looking for pp amp schematic without nfb

[robertc]

anyone can show me where i can find a schematic or how to build a pushpull power amp without global nfb. by using el34 or kt88. i heard that amp without nfb is more natural sound than with nfb. is that correct?

[ilimzn]

Originally posted by robertc
i heard that amp without nfb is more natural sound than with nfb. is that correct?

Don't believe everything you hear. Such sweeping statements are bound to be overturned by a single instance where they are proven not to be true. Last I looked, there were many such instances. The absense of NFB is by no means any guarantee of 'natura sound'.

[robertc]

hi,ilimzn
i got 3 diy preamp, i love the one with out nfb. so i like to try the power amp without nfb. i also been told that single ended power amp that without nfb have a very good sound.

[kevinkr]

Take a look at my 300B PP design, originally published in VTV some years back, available on my site now. You can sub a pair of triode connected KT88 quite easily in this design, all that is required is a minor adjustment to the output tube bias setting pot resistor strings.

Http://www.kta-hifi.net

This design has been built by a fair number of people worldwide and was also an inspiration to tubelab who built an "interpreted" version of the amplifier. (different tubes in the driver stage)

Triode connection will give you about 30W with KT88 in this design.

[robertc]

thanks Kevin
i checked your site but i don't see any schematic there.

[phn]

I have this on my HDD. Found it on the Net somewhere.

[phn]

Here's the PSU.

[Eli Duttman]

Relatively reasonable in cost and an EXCELLENT performer, Poindexter's "Musical Machine" is WORTH looking into.

Check the "Musical Machine" out at Poinz's site.

[tubelab.com]

I went to Kevins site and found the schematic here:

http://www.kta-hifi.net/projects/amp...0Amplifier.pdf

As Kevin mentioned, I used this magazine article as the basis for my own 300B push pull amp. There is no feedback anywhere in the design, and it sounds great. It does not however sould like any of my non feedback SE amps. It has a much more dynamic, punchy sound, but lacks some of the detail associated with SE amps (may be the cheap transformers).

I have also built other push pull amps with no feedback, and a couple of unique amps with adjustable global feedback. One of these amps (KT88 P-P) was loaned to several people. This resulted in two things, The amp was sold, and one of the borrowers had me disconnect the feedback in his Cary KT88 amp.

I would attempt to listen to as many amps as possible to find out what you like. A lot depends on your speakers and how much damping they require. Feedback raises the damping factor, which often improves the speaker control in the bass region.

Kevins amp works great with 300B's. Some of this is due to their low plate resistance. Building this amp with KT88's may result in a slight loss of damping. This may or may not be an issue in your case.

All feedback is not evil. A little feedback can tighten up the sound of an amp. Too much will kill the sound. There were formulas in engineering school that reveal that if enough feedback is applied to an amp (any amp) the amp will take on the characteristics of the feedback network. This means that with enough feedback, all amps will sound the same. Have you been to a big box stereo store lately?

Whatever amp you build, allow some extra gain so that some feedback can be applied. If you don't like what it does to the sound, you can always remove it. Just don't get carried away. Most P-P amps may sound beter with a little feedback.

[Miles Prower]

Originally posted by robertc
anyone can show me where i can find a schematic or how to build a pushpull power amp without global nfb. by using el34 or kt88.

EL34s and KT88s are pentode types. These won't sound very good at all without gNFB. Bass will be too sloppy due to the high Zo of the final, and pentodes tend to add considerable, annoying "stridency" to the high end. Triodes such as the 300B, 45, or the vertical deflection amps (6CK4, etc.) might be useable without gNFB.

i heard that amp without nfb is more natural sound than with nfb. is that correct?

Don't believe everything you hear. This certainly is not correct, as NFB, either local, global, or both are highly effective methods for overcoming inevitable active device and xfmr nonlinearities. It's the misuse of NFB that's given it a bad rep in some audiophool circles. If you start off with a fundamentally good open loop design, gNFB can only improve it if done right.