Zero Feedback Impedance Amplifiers

[Susan-Parker]

Hi darkmoebius,

Thank you for your post.

Originally posted by darkmoebius
For those who newbies (like me) who already have RCA or "single-ended" preamps, cd players, etc and still want to try Susan's Zeus amp without a major investment to convert to balanced/differential inputs - digi01 has started an excellent thread based around this single chip design that only uses 2 resistors and capacitors. Total parts cost including the regulators and power supply should be less than $10. The best part is that it may be possible to simply place this tiny SE to Differential converter inside the Zeus amp at it's inputs.

Had a look, couldn't see my name anywhere

Okay, back to the regular programming

Please remember that the drive to the Zeus amplifier is not "just" 600 ohms, or 150 ohms, but that the mosfet gate load is reflected back through the transformers.

As such not all line driving stages are suitable even though they might quite happily drive a standard 600 ohm resistive impedance.

I don't want to put people off, but it is something that one should be aware of as it isn't guaranteed that any old op-amp can walk this particular walk.

Hence the OP275s which do work well, and the TI part that drives that as well as much lower load impedances quite happily.

Also if there is anything specifically to do with the Zeus amplifier then please keep it here as otherwise it will get lost very rapidly (I don't imagine the DIYaudio list people, wonderful as they are, will give us our own section to keep things organized).

Some of us don't get to skim other threads much, myself included, as I am spending 20 to 40 hours a week just supporting this thread (someone asks a question - I go off and research it, maybe order some parts from RS - come back hopefully with an answer or a suggestion).

Re the DRV134 I haven't tried this particular part, so can't comment on it's actual use. However I would fit series resistors of around 22 ohms on each output. And use very good decoupling caps right up close to the pins.

In case anyone was wondering I did test out the SSM2142 and regretfully (as I have a full tube of the things) that gets the thumbs down as far as transformer input stage driving goes.

Look forward to receiving any feedback on how the DRV134 part works out.

Best wishes,
Susan.

[darkmoebius]

Originally posted by Susan-Parker
Had a look, couldn't see my name anywhere

Aaah, but you're always in our thoughts

Please remember that the drive to the Zeus amplifier is not "just" 600 ohms, or 150 ohms, but that the mosfet gate load is reflected back through the transformers.

As such not all line driving stages are suitable even though they might quite happily drive a standard 600 ohm resistive impedance.

Funny that you should mention this (again) because I took the liberty of posting on the forum of Lundahl's USA distributor, K&K Audio, asking about the input imped. & FR of the LL1689 or if Lundahl has something more appropiate . I supplied your schematic, the Sowter #8160 specs and a link to your website.

Kevin Carter(owner) responded:

I could find no data for the Sowter 8160 on the Sowter webpage. While the ratios are similar, the inductance data for the 8160 are very different than those for the LL1689/PP. I'm not sure what to make of that, frankly. MOSFETs are very high impedance devices, so presumably the input impedance is set by the reflected terminating resistance. I am curious to know what kind of a circuit you intend to use to drive the input transformer. That's a pretty challenging input impedance.

Kevin Carter
K&K Audio
www.kandkaudio.com

I'll let him know R-Term is 120 KΩ and see what he thinks.

Re the DRV134 I haven't tried this particular part, so can't comment on it's actual use. However I would fit series resistors of around 22 ohms on each output. And use very good decoupling caps right up close to the pins.

Look forward to receiving any feedback on how the DRV134 part works out.

Surprise, surprise!!! The UPS man dropped off four DRV134 this morning. I need to assemble a nice, quiet, 15V supply for a mockup. I also have quads of AD815 & AD826's on the way for testing. We'll see how they stand up to the stress.

I need to tackle the Zeus input and output transformer issue this weekend. I've got the PS transformers and matched Hitatchi 23j49 MOSFETS covered from Bear, although I won't be using those until the circuit is up and running smoothly for a few weeks on much cheaper IRFP150N's.

[wrenchone]

To Susan

When I looked at the inductance of my toroidal transformers, I used an HP4194A analyzer in L/Z mode, swept from 100Hz to 20kHz. Excitation voltage was around 400mV. The inductance on either side of the secondary CT was about 760 uH (matched to better than 1%!). The inductance steadily fell with frequency until resonance at about 2kHz - not very pretty, but I suspec, typical. Tomorrow I'll look at a 28-0-28V transformer (I have scads of them) if I can remember to slip it in my briefcase tonight. Just from the higher output voltage, it should have higher inductance (I hope). I suspect that the resonance situation will be no better, though.

[tubesguy]

Hi folks -

I just realized that I hadn't gotten back to this thread with news about Warren Gilson, the patent holder referred to earlier in the thread.

I had posted that he was listed in the local phone book, with an address just blocks from my home. I had copied the schematic from the first post in this thread and I was going to stop in and show it to him.

Alas, the phone book that I had here in the office was several years old, and when I checked the current one, Mr. Gilson was no longer listed. His prior address had been the location of either a nursing home or other retirement facility. I fear that Mr. Gilson is perhaps no longer with us.

- Pat

[darkmoebius]

Torid Corp of Maryland offers them in 5 standard configurations(all 117V primary). They supply everything needed to wind the transformer.

Here's a picture of the winding jig:

The two possibly relevent to this design are:
80VA, 60W rating, 0.13V per turn, $27 US
200VA, 140W, 0.23V per turn, $38

Each kit contains:

* A toroidal core of grain oriented steel
* Mylar taped for insulation of secondary windings put on by the customer
* Built-in thermal fuse rated at 130 ¹ C
* Manual with step-by-step instructions on how to design and wind the secondary windings.
* Metal washer and 2 rubber pads for mounting the transformer
* Guide to calculate proper wire size and length required to generate your calculated output data.

If someone could help figure out the winding count for our purposes, I'd be willing to give it a try.

For those in the USA, mabe this is an option:
Torid Corp of Maryland

[Susan-Parker]

Hi Pat,

Originally posted by tubesguy
Hi folks -

I just realized that I hadn't gotten back to this thread with news about Warren Gilson, the patent holder referred to earlier in the thread.

....

- Pat

Many thanks for your post.

Best wishes,
Susan.

[Susan-Parker]

Hi wrenchone ,

Originally posted by wrenchone
To Susan

When I looked at the inductance of my toroidal transformers, I used an HP4194A analyzer in L/Z mode, swept from 100Hz to 20kHz. Excitation voltage was around 400mV. The inductance on either side of the secondary CT was about 760 uH (matched to better than 1%!). The inductance steadily fell with frequency until resonance at about 2kHz - not very pretty, but I suspec, typical. Tomorrow I'll look at a 28-0-28V transformer (I have scads of them) if I can remember to slip it in my briefcase tonight. Just from the higher output voltage, it should have higher inductance (I hope). I suspect that the resonance situation will be no better, though.

Thanks for the further info.

Probably best to put on a termination resistor with a 10k twiddle pot and do a quick sine sweep and adjust to flaten out the kick up.

The mains toroids as being used are not going to be the best one can do but hopefully good enough to try things out and get a bit of hands on.

You should be able to run a pair of 28-0-28 transformers together to give you a balanced center tap output on the 115 + 115 windings.

Best wishes,
Susan.

[Susan-Parker]

Hi darkmoebius

Originally posted by darkmoebius


...

I'll let him know R-Term is 120 K& and see what he thinks.

Okay, will be interesting

Surprise, surprise!!! The UPS man dropped off four DRV134 this morning. I need to assemble a nice, quiet, 15V supply for a mockup. I also have quads of AD815 & AD826's on the way for testing. We'll see how they stand up to the stress.

Always a nice surprise - the TI chips arrived here in England before I go the email notification that my sample order had been accepted and a shipping number!

I need to tackle the Zeus input and output transformer issue this weekend. I've got the PS transformers and matched Hitatchi 23j49 MOSFETS covered from Bear, although I won't be using those until the circuit is up and running smoothly for a few weeks on much cheaper IRFP150N's.

Looking forwad to hearing of your progress (as I do for everyone who is having a go at this amplifier).

Prudent to use the IRFP150N to start with. These are the parts I have been using so I do know that one can get good results with them.

Don't forget the fuse after the smoothing caps

Best wishes,
Susan.

[Susan-Parker]

Hi darkmoebius,

Thank you for the post...

Originally posted by darkmoebius
Torid Corp of Maryland offers them in 5 standard configurations(all 117V primary). They supply everything needed to wind the transformer.

Here's a picture of the winding jig:

... and this information.

The two possibly relevent to this design are:
80VA, 60W rating, 0.13V per turn, $27 US
200VA, 140W, 0.23V per turn, $38

Also the 400 VA, 260W $54.00

For power ratings (i.e. flux density) one needs to derate for both frequency and also to keep lower down the flux squiggle as mains transformers are usually rated at 80% to 85% of saturation and the toroids curls over faster than EIs with their less contained magnetic circuit.

So for output transformers the 200 VA (at 60 Hz) is for the sort of powers that I use i.e. 35 watts (here in the UK we have 50 Hz so the equivalent VA rating is about 20% higher).

The 400 VA core is good for powers up to 75 watts with a bit of a decrease in margins.

Each kit contains:

* A toroidal core of grain oriented steel
* Mylar taped for insulation of secondary windings put on by the customer
* Built-in thermal fuse rated at 130 ¹ C
* Manual with step-by-step instructions on how to design and wind the secondary windings.
* Metal washer and 2 rubber pads for mounting the transformer
* Guide to calculate proper wire size and length required to generate your calculated output data.

If someone could help figure out the winding count for our purposes, I'd be willing to give it a try.

For the output transformer one doesn't need the mains winding so maybe the company will supply the cores unwound, otherwise they should be removed.

Quad-filar wind 0.8 mm enameled magnet wire (200 VA core) or 1.0 mm wire (400 VA core) with 120 to 150 turns.

For those in the USA, maybe this is an option:
Torid Corp of Maryland

Looks good

Many thanks for looking into this.

Best wishes,
Susan.

[darkmoebius]

I checked with Sowter for the US price(including VAT & shipping) on the 8160 input transformer - 124 UKP or $232 US.

Not cheap! But, hey, people drop a hell of a lot more than that on high-end interconnects. Not to mention NOS output tubes.

I'm still trying to track down an E-I kit, but there doesn't seem to be a lot of companies in the US offering such. Lots of Google hits for the UK, though.

As for the toroid kit, is there any special core type to request? Something the equivilent of M6 Othosil?