Zero Feedback Impedance Amplifiers

[nemestra]

Hi Susan,

used a signal generator and scope to check the phase of all the windings. Have now got a working Zeus. Thanks very much for your help.

I'm just starting to do a few measurements. With 32V rails, a 50ohm source, 32 ohm load what voltage gain would you expect the Zeus75 to have? I've wired the input transformer in series and the output has the 4:1 configuration, i.e. four output windings in parallel.

Thanks,

James

[Sheldon]

It looks like the low temp. solder will work just fine for bonding these little packages to a more substantial heat sink. I used Tix solder (an indium alloy) and Tix flux. It's sold here by jewelery making supply houses, gunsmith supplies, and some other hobby suppliers.

I cut some squares of sheet copper about 3mm thick to the dimensions of a T0-247 package. I brushed a bit of flux in the center of the piece and clamped across the piece in a vice. The surface was facing up at this point. I put a small u-shaped piece of the Tix, a little less than 1cm total on the copper. I carefully tinned the back of the JFET with the Tix. I only had to touch the iron to the piece, as the solder melts at about 135 degrees C. I set the FET on top of the solder U, and applied a pencil torch to the back of the copper. As soon as the FET started to move, I pushed it down on the copper and removed the torch. It hardened before my finger got too hot - within seconds. The bond looks good and tight.

Sheldon

[jkeny]

Hi Sheldon,

See your making progress with the Lovoltech fets - how will you fix the jfet with copper pad attached to a heatsink - using bolts ? - is there enough room for bolt holes in the copper?

No deformation of copper took place in the heating - jfet is flat to copper surface? I assume you tested fets after & they are ok.

I am waiting for my fets & will be keeping an eye on your progress - please post here your steps so we can all benefit from your experiences.

Regards
John

[Sheldon]

Originally posted by jkeny
Hi Sheldon,

See your making progress with the Lovoltech fets - how will you fix the jfet with copper pad attached to a heatsink - using bolts ? - is there enough room for bolt holes in the copper?

No deformation of copper took place in the heating - jfet is flat to copper surface? I assume you tested fets after & they are ok.

I am waiting for my fets & will be keeping an eye on your progress - please post here your steps so we can all benefit from your experiences.

Regards
John

The copper is thick, so no way it will bend. I ground the surface flat before I soldered too. The copper is about as thick as a regular T0-247 package, and similar dimensions, so I can use a standard mica insulator. I'll drill a hole in the copper so that a standard insulating washer fits and mount the assembly just like one would mount a T0-247 power transistor. So far, I just tested resistances. No change from before soldering.,

Sheldon

[Zhan]

Hi Susan, thanks a lot for sharing your excellent and easy-to-build design! One question, if I were to build an amplifier system including the PP line drive and the power stage, fed by a CD player, where is the best place to put in a volume pot in your design? Your opinion?

Best Regards

Mark

[apelizzo]

Hi

1060

Thanks for your advice. I got lost with line drivers. I read about the use of AD815 which is quite expensive device and instead I'm trying with 2x OPA552 as in Fig.53 of AD815 Datasheet but I'm still not having good results. Can you please help me?

Best regards,

Alfonso

[Susan-Parker]

Hi James,

Originally posted by nemestra
Hi Susan,

used a signal generator and scope to check the phase of all the windings. Have now got a working Zeus. Thanks very much for your help.

I'm just starting to do a few measurements. With 32V rails, a 50ohm source, 32 ohm load what voltage gain would you expect the Zeus75 to have? I've wired the input transformer in series and the output has the 4:1 configuration, i.e. four output windings in parallel.

Thanks, James [/B]

Simplest setup:

HP8903B at 1KHz 50 ohm source impedance
Input transformer 1+1:10+10 Sowter
Output Transformer 2:1 and 4:1 configurations
Load 8 ohms.

Set level of HP = 1.00 Vac
Across input transformer primaries - series = 0.955 Vac
Across input transformer secondaries = 9.47 Vac
Output transformer 2:1 = 4.21 Vac
Output transformer 4:1 = 2.29 Vac

Notes:
======
50 ohm output impedance results in small signal drop, however this source impedance is necessary to match the Rterm value used across the input transformer's secondaries. Mismatch here results in either restricted bandwidth or peaking just before rolloff.

Input transformer gain is 10x across both secondaries. Using headphone type driver with primaries in parallel the gain here is doubled (Rterm may need to be changed).

Output transformer "gain" is 2:1 or 4:1 but actual output is reduced somewhat by the load and the follower "less than unity" (which is one of the key stabilization mechanisms of the amplifier when driving the reactive speaker load).

I use an 8 ohm load, for 32 ohms the reduction will be less.

Hope this helps.

Best wishes,
Susan.

[Susan-Parker]

Hi Mark,

Originally posted by Zhan
Hi Susan, thanks a lot for sharing your excellent and easy-to-build design! One question, if I were to build an amplifier system including the PP line drive and the power stage, fed by a CD player, where is the best place to put in a volume pot in your design? Your opinion?

Best Regards, Mark [/B]

Thanks for your interest.

For the PP line driver are you are thinking of using the Sowter 9063, or some other transformer? The Sowters are expensive but they are a huge improvement over the previous transformers which in themselves were good.

The VMOS PP was envisioned for people who already have a favored preamp, probably tube, perhaps an Aikido or some such.

If the CD player has 24 bit DACs can you use digital attenuation (not recommended for lower resolution parts)?

Otherwise in the PP circuit one can use a balanced attenuator per channel (switched resistors to maintain the accuracy of the two phases).

Something like the DACT switches (which they refer to as balanced stereo)...

http://www.dact.com/html/attenuators.html

... probably the 500K version to give a 60K input impedance.

Not sure how the matching or the high impedance is going to affect the performance as I have not tried this particular configuration.

One could try with a conventional ganged pot to check on the basic operation and ensure bandwidth is not affected (I only have 20K volume pots to hand but can try some fixed resistor divider combinations).

Putting a pot in series with the input would also work as a volume control, but the rising impedance will affect the bandwidth (however this may still be enough particularly as you are using CD as the source).

Other option is SE and switched attenuator, I hope to have a chance to try out the VMOS parts in this configuration next weekend.

Best wishes,
Susan.

[Susan-Parker]

Hi Alfonso,

Originally posted by apelizzo
Thanks for your advice. I got lost with line drivers. I read about the use of AD815 which is quite expensive device and instead I'm trying with 2x OPA552 as in Fig.53 of AD815 Datasheet but I'm still not having good results. Can you please help me?

Best regards, Alfonso

Okay, looking at Fig 53 one should add a 25 ohm resistor in series with each output as per Fig 2 to decouple the load capacitance from the feedback.

For the Sowter 1+1:10+10 transformers most standard op-amps don't have the drive capability even though they might nominally be shown driving 600 ohm circuits in their app notes.

I use OP275s which work although the bass is a bit light (which wasn't a problem as I was driving the low bass separately).

But the OPA552 has high drive capability so should work okay.

From the TI/BB datasheet I note that the OPA552 is designed for use in configurations with gains of 5 or greater - from which I assume they are a little "twitchy".

I have just got my hands on some of Nat Semi's super dupa new LM4562NA opamps which I will try out this weekend.

Otherwise the TI headphone driver works well particularly if using toroid transformers for experimenting.

I must admit to still be a little uncertain as to your exact implementation, is it possible for you to show us a schematic?

Thanks.

Best wishes,
Susan.

P.S. I have ordered a pair of AD815s to try out.

[apelizzo]

Hi Susan,

thanks again for your help

From the TI/BB datasheet I note that the OPA552 is designed for use in configurations with gains of 5 or greater - from which I assume they are a little "twitchy".

You are right, they are "twitchy" so I will put them away and I'll try not to make Zeus unnecessarily complicated.

I must admit to still be a little uncertain as to your exact implementation, is it possible for you to show us a schematic?

I'm reviewing my approach to the driver section. One of the guidelines in building Zeus is what you mentioned at the beginning "the wire with gain" and therefore it has to be as simple as possible and also I prefer not to use "special" parts. I'll follow your design of two stages amp using 10K - 10K input transformer - pp mosfet - output/input transformer - pp mosfet - output transformer. I will use a 38VCC for this amp and so I assume that in order to drive it properly the pre-amp has to provide ~30V to the power stage. I assume to have 1Vpp signal at the input from a mixer output, the input transformer has gain 2 and the second transformer should have 1:15 or 1:20 ratio.
I would appreciate if you can correct me with this approach.

Best regards,

Alfonso