anyone build an acoustic amp?

[cheeseman]

i'm thinking of building an acoustic amp just for sh*t's and giggles. i've been reading a fair bit on roll off points, speaker configurations and cabinets and such, but it would be nice to hear and advice or opinions from the huge knowledge base here before starting just so i don't hafta reinvent the wheel.

thanks in advance everyone!

[Wally]

Tube amp, right? the best tube amps I have heard for acoustic have Ultralinear output sections. My Ga-100 with UL output and tube compression is marvelous with an acoustic. I had a Sano with hi-fidelity output, 1X15" and 2X5" tweeters with a paossive crossover that worked very well. A SFTR Ul135 watt is a good acoustic amp..

[cheeseman]

thanks wally! some great advice there. i have tried some solid state units in the local shop and they leave me...cold. i know that solid state seems to be the trend, but damn it, i just can't go that way! i built a couple of 35 watt monoblocks for my home stereo, and the even make cd's sound better in a/b tests!

[David Barnett]

I've been thinking about this as a project too, since I've got an extra cabinet and blank chassis to stuff.

I was thinking about just using a monolithic SS power amp chip for the power section (http://www.guerrillaaudio.com/store/...3875Chip.html), but I like the suggestion upthread to do it as a tube hifi amp. Maybe push-pull 6550s, like a Dynaco MkIII, that'll give 60 clean watts.

For the front end you'll need something with a high input impedance that won't load down a piezo or other acoustic pickups. Low pass and high pass filters, and a few bands of Parametric EQ to notch out feedback frequencies. I haven't looked around yet, but surely there's someone who has Parametric EQ kit boards that could be incorporated.

Anyone got any thoughts about speakers? I was thinking about a 12" co-ax, but don't really know what's out there.

[cheeseman]

i love the looks of the dynaco you mentioned dave. in fact, looking at it quickly, i'm thinking two of those circuits for a stereo amp. very interesting...

here's the speakers i'm looking at:

eminence acoustinator cx2008...

[cheeseman]

i've been reading and drawing and calculating. i've made a little progress, but it's been rough going.

i've decided to go with the dynaco mark III suggestion. i like the economy of the circuit. in fact, it's so straight forward i've decided to go with a stereo approach. here's the topology so far:

single solid state power supply
dual discreet mark III driver circuits on tag boards
dual (stereo) effects loops
sweepable notch filter
eminence acoustinator speakers

i haven't made any concrete decisions yet but that seems to be where i'm heading. i've laid out my tag boards and should have 'em stuffed in a week or so. i've got a note into my friendly neighbourhood eminence guy asking the usual questions about the speakers. they're pretty cheap, so i may get one of each and try em out. there's even a possibility of mix and matching them into one combo. not sure about that yet.

i was hoping someone may be able to help me with the dynaco transformers. i can't seem to find any info on the output transformers apart from them being legendary. if anyone has specs i would be grateful.

does anyone know what the input impedance of the mark iii is? i can't seem to find that info either. perhaps i'm looking too hard...

[David Barnett]

470K I think

[bradpdx]

Heavens, that's a lot of work, but if you have the time and inclination by all means give it a try. I used to have several Dynaco Stereo 70s that I modified to monoblocks for hifi use - they are really nice amps, a great design.

Good luck finding those transformers. I have a feeling they won't come cheap.

An acoustic amp is really a small PA system - fidelity and control are the goals.

I do a lot of acoustic work. For years I carried a small PA system as an "amp" because it was an easy way to use one of my EV floor monitors for the guitar, behind me. I wanted to continue to use the monitors as monitors, and so I didn't build the whole thing into a box. It was a pain but it sounded nice.

But times change and the acoustic amp scene is much better than it was 10 years ago. These days I carry a Fishman Loudbox and I am very pleased with the results. It has a beautiful, warm sound that really works on stage with great EQ and many useful options for DI to the board. Loads of power, I can't clip it, and at 35lbs it is ever so easy to carry.

At most shows, no additional EQ is needed at all; it just works terrifically and is easy to carry at that. Last Saturday I played for about 75 folks using just that amp and no PA, and it worked like a dream and wasn't breaking a sweat.