Zero Feedback Impedance Amplifiers

[Netlist]

Originally posted by john curl
This design is almost exactly the circuit with germanium transistors that I have in my almost 40 year old Telefunken Bajazzo portable radio.

I happen to own the schematic from the Bajazzo and scanned the relevant part for your interest and pleasure.
Note the details about voltage and current supplied in those days.
If anyone wants the whole schematic, mail me. It's only a few pages.
I found another Telefunken portable tape deck that uses this topology: the M300 /M302

/Hugo

[Susan-Parker]

Hi Patrick,

Thank you for the positive comments.

Originally posted by EUVL
Susan,

I have only one suggestion to make :

I agree totally with you that the only way to assess a design is to build and listen. So to put the other guys to task who says no before trying, may I suggest you make some enquiries with your suppliers and offer us a set of transformers at a reasonable price (whatever that means).

I guess if it is not too unaffordable, you would not have too much difficulty finding enough 'customers' for a batch of 10 or even 20, which helps to keep cost down.

Once there is feedback from different corners, the sceptics would shut up.


Me, I am not qualified to comment. I have not built one. But I also do not rule out that it can sound pleasant. It is an interesting design in any case, and the audio world is definitely a not a poorer place with it.

Patrick

... and everyone else who expressed an interest in buying.

I can only get at most 10% discount from Sowter and that would not cover the cost of repackaging and reposting to people. Also I am not set up to take CC payments, etc.

The tricky (relativly) part to make is the input transformer, as it needs to be exactly matched for number of turns in each section and as this has lots of turns with fine wire it is difficult to do without a proper coil winding setup. Plus it needs the right laminations etc.

So I would recoment buying directly from Sowter for anyone who is really interested in building this design.

http://www.sowter.co.uk/

Unfortunantly they are about UKP 40 a go, plus VAT and postage, so I hesitate to be too enthusiastic because I would not want anyone to feel that I had "done a dirty" on them if they are unhapy with the results acheived.

Also with Sowter one could order the special lamination mix that they are using for some of the attenuator transformers - although I can't comment on the exact resuits as I haven't bench tested any.

As to the output transformers, they can be made from good quality mains transformer kits - just ensure that the laminations are M6 type not "pig iron".

I can ask Sowter to quote for ready made, but these are something that you might want to experiment with in terms of windings, size and turns. They are probably going to be quite expensive as well, given the weight of laminations (which are charged for by the kilo).

These would probably be around the UKP 100 each mark, plus VAT and shipping. A 200 VA transformer kit from RS is UKP40, although I don't know what lamination material is used for this. (I note that these type of kits are not as readily available as they used to be.)

Please remember though that the output transformers are special and MUST be made with multi-filar windings. They are NOT the same as standard valve output transformers, so don't let anyone try to convince you that my winding topography is wrong as the resultant output transformer if made with sectioned windings WILL NOT WORK!

I have measured the inductance between the primary and the secondary of the output transformer and my LCR meter tells me that this is:

1.95 H @ 1 kHz
148 H @ 120 Hz

Whether this is meaningfulI don't know.

I hope the above helps.

===========

Hi Dave,

Originally posted by planet10
Susan,

A very elegant design... a refreshing idea...

I too was thinking group buy as i read thru this thread...

and some concrete ideas on matching the input requirements with stuff more typically found lying around.

dave

Thank you also for the positive comments.

I should point out that I have deliberatly split the amplification stages differently to normal, and in this regard it is closer to RF practise.

The final power stage - i.e. my amplifier - is designed to sit next to the load - i.e. the speaker - it is driving.

The pre-driver stage is at the source/pre-amp end (and could be part of it) which drives the ballenced line XXX yards to the power amp.

This split provides the optimum signal conditions with isolated differential ballenced line drive that is immune to interferance and ground loops, and a short high current path to the loudspeaker.

Obviously one can put it all together in one box. However it becomes very heavy with all the transformers - particually for a 6:2 surround system!

Good perceived value though!

--------------

Best wishes,
Susan.

[Susan-Parker]

Originally posted by Netlist

I happen to own the schematic from the Bajazzo and scanned the relevant part for your interest and pleasure.
Note the details about voltage and current supplied in those days.
If anyone wants the whole schematic, mail me. It's only a few pages.
I found another Telefunken portable tape deck that uses this topology: the M300 /M302

/Hugo

Hi Hugo,

Thank you for the reference.

However, despite the way they are drawn, the AD155s are PNP transistors and are therefor operating as amplifiers, not followers.

AD155 PNP UcE:15V PT:6.0W C-2D3 SOT-9 AF Output Stage

The input transformer Tr 401 is something like 3.5:1+1 or 2:1+1 step down impedance matching and the output transformer Tr 402 is probably also a step down about 2+2:1 (ish) to match the speaker impedance.

Therefor this circuit is not the same as my amplifier - although this is the topography used in many RF power amplifiers.

I also have similar designs in my Mullard Reference Manual of Transistor Circuits - First edition 1960.

However many thanks to those who have spent time and effort looking up these references, it is all much appreciated.

Best wishes,
Susan.

[dimitri]

push-pull projects by J.Hiraga

[Netlist]

Susan,

You never expected this overwhelming positive response didn’t you?
Thanks for the technical info, I believed John Curl when he said he recognized the same design.
So this means your design would be unique.
I can’t wait to build it. It’s on the list of future projects.

Originally posted by Susan-Parker
I know this because I built my own CD player using a CD-ROM drive and striped out the digital audio data from the signal processing chip to feed into my own Burr Brown DACs to then drive my amps directly.

You should start a new thread on this one too, it’s enlightening to read your posts.

Originally posted by Susan-Parker
... but I am beginning to repeat myself, which is also boring. [/B]

Don’t worry about repeating yourself, we have a tradition here to fabricate monster threads.

/Hugo

[Susan-Parker]

Hi everyone,

Just for your information this is why I built my power amplifiers:



http://www.susan-parker.co.uk/susan-speaker-sphere.htm

and I designed these because I wasn't happy with the sound of my LS3/5As (or the alternatives).

The page text is a bit promotional as I was trying to commercialize the design, but with as much success as with my power amplifiers - i.e. NONE.

Also why I am not trying for 100 Watts! I have a separate bass speaker for that - which amongst other reasons is to minimize the Doppler effects.

Many thanks for everyone's positive comments and encouragements.

Best wishes,
Susan.

[zinsula]

Originally posted by Upupa Epops
Whatabout damping factor, Susan ? Do you think, that several ohms is good ? By this value is bass like snowball throwed on tent .

Doesn't this depend frome the driver too? Note that Susan seems to use line array speakers, each driver being 2".

Maybe splitting bass (driven with SS amp) and the rest would be a solution? But then throwing away the required phase coherence due to the required HP filter?

Tino

[Susan-Parker]

Hi Dimitri,

Thank you for the schematics.

Originally posted by dimitri
push-pull projects by J.Hiraga

I note the following.

1. Uses negative feedback.
2. Has capacitors in the audio path.
3. Uses MOSFETs as amplifiers.

Remember that a transformer couples ALL ways, so load effects not only get fed back round on the feedback to the phase splitter, but also directly into the mosfets themselves. Wibble, wobble!

The Hiraga circuits with mosfet amplification in the final stage (without the feedback) in RF power amplifiers can self destruct if fed into a mis-matched load.

In my amplifier design using followers the mosfets have a gain of just below unity. This is a "good thing" in my book.

It is therefor very tolerant of miss matched loads - which is why I was looking to use it as a wide band RF amplifier for driving CDN units for EMC/RFI compliance testing as the resultant load is anything but a nice stable 50 ohms.

Many thanks for the time digging these schematic out for me, these are all of interest even if not the way I am doing things.

Best wishes,
Susan.

[Audiofanatic]

Originally posted by Susan-Parker
Hi everyone,

Just for your information this is why I built my power amplifiers:



http://www.susan-parker.co.uk/susan-speaker-sphere.htm

and I designed these because I wasn't happy with the sound of my LS3/5As (or the alternatives).

The page text is a bit promotional as I was trying to commercialize the design, but with as much success as with my power amplifiers - i.e. NONE.

Also why I am not trying for 100 Watts! I have a separate bass speaker for that - which amongst other reasons is to minimize the Doppler effects.

Many thanks for everyone's positive comments and encouragements.

Best wishes,
Susan.

Hi Susan,

These are one of the cutest (little) speakers I've seen in my audiophile life. WOW!!!

Audiofanatic

[PMA]

Susan,

how about all of that electronics before your impedance amplifier. Does not it destroy the sound?