John Curl's Blowtorch preamplifier part II

[john curl]

Interesting, maybe brilliant!

 

[john curl]

This is the problem with discussing engineering: It leaves so many people out of the conversation.

 

[EUVL]

> maybe brilliant!

Indeed.

One can play with the values of R3 (=0 in barso's original schematics) & R7 to get distortion down, as shown in the "Line Level" asc file.


Patrick

 

[31697B]

Interesting, maybe brilliant!

😎🙂



THx-RNMarsh

 

[anatech]

Hi John,

This is the problem with discussing engineering: It leaves so many people out of the conversation.

You can never improve at chess without playing those who are much better at the game. So too with circuit design.

-Chris 🙂

 

[bcarso]

I wish floating supplies were less cumbersome. At least biasing high-Z nodes can be done pretty easily.

 

[EUVL]

> use batteries---and the latter are going to run down in a hurry with tens of milliamperes being pulled all the time.

This circuit uses 20~40mA.
AA size NiMH are usually >2000mAH these days.
You'll need 20 of them at least, but they will let you play 60+ hours on one charge.
Even AAA size with 800mAH will be more then sufficient.

http://www.reichelt.de/ENELOOP-5AAA...E=108067&artnr=ENELOOP+5AAA+1Z&SEARCH=eneloop

You can also use RC model LiPo packs, around 7.4V 2400mAH.
4 packs will do nicely.
We use them for headphone amps with total current consumption of 250mA.

http://www.conrad.de/ce/de/product/239510/Modellbau-Akkupack-LiPo-74-V-2400-mAh-20-C-Hacker-EC3


Patrick

 

[bcarso]

> use batteries---and the latter are going to run down in a hurry with tens of milliamperes being pulled all the time.

This circuit uses 20~40mA.
AA size NiMh are usually 2500mAH these days.
You'll need 20 of them at least, but they will let you play 60+ hours on one charge.
Even AAA size with 800mAH will be more then sufficient.

You can also use RC model LiPo packs, around 7.4V 2000mAH.
4 packs will do nicely.
We use them for headphone amps with total current consumption of 250mA.


Patrick

Thanks!

 

[john curl]

Come on guys, you have to do better than that if you want something practical. Perhaps just a resistor is the best compromise. '-)

 

[EUVL]

Very fruitful discussion. 🙂
I hope we can have a lot more of similar here.


Patrick

 

[EUVL]

> Perhaps just a resistor is the best compromise.

A resistor and poor PSRR is just moving the complexity somewhere else, not true ?


Patrick

 

[EUVL]

I made a quick attempt to power the entire circuit from a single floating supply.
It works quite well in Spice, but probably less well in reality, as the 2x 2SK30ATM are never identical.

Next question is can we apply the same to a balanced (BT-like) circuit and still use one battery ?


Patrick

.

 

[EUVL]

Seems to work also for balanced with a single supply.


Patrick

.

 

[Elvee]

Elvee, the second two Jfets are cascodes, with their drains at the output. So the output current comes from the input Jfets, not from the resistors.

Here is what I'm thinking.

OK, I get it now. The symmetry of the FETs put me on the wrong track.

 

[Esperado]

I wish floating supplies were less cumbersome. At least biasing high-Z nodes can be done pretty easily.

Come on guys, you have to do better than that if you want something practical.

In an industrial project, you can ask for an other separated coil in the power trafo.
But, again, who cares of pre-pre in a digital world, apart microphone amplifiers in mixing desks ?

 

[EUVL]

Perhaps you should be frequenting the Digital & Class D Forums instead ?
It is definitely more "modern" than us dinosaurs in extinction ??

😉


Patrick

 

[Esperado]

Perhaps you should be frequenting the Digital & Class D Forums instead ?
It is definitely more "modern" than us dinosaurs in extinction ??

I fear I'm myself a dinosaur in extinction. But that not prevents me to hope that one of my fellow invents the wheel, since the time I walk the same trails ;-)

 

[SY]

John, one practical question- on the back of the Vendetta PCBs, there's some jumpers and tacked-on resistors. Was that "factory" done or did someone go into this unit and try to mod it?

 

[jacco vermeulen]

In an industrial project, you can ask for an other separated coil in the power trafo.

Maybe you have not been in DIY long enough, plenty of transformer winders willing to do a custom job for you.
Added windings, foil screen between primary/secondary, metal screen under the outer foil wrap, one-off secondary voltages, even if you require only two.
I can show you a crate full of examples if you wish (me dinosaur).

 

[anatech]

Hi Stuart,

on the back of the Vendetta PCBs, there's some jumpers and tacked-on resistors.

Oh oh.

Was that "factory" done or did someone go into this unit and try to mod it?

Now, there is a critical question! The changes could be mapped out on an original schematic, but that takes time and a lot of effort to do.

What you need is a response from John, or let us know he contacted you off the forum. Otherwise, you need a known original factory Vendetta. Even a picture with reasonable resolution would help you answer that question. I've been in this position many times.

Wishing you luck or a response!

-Chris