I thought Sziklai pairs were a unity gain configuration and that you had to incorporate a method of reduce the 100% local negative feedback of the output stage to achieve gain.
I can't quote anybody that's just what I heard somewhere and it makes sense if it's truly 100% local nfb.
Wouldn't the effect, if thier is one, be the same for all output devices used with in reason of the transistor parameters?
I can't quote anybody that's just what I heard somewhere and it makes sense if it's truly 100% local nfb.
Wouldn't the effect, if thier is one, be the same for all output devices used with in reason of the transistor parameters?
Easyamp said:
you are right: more like 90%+ due to the Vbe loss on the driver transistor.
the only case where I heard transistor-specific "coloration" is in a jlh1969, output using tip41 vs. irf540, especially driving a 0.47uf capacitor in parallel with a speaker.
I have not found much (any) sound difference between various onsemi, toshiba and fairchild transistor. Then I haven't looked hard enough.
anatech said:
Yes this is true. --------
This circuit is beta dependent and probably somewhat unstable with low Z loads, particularly because the input imeadance of a feedback pair connection is usually high. Here it is set way to low, also feedback pairs made of unmatched and different types of transistors are sometimes not linear. If you use different componants even with the same # it may not work anymore. This circuit might work if the devices are matched precisely, and the diff. amp has a super high freq. responce and gain. And if the load Z wasn't to low.
The collector output is not as current efficient as emitter follower.
You need some thermal biasing compensation. Thermal runaway?
you might try disconnecting the emitters of the drivers directly from the collectors of outputs, and putting in emitter resistors---maybe 30-40 Ohms or so each, and connect them together. Then connect a resistor maybe 1k or so from between the emitter resistors and the collector outputs. connect another resistor from the point where the three come together to ground, 100 Ohms or so.
may want to add a DC blocking cap
Add base resistors to the drivers, and then make a bias servo circuit (between your VAS and current source, accross the bases of the drivers), mounted with output transistors.
Just some thought on how to combat these minor issues. The overall topology should work though.
There is gain, depending on the bulk emitter resistance. No emitter degeneration makes it very non-linear. So there is 100% feedback, but that doesn't mean distortionless. The varing load impedance will also load the VAS to varing degrees. This may change the sound.
Anyhow, the transistors you like best would be the best in your application. I was just trying to explain a couple mechanisms that may change the sound.
Generally, if you can hear it, there is a big change in circuit operation. Usually transient & hard to measure.
-Chris
Anyhow, the transistors you like best would be the best in your application. I was just trying to explain a couple mechanisms that may change the sound.
Generally, if you can hear it, there is a big change in circuit operation. Usually transient & hard to measure.
-Chris
Thankyou for all your input, the amp works well as a test bed I never built one like this past the proto board.
I chose the this setup just so that I could get away without having a bias tracking transistor, to ease in swapping out power bjt's and heatsinks and what not.
I varied the bias resistor to acheive roughly 80~100ma between the power bjt's collectors before listening.
You don't need thermal compensation with a Sziklai pair, well at least not for the output stage.
Cheers
I chose the this setup just so that I could get away without having a bias tracking transistor, to ease in swapping out power bjt's and heatsinks and what not.
I varied the bias resistor to acheive roughly 80~100ma between the power bjt's collectors before listening.
You don't need thermal compensation with a Sziklai pair, well at least not for the output stage.
Cheers
Re: Re: Commentable Thoughts
Mr.Millwood
I had never made any single Blanket Statements so far throughout my Professional Carrier in amp manufacturing.
I have just stated the superiority of Mosfets Over BJTs in professional world so far.
So be more consistant in making statements to others.
Regards,
Ampman
millwood said:
Mr.Millwood
I had never made any single Blanket Statements so far throughout my Professional Carrier in amp manufacturing.
I have just stated the superiority of Mosfets Over BJTs in professional world so far.
So be more consistant in making statements to others.
Regards,
Ampman
"I have just stated the superiority of Mosfets Over BJTs in professional world so far"
Let me do something really stupid and try to abritate this.
I don't think the original statement indicated "professional world"; that's not an insignificant qualification. I think it is valid for pro-equipment, especially touring, if only on the evidence that MOSFETs are used almost exclusively for this pupose. I expect that if pressed most puveyors of high or very high power touring amps would cite MYBF and warranty returns as one of the mpst important considerations.
For home audiophile use, there are enough good sounding high power models using BJT's, J-FETs or L-MOSFETs that I doubt there is any significant advantage. In this realm the characteristics deemed desirable are different from those of professional. There isn't even much agreement by customers on what these alternative characteristics are. This supports a situatio where there are multiple and varied "solutions".
Let me do something really stupid and try to abritate this.
I don't think the original statement indicated "professional world"; that's not an insignificant qualification. I think it is valid for pro-equipment, especially touring, if only on the evidence that MOSFETs are used almost exclusively for this pupose. I expect that if pressed most puveyors of high or very high power touring amps would cite MYBF and warranty returns as one of the mpst important considerations.
For home audiophile use, there are enough good sounding high power models using BJT's, J-FETs or L-MOSFETs that I doubt there is any significant advantage. In this realm the characteristics deemed desirable are different from those of professional. There isn't even much agreement by customers on what these alternative characteristics are. This supports a situatio where there are multiple and varied "solutions".
I think Millwood may have a case.
It's horses for courses. It's easier and cheaper to make a rock-band proof, high power ear blaster using mosfets. I'm interested in purity of sound quality at domestic listening power. I think you can make something exceptional using any power device if you know what you are looking for.
Better bass slam
Yes, they have, but only in comparision against simply Fet's connections. If is used Hawksford / Cordell error corection, result is quite different. In this case have fets too much more clear trebles and " natural " feeling of reproduction is much better. All is about distortion and this is in this case realy low. In my amps, where I'm using it, is distortion on low and middle levels below noise ( - 124 dB ) and approximately 0.003 % at 1 dB below clipping ( 20 kHz ). This amps are good slaming too .
Yes, they have, but only in comparision against simply Fet's connections. If is used Hawksford / Cordell error corection, result is quite different. In this case have fets too much more clear trebles and " natural " feeling of reproduction is much better. All is about distortion and this is in this case realy low. In my amps, where I'm using it, is distortion on low and middle levels below noise ( - 124 dB ) and approximately 0.003 % at 1 dB below clipping ( 20 kHz ). This amps are good slaming too
.
Although there are all-FET amps, the majority of SS amps seem to be BJT based as far as input and voltage gain are concerned. Frequently if you were to conceal the choice of output device there would be few clues to the amp's nature from looking at the rest. Well, perhaps you should conceal the biasing scheme and V-I limiting, too.
Anyway, when comparring just the power sections BJT vs FET you are looking at only one or two sources of performane (measured or subjective) differences out of many. Gain and feedback factors, slew limits, PSRR, noise floor are influenced greatly by the small signal stages which can be designed almost (but not quite) independently of the choice of output device.
To my mind, the two principal advantages to MOSFETs (L-MOSFETs in particular) are simplification of biasing and output circuitry (ability to eliminate drivers and use simpler V-I limiting) and secondly the non-critical nature of biasing and thermal tracking. The pricipal disadvantages is a tendency to run hotter, slightly lower power for a given power supply and a bit more noise and distortion.
I don't think it is a question of adding up all the factors and choosing the one with the most "point", but rather knowing that you have a different set of problems to deal with in each case.
Anyway, when comparring just the power sections BJT vs FET you are looking at only one or two sources of performane (measured or subjective) differences out of many. Gain and feedback factors, slew limits, PSRR, noise floor are influenced greatly by the small signal stages which can be designed almost (but not quite) independently of the choice of output device.
To my mind, the two principal advantages to MOSFETs (L-MOSFETs in particular) are simplification of biasing and output circuitry (ability to eliminate drivers and use simpler V-I limiting) and secondly the non-critical nature of biasing and thermal tracking. The pricipal disadvantages is a tendency to run hotter, slightly lower power for a given power supply and a bit more noise and distortion.
I don't think it is a question of adding up all the factors and choosing the one with the most "point", but rather knowing that you have a different set of problems to deal with in each case.
operating speed??
In the case of MJ802/MJ4502, they are considered slow. If the device is linear up to 7A and 7A is the maximum peak current it will have to conduct, then why does the device have to operate at RF frequencies? It should not have many odd harmonic additions because 7A is in linear operation. (the generic version of this device I can't varify.)
Chris
In the case of MJ802/MJ4502, they are considered slow. If the device is linear up to 7A and 7A is the maximum peak current it will have to conduct, then why does the device have to operate at RF frequencies? It should not have many odd harmonic additions because 7A is in linear operation. (the generic version of this device I can't varify.)
Chris
Earlier in this thread someone mentioned a couple of Toshiba TO-3 devices that had pretty constant hfe and I searched the Toshiba website and could not find those devices (maybe they are discontinued).
The problem is that I could not figure out how to search their site for TO-3 cased devices. It returned zero results when I tried. Has anyone used their site enough to know how to find what they offer in that case style? Is there another name for TO-3?
I am interested in finding a device with relatively constant hfe up to about 2 or 3 amps of Ic since I plan to bias a class A output at around 1 to 1.5A.
The problem is that I could not figure out how to search their site for TO-3 cased devices. It returned zero results when I tried. Has anyone used their site enough to know how to find what they offer in that case style? Is there another name for TO-3?
I am interested in finding a device with relatively constant hfe up to about 2 or 3 amps of Ic since I plan to bias a class A output at around 1 to 1.5A.
Methinks mashaffer,
You will have to use the plastic case TO-3. Most Japanese audio outputs went the the plastic case for new product. 2SD424 / 2SB554 and their kind have been discountinued for years. Anything new is very likely to be a fake, or a remark.
The new Motorola (ON) audio devices share that linear characteristic you are looking for. They can be had in either case style. You may as well use the T0-3P case (plastic) for future replacement. Be aware that Motorola has imposters floating around also. Buy from a reputable vendor that can trace the product back to Motorola. That means, run from prices that are too low. Look carefully at the printing on the part(s) when you buy them.
-Chris
You will have to use the plastic case TO-3. Most Japanese audio outputs went the the plastic case for new product. 2SD424 / 2SB554 and their kind have been discountinued for years. Anything new is very likely to be a fake, or a remark.
The new Motorola (ON) audio devices share that linear characteristic you are looking for. They can be had in either case style. You may as well use the T0-3P case (plastic) for future replacement. Be aware that Motorola has imposters floating around also. Buy from a reputable vendor that can trace the product back to Motorola. That means, run from prices that are too low. Look carefully at the printing on the part(s) when you buy them.
-Chris
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