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Old 6th February 2003, 12:17 PM   #1
Sheriff is offline Sheriff  United Kingdom
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Default Thoughts on a mosfet amp with low listener fatigue and full, deep bass??

Dear All,

As a new boy to the forum, i would like to gain some opinion as to what has the most significant affect on the deepest bass and general listener fatigue on a power amp using a MOSFET O/S.

I am using 5 pairs of magnatec single die plastic FET's per chanel with matched (within a few %) VGS on. The FET are used in the 'enhanced mode'.

The PSU has tightly regulated rails, +/-70, +/-35V and + and - 10V (with respect to the 70's) for enhancing the FET's.

Input stage and VAS are similar to self's blameless amplifier.

The main area of interest to me at this moment is the configuration of the O/S and views on O/S feedback.

Any thoughts on the above would be appreiciated.


Cheers

Sheriff
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Old 6th February 2003, 05:06 PM   #2
halojoy is offline halojoy  Sweden
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Default Re: Thoughts on a mosfet amp with low listener fatigue and full, deep bass??

Quote:
Originally posted by Sheriff
Dear All,
As a new boy to the forum, i would like to gain some opinion as to what has the most significant affect on the deepest bass and general listener fatigue on a power amp using a MOSFET O/S.

I am using 5 pairs of magnatec single die plastic FET's per chanel with matched (within a few %) VGS on. The FET are used in the 'enhanced mode'.

The PSU has tightly regulated rails, +/-70, +/-35V and + and - 10V (with respect to the 70's) for enhancing the FET's.

Input stage and VAS are similar to self's blameless amplifier.

The main area of interest to me at this moment is the configuration of the O/S and views on O/S feedback.

Any thoughts on the above would be appreiciated.
Cheers
Sheriff
I do not know for certain which can give deep bass.
And to know what gives less listening tiredness
I wouldn't think anyone knows really.
-------------
Bass needs a good PSU - not too small transformator.
To limit the upper frquency could eventually
give a "softer" sound. And that could possibly make you less tired of listening.
--------------
Think you are to build a quite good amplifier.
You have good knowledge, and the details
will be worked out.

Make a first version. Then test and listen.
Then you can make changes and trim with capacitors.
You will get good advices here.

/halo - watching an amplifier being born
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Old 6th February 2003, 06:17 PM   #3
Sheriff is offline Sheriff  United Kingdom
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Default Halo, the story so far...

OK Halo,

The PSU works really well.

It is a PFC based half bridge switcher using current mode control.

I have synchronised the PFC, downconverter and aux supplies to avoid beat frequencies.

Ripple and noise is very low for a supply of this type and the thing well shielded, power limit is 1600W- a good start.

There has been a lot of people playing around with open loop output stages, but i would imagine the THD etc to be poor.

Maybe some kind of forward error correction a la hawkesford should be in order.

Caps are another interesting one. the closed loop switcher gives a very low output impedance, and quick refreash rate (70KHz) but i will play around with the value of the caps on the amp board. I have heard that the 'gold tune' nichicon electrolytics are pretty good. Have you any experience with these?

This should be ok as the PSU loop stablity will increase if i up the C a bit (800uF decoupling close to the MOSFET's per rail currently)

Just after some comments on what people have tried, kind of correlating opinions really.

Cheers

Sherrif
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Old 6th February 2003, 07:24 PM   #4
halojoy is offline halojoy  Sweden
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Default okay

PFC, I have never heard of that name.
But I understand it is a switching power supply thing.
--------------------
I have never used any other Caps then ordinary Elyts.
Rifa and ROE.
I think the value is more important than the Letters on the Label.
But those small caps we need to put into an amplifier,
at the last trimming, is what I am thinking of.
Compensation and such. It can of course involve some decoupling, too.
-----------------------
In many MOSFET output stages, I have seen fairly large Caps
Next to the transistors. Like 1000uF.
So I think that is a good idea.
------------------
I am right now searching the web.
To find out about Malcolm Hawksford (without "e"!!!).
He seems to have done some research about high speed digital
cables. I do not know if this can be used for Low Frequency Audio applications.
But any knowledge is good.

This is attributable to the intermodulation of Sonic's master clock by signal-correlated jitter during the loadin. During playback, since the source is no longer feeding digital audio, even though the DAW is still locked to the external clock, the external clock is much more stable from the DAW's point of view. Its PLL is no longer modulated by the digital audio that is combined with the external clock. This well-known phenomenon, known as signal-correlated jitter, has been documented by researcher Malcolm Hawksford. See AES preprint titled "Is the AES/EBU S/PDIF Interface Flawed" AES Journal 1995. This form of jitter is quite audible.

/halo - is hungry - should have something to eat now
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Old 6th February 2003, 07:35 PM   #5
Sheriff is offline Sheriff  United Kingdom
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Default PFC and FEC

Hi Halo,

M Hawksford, developed a forward error correction stage for MOSFET output stages.

Reducing distortion of this stage to as little as .008% i recall.

Sorry, PFC stands for power factor correction. Where the PSU takes energy from the mains through about 90% of the cycle rather than just grabing current from mains 50 or 60 times a second. The PSU presents a pretty constant resistive load to the mains.

The PSU drawing from the mains on a near continous basis is pretty cool for audio and runninf an amp in areas where the mains conditions are poor.

Again, any help is gratefully received.

Thanks


Sheriff
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Old 6th February 2003, 10:12 PM   #6
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What manufacture is the switching PSU?

Sam
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Old 6th February 2003, 10:44 PM   #7
mlloyd1 is offline mlloyd1  United States
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Default Re: PFC and FEC

Tandberg and Akai (not in US market) released amps using Hawksford's output stage error correction tech. I never heard them but the schematics were fun to study.

mlloyd1

Quote:
Originally posted by Sheriff
...
M Hawksford, developed a forward error correction stage for MOSFET output stages.
...
Sheriff
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Old 7th February 2003, 01:07 AM   #8
Sheriff is offline Sheriff  United Kingdom
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Default The PSU

Hi Sam,

The PSU started life as a forward converter, it has been developed into this. It is of our own design.

I have a picture if anyone is interested.

It has taken a long time and a lot of $$$ to develop.

But the results are worth it. (thank god...)

The FEC stuff has been used in a few commercial designs that i know of like the halcro units.

Cheers


Sheriff
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Old 7th February 2003, 01:39 AM   #9
Jeff R is offline Jeff R  United States
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Getting deep bass, I would think, would have more to with your speakers and crossover network than with your amplifier.
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Old 7th February 2003, 02:05 AM   #10
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Hi Sherrif,

Lets take a look at those pictures.

Sam
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