schematic/kit recommendation for low gain output stage

[gary h]

Hi all,

I rediscovered my Aikido preamp recently. I never implemented it because after building I couldn't get rid of the hum in my system. I hooked it up to a 15 watt amp and much less sensitive speakers the other day and realized that the hum was probably due to excessive amplification of the noise floor. The idle voltage at the the output is a few millivolts.

Hooked up to some not-so-efficient 8 ohm speakers and a cheezy little 15W power amp (also turned way down) I discovered that using the Aikido, I really only need a gain of 2x to get desired dBs in a smallish room. (I can hear the chuckles...)

Much attention on this site is focused on improving power, fidelity, efficiency, etc. I'm looking for a tube output stage that can provide me with modest amplification while maintaining the quiet and pleasant sound of the Aikido. (And no offense to those who disagree, I noticed some posters are not fans.) I guess I'm looking for less, not more. Any suggestions? I suppose I should also stipulate a build complexity and price commensurate with my (probably obvious) novice level and budget respectively.

Thanks for the suggestions

 

[vinylkid58]

What tubes were you using in the Aikido?

jeff

 

[gary h]

good question. 6sn7s.

 

[Koifarm]

Use 4 a 5 times voltage devider at output of the akido. Humm will be 5 times lower. Or if you know the input impedance of your amp mutiply this resistance 4 a 5 times and place this resistance in serie in the signal path between output akido and input amp.[book=]%[/book]

 

[Eli Duttman]

Gary,

As you've found out, the "Aikido" has substantial gain. Therefore, you need power amplification of low sensitivity. IMO, it makes little sense to pad the line stage's I/P only to raise the signal level back up. There goes good S/N performance.

Dig into the archives for the "El Cheapo" schematic. Use a 6J6, instead of the 12AT7 shown, as the splitter/driver. Don't alter LTP operating conditions. Heck, a 12AU7 might yield a satisfactory overall gain structure, but you would have to work out the operating conditions. Any 6V6 family O/P tube works with the parts values shown. Triode wired "finals" yield approx. 6 WPC, while UL mode yields approx. 12 WPC. If at all possible, use Dyna Z565 O/P "iron" in either original or "clone" form. The guitar O/P trafo shown in the graphic is only for triode and is marginal in performance.

 

[gary h]

Gary,

IMO, it makes little sense to pad the line stage's I/P only to raise the signal level back up. There goes good S/N performance.

That's what I was thinking, and to my ears it sounds nice with the Aikido seeing a less attenuated line level signal coming in.

I found the schematic. Is there a pcb and/or group buy kit option available for this design?

thanks for the help

gary

 

[Eli Duttman]

I found the schematic. Is there a pcb and/or group buy kit option available for this design?

No. However, Jim McShane offers parts kits for the project. Obviously, you don't need a volume control. You supply a chassis and magnetics. A heavy walled, commercial kitchen, aluminum baking pan makes a cost effective chassis. A complete set of power magnetics can be purchased from Allied Electronics. I'll provide stock numbers, if you ask.

If you use Z565 O/P "iron" definitely increase the I/P cap. value. High pass filtration is present to prevent O/P transformer core saturation. Better "iron" allows the "corner" freq. to be lowered. 0.68 μF. brings the pole below 25 Hz. Good "iron", like the Z565, allows settling in the 16 to 18 Hz. range. Just keep in mind that the lowest note a "standard" double bass plays is 41 Hz. With a few exceptions, "flat" down to 30 Hz. should prove sufficient.