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Balanced Monoblock F4s with very high Bias
Balanced Monoblock F4s with very high Bias
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Old 4th December 2018, 04:06 AM   #1
syracuze is offline syracuze  United Kingdom
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Balanced Monoblock F4s with very high Bias
Default Balanced Monoblock F4s with very high Bias

First, a big thanks for NP for sharing so many great designs. I have recently built a balanced mono ACA’s and F5, and I am quite happy with them.

After reading through NP’s 2 great papers (more than papers, kind of design manifestation actually );
1) Leaving Class A - http://www.firstwatt.com/pdf/art_leave_classa.pdf
2) Audio, Distortion and Feedback - http://www.firstwatt.com/pdf/art_dist_fdbk.pdf
And also seeing the distortion cancellation in my ACAs between single ended and balanced modes through a QA401; I have been thinking to build a balanced, no-feedback, very high Class A bias amps.

With its massive simplicity and universal positive feedback F4 looks just perfect.

As a starting point, recapping 6L6’s measurements for distortion (1kHz, 4 ohm, 3V, ~2Watts) for different Bias settings (part of his great build guide - A guide to building the Pass F4 amplifier.)
100 mV Bias - 0.065%
200 mV Bias - 0.016% (Default setting)
270 mV Bias - 0.008%
370 mV Bias - 0.0045%

Well, in the spirit of overkill , if one goes for say 470 mV Bias, some very rough extrapolation would give around 0.003% distortion

And, on top, given that this distortion is predominantly 2nd order, running two output stages in a balanced setup would yield a good chunk of distortion cancellation; if we assume 1/5 for this effect, then we might potentially end up having 0.0006% distortion!

Just to make it clear, I have no intentions to delve into sound quality vs. distortion measurement, yet as crazy as this number sounds for any amplifier including Class D’s etc, having a very simple, no feedback amp reaching this level is just insane.

I wonder whether these assumptions are sound, and/or anyone else built/measured similar very high biased balanced F4s?

Last edited by syracuze; 4th December 2018 at 04:17 AM.
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Old 4th December 2018, 04:14 AM   #2
syracuze is offline syracuze  United Kingdom
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Balanced Monoblock F4s with very high Bias
Default Feasibility

Btw, in addition to this theoretical performance, if one thinks about the feasibility;

1) Mosfet power dissipation
At 470 mV Bias with suggested 0.47 ohm source resistors, every output device will draw around 1A. And with suggested 24V rail voltage, each will dissipate 24W. So, it is still lower than 30W value that NP suggests, and should be OK!

2) Total power dissipation
With 6 output devices per side and 12 per channel for balanced mono, we have 12 x 24 = 288W dissipation per channel. It is on the higher side, but still feels quite doable with heavy heatsinks; probably Big 5U chassis with 40 cm deep per channel will be just fine.

3) Input JFETs current drive
Not sure whether they will be able to drive the circuit due to much higher class A bias, any ideas?
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Old 4th December 2018, 11:01 AM   #3
Zen Mod is offline Zen Mod  Serbia
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Balanced Monoblock F4s with very high Bias
1. ok
2.5U/500 Modushop , as minimum
3.no worries ; Iq is not changing drive demands
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Old 4th December 2018, 11:14 AM   #4
JeffYoung is offline JeffYoung  Ireland
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Balanced Monoblock F4s with very high Bias
Where did you see the 30W recommendation? The 240/9240s are rated at a maximum of 150W so I'd think they'd have a good lifespan at 50W, and probably even a decent one at 75W. Given sufficient heat-sinking, of course.

When you say "mono" I assume you mean 1 5U chassis per channel (and not just mono PSUs)? That should do the trick.

My understanding (admittedly weak) is that the drive requirements from the front end have more to do with the output FET gate capacitances than their bias.

Cheers,
Jeff.
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Old 4th December 2018, 11:15 AM   #5
JeffYoung is offline JeffYoung  Ireland
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Balanced Monoblock F4s with very high Bias
Oops, cross-posted with ZenMod.

Well, at least I didn't contradict anything he said.
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Old 4th December 2018, 11:34 AM   #6
Zen Mod is offline Zen Mod  Serbia
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Balanced Monoblock F4s with very high Bias
oh yes , you did

unless you have significant experience with mosfet dissipation , don't be brave on someone else's expense

I'm using IRFP150 (practically 2 x IRFP240 in one case) on 50W heat , but gooood heatsinking and keratherm is a must

IRFP240 .......... sane is 35W , 40W is still sane , while more than that is strictly your own responsibility

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Old 4th December 2018, 12:20 PM   #7
Dennis Hui is offline Dennis Hui  Canada
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Balanced Monoblock F4s with very high Bias
Hi Jeff,

On the 30W recommendation, please see BA2 article:

"...If you have enough heat sinking, you can consider as high as 30 watts per device, allowing the reliable use of fewer devices.

If you decide to higher, remember that Mosfets are better at higher current until they catch fire and fulfill the promise of the amplifier's name."

http://www.firstwatt.com/pdf/art_ba2.pdf

Cheers,
Dennis
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Old 4th December 2018, 12:33 PM   #8
JeffYoung is offline JeffYoung  Ireland
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Balanced Monoblock F4s with very high Bias
He he... I do remember that quote, but I took it as encouragement.
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Old 4th December 2018, 02:37 PM   #9
syracuze is offline syracuze  United Kingdom
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Balanced Monoblock F4s with very high Bias
Default Thanks Zen Mod!

Quote:
Originally Posted by Zen Mod View Post

I'm using IRFP150 (practically 2 x IRFP240 in one case) on 50W heat , but gooood heatsinking and keratherm is a must

IRFP240 .......... sane is 35W , 40W is still sane , while more than that is strictly your own responsibility

I did not know that there is a much powerful variant of irfp240. This sparked a new thought!

What happens if instead of 3xirfp240 per side per channel, I use only 1 irfp150?

My understanding is;
Downside;
= I will loose the 1/3 of current drive (as essentially moving from 3 devices to 2)

Upside;
= Great work around for the joy of the MOSFET matching, with just one device p or n device per side, no need to worry about it at all (p vs n matching is dealt with trimming pot)
= I can decrease the source resistors as NP mentiones. Actually, with only 1 device, do I even need a source resistor?
= It will also lower the total heat dissipation by 1/3, from 288W per channel to ~190W, much easier to accommodate

Btw, fully agree on the need for the proper heatsinking. I checked my F5 based on 4U case 300mm, while the case is not warm, looking with a thermal camera output devices are not efficiently using the full case capacity. I was thinking about having copper plates between Mosfets and aliminium heatsinks to a) better dissipate heat from MOSFETS and b) more uniformly distribution temperature across the heatsink

Thoughts?

Last edited by syracuze; 4th December 2018 at 02:41 PM.
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Old 4th December 2018, 02:45 PM   #10
JeffYoung is offline JeffYoung  Ireland
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Balanced Monoblock F4s with very high Bias
What impedance speakers are you driving? You've probably got more current drive than you can make use of anyway.
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