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Old 30th June 2008, 12:42 PM   #1
tryalx is offline tryalx  United States
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Default MJR-7 Mosfet Amplifier of M. Renardson

Hi,

I would like to test the MOSFET amplifier designed by M. Renardson, from
http://www.angelfire.com/ab3/mjramp/amp7.html.
http://www.angelfire.com/ab3/mjramp/construct.html
http://www.angelfire.com/ab3/mjramp/simple5.html

Did anybody experiment with it? How does it sound?

I would like to ask some advice regarding the implications of increasing the voltage from 60V to 90V and eventually adding a second pair of MOSFETS for extra output power.

Besides choosing capacitors of higher voltage for the power supply and for the 4700uF output (100V instead of 63V), do we also need to increase the output capacitor from 4700uF to 6800uf or higher in order to accomodate the extra power?

If we increase the value of the output capacitor, since it is part of the feedback loop, do we also need to adjust the values of any other components ?

Do we need to increase the value of the power supply capacitors from 12000uf to let say 20000uf?

To add a second pair of MOSFETS, I assume that I would need to add another pair of resistors (330R) and 12V zener diodes to drive these MOSFETS. Is this correct?

Any other advice is welcome :-)

Thanks!
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Old 30th June 2008, 04:43 PM   #2
Mooly is offline Mooly  United Kingdom
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Hi,
As a huge fan of Lateral FET's I have to add my two penneth
Easy bits first, yes just add an extra pair of 330 ohms and parallel the outputs. You could always use double die MOSFET's instead such as BUZ900D and BUZ905D (TO3 Package). The 12 volt zeners, it's not usual to use back to back zeners, normally just a zener and series In4148 if you want to include them at all !! I normally do not.
The value of the output cap is unrelated to power output.
I imagine it sounds -- well -- very nice

Any reason why this particular design ?

Regards Karl
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Old 1st July 2008, 12:23 AM   #3
tryalx is offline tryalx  United States
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Hi Karl,

Thank you for your feedback.

I looked at this design because of its simplicity, yet very low noise/distortions and good linearity.

I would like to build a multi-channel home theater amplifier, and eventually pair it with one of these speakers: the Gallo reference speakers (http://www.roundsound.com/reference-speakers.htm) or the Druid MK4 (http://www.zuaudio.com/druid_2.asp).

I think that increasing the voltage I may be able to get at least 80W of power, which should be enough. Maybe I would need a more powerful amplifier for powering the woofers' second second coil of the Gallo speakers.

What do you think about a design that uses symetrical voltage? Could we get similar performances using only 7 transistors? Such a design may have an advantage that the capacitors will need lower voltage, and I could also get more power, especially for the woofer speakers.

Regards,
Alexandru
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Old 1st July 2008, 12:23 AM   #4
tryalx is offline tryalx  United States
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Hi Karl,

There is something more... I looked again at the schematics, and the zeners are after the 330 ohms resitors, which makes it more difficult to add another pair of transistors, if we keep the zeners. Therefore your idea to use the BUZ900D and BUZ905D MOSFETS would be better.

Do you know where to buy a matched pair of BUZ900D and BUZ905D?

Thanks,
Alexandru
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Old 1st July 2008, 01:02 AM   #5
lineup is offline lineup  Sweden
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Default Re: MJR-7 Mosfet Amplifier of M. Renardson

Quote:
Originally posted by tryalx
Hi,

I would like to test the MOSFET amplifier designed by M. Renardson, from
http://www.angelfire.com/ab3/mjramp/amp7.html.
http://www.angelfire.com/ab3/mjramp/construct.html
http://www.angelfire.com/ab3/mjramp/simple5.html
--------------
Any other advice is welcome

A general comment on Mike J. Renardson.
The guy obviously knows a lot about amplifier designs.
As a matter of facts, I was browsing his website a couple of weeks ago
and downloaded some of his best stuff .......

I would say his later amplifiers, like MJR-6, MJR-7 are 'his best'.
( Because usually people progress, no matter from which starting level )

He has got a lot of interesting knowledge Articles, besides amp projects:
Quote:
DESIGNING AUDIO POWER AMPLIFIERS
http://www.angelfire.com/ab3/mjramp/designing.html
# EFFECT OF FEEDBACK ON HARMONICS. Updated 13-May-2006.
It is well known that a square-law amplifier adds only second harmonics with no feedback, but many high order harmonics with feedback applied, but this is almost entirely misleading, as explained here.
# INPUT STAGE DISTORTION.
Seven of the most common one and two transistor input stages are compared by calculating their levels of intermodulation distortion under identical stage output current levels.
# SLEW RATE AND T.I.D. (Updated 12 June 2006)
The maximum slew rate requirement and the design techniques for low t.i.d. are investigated. Now includes measurement results for maximum CD slew rate.
# MEASURING DISTORTION.
To test our low distortion designs an alternative to buying expensive distortion measuring instruments is to use the bridge-nulling method in which the input and output are directly compared.
# PHASE INTERMODULATION.
# Explanation, reduction and measurement of phase intermodulation distortion.
# COMMON-MODE DISTORTION.
Common-mode distortion is examined, a test method described, and the performance of typical input stage transistors tested and compared.
# MEASURING CAPACITOR DISTORTION. (Updated 22-Apr-2006)
Capacitors are known to add a little low order harmonic distortion, but does transient testing reveal something a lot worse than this?
# ERROR FEEDBACK AND NESTED LOOPS.
An examination of how effective these methods can be compared to overall negative feedback.
# SYMMETRY. (Updated 01-May-2006)
Symmetry can be good or bad, but just aiming for maximum circuit symmetry may add unnecessary complexity with no real benefit. It is possible in principle to achieve zero distortion in some circuits with correct use of symmetry.
DISTORTION - HOW LOW CAN YOU GO?
Is there any limit to how far power amplifier distortion can be reduced?
The startpage of his website, with amplifier project and articles
is:
http://www.angelfire.com/ab3/mjramp/index.html


Regars Lineup
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Old 1st July 2008, 02:21 AM   #6
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Default Re: MJR-7 Mosfet Amplifier of M. Renardson

Quote:
Originally posted by tryalx
Hi,

I would like to test the MOSFET amplifier designed by M. Renardson, from
http://www.angelfire.com/ab3/mjramp/amp7.html.
http://www.angelfire.com/ab3/mjramp/construct.html
http://www.angelfire.com/ab3/mjramp/simple5.html
Any other advice is welcome :-)

Thanks!

Looks very similar to an old Maplin 75WRMS amplifier.

There seems to be quite a few more components than is absolutely necesary in this design. Typical of a designer who has been around a while !

I prefer to stick to simple designs. I dont use zeners on the MOSFET gates. I lose the output inductor. I keep the zobel network tho. I would only use one low pass filter on the input.
I probably wouldnt use the 1000uF on the output stage.
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Old 1st July 2008, 06:48 AM   #7
Mooly is offline Mooly  United Kingdom
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Hi
Have you checked out my design on this forum.
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Old 1st July 2008, 06:52 AM   #8
Mooly is offline Mooly  United Kingdom
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I saw you were online. I haven't figured out how to link to other threads -- it never works for me !!!
Look in the solid state forum, it's called "My MOSFET Amplifier designed for music". Have a read
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Old 1st July 2008, 06:59 AM   #9
Mooly is offline Mooly  United Kingdom
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Check www.profusionplc.com/subsites/exicon.htm
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Old 1st July 2008, 08:58 AM   #10
h_a is offline h_a  Europe
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Quote:
Implication 60V to 90V
Now that's a drastic change. You would at least need to check the biasing and check the SOAR of all transistors. I don't have the time at the moment to have a more closer look to the schematic, but from experience I doubt that you can increase rail voltage by that amount without reworking the circuit a bit.

Have fun, Hannes
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