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Simple High Performance DC Coupled Class A HPA with sub PPM THD

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#304
Perhaps I didn't explain it clearly in my previous post.

I am proposing to move the 2K2 resistor to the gate, as a gate stopper. In front of it, the two capacitors will act as high pass and low pass filters.

Is there an issue doing this with a FET input?
 
#305
Harry3,
Can you mark up the schematic with what you are proposing?
685372d1528348988-simple-performance-dc-coupled-class-hpa-sub-ppm-thd-tims-simuation-schematic-v3-png


Do you mean make R13 2k2 and get rid of the 100R? The 2k2 acts with C4 as a filter, whereas the 100R is a gate stopper, which probably isn't needed as the 2k2 is already there. However, it matters where the node to the filter cap is located. I think you can leave R13 where it is but ok to get rid of the 100R at R4.
 
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#309
Hi XRKaudio , I was wondering the status of my order - I received your invoice on Oct.2nd and 63.00 was sent on Oct.3rd by PAYPAL for -- a Simple DC Coupled Class-A HPA PCB Panels , Qty -2 . Hope all is well and postal company hasn't lost it somewhere.
 
#314
xrk971 said:
2. For the main MOSFET Drain resistors R117/118 and R137/138, please use high quality metal thin film as well. I only had thick film on hand, so once again, you may see significant improvement in distortion over my verification build if you choose this. I would start with the 2R7 values to get 115mA. If you really want 145mA then use 2R2 and that will be the maximum capability of the Traco Dc-DC (315mA for both channels).
Click to expand...

X - found an inconsistency.
Thin or thick film for these ones (R117/118 and R137/138)? BOM lists thick for these (and that's what I ended up ordering), but you advise thin in the quoted post.

Thanks
 
#316
IMO - thin film resistors are generally the best you can get. They are stable, tight tolerance, low PPM, non magnetic and are made of good quality raw material etc etc...

Some people like carbon resistors because they have their own "signature" sound.

You choose your poison😀
 
#317
On the feedback resistors (R16/107) and MOSFET source/drain power resistors (R115/116/117/118) especially, use either carbon film or metal thin film. If you use metal oxide thick film here, it tends to generate the unpleasant third order harmonic distortion. I have measured it and it is not insignificant. But only in places where there is a significant amount of AC-signal current induced heating. Some people argue that gate stoppers (R113/114) for MOSFETs should be carbon as well.

To be safe, use all metal thin film if you want. It will cost a bit more but will be safe and low distortion. The carbon thin film on the key feedback and ground shunt resistors will have a smoother ("less etchy metallic") sound.

685373d1528348988-simple-performance-dc-coupled-class-hpa-sub-ppm-thd-timss-final-schematic-prod-png
 
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#318
ahh gotcha - thanks X thanks Harry3!
Guess I'll be ordering some thin film / MELF 2R7 & 180R carbon resistors to muck about with.
2R7 backordered @ RS until March, so I guess I'll have it running on the thick film until then, carbon TH available for next business day delivery 😀
 
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