You may have difficulty beating the L.F. noise level of Nuvistors with any normal solid state device, but above 1Kc/s there are many solid state devices that are as good as the best valves.
The circuit design is critically important, especially the method of matching the input impedance to the source impedance. If you throw away signal at that point, no first stage valve or transistor, however exotic, will achieve the best S/N ratio.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
...
Why not? They're 100% efficient space heaters. That's as efficient as it gets, riiiight?
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Right, but that's not such a great thing in August around here.
Some of those things could constantly blast half a kilowatt of heat into the lab. When I got a 7704 I was thoroughly surprised that it contained a very efficient primary-side switcher. Maybe it was designed during the energy crisis.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
They weren't afraid to use switchers -- HV supply in those is usually a 6AU5 or such blocking oscillator (g2 controlled, of course), with a multiplier hanging off one end for the 2nd anode (~7kV). It's just that the high voltages and massive waste of heater power kind of drowns out everything else in terms of wattage. :)
It's pretty funny to think, really, high performance tubes work at the same currents, and nearly the same impedances, as transistors. They just have to be biased with >10 times the voltage, plus heat.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
In some direct conversion receivers with the SBL-1 diode ring mixer, quite interesting audio preamplifiers are used. For best performance, the mixer "IF" (in this case audio) port had to be terminated into a
50 ohm load. Often a common base amplifier stage was used, sometimes consisting of multiple small signal transistors in parallel or using a big power transistor for best noise performance.Perhaps such amplifiers could be used with low impedance microphones as well.
The microphone characteristics, such as frequency response is usually specified for some standard load impedance, loading the microphone for power match may alter these characteristics, so some corrections may be needed in the following stages.
Impedances between a few ohms and a few tens of ohms are common in moving coil stereo record pickups. The common-emitter configuration is often used in their pre-amps, with many paralleled large-area switching transistors (the ZTX series being particularly popular).
[Doug Self's design]
That is true, although the difference with modern designs is usually quite small. Some older designs relied heavily on the correct electronic damping.
By using two independent feedback paths it is possible to make the termination appear different from that of the active device alone. One path drives voltage into the reference terminal of the first stage to set the voltage gain; this raises the apparent input impedance above that of the active device. Another path drives current into the input terminal, which lowers the apparent input impedance - and because the current is supplied through a feedback loop, the noise of the resistors in that pathway is much reduced by the effect of feedback.
The two feedback signals need to be in opposite polarity, this can be achieved with a simple inversion stage as in the circuit below:
Care has to be taken that the extra phase shift of the inverter does not cause loop instability. Alternatively both signals can be of the same polarity but the voltage feedback can be applied to the 'earthy' end of the input transformer or the input device. This means that several parameters of the transformer or input cabling which are often ignored, must be tightly specified - in particular capacitance between the 'cold' end and earth.
If a transformer is used in the output stage, it is much easier to derive two different polarities of feedback.
This elegant BBC circuit does that by running a floating feedback winding through resistors to earth at both ends (R11 & R16) - and manages to include stepped gain as well:
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
What are the voltage and current noise densities?
John
te
hem,
y d=A0 =A0 ...Jim Thompson
Do you still have the schematic of your amp? Also, did it have a mic level pre? or just a line level input?
Chris
Also, did anyone get a chance to listen to the mp3 I posted? I thought the end result was pretty nice. A cheap tube pre (I would bought a SS if they sold any), and a ~$100 PCM recorder.
Chris
Or heavy metal. ;-)
-- Lead free solder is Belgium's version of 'Hold my beer and watch this!'
Afraid of switchers, that's a good observation. I find that at times and it takes some convincing along the lines of "It's going to be ok, we won't crash and burn". Then when the design is done, and there is no heat sink, and nothing gets hot, that does the ultimate convincing.
That's why I think it is good that you experiment with switchers while still at university. You'll probably have tons of work once you enter the workforce.
[...]-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
If you listen to music from the Scorpions the noise figure does not make one iota of difference :-)
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Speaking of school, I recently tested part of my final project.
Now I just need to get rid of that masking tape! ;)
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
I agree. With electric guitar/Bass amplification is like this: you own a piece of wood that sounds like crap without proper amplification. Try to play a electric guitar not plugged in and listen to the sound. Now try two different amplification methods, 1 mike the guitar, 2 play it through a solid state amp. You will find that if the solid state amp is built for low noise, low distorion, you'll hear the same sound you heard with the guitar not plugged in, only amplified. The miking method should sound even worse since the mike is usually worse for guitar than the pickups.
This happens so because the transistors are more ideal amplifying devices than tubes are...so the signal that comes in is the one that comes out, with as little distortion as possible. Since the tubes are less ideal, they add different components to the original signal... this have turned out to be more pleasant to the human ear.
If you own an acoustic guitar you'll notice that it has a sound hole, and the wood is very different from it's electric counterpart. This is an acoustic method for making the instrument take it's original "pleasant" sound. You could call the tubes the analog of the "sound hole".
UC3842? How did you get PFC and regulation done with just one chip and one inductor?
Whoops, did it bake on?
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
I would disagree with this statement. The curvature of the transistor characteristic is much sharper than many valves and the harmonics generated are much nastier-sounding. Valve stages with no feedback are quite common and the distortion they generate with moderate signal voltages is quite tolerable, but transistor stages with no feedback are virtually unusable for audio.
Transistor amplifiers can contain more devices in less space for the same price and that allows them to employ much more feedback than valve ones, not just overall but within individual stages; this is what gives the improvement in overall linearity which you have erroneously attributed to the devices themselves. The down side is that when they eventually overload, they do so much more sharply and generate higher harmonics, which sound vile.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Unless you build in "soft" limiting and make 'em sound toob-like :-) ...Jim Thompson
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The only thing bipartisan in this country is hypocrisy
Try running a 12AU7 with a plate supply of 45V and grid-to-cathode voltage of about -1.5V (tweaked for zero grid current).
A lot better than my record for a tube amp at 100W RMS sustained, with total hum and noise under -60dBmW. Do not remember how far down it was, but cranked up all the way it made no difference open input or shorted input, one could not hear noise and/or hum with ear right at speaker.
I listen to traditional Country music, Bluegrass & Southern Gospel.
-- Lead free solder is Belgium's version of 'Hold my beer and watch this!'
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