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KMG SS Poweramps

Started by THChrist, December 28, 2010, 11:02:15 PM

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KMG

Quote
But I was actually reffering to *line driver transformers* as used in buildings where very long wires are used to link many speakers.
If anyone is interested I'll try to find more info.
Phil.
Found at least one type
http://www.rcf.it/en_US/products/commercial-audio/attenuators-transformers/td-105
Primary 100V with center tap

phatt

Yep they are the type that will work well.
Tricky for the novice to understand how to convert the taps to something meaningful.

This gives some info on how to wire-up the taps
http://www.ozvalveamps.org/optrans.htm
Phil.

KMG

#47
Results of testing Xicon transformers in micro PA (instead of russian trafos, that hard to find).
Schematic

Voltage frequency response on resisitive load with Presence & Depth action (real device).

Comparative voltage frequency response on resisitive load & cabinet with Presence & Depth = min.

Sim (LtSpice) project with measured parameters of TM008
http://milas.spb.ru/~kmg/files/projects/fetpa/micro/xicon/sim/
It gives about 100mW of power, really audible at 4x12 cab & suitable for "night" playing.
For line recording I plan to use this kind of load to simulate speaker impedance for adequate interaction of PA with load.

And frequency correction after load to simulate cabinet/mic response.

PS Sample:
Fet JCM800->Micro PA->4x12 cabinet
http://milas.spb.ru/~kmg/files/projects/fetpa/micro/xicon/s/8-Southern%20LP.wav


joecool85

KMG, I haven't had a chance to listen to your latest samples, but seriously, is this what you do for a living?  It seems you're working the hell out of this circuit.  Are you planning on selling PCB kits?  Full built amps?  Anything?
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

KMG

Quote from: joecool85 on October 21, 2011, 03:46:52 PM
KMG, I haven't had a chance to listen to your latest samples, but seriously, is this what you do for a living?  It seems you're working the hell out of this circuit.  Are you planning on selling PCB kits?  Full built amps?  Anything?
No, it's my hobby to change my thoughts from main work.
Only one item now in production - 5W fet PA kits (assembled & tested board).

In plans home recording box - micro PA+load equivalent+cabsim.
You can use micro PA output on cab for night playing with recording from sim output.
Or you can pass singal to sim input form big amplifier output (in parallel with cabinet) for recording without mic.

KMG


KMG

Micro PA, load equivalent & speakersim assembled in box.

KMG


Evil_Food

#54
I don't understand this at all.

In my view the whole point of solid state amps is to save money. The most expensive components in tube amp is usually the output transformer, which usually costs several times the rest of the components. If you just look on sowter's website, the cheapest one is about £100 http://www.sowter.co.uk/acatalog/SOWTER_TRANSFORMERS_PUSH_PULL_OUTPUT_16.html (guitar OTs are cheaper though).

If we disregard the tone effects of an OT (I haven't read a sophisticated article regarding this subject so far), there are two aspects of a tube power amp - overdrive behaviour and output impedance.

The impedance is so EASY to implement in a SS amp (I bet someone's already posted a link to this http://sound.westhost.com/articles/guitar-amps.htm#s3) and is actually widely used.

The other aspect is overdrive behaviour. I've read articles saying that FETs operate like Tubes (which is a stupid statement) and therefore valve circuits can just be copied and components replaced.

What's similair between the two types of devices is that they have very low transconductance and, unlike BJTs, it isn't as strongly dependent on quiescent current (in a BJT gm=Ic/VT and in a MOSFET, for example, above the threshold voltage it is fairly constant).

I tried making a MOSFET gain stage using a BS170 biased with a quiescent current of about 150mA (which is quite high for a preamp). The output waveform (on an oscilloscope, not SPICE) looked pretty much like that of a tube gain stage. But when I played a guitar through it it didn's sound good at all. I decided to look at the spectrum of the output from the gain stage. What I noticed is that there was a strong second harmonic (which everybody goes on and on how desireble and tube-ish it is), but there was also a noticeble 9th harmonic. I tried to find an ideal overdrive value, but there wasn't one. It just doesn't work with single discrete mosfets.

I've wandered off a bit. My main question is - what's the benefit of all this?

phatt



@ Evilfood,
"I've wandered off a bit. My main question is - what's the benefit of all this?"

Answer is IT Works.
I agree that Transformers can be expensive but you can substitute 100 Volt line units which can often be purchased for VERY Low $$.
Phil.

J M Fahey

Dearevilfood.
Generally it is as you say. I agree.
In fact, I HATE the assumption so much believed in a couple famous Forums (won´t give names but everybody knows them) where they foolishly take some classic tube amp, just substitute the tube with some FET, use the same biasing resistor values, power it with 9V and happily "think" it´s the same.
Ugh !!
They even post buzzy horrible sounding clips and find them "the same as the originals"
That kind of "design" is so horrible that they *need* a trimmer on *every* FET to make it work (sort of).

When I first read about these, I must confess my first gut reaction was the same as yours, in fact my first answer to the topic said something similar.
But Mr KMG blew me away, simple as that.
He did his (very extensive) homework, advanced slowly, step by step, and has made everything necessary to capture Tube sound and behavior.
I invite you to read all his posts, from the very beginning ...... then re-read again, a couple times.
Much more important: listen to the MP3's , the REAL proof.
What surprises me even more, is that he generously shares everything here, even PCB designs, he hunts affordable parts for us to build his designs, answers doubts, etc. , when in my opinion he might (and should) be making big $$$$$ with this.
In other high level Technical forums he is much praised by very knowledgeable and experienced guys.
And, as I said before, the proof of the pudding lies in the eating, just listen to the MP3s.

KMG

Preliminary (test) single board version of "recording box" (Micro PA, load equivalent & speakersim).

KMG

Link to full project (microPA, eqivalent, cabsim)
http://milas.spb.ru/~kmg/index_en.html
Micro power amplifier for line recordimg ver. 2

phatt

Plus x 10 on what Mr Fahey stated above.

Beyond doubt this is very clever.

What I Would like to mention is the issue of *Surface mount*
This more than any other problem will slow the progress of the concept.

I'll even stick my neck out here;

The best thing that could happen NOW is have folks build it and report back with Positive findings.
The faster that happens the better chance of success.

So @ KMG,, I hint,

If you want this to take off You need to present it as a Normal single sided board layout.

I have no doubt your ideas are worth building but surface mount is not for the average home builder. xP

Wait till some of you chaps hit 50 years plus and your eyesight starts to let you down and you will see the sense in SANE layouts.

I refuse point blank to work on things I can't even see. :trouble

Not to mention that slowly but surely the experts are starting to realize that the Lead Free Solder stuff is not as good as it first looks.

Just some thoughts and observations from an old bugga in the  Land down under,,, Phil.