Everybody has a different take on what's reasonable.

You might be surprised

Beware that the fan's voltage, current, CFM and static pressure ratings need to be very close on any fan that uses feedback speed control. These parameters are an important part of how well the fan speed tracks temperature. It's possible to really screw things up by focusing on the quietest fan, resulting in an amp that will either shut down (if you are lucky) or fail (if you are unlucky).

As power levels increase and size decreases, fan cooling becomes more and more necessary. This is something that the pro audio industry has dealt with for decades. There are good implementations with fans and poor implementations with fans, noise level being the most objectionable issues. Fan cooled amps do not need to be noisy.

That's how that designer chooses to do it, in general it's the exception to the rule. It's no easier or harder to do it pre or post gain control, it's purely the designer's choice, which may relate to what else (voicing filters, effects loops, etc.) they may wish to include in the pre-eq signal path. I'm just suggesting that most choose to take the signal post input buffer and pre gain stage. If it's an issue to have the signal pre or post gain stage, it's good to know this before buying the amp.

It it's switchable, then in the pre-eq position it will almost always be BEFORE the gain control, not after.

Which one is this?

This is generally correct, gain is gain as far as gain before feedback is concerned.

For post-eq DI, that would be always after the preamp. For MOST pre eq-DI's it's before the gain stage. Of course there can be exceptions, but my experience it's about 95% give or take.

On most amps, this is unlikely to be much of an issue unless the gain is really low. Also, if you take the DI pre eq, most amps pick off the signal pre gain control, so this won't be an issue at all. This could only be an issue using a post eq DI out.

Probably not.

If you are hearing "fur" or distortion, then you are clipping something in the preamp. Turn the gain down a bit and you will be fine. Noise is independent of power amp topology or class. Designs (at least many) have gotten better in this regard.