Bass performance differences between different amp types?

[Racket Scientist]

Looking around at options for an amp to drive my two pro audio subs that I've added to my system. They are sealed boxes with a single EV 18" driver each rated for 300 watts RMS. In mulling it over I began to wonder if there are percieved differences particularly for low bass in the performance of different solid state amp types. Obviously its easier to get higher RMS output with class D, but aside from that, would you tend to pick a bipolar transistor, MOSFET, or class D design for low frequencies?

[Rikard Nilsson]

Shouldn´t matter really, it depends on what you refer to as good bass performance?

[Racket Scientist]

The adjectives I've read most reviewers use when describing good bass performance are "authority, slam, and control". I do notice some significant increase in "authority and control" when I've put my class D amp in there (it puts out around 300 watts at 8 ohms) as compared to my small Crown amp that puts out around 40 watts. Of course with that difference in wattage you'd expect that. On the downside, there's that pesky loud fan in the class D amp.

[Charles Darwin]

First off I know next to nothing about the insides of amps.
That out of the way more often than not when commercially available amps are renowned for good bass performance they tend to be bipolar types in my limited experience.
The best bass to me I have ever heard came out of MC2 Audio MC series amps. These are bipolar but MC2 claim the bass performance is due to a "unique current driven driver stage". Again I don't know enough about electronics to confirm or dismiss this…
Myself I am using a MC750 for bass duties and couldn't be happier. And a MC450 for mids plus a T500 for treble so I could be prejudiced! ;-)

[Racket Scientist]

Thanks for the input, and don't worry too much about your limited electronic knowledge - your contribution of the theory of evolution is quite adequate!

The amp that I'd planned to use in that spot is an old Audio Centron PA amp with bipolar transistors (MJ15022, MJ15023) fed by stages containing op amps. Unfortunately its currently not working. So far I've checked the power supply bridge rectifier and power transistors and they seem okay. I guess I'll be pulling the boards and troubleshooting the rest, and probably will upgrade some of the components along the way. I just hate to do all that work, especially if I could get a better result inexpensively. Anybody know of any inexpensive "generic" boards that would be a more modern design that I could replace the front end and driver circuits in this amp with? It would be nice to just pull out the old stuff and replace it with a better sounding and simpler circuit but reuse the old power supply and power transistors. The other courses would be to just toss this one and buy a Peavey IPC1600 or pull everything but the power supply and put in two class D modules.

[Rikard Nilsson]

The maximum wattage is not likely to be used frequently enough, and therefore not have any impact, until you reach the maximum SPL level, so even if the amp can put out 1kW I would seriously doubt it would sound any better at lower levels. However, if the damping factor is much better for one amp compared to another, it could support better control of the speaker which would be possible to hear. Not claiming to be an expert at all, I would say the simplest way to obtain high damping factor is to use large amounts of negative feedback. If done properly this should not cause any other problems, but even if done poorly you are not likely to discover the downsides of it ín the subwoofer frequency range. The human hear is rubbish at these frequencies anyway.

Hope you can get your amp fixed, otherwise why not use the PSU, output trannies and just put together a simple input diff stage, VAS and make a new amp? There are loads of suitable schematics on this forum.



/Rikard

[Racket Scientist]

Thanks.
Do you have a suggestion for a suitable amp that puts out > 250 wpc RMS into 8 ohms and uses a rail supply of around +/- 75 volts? If so can you direct me to it? I'd probably be ahead to just gut this amp and start over.

They're a bit more than I'd like to pay right now, but one option I've considered is a pair of these ColdAmp modules:
https://www.coldamp.com/store/sonora-s750.html

[dave_gerecke]

Here's one possible design. I haven't tried it, but it seems it would do for your application. And if I remember the app note correctly, it is scalable. So you can add more output transistors (up to a point) to increase the maximum available power output.

http://www.national.com/pf/LM/LM4702.html#Overview


Peace,

Dave

[Racket Scientist]

That looks perfect. I've never done a project like this before - I'd assume what I need to do is find someone who makes a PCB using an LM4702, get the IC and populate the board, remove everything before the output stages from my amp, replace it with this board, hook everything up, run some diagnostic tests, and hopefully be off to the races? I'm guessing thats it in theory, then once I get into it I find there are various problems to solve, and hopefully at the end I have a working amp without having spent a fortune. I have rudimentary electronics knowledge, some soldering skills, and have modded tube amps successfully. I've never done much with solid state stuff.

[dave_gerecke]

Hello Racket Scientist, that's one way to do it; but I don't know who is making a PCB for the 4702 driver chip. Depending on the pinout, perhaps you could adapt the PCB that either: audiosector.com or chipamp.com does for the LM3875 or LM3886, but I'm not sure. The other way would be to build your own driver stage, following the schematic in National App Note AN-1490 and the LM4702 datasheet, on a piece of perf board. Then if the output transistors are still good in your amp, use them. Otherwise, ditch the entire circuit inside of the amp, and just reuse the case, heatsinks and power transformer. You should also be able to use the existing rectifier, but I would think it would be better in the long run to get new filter capacitors for the power supply section. Besides, if you really want to get crazy, you could have separate bridge rectifiers and filter stages for each channel of the amp.

Peace,

Dave