F5 power amplifier

[jackinnj]

I am still intrigued by using 2 different resistors/voltage dividers on the gates of the JFET's as well. You can go as low as some 10's of ohms here.

Here's the results using 100pf, 160pF and 470pF cap on the front end. I note that Borbely used 560pF on the input of the Servo-50 and the DC-102.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

 

[jackinnj]

Here's how the amp looks with 3.3nF compensation caps on R5/8 and no input capacitor:

An externally hosted image should be here but it was not working when we last tested it.

 

[EUVL]

What is dark blue and what is light blue on the oscilloscope plots ?

And what is the vertical scale (dB / div) on the freq response plot ?
Is it possible to see the response a bit further upscale (say to 5MHz) to see what happens now with your previous peaks and valleys ?


Patrick

 

[jackinnj]

Dark blue is the sq wave oscillator output (input to JFET). Light blue is the amplifier output.

The network analyzer output is 5dB/div from 10kHz to 5MHz -- the absolute reference isn't relevant as I have an attenuator on the "A" channel -- but overall the amplifier gain is 15.4dB.

The real screw up on my part was not recognizing that the Vcc/Vee at the trimpot nodes had to be bypassed right then and there.

I'm leavin' for a bike ride ...

 

[Nelson Pass]

I am still intrigued by using 2 different resistors/voltage dividers on the gates of the JFET's as well. You can go as low as some 10's of ohms here.

One limitation here is that you can't assume too much about
the output impedance of the preamp that will drive it. Most
products are a couple hundred ohms to a Kohm or so.

 

[steve jones]

Heatsinks.

Did a search, but not much help.

I've got the parts for a F5, and the Plitron xformer is on its way.

All I need is some heatsinks.

Does anyone have any current recommendations on where to get heatsinks with part number for the F5 ?

TIA,
-Steve

 

[MEH]

One limitation here is that you can't assume too much about
the output impedance of the preamp that will drive it.

Uhhhh... maybe you can't, but those of us who know exactly which preamp will be driving it certainly can.

 

[SCD]

Heatsinks

Hello Steve Jones:
I recommend the Conrad heat sinks. There is a group buy getting started right now. There are some troubles at Conrad that are getting sorted out. Once things get going these sinks are the stuff. For me at least.
Which Plitron transformer did you order?

 

[steve jones]

Hi SCD,

Another DIYAUDIO member has emailed me offline with some surplus 8X12x1 heatsinks. So I believe that problem will be solved.

I ordered the Plitron 18V+18V@8.33A (300VA) p/n 077014201.

I'll be sure to check out the Conrad heatsinks when I build the F6. Thanks for the tip.

 

[vdi_nenna]

Some heat sinks here: www.apexjr.com

http://www.apexjr.com/images/GOLDHEATSINK3.JPG

Ask for matching color heat sinks.

 

[tms0425]

I used the largest Conrads, but these should work if tall enough or use two 8.5" sections side by side and 8-12" tall.

Ebay heatsinks cut to order

I also used that Plitron and it's a nice transformer for this amp.

Tom

 

[vdi_nenna]

Transformer Question

Do you guys use 1- 18+18v, 8 amp transformer for 2 channels?

 

[tms0425]

I plan to use Plitron 18V+18V@8.33A (300VA) p/n 077014201 also for 2 channels. Currently I have 2 of those in my F4, but one actually works just fine. I think Nelson indicated awhile back in the thread that I should be ok with one of those for a stereo F5.

Tom

 

[vdi_nenna]

You guys don't think there will be cross-talk between the 2 channels? For example, the Zen amp require only a + supply, so using 1 transfo with dual supplies worked ok. The F5 requires a + & - supply. Just want to get your thoughts on this.

Thanks,

Vince

 

[DaveM]

There should be no problem with this at all.

 

[Nelson Pass]

The crosstalk I got was so low that I couldn't measure it
reliably.

 

[Manu]

Ah!

What is generally the main culprit for crosstalk?

or should I say : what make F5 immune for xtalk?

Manu

 

[Dr. ODD]

or should I say : what make F5 immune for xtalk?

Class A ?

 

[Nelson Pass]

The channels are well separated, and the grounding is good.

 

[Zen Mod]

Ah!

What is generally the main culprit for crosstalk?

or should I say : what make F5 immune for xtalk?

Manu

weed?



(every new user of F5 will find a small bag in it ...... so in case of selling it ( God forbid!) , remember to pay it forward ......)