Zero Feedback Impedance Amplifiers

[Susan_Parker]

Hi,

Originally Posted by apelizzo

I can try with DC but I try to avoid it as I prefer the valve sound with AC heater. With DC heater IMO the the cathode generates a space charge that is polarised and at the end the valve sounds dead. This is just my opinion.

Whilst space charge may have an effect on low current amplification (particularly for directly heated tubes), for an indirectly heated 6C33C running at almost 300mA on a 37 volts anode supply any effect is going to be way below the noise floor.

What is different is the AC fundamental (at 50/100 or 60/120 Hz) and it's harmonics and the intermodulation effects these have with the music, compared to a steady DC state.

That could well be audible as a difference to the sound and the lack of these intermodulation products could indeed make the sound appear less "alive".

See examples.

Best wishes,
Susan.

P.S. I am not completely convinced that there isn't a degree of thermal modulation "punch through" from the AC heaters, even on the indirectly heated ones. This is an area that I have been intending to look at, but just haven't had the time.

[EWorkshop1708]

I'm in the process of building a transformer/transistor amp with PNP transistors simply for the reason of being able to ground the collector leads (metal tabs) to the heatsink chassis without insulators, and also for the (hopefully) simple biasing setup.

Regardless of MOSFET or Bipolar or Tubes, I'm wondering about the paths of current flow in the input transformer of the circuit. Where is the return path, the CT or from the other transistor? Could this be modified without CT? Do I need the opposing diode and capacitor to sustain signal flow if the required driving current is higher than what is provided by the biasing network?

[apelizzo]

Originally Posted by Joao@AltheaMusica

If you have no noice, hassle with ac-filaments then it's ok and why then using dc... What core material you are using?

@Susan
We discussed in a fewer post of this thread a phono stage using interstage trannies. I have designed one in single ended with IS - trannies in the drain - and LCR-RIAA. Sounds impressive.

The core material is M6

[Susan_Parker]

Hi,

Originally Posted by apelizzo

The core material is M6

From my measurements the line driver and output transformer EI-120 size laminations with the high current low impedance drive of the followers in M6 works well.

For the 3/4" (or thereabouts) sized input and step-up transformers which are higher impedance devices I can see a measurable improvement using MuMetal laminations in the mid-band.

The LF end is not so good with MuMetal at higher levels for the power amp input transformer, but the amounts are still small compared to the loudspeaker distortions down at the LF end.

M6 is still fine to use, but if there is an option to add 50% (or 100% as I do) MuMetal for not too much extra then I would say it's certainly worth considering.

For a 5.1 or 7.1 system I would suggest that the Front Left, Front Right and Front Middle would be the ones to use MuMetal, and the other channels and the bass should be fine with M6.

Best wishes,
Susan.

[EWorkshop1708]

I finished it!!!! It lives, and is loud!!!!

The Bass response is really strong, this thing plays lows with ease and really moves the speaker cone effortlessly. The highs are a bit lacking, but it plays clear, adding bias picked up the highs better some.

It plays loud on only 12V! So far I have no heat at all, it runs really cool.

I finally sourced my input transformers, I found two identical 240V/24V 30VA 50/60 Hz transformers from scrap air conditioners. I run them both in series to have a CT.

I tried using the HV (old primary) winding on the output transformer and noticed lack of frequency response and excessive current draw, so I'm using the speaker directly across the CT winding driven by the transistors. Connected direct gives good frequency response and driving capability.

I'm still tweaking the bias before I settle on a value and stick with it. I will post an updated schematic with all the parts values. I'm still having fun playing with it!!!!

May make a good PA amp for simple use........

[Steve Eddy]

Cool beans!

Have any photos you can share?

se

[EWorkshop1708]

Here it is! Finally a loud and simple 12V amp that does not require an SMPS!

This is the final changes to the circuitry. Feel free to enjoy, copy, modify, or make improvements as needed. Thanks to everyone here @ diyaudio for keeping this discussion going and for the tips required to get it all right!

Also uploaded some pics. All PTP, no PCB! As you see, this is a very compact and heavy little amplifier. The output transistors are on the heatsink, concealed by the 3 transformers.
Don't mind the big ball of electric tape with the power wires, it's just covering up my 35V 4700uf Capacitor and Ferrite Toroidal DC filter.

Now about the Amp.............
Bass Response is excellent. Audio on this amplifier even seems bass-boosted.
Treble is good now, I find a trick is to put a 4.7 or 10 ohm resistor in series with input to boost upper highs.

PNP transistors work very good, screwed directly to negative-grounded heatsink with just thermal compound, no insulator required! Devices rated @ 12A, 130W each, genuine Toshiba.

This amp is STRONG! It plays 4 and 2 ohm loads with ease! Amp only gets hot playing 2 ohms, just warm with 4 or 8 ohms. Distortion comes on easy, but sounds good still unless driven to severe clipping.

Amp has High-Gain. Not much needed to get loud!

I have no problem driving asymmetrical loads. I have antiparallel LED's across the speaker output to see them light according to positive or negative swings, and lots of modern audio (esp dubstep) has signals that will drive just one of the LED's for certain notes, but the amp doesn't distort or have any trouble. I still find however, that rock music, or music with heavy guitar or drums sounds best.

Can't wait to try MOSFET on the next one!

[Susan_Parker]

Hi there

Originally Posted by EWorkshop1708

Here it is! Finally a loud and simple 12V amp that does not require an SMPS! .....

Super, great to hear your news.

Thanks for the post and the schematic and pictures.

Best wishes,
Susan.

[Steve Eddy]

HA!

Zip ties, electrical tape and wire nuts. I love it!

Good job and thanks for sharing!

se

[Parasonic]

but don't let anyone see you carrying that baby around!