Zero Feedback Impedance Amplifiers

[Susan-Parker]

Hi John,

Originally posted by jkeny
Susan, sent you some emails but don't know if you are receiving due to your computer problems, so this is just a note to see that all is ok at your end?

Computer problems now resolved, thanks.

Best wishes,
Susan.

[jkeny]

Good to hear that Susan - it took a while - I will be in touch via PM shortly

John

[padamiecki]

"I don't think that that the Aikido preamp would be able to drive the 600 ohm load of a Zeus power amp stage with the 5687/6SN7/6AS7/etc. type tubes specified..."

Dear Susan,
Aikido preamp with 6SL7 at the front and 6SN7 at the end has voltage gain ~35 and output impedance ~800 ohm...
It seems to be perfect to drive Zeuses!

[Upupa Epops]

Output impedance isn't the same like load impedance.... But if you like distortion, so go on....

[padamiecki]

Originally posted by Upupa Epops
Output impedance isn't the same like load impedance.... But if you like distortion, so go on....

So what should be output impedance of the preamp to easy drive 600ohm loads?

[Upupa Epops]

Easy drive 600 Ohm ? You can drive Zeus with this preamp, but question is, what is distortion of this amp in this case... I'm afraid that will be too high...

[padamiecki]

Originally posted by Upupa Epops
Easy drive 600 Ohm ? You can drive Zeus with this preamp, but question is, what is distortion of this amp in this case... I'm afraid that will be too high...

agree,
but what output impedance pf the preamp would be enough?
anyway, no matter,
the best to drive Zeus from power output amplifier!

[Upupa Epops]

Is common rule, that output impedance should be minimal ten times lower, than load impedance...

[Sheldon]

Susan,

As an amp builder neophyte, I've enjoyed the thread on this amp because of its unique approach. I'd like to try it, but with a different driver and have a few questions. First I'll explain my mad plan:

I've been building a speaker system that has four drivers per side and will be powered by four amps per side. From about 350 Hz down, I have two solid state amps (Symasym that were from Mike B. here). Above that range is a Unity Horn that has a compression tweeter for about 1100 Hz up, and four mids for the range between 1100 and 350 Hz. The tweeter and mids are about 105-110 dB efficient, so for modest listening levels, I'll have two stereo 801 amps (one built so far) based on a design from Steve Bench: http://members.aol.com/sbench/inp_dri.html, my version here: 801 low power SE amp It does sound very nice, but since it only puts out about 0.5 watts into 8 ohms, it's obviously not going to be able to keep pace if I want to crank up the volume. Sooo..., as it was actually designed as a driver, I thought; why not build a separate output stage that can be added in when needed?

Your amp has many features that make it a good candidate. It has enough power for my needs. It's transformer coupled, which means I might get by without a tweeter protection cap. And, it might well retain the sound character of the driver stage. So my questions, should you be so inclined as to participate in this quixotic quest, are:

As I understand it, the Mosfet end of the circuit doesn't provide significant voltage gain. Since my amp power/voltage output is so low, I would need the input step-up transformer. I could just use what you show, but I could also bypass the opt's on the 801 amp and use about a 1.2k transformer instead of the current 12k. This would mean one less transformer in the signal path, which seem a good idea to me. Your thoughts?

If I do bypass the regular 801 opt, I have a couple of choices. The easiest would be to use the same cap coupled transformer output, just a different transformer. This should work without changing any of the operating points of the 801 amp. It is possible to bypass the resistor load and make a regular single-ended transformer coupled output. I'm leaning against this, because it would likely change the sound, and I'd have to adjust the bias voltage between the two systems. Assuming I get a separate transformer to assume the role of the Zuess input transformer, do I need the center tap for the mosfet bias supply, or can I make a resistor divider pseudo center tap?

With four stereo channels needed, size is an issue. I don't really need 75 watts, certainly not in any continuous fashion, though good peak output can't hurt. I think 15-30 would do nicely, and I don't need frequencies below 300 or so. Can I have smaller transformers wound? I didn't find the winding info on your site that I saw some time ago.

Thanks,
Sheldon

[Sheldon]

Originally posted by Sheldon
[B I didn't find the winding info on your site that I saw some time ago.
[/B]

Found it: http://www.susan-parker.co.uk/zeus-out-tx.htm

Sheldon