Power amp under development

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Samuel Jayaraj

Member
Joined 2001
Andrew, please don't be offended. It is not my secret but someone elses who has a commercial interest. I can easily take out that sub-circuit and report what I find. By the way, while I am seeking some information from those like you who are experts, I am also trying to contribute for the benefit of those who may want to try the NMOS series with their junk box components or indeed, those who may not be able to find the specified output devices.

I have already stated that I already have an NMOS stereo amp working off +-63volts and using 3 pairs of IRFP450s.
 
keypunch

keypunch

Member
Joined 2004
Samuel Jayaraj said:
The Vas buffer circuit was given to me by a commercial designer who is also a member of this forum. Since he may be using this in his commercial designs, I am afraid I may not be able to share.
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Darn, such a shame. I would have liked to try the idea and see if it improved or stabilized matters even just a tad.

There are ofcourse, several discussions on this forum regarding the capacitance presented by Mosfets to the preceding stage. It has also been shown that the effective capacitance presented varies with supply voltage. This may be the reason as to why the Quasi amps have some issues with lower voltage operation.
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Interesting the capacitance varies with supply voltage. I had just assumed, possibility incorrectly, that the MOSFET capacitance presented to the driver stage was a function of the audio output signal voltage. It sounds like this assumption is only partly true and that the rail voltage factors into the characteristic capacitance of the MOSFET as well.

Does anyone know how easy it is to actually measure the capacitances at different supply voltages?

Whether it is called NMOS350 or 500, with 3 pairs of IRFP450s and +-63 volt supplies, I set Iq at 115mA which gave me the best subjective performance.
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I did alot of searching on the internet as my question about Iq was to clarify ther terms use. Sadly I did not find much to clarify. It seems Iq can be used in many different contexts. What I am deducing from your reply is that you used Iq in the context of the total idle current you set with the bias. So for a 3 pair version of the quasi NMOS amp each MOSFET pair has about 38.3ma per your Iq setting of 115ma.

But with the single pair of OPs being IRFP460 and rails of +-39 to 45 volts, only Iq of 160mA seems to help, whether R21 is 100E or replaced with LED/Diode string. As to the over-sensitivity of the multi-turn bias trimpot with the Diode string, I have no clue.
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Interesting not so much that a single output pair needed more Iq than the three pair, but that it was not even close to the Iq used for 3 pairs of IRFP450s. Do you think because the rail voltage was lower this required a higher Iq setting, that the higher Iq setting was because of the specifications of the IRFP460, or for both reasons?

Perhaps someone can shed some light on why the diode string replacement for R21 caused the bias setting to me over sensitive?

Though I have not faced any thermal issues, I feel that the Vbe multiplier circuit should be tweeked. With the successful 3 pair boards, when I increased the 0.1uF cap across Emmiter and Collector of the Vbe multiplier, increasing bias was just not possible. Normally within reasonable limits, this should not be the case.
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Interesting observation. Personally I have felt the Vbe should be MOSFET based simply because the output stage is MOSFET and the thermal tracking device should be the same type, rather than BiPolar, in order to mirror the thermal characteristics of the output devices. To be clear this is my personal opinion as I do not have the technical knowledge to know if there is any basis of fact for my personal opinion in this regard.

I will be making a separate posting later today or tomorrow with some interesting links I found while trying to find out the definition/context of Iq. It is interesting how Iq has a number of related elements in a design and the effects including distortion and crossover distortion in MOSFET designs.


In Quasi's NMOS350 MKII note that the real changes are in the current flowing through the front end. The Vas is now cascaded. The cascade presents an easier load to the LTP. The changes I have been trying all along have to do with the same two stages. Just looking at the MKII schematic, I suspect that the amp will sound more detailed but may sound a bit bright and metallic. It is just my estimation. Someone building it can report on the subjective aspects.
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Can you explain why you feel the MKII may be more detailed, yet may sound a bit bright and metalic with these changes? Would gate resistors being removed in the MKII be a factor to your opinion before hearing the MKII?

Samuel Jayaraj said:
Andrew, please don't be offended. It is not my secret but someone elses who has a commercial interest. I can easily take out that sub-circuit and report what I find. By the way, while I am seeking some information from those like you who are experts, I am also trying to contribute for the benefit of those who may want to try the NMOS series with their junk box components or indeed, those who may not be able to find the specified output devices.

I have already stated that I already have an NMOS stereo amp working off +-63volts and using 3 pairs of IRFP450s.
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I just like to add Samuel was first I know of to use the KSE rather than the MJEs in quasi's design. I know Samuel used his own PCB design, but I think perhaps Samuel's initial findings may have, in part, been related to using the KSE's. That is not a negative comment, just a possible consideration and one that I am grateful for knowing as well as Samuel's other findings and solutions he has found in his version of quasi's design.

I personally have not had the time yet to mock up a base design and test all the combinations and ideas in my mind as I have been limited in what SPICE programs I can use as a initial sense of what the results of the various ideas I like to try. I know a SPICE simulation is not the same as the real thing, but I wanted to see if I could shortlist some ideas and their permutations before I figured out the best breadboarding mock approach to use for real world testing of these ideas. This means the results and ideas like Samuel's, Radionman62, Quasi, Andrew's and may others I cannot remember all of them off the top of my head) that have replied to my postings or other builder's postings has been very educational and inspired me to think and better understand not just the quasi design, but many other elements to consider in an audio foodchain.


Regards,

John L. Males
Willowdale, Ontario
Canada
08 March 2008 (08:25 -) 09:41
Official Quasi Thread Researcher

08 March 2008 09:48 Typo corrections. jlm
 
J

jethari

Member
Joined 2002
quasi said:
Hi folks,

A fellow DIY'er asked about a TO3 version of this amp series. It seemed quite easy to modify the bi-polar version N-Bip300 or "Brother of Quasi" to take these FETs, so now we have the Nmos400 the TO3 version.

I guess just about all needs should be covered now. Details on my web site under Nmos400.

Cheers
Q
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Quasi
Can I use two pairs of IRF452 or equivalent with a supply of +/- 60V dc and a load of 8 ohms?
Hari
 
quasi

quasi

Member
Joined 2004
sites.google.com/site/quasisdiyaudiosite/
jethari said:


What about T9/T10, can I stick with MJE340/MJE350?
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The pin-outs are different unfortunately and further PCB changes would need to be made. Use of the TO220's gives an opportunity to reduce the 2 x 220 ohm gate voltage drive resistors to lower values and this should improve the performance of the output stage.

Example: These could be reduced to 100 ohms, doubling the current through T9/10. If this was done then the TO220's must be used.

Cheers
 
J

jethari

Member
Joined 2002
quasi said:



The pin-outs are different unfortunately and further PCB changes would need to be made. Use of the TO220's gives an opportunity to reduce the 2 x 220 ohm gate voltage drive resistors to lower values and this should improve the performance of the output stage.

Example: These could be reduced to 100 ohms, doubling the current through T9/10. If this was done then the TO220's must be used.

Cheers
Click to expand...

The pin outs will not be a problem since I plan to use the same layout that I designed for the NMOS350 which in turn is an adaptation of the NMOS200 layout
hari
 
S

spind

Member
Joined 2003
Paid Member
quasi said:


so now we have the Nmos400 the TO3 version.

I guess just about all needs should be covered now.
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Hi quasi,

Thank you so much for all your continued efforts! In the 'when will they ever be happy category' of requests......how hard would it be to convert this T0-3 version to accept T0247 outputs?. It would be good for us that have longer heatsinks to spread the output devices out a bit. I like the 4 pairs of outputs and the long layout---it makes mounting the outputs a bit easier. If the pads were right at the edge of the pcb then the outputs could be mounted flat on the heatsink (ie at right angles to the pcb). Alternatively, an angle mounting bracket could be used and the outputs could be mounted like was done in the Nmos200 if some pbc material was left on the 'far' side of the board when it was being etched (the choice could be left to the person doing the etching).

Don't worry if it's too much trouble! I'm working on an Nmos200 right now and if I ever get it done :smash: I'll look into trying to learn to modify the tracks myself.

Thanks again quasi!
Steve.
 
S

superseadragon

Member
Joined 2007
Nmos350 parts and suggestions

On quasi's Nmos350 the schematic on the webpage says there are 0R33 resistors on the output stage, the Nmos350 parts for sale have 0R39, and the pcb says 0R47. Which to use?

I am also having a great deal of trouble finding 2sc1845 as they are discontinued i think. Are there any equivalent transistors out there?
also for the dc detect circuit Quasi has the bipolar 22uF caps, I cannot find any ceramics that high. Is there a certain type i should be looking for.

My application is to create a bass guitar amp. Quasi has already advised me to derate the power supply to +/-60V for this application. Any have any extra insight?

Thanks!
 
P

pejinm

Member
Joined 2005
Hi superseadragon,

For output stage resistors, any value between 0.33-0.47 will do fine.
Instead 2sc1845 I'm using 2sc2240 and it works fine on my quasi amp.
The caps on dc detect are not ceramic (nonpolar), they are bipolar. Big difference!
wiki:
Bipolar electrolytics contain two capacitors connected in series opposition and are used for coupling AC signals. Bad frequency and temperature characteristics make them unsuited for high-frequency applications.

Yes, this amp should be one great bass amp!

Best regards,
Miodrag
 
S

superseadragon

Member
Joined 2007
Thanks zlast and Miodrag!! Now I can get the rest of my parts!!

Another question: I am looking for a toroidal transformer for this setup anyone have any suggestions of where to buy such transformers?

Thanks again guys!

I forgot to ask about the output inductor and the resistor and cap in series right before it on the schematic. What should be the watt rating on the resistor, voltage rating on the cap and the current rating on the inductor?
 
S

superseadragon

Member
Joined 2007
OK I am ordering the remaining parts that I need but I want to be sure about them. The relays for the DC detect and soft start are 24v but the dc detect is NC and the softstart is NO correct? I was thinking of getting a 6A relay for the dc detect and a 8-10A for the softstart. It is a bit extra overhead on the ratings but hey couldnt hurt.
 
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