If Wishbass made an amp...

[fdeck]

... it might look like this. Here it is at last, my DIY amp rig. Bear in mind that this is strictly experimental at this point. I will share the intimate design details soon. What you see is:

1. Homemade battery powered preamp, of my own design. Two channels, gain & phase on each input, bass & treble, mute switch, master volume, main and direct outputs.

2. Semi-homemade switching power amp. Started with an AMP1 kit from 41 Hz Audio (www.41hz.com), added a conventional power supply. This puts out roughly 130 Watts.

3. Typical DIY speaker, 12" Eminence 121895 driver from Madisound (www.madisound.com) in a small rear-ported box.

I have gigged with the preamp and speaker for several months. The power amp is the last piece of the puzzle. I have run it at fairly high power into a dummy load, but tomorrow at a big band rehearsal will be its first real test.

[fdeck]

First real use of the DIY amp. I think the amp did just fine, but it may be powerful enough to challenge my speaker. I did not want to play it really loud in between tunes at the rehearsal, so I left all of the knob settings alone, and will try it at home tomorrow during my lunch hour when nobody's around. I am suspecting either port chuffing or maybe excursion limiting.

This is the loudest band that I play with, so it is a pretty good torture test for a rig.

If it turns out I really need more sound on a regular basis, I have my little 1x15 cab in reserve, and it can definitely take the power.

Life's an experiment.

[seamonkey]

That is outstanding! Very classy. Great job.

I'm looking forward to your design specs.

It looks like the speaker is up on a stand?

I've had my eye on the 41hz amps, but the surface mount components scare me.

[robgrow]

Fdeck,

Very nice job. Having built a lot of stuff over the years, I can appreciate how much work went into this. I'd love to see the circuit for your preamp design. I'd also like to hear your views about the 41Hz Amp1 kit.

Do you have some way to filter out subsonic information in your signal path? I'm wondering if your speaker is over-excurting trying to reproduce this stuff. I've found completely flat response to be impractical, especially at higher volume levels. Rolling off the lowest freqs really helps a lot for me.

[fdeck]

Thanks for the kind words. I will wrap up my little document on the preamp this weekend, and put it up on my website.

Today during my lunch hour, I tried out the amp with a speaker that will not over-excurt at the wattage delivered by the AMP1. The problem remained -- when the amp is driven into hard clipping, it puts out non-harmonic "hash" rather than mere harmonic distortion.

A problem with musical amplifier design is that unless you have ample headroom (e.g., a 1200-Watt head for a cocktail jazz gig), how the amp responds to overload signals is crucial. It has to be musically forgiving. The AMP1 does not seem to be. Yet all is not lost. One option that I will now consider is to add a limiting stage before the final power stage, so I can control the limiting behavior. This will be at the sacrifice of a few Watts of ultimate power, but I would rather have (for instance) 120 well behaved Watts than 140 ill-behaved Watts.

Fortunately, the power amp is the least interesting part of my system, since it is just a "wire with gain" after all. It would not break my heart to us a commercial power amp, but I am having fun learning some things I did not know about switching power amp design. One lesson is to carefully test how the amp behaves when you push it to the limits.

[fdeck]

Now I feel pretty dumb. The problem was in my pickup cabling. The amp is fine. I can push it to levels of distortion that are probably not musically useful.

In doing so, the amp activated its current limiting. On this particular chip amp, current limiting consists of going into standby mode until you shut down and reboot.

The tone quality of this amp is stunning, but I can't say that it is gig-ready until I find out how to keep it from shutting down. Thus it may still boil down to limiting the input signal.

Update: The AMP1 board is based on a Tripath TA2022 chip. This chip has a current limit trip point of 8 A. That would not be a problem. The real problem is that exceeding this limit, even briefly, results in the amp going into protect mode. Such a condition could occur if you are driving the amp into hard clipping, at which point it's basically a straight path from the DC supplies to the DC resistance of the voice coil.

In order to be stable, a power amp has to have a current limit that simply clips the signal temporarily, or it has to have sufficient current reserve that limiting never occurs. This would be the case with an amp that has low-impedance drive rating, e.g., an amp driving an 8-Ohm speaker but capable of driving 4 Ohms.

My tentative conclusion is that the TA2022 handles the "holy trinity" of hi-fi amp specs with aplomb -- flat response, low distortion, low noise. But a musical instrument amp needs to handle temporary overload conditions. Unless I can add some active current limiting, or lower my expectations for output power, the TA2022 is not a bass amp.

But it was fun trying.