Zero Feedback Impedance Amplifiers

[Circlomanen]

Yes, I tried 100% Mumetal in the input laminations.

Some improvement in the 200 Hz to 1 kHz band, but considerably more saturation in the low bass at higher levels.

Why not use a bigger laminationstack? That should lower distortion a lot, especialy at higher powerlevels! Same should work for the output-transformer. ???

[Susan-Parker]

Hi Circlomanen,

Thank you for your post.

Originally posted by Circlomanen

Why not use a bigger laminationstack? That should lower distortion a lot, especialy at higher powerlevels! Same should work for the output-transformer. ???

In principle this is a good idea and would give the benefits as you suggest.

Unfortunately Mumetal is not available from Sowter in larger than 3/4" lamination sizes, and in this instance I was looking to see if there was any advantage to using Mumetal instead of the M6 silicon steel specifically in my input transformer.

Given that I am chasing 0.001% ths distortion levels the improvement of using a 50/50% mix would seem to be worthwhile for the relatively small additional cost in this size. Going for the 100% Mumetal is still economic but does have the drawbacks as I mentioned.

If one is intending to have a separate amplifier for lower bass (as I do) then a 100% Mumetal core isn't a problem.

An improvement could be obtained by having a deeper lamination stack in the same lamination size, but this would involve a redesign of the transformer. My current input transformer has a 3/4" stack, I believe bobbins are available for a 1-1/4" stack.

Unfortunately the high cost of Mumetal becomes prohibitive for larger lamination sizes (if they are available) that one would use for an output transformer. In this case I would be looking to move from M6 to M3 or specialty C cores.

Best wishes,
Susan.

[Bill F.]

Susan,

You may have already mentioned this and I missed it: Where do you source your mumetal laminations, and what's the approximate cost?

[Susan-Parker]

Hi Bill,

Originally posted by Bill F.
Susan,

You may have already mentioned this and I missed it: Where do you source your mumetal laminations, and what's the approximate cost?

I bought my 3/4" Mumetal laminations from Sowter as I already have transformers from them (# 8160).

http://www.susan-parker.co.uk/zeus-in-tx.htm

It costs about 40 to 50 USD per inch, depending on if one has to pay sales tax.

Best wishes,
Susan.

[Circlomanen]

An improvement could be obtained by having a deeper lamination stack in the same lamination size, but this would involve a redesign of the transformer. My current input transformer has a 3/4" stack, I believe bobbins are available for a 1-1/4" stack.

Unfortunately the high cost of Mumetal becomes prohibitive for larger lamination sizes (if they are available) that one would use for an output transformer. In this case I would be looking to move from M6 to M3 or specialty C cores.

Can I use a larger M-6 laminationstack and get better results than a smaller mu-metal stack??

If I use a EI-180 stack in an 40 watt version (as a output-transformer) , wouldnt that have lower distorsion than an EI-96??
What happens if I use a 10 inch (25 cm) thick stack of EI-180 laminations??

[Susan-Parker]

Hi Circlomanen,

Originally posted by Circlomanen

Can I use a larger M-6 laminationstack and get better results than a smaller mu-metal stack??

If I use a EI-180 stack in an 40 watt version (as a output-transformer) , wouldnt that have lower distorsion than an EI-96??
What happens if I use a 10 inch (25 cm) thick stack of EI-180 laminations??

The problem is that M6 has higher losses than Mumetal and so there is an intrinsic point where performance can't increase for a given driving impedance.

As I am driving my output transformers from a low impedance source I am operating under nearly ideal conditions.

What is interesting is to look at the possibility of going for a hugely oversized core and then using an air gap to end up with a similar inductance to the current output transformer. Depending on what one reads this will make performance better or worse!

I have some M6 EI 2-1/8" laminations for this purpose and 3" bobbins to match. Wire for this is going to be over UKP 50. And the transformer will be heavy!

Just adding core size on it's own doesn't necessarily improve matters as the inductance goes up (which affects the HF response). So one uses thicker wire with less turns to compensate. Inductance is reduced but the DC resistance also goes down. So the mosfets become more twitchy to bias into their "nice" zone.

Adding the gap reduces the inductance so allows one to use only slightly heavier gauge wire with more turns. I aim to have a DC resistance of about 0.5 ohms or a little higher for each primary arm.

Whether this transformer will actually result in lower distortion will be interesting to see. I suspect for PP it won't be as good but for SE it will probably be better.

Best wishes,
Susan.

[Circlomanen]

Thank you Susan!
Your help is very much appreciated!

[bobo1on1]

I just got my hands on a big circuit board containing 48 HUF76143P3 mosfets, I think its old military stuff from a huge power inverter to get ac from 24 volts.
Here is the datasheet, although I don't understand everything it says it seems like it has very high gain, although I don't know if the same applies at these low currents, I don't know if its linearity is good or bad either.

So I just build it with a 14 cm² core transformer having 70 turns for each winding, it sound surprisingly good.
Now all I need is a good gain stage because I don't want to use a transformer at the input.

Idle current is set at either 100 mA or 500 mA, my multimeter shows different values at different settings lol.

[Susan-Parker]

Hi Bobo1on1,

Thanks for your post

Originally posted by bobo1on1
I just got my hands on a big circuit board containing 48 HUF76143P3 mosfets, I think its old military stuff from a huge power inverter to get ac from 24 volts.
Here is the datasheet, although I don't understand everything it says it seems like it has very high gain, although I don't know if the same applies at these low currents, I don't know if its linearity is good or bad either.

I have found relatively small difference between mosfet types. Its worth matching pairs if one can. And some are a bit better than others but there does not seem to be a huge difference except for the top end when the part is approaching the power rails.

The comparison graphs I did for the SE version show this.

I assume you are using quite a low voltage power supply as these parts have a 30 volt VDSS?

[QUOTE]So I just build it with a 14 cm² core transformer having 70 turns for each winding, it sound surprisingly good.[QUOTE]

Good

Are you using the speaker across the primaries, or with a secondary?

Now all I need is a good gain stage because I don't want to use a transformer at the input.

If you are running on a low supply rail one can use an offset +18 -15 volt supply (e.g. 7818 & 7915 regulators) and reference the input to get the bias (may need a capacitor in the audio path).

Within these limits something like a DRV134 differential line driver could be used instead of a transformer.

Idle current is set at either 100 mA or 500 mA, my multimeter shows different values at different settings lol.

I use a 0.1 ohm resistor in series with the power supply line and measure the voltage drop across the resistor (100 mV = 1 Amp) for that reason.

Any pics?

Best wishes,
Susan.

[bobo1on1]

Here's a webcam pic of how its sitting on my desk right now, not much to see because of all the wires.


The speaker is connected directly to the primaries, I wound the transformer with 4 strand telephone wire but when I use the secondairies the highs don't sound as nice.
There is also a ptc in series with the 5k pot for thermal stability, the ptc is about 60 ohms at room temperature and about 700 ohms when the heatsink gets really hot, idle current only goes up a few miliamps.
It's all running off a 16 volts rail, including the pot for bias.