Simple Schade feedback without transformer

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#23
John Broskie's Tube-Cad site is first rate
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Lots of fun ideas to play with.

Just want to say thank you by posting on YouTube a clip containing your SPICE.
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Thanks for testing my ideas and playing with the IRFP7430. It is a fun mosfet, even though it can have some serious attitude and character if used in a wrong way.

Your amp seems to sound very large, vivid and alive with the PCF.
In my experience PCF seems to work like a true "presence"-control in more ways then just increase the level of the midrange.

There seems to be a quite dramatic increase in distortion when you apply PCF to the SE-amp with the IRFP7430. How much PCF do you use? If pushing things far into negative output-impedance territory distortion will skyrocket and the speaker might start to sound very strange.

PCF is a very useful tool to increase control over the bass and to let the amp have some muscle and authority, but if overdone will lead to some strange phenomenon.
I have had my speaker displaying a hollow and metallic "twang" whenever subjected to any kind of transient. I guess it was just at the verge of oscillation. The speaker sounded like a broken 3 inch fullrange driver at the bottom of an empty tincan.

What do you think of the IRFP7430? Have you compared it to any other more "normal" mosfet?

Cheers,
Johannes
 
#24
my amp is a transformer PCF, still waiting for few P Mos
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I would recommend IXYS IXTH140P05T or something similar. A normal IRFP9140 does not sound very good in a Bastode circuit. It seems to need more voltage between the drain and source. Low voltage Ixys p-channel mosfets sounds very nice with Vds = Vgs.
I have tested an Ixys IXTP 32P05T with as little as -2 volts Vds and it still sounded much cleaner and better then an IRFP9140 or IRF9530 with -4.2 volts VDS (Vds = Vgs).

Cheers,
Johannes
 
#25
Circlomanen said:
I would recommend IXYS IXTH140P05T or something ...
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I already placed an order for ALF08NP16V5 with an intention to use PCF circuit as driver for ALF in AB class with air coiled trifilar output transformer in purpose for odd harmonics reduction. That would save a tone of aluminium fins.
Talking about myself, I had stopped diy audio 40 years ago due to lack of interest. Somehow last winter (here is in June-July) after a month in Western Australia tracking the BIBBULMUN
https://www.bibbulmuntrack.org.au/
During the bush walking, I suddenly developed an urge to build a real good audio amplifier. Then returned home, lot of reading and searching, I found your threads.
Back to PCF subject, on my last experiment, I had done only a switching PCF in and out to see how it affects the harmonics of the content

https://youtu.be/tfM3nDKFAIE

My next step will do a variation of the positive feedback amount (voltage divider at 8 ohm load).
On your SPICE, I realised the limit is 7.5 ohm / 0.5 ohm before oscillation.
On my circuit, it will be a simple 500 ohm pot with a cursor to play with.

Cheers
Pascal
 
#27
Hi wdecho,

Sorry for late reply,
I haven't done any test yet, still building an interface for the sound card to have a much cleaner spectrum display.
Related to your question, I built the PCF amp with transformer at IPS, aiming to a circuit without it.
Johannes was kindly provided the SPICE's file to the forum, it was on my screen when I made a video, not related to my built amp.

Cheers
Pascal
 
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