Zero Feedback Impedance Amplifiers

[Bill F.]

Thank you, Susan! Just the kind of feedback I was hoping for.

Yes, 10 Hz to 50 kHz would be plenty of bandwidth.

I hadn't realized the ribbon output level was similar to that of an MC phono cartridge. So an MC phono preamp (minus the RIAA EQ, plus balanced output) could serve well as my ribbon head amp? Or would the execution be somewhat different?

Cheers,

Bill

[Susan-Parker]

For those who are wincing at the cost of the transformers for the Zeus VMOS Line Driver...

Further to my previous post the AD815s have arrived and I have lashed something together (not pretty, but it works).

I use the circuit as per the front page of the AD815 datasheet, but all three gain resistors are 1K0s. Plus 27 ohm series resistors on each output.

For simplicity to get the balanced input I have used a SSM2142 as a splitter/ driver.

Regret it is too late in the evening to start trying out with the amp, have to start tidying up.

More on this soon.

Best wishes,
Susan.

[Susan-Parker]

Part II

Initial testing showed the AD815 had LF insability so I have moved the two feedback 1K0 resistors from the output (as shown on the datasheet schematic) to the far side of the 27 ohm series resistors. Now drives ruler flat down to 20 Hz.

The SSM2142 splitter/driver needs to be driven from a low impedance (lower than I was expecting) otherwise it is very noisy, so I am adding a buffer (variable gain) stage before it.

Initial testing here:

http://www.audiophonics.com/audiopho...r-ad815-1.html

... which shows line driver and Zeus75 combo.

N.B. The two graphs further down the page show the single ended "hot" EMu 1212M source (I normally use this output balanced). There is significant noise which is clearly visible on a scope with normal signal levels.

I have left the basic EMu loopback tests at the bottom for comparison.

Best wishes,
Susan.

[Susan-Parker]

Dear All,

I am please to be able to say that all current Zeus transformers available from Sowter can now be ordered online...

http://www.sowter.co.uk/acatalog/SOW...MPS_15.html#a6

as well as the 9063c type...

http://www.sowter.co.uk/acatalog/SOW...RS_11.html#a59

Hope this will help.

Best wishes,
Susan.


P.S. I used their online ordering this afternoon for a pair of 9063s for the 6C33C triode version of the VMOS PP Line Driver. The system works.

[Susan-Parker]

Hi Bill,

Originally posted by Bill F.
Yes, 10 Hz to 50 kHz would be plenty of bandwidth.

I hadn't realized the ribbon output level was similar to that of an MC phono cartridge. So an MC phono preamp (minus the RIAA EQ, plus balanced output) could serve well as my ribbon head amp? Or would the execution be somewhat different?

If I have done the sums right (and not misscounted the fingers and toes) then as far as I can see the basic amplification chain could be of a similar topology (although there are allowances for losses in the filtering which we don't need to worry about).

I am not sure about hacking up an existing MC phono-preamp without seeing the schematic as I am sure there are going to be a few wrinkles (of the type that can be easily overlooked in bold sweeping statements) in that.

Simplest test if you happen to have a MC preamp handy is to connect up the ribbon element and see how much signal you get (making allowences for the RIAA curve).

Best wishes,
Susan.

[Bill F.]

Thanks again, Susan.

I'll be interested to see how your phono pre turns out.

Happy designing!

Bill

[Susan-Parker]

Part III

The SSM2142 splitter/driver is now driven from a LM4562 buffer (variable gain) stage before it. N.B. needs a unity gain stable opamp.

Picture, schematic and further testing added:

http://www.audiophonics.com/audiopho...-ad815-1.htmll

... which now shows basic line driver performance as well as the line driver and Zeus75 together.

I have also added further open/short and EMu loopback tests for comparison.

Best wishes,
Susan.

[powerbecker]

Hello Susan,

your Zeus is a nice and simple design... alas not quite cheap
if one buy the transformers.

Do you know his ancestor from Dynacord?

http://www.audiophonics.com/audioph...r-ad815-1.htmll
this link seems not to work!
Regards
Heinz!

[Susan-Parker]

Hi Heinz,

Thank you for your post.

Originally posted by powerbecker
[B]Hello Susan,

your Zeus is a nice and simple design... alas not quite cheap
if one buy the transformers.

Thanks. I agree the transformers do addup if one has to buy them all at once. However one can tryout the principles using mains power toroids and a headphone output which is very cost efective.

This gives a easy introduction and enables one to get a feel for the "Zeus sound".

Do you know his ancestor from Dynacord?

No, never seen this one before. Thanks for the schematic.

http://www.audiophonics.com/audioph...r-ad815-1.htmll
this link seems not to work!

Ops, seems to have an extra l on the end.

Should be:

http://www.audiophonics.com/audiopho...r-ad815-1.html

Thanks for pointing that out.

best wishes,
Susan.

[m8o]

Wow, 109 pages of posts!

Hello Susan, thank you for this addition to the DIY world. I've recently come upon it after thinking I wanted to build something similar to the First Watt F2. I have been reading your website (old and updated) intently over a few days, as I am intrigued by this wonderfully clean and simple design.

I'm wondering if your design can be adapted to serve as a high output impedance power source simply enough by choosing a different (step-up?) output transformer? I'm hoping for at least 40 ohms, but a bit higher would be fine too. I'm sorry if this was answered already ...but 109 pages of posts! whew.

p.s. for others: I modeled the circuit in LTSpice, but can't seem to get any current through the MOSFETs to the output xformer, even though I can get voltage if I raise the biase to 4.1v or over. All I get is super low level noise. Anyone use LTSpice? (I know you use B2 Spice v5 Susan. )