OTA - One Transistor Amplifier

[vizion]

Can the OTA be turned into a Power Follower ?

[Mad_K]

It is a power follower

[VladimirK]

Originally Posted by inertial

(sorry for my english)
Hi Mad_K,
I have built the " power follower 99" ( Andrea Ciuffoli design) and
have one observation to do.
After a long experimentations ( two years) of listening tests I have found this amp sounds wonderful on the mid and high but sounds also
without any type of punch / slamm on the bass. Big delusion.
This is not a problem of power, I am sure . I listen at low levels with "easy" speakers ( in mid-field).
I have compared it with a aleph30 and the result is very similar to my ears. Monotriodes also.
Compared with a X-250 the difference is total, another world! One of the best bass I know.
Again I repeat I listen a very low levels.
Is it a sort of "genetic trade/ carachter" of the SE (only output stage
I suppose)?

All the comments are appreciate
Cheers,
Inertial

Although late for answering this post, I would like to add my 2 cents.

First, guys, forget about 4700uF output cap. Decent solution for an output cap, is three "caps" in parallel.
1-st cap: 33...68uF polyprop cap (closest to the schematics)
2-nd "cap": 16...20pcs of paralleled 1000uF Nichikon KZ (or Elna Silmic)
3-rd "cap": 1 or 2 paralleled 10000uF average quality caps in series with 50mOhms resistor (two 0,1R resistors in parallel).
Whatever your impedance calculations say, adding of the 3-rd "cap" is listenable at bass domain.

Next, NoNFB designs are very sensitive (from BASS point of view) to a kind of output transistors. Bass is getting better along this line: lateral MOSFETs, vertical MOSFETs, silicon BJTs, Ge BJTs. The laterals are "afraid" of current, while BJTs and especially Ge BJTs are "glad to accompany" the current. There is a ground to the fact, that Charles Hansen uses BJTs in Ayre amps. Even if Zout of designs with various kinds of transistors are close to each other, bass signature differs substantially.

Deep NFB designs are also not free from effects of output transistors used. With lateral MOSFETs, even design with 60dB of NFB at 20kHz gives a bit weak bass. Vertical MOSFETs and BJTs are better for bass domain.

PF99 is a great IDEA for SE output stage, but for extracting all potential from it, I have ended up with necessity of some pre stages and some so called "current NFB", with 300MHz BJTs at the output. This allows for great sound.

[lineup]

Originally Posted by VladimirK

Although late for answering this post, I would like to add my 2 cents.

First, guys, forget about 4700uF output cap. Decent solution for an output cap, is three "caps" in parallel.
1-st cap: 33...68uF polyprop cap (closest to the schematics)
2-nd "cap": 16...20pcs of paralleled 1000uF Nichikon KZ (or Elna Silmic)
3-rd "cap": 1 or 2 paralleled 10000uF average quality caps in series with 50mOhms resistor (two 0,1R resistors in parallel).
Whatever your impedance calculations say, adding of the 3-rd "cap" is listenable at bass domain.

Next, NoNFB designs are very sensitive (from BASS point of view) to a kind of output transistors. Bass is getting better along this line: lateral MOSFETs, vertical MOSFETs, silicon BJTs, Ge BJTs. The laterals are "afraid" of current, while BJTs and especially Ge BJTs are "glad to accompany" the current. There is a ground to the fact, that Charles Hansen uses BJTs in Ayre amps. Even if Zout of designs with various kinds of transistors are close to each other, bass signature differs substantially.

Deep NFB designs are also not free from effects of output transistors used. With lateral MOSFETs, even design with 60dB of NFB at 20kHz gives a bit weak bass. Vertical MOSFETs and BJTs are better for bass domain.

PF99 is a great IDEA for SE output stage, but for extracting all potential from it, I have ended up with necessity of some pre stages and some so called "current NFB", with 300MHz BJTs at the output. This allows for great sound.

I am not convinced about your 3 Caps concept.
My opinion is that one good quality 4.700uF cap is alright for 8 Ohm speaker.
Are you sure you get better results with your way?

[alant4321]

Originally Posted by lineup

I am not convinced about your 3 Caps concept.

Hi Lineup,
Long time no talk.
Capacitive decoupling (i.e. with capacitor) and direct coupling (i.e. without capacitor) perform different sound quality which is subject to individual taste of sound. If capacitor is used in input/output of circuitry, it must form an analog filter, common RC filter, or even LRC filter at output of power amplifier. (e.g. High pass, low pass, full pass, etc…) Becareful to use it as human audible frequency 20Hz-20KHz cannot be cut off. I can see many people repeat and repeat to misuse it, I have no time to comment anymore. Lastly, becareful to use capacitor!