25 WPC stereo amp design, help? (Solid state)

[cmjohnson]

I'm looking for some existing designs for a 25 WPC stereo power amp.

This is kind of interesting in that it can't be MORE than 25 WPC, since the speakers to be used are known to blow if you exceed that power level.

The amp is to be used as the power amplifier for a Behringer digital modelling guitar amp, which only has line level outputs.

It will be driving a pair of 25 watt rated Celestion greenback speakers.


I've been looking around and it seems that my options for buying such an amp, the way I want it, are limited.

My requirements are 25 WPC, stereo, push-pull, and it has to fit into a 1 RU height rack enclosure.

Additionally, since I have the components on hand and want to put them to good use, I would prefer to use the following devices:

Drivers: MJE 15034 and MJE 15035

Outputs: MJW 21195 and MJW 21196.


Class of amplification: I don't think that in this case it's really all that important, as this amplifier will be amplifying a guitar amp signal that will typically be heavily distorted.

And, I want it to be a simple design, easy to build.

Making up a PC board is easy enough once I have the design. I have access
to board makers and I have my own small machine shop, so all I need is
a good circuit to start from.

I freely admit, while I'm a decent technican, I'm no designer. That's why I'm posting here. And, someone here probably already has a ready-made design that fits the bill nicely or can be adapted to it.


CJ

[Tajzmaj]

I will check what I have but are those greenbacks 8 or 16 ohm ? So 1 speaker per each channel?
Regards
Taj

[cmjohnson]

These speakers are 8 ohms, and will be used 1 per channel. Thanks.


CJ

[Bone]

The trouble in saying that it cannot be more than 25 watts, is that if the guitar signal is driven into distortion it is easy to generate more than 25 watts from a nominally 25 watt amp. Maybe you should be looking at a 10 watt amp instead?

[AndrewT]

The operator is generally responsible for blowing up gear.

It is extremely rare for the gear to blow up without the operator abusing said gear.

[lineup]

You should look for an amplifier with +/-25 Volt DC supply rails
See:
http://sound.westhost.com/project3b.htm

[cmjohnson]

Distortion does create more energy vs. RMS sine waves, that's true.

This should be looked at from a voltage limiting perspective.

It is also expected that the 25 watt rated speakers will handle the output of a 25 watt (undistorted) amplifier and also handle its output with any amount of hard clipping.

It's not at all unusual for a set of four of these speakers to last a lifetime when connected to a 100 watt Marshall all-tube amp cranked all the way, with extreme levels of distortion. So if four of these can handle 100 watts plus distortion, then one can handle 25 plus distortion.

However, the classic Marshall amp that they'd normally be used with has a tube output stage, transformer coupled, and runs a quartet of EL34s, 6550s, or 5881s.


It's true that the operator is generally responsible for blowing up equipment, but
in this particular application, it's considered normal to be able to run the amp at full
output in heavy distortion and nothing should blow up.

Now, I'm not going to be driving the solid state power amp to clipping. It needs to have
enough headroom to avoid that. But it does have to have enough headroom to handle
the full output level of the preamp and also be able to drive the speakers to their design
limit of 25 watts RMS.

CJ

[digits]

As far as I know its actualy normally better to have an amp several times the power rateing of your speakers.... with judicial use of gain and volume control.

[AndrewT]

Originally Posted by cmjohnson

It's true that the operator is generally responsible for blowing up equipment, but in this particular application, it's considered normal to be able to run the amp at full output in heavy distortion and nothing should blow up.

This does not sound like Audio Signal Reproduction.

This is an Audio DIY forum.

[CBS240]

Originally Posted by digits

As far as I know its actualy normally better to have an amp several times the power rateing of your speakers.... with judicial use of gain and volume control.

I agree. As a general rule, I like to run speakers at about 60% rated RMS power as the maximum. IOW, ~90Wrms into a 150Wrms speaker. This prevents the destruction and quick wear of the reflex and spider. Besides, once you reach this level of diaphram motion, it becomes non-linear and creates its own distortion regardless of the signal it is receiving.