Zero Feedback Impedance Amplifiers

[jkeny]

Yep Sheldon,
Seems like these devices tick all the boxes for a nice simple circuit in a simple & unique circuit - wonder if Susan is listeneing in? Interesting to hear how it will sound - reports seem exemplary

John

[wrenchone]

The only problem with the Lovotech devices is their low breakdown voltage. This will limit the output power available unless the parts are operated as the lower half of a cascode.

[jkeny]

Wrenchone,
When operating them from 12V SLA this should be OK I think?

John

[wrenchone]

Just barely...

[Sheldon]

Originally posted by wrenchone
Just barely...

Especially because a fully charged SLA battery is a little higher than 12 V.

Sheldon

[jkeny]

Oops might need to order more than 2 matched pairs then - will probably regulate voltage anyway and while I'm at it keep it down to 10V to be a bit safer - is there any other way of using these devices in this amp config which will result in safer operating area?

John

[bear]

Originally posted by Sheldon


I'll let you know if I go this route, and I'll try to decide soon. No big gotcha's so far? And the simple biasing scheme is very attractive. I'm very new to ss device characteristics, but it looks like these FET's have a negative temp. coefficient, so, unlike the mosfet's, no special measures are required to avoid thermal runaway. Am I reading that right? If so, that's another nice feature.

Sheldon

Mosfets have a negative tempco.

Until you get them very too hot, then they give up the neg tempco idea and run away... In fact it is difficult to bias up Mosfets past a certain standing current unless you can keep them very very cool, they try to track back to where they want to sit...

_-_-bear :Pawprint

[Susan-Parker]

Hi All,

I trust everyone is keeping well.

I have been busy with work stuff so not so much time for audio these last few months.

Originally posted by jkeny
Yep Sheldon,
Seems like these devices tick all the boxes for a nice simple circuit in a simple & unique circuit - wonder if Susan is listening in? Interesting to hear how it will sound - reports seem exemplary

John

Who - me?

Originally posted by wrenchone
The only problem with the Lovotech devices is their low breakdown voltage. This will limit the output power available unless the parts are operated as the lower half of a cascode.

I did look at these parts but as the standard mosfets are giving below 0.01% THD I didn't see the point of the additional complexity.

However I have used Supertex DN2535s for a Zeus PP line-driver

http://www.supertex.com/pdf/datasheets/DN2540.pdf

see some preliminary testing at:

http://www.audiophonics.com/audiopho...vmos-pp-1.html

They give good results.

Note however that the termination of X1's secondary has to be matched to the driving source impedance otherwise there can be significant HF peaking before rolloff.

This circuit always operates in Class A, so the auto bias method is okay. Voltage rating is 350 or 400 volts, so no problems here with a 24 volt supply.

The DN2535 has a max current of 500mA and 15W so would need to be paralleled up for an output stage driver. And I would not auto bias (unless always in Class A with a big step down ratio) so having to do a separate negative bias plus multiple devices somewhat increases the complexity and makes for a potential failure mode if the bias doesn't come up properly.

Using standard MOSFETS removal of the positive bias voltage effectively puts the amp into standby, which is a useful feature for energy savings when the amp is not in use.

Best wishes,
Susan.

[Sheldon]

Nice to see you here. I did try one channel with the regular Mosfets, the 801 driver, and an autoformer for the input. Got tunes out and sounded fine on a small speaker. I've got an RIAA project to finish first, but then I'll play some more with this one.

Originally posted by Susan-Parker
I did look at these parts but as the standard mosfets are giving below 0.01% THD I didn't see the point of the additional complexity.
Susan.

By added complexity, I assume you mean the cascode idea. I was thinking of trying them in a low power version as a self biased sub for a mosfet. As for the point, yes you are correct, but hey this is just for fun.

Originally posted by Susan-Parker
This circuit always operates in Class A, so the auto bias method is okay.

This amp is a little different to figure. Using the Lovaltech devices, what conditions would meet this requirement? I was looking at something on the order of 1amp bias current for a 10 volt supply and maybe a 2:1 output.

Sheldon

[Susan-Parker]

Hi,

Originally posted by Sheldon
Nice to see you here. I did try one channel with the regular Mosfets, the 801 driver, and an autoformer for the input. Got tunes out and sounded fine on a small speaker. I've got an RIAA project to finish first, but then I'll play some more with this one.

Thanks. Please let us know how you get on.

By added complexity, I assume you mean the cascode idea. I was thinking of trying them in a low power version as a self biased sub for a mosfet. As for the point, yes you are correct, but hey this is just for fun.

Understood. And Agreed. This is hobby and DIY and meant to be relaxing etc. with personal enjoyment, satisfaction and learning.

This amp is a little different to figure. Using the Lovaltech devices, what conditions would meet this requirement? I was looking at something on the order of 1amp bias current for a 10 volt supply and maybe a 2:1 output.
Sheldon

I would probably look at using a standard MOSFET to give a reasonably regulated 11.75 volt supply as the extra couple of volts will make a big difference to the output watts possible.

Note from my VMOS schematic that the gate zeners are reversed in polarity. And here should probably be reduced to 9 volts.

Also don't forget the current capability of the transformer windings, as with these high biases there will be some heating.

One of the problems seems to me to be the package as it isn't overly big for the types of currents and wattages proposed.

I know that it is theoretically capable of 65 watts but once one has derated at higher temperatures e.g 80 degrees C it is only about 30 watts and the package has a thermal resistance junction to case of 1.8 degrees/watt - compared to 0.69 for TO247

But high bias with low voltage is certainly one way to go.

Per STW34NB20 I use 750mA at 34 volts supply in my standard power stage.

Will be interested to hear how this version fairs.

Best wishes,
Susan.