Wednesday, April 15, 2015

Cheap and easy excellent tube amps


The Akai M-8 reel to reel and its re-badged versions (Roberts 770) happen to contain a completely independent pair of very high-quality tube preamps and amps which may easily be removed from the original case and with the addition of one cap value between two points, be turned into really excellent low-power single-ended tube pres/amps. The current price of these units in great condition is around $300 and you can find them for much less as well. Many of them were manufactured and I've read that many were brought to the States by soldiers purchasing them while overseas.

Mind you, we're talking 4 watts per channel at 8 ohms, but if your speakers are 90dB efficient for 1W/meter or 2.83V, and you sit near them at a desk (less than a meter from them), this is a superb project. I'm talking no hiss or hum, and the pre can be used for recording via the headphone out. There are another pair of pres in there, for phono (lacking RIAA correction) and for the tape section, which may also be brought out separately. The pre and the amp can be overdriven for special effects when recording.

We're talking single-ended design, known as being very clean and musical. Modern enthusiasts have discovered the 2A3 and rediscovered the 300B triode and there are many kit and boutique amps utilizing them from decware, tubelab, and more. 2 to 8 or so watts per channel, called "flea powered" amps, but with real detail and musicality. I built a solid-state Chipamp using tantalum resistors and it sounds fine, extremely quiet, etc. at some 70WPC. After getting these amps up and running, I've boxed and forgotten the Chipamp, only missing it for those times I really want to peel the pain off the walls.

If you use a subwoofer and highpass the audio input into these amps, 4 watts above say 150Hz or higher will be more than if loading them with full-range signal. Will be attempting this at some point.

Don't be fooled by the unit in stock condition; monitoring the Mic/Line inputs through the Speaker outputs (all 1/4" jacks) will result in quite a treble boost which is pre-emphasis for the tape section. A couple of high-voltage Mica caps totalling 375pf (used 220 + 150, parallel) across two points flatten the response (and the tape speed tone selector's 17/8 IPS setting will then provide a slight treble boost. I imagine that you could use a switch to bring in and out one of the caps which reduce the treble pre-emphasis, to act as switchable treble boost controls if you need more than provided for with the 17/8s switch.

Instructions on the net regarding disassembly come off as more complex than what I found to be the reality. Perhaps my unit was a later version because no wire clipping was required whatsoever for the basic removal and use modification. There are several multi-pin plugs which must be removed between the amps and tape section, as well as a pair of RCA jacks. One set goes to a DIN jack on the side of the cabinet; I believe this is to tap the phono output...I'm not sure as I've not concentrated upon that part so read up. I've simply disconnected it.

If you're stuck with less than 90dB efficient speakers or have a large listening space, remember that the speakers of yesteryear were designed to be loud when used in combination with such small-wattage tube amps, as are often found in vintage stereo consoles (another goldmine for tube gear, can sometimes be found for very cheap...I missed a Grundig with a reel-to-reel for $30 a few months ago).

Pioneer made a line of highly efficient speakers for hobbyists at one time, the PAX series, which save space and add to coherence by virtue of coaxial mounting. Although the frequency response of most of them will leave you needing to add a graphic EQ in order to tame them, we're talking 100+dB speakers. Put them in a 1/4 wave enclosure or full transmission line...even though they're not indestructable (20 or 30 watt limits), it'll do. Altec also made significant highly efficient coaxial designs if you're super serious about this. One company is even re-manufacturing them at the time of this writing. (Great Plains Audio.)

It bears stating that this preamp/amp combination does not impart some magical glow upon the signal. It's also not overly crisp. It's not "warm" because the treble is slightly rolled off. It just gets out of the's lacking some things which are unpleasant about less well-designed solid state gear.