Can any direct me to any schematics that use valve output stages but soild state front ends?
http://www.lenardaudio.com/main/051_opal_const.html if you have a look half way down this page you will see these speakers amps, which are push pull amps using KT 88s which have solid state driver stages.
Something around this sort of power (100watts) is what I am after really.
http://www.lenardaudio.com/main/051_opal_const.html if you have a look half way down this page you will see these speakers amps, which are push pull amps using KT 88s which have solid state driver stages.
Something around this sort of power (100watts) is what I am after really.
That's an interesting link. I can understand their preference for a low-noise solid state front end and I agree that it's the output stage that gives it the 'tube sound'. (I could never understand why some people want to put a tube front end on an SS output stage.)
I think they're a bit confused over Classes AB1, AB2 and B.
I think they're a bit confused over Classes AB1, AB2 and B.
National Semiconductors launched the LM4702 some time ago, which could probably be used as a single stage driver for high power output tubes.
http://www.national.com/pf/LM/LM4702.html
In the chipamp forum there are some people who already built PCB's for the LM4702 to operate as driver stage for SS output...surely someone has already tried it to drive tubes!
Erik
http://www.national.com/pf/LM/LM4702.html
In the chipamp forum there are some people who already built PCB's for the LM4702 to operate as driver stage for SS output...surely someone has already tried it to drive tubes!
Erik
Ray, the "low noise" thing baffles me. A tube front end can be less noisy than the thermal noise of the preamp feeding it. An ECC88, for example, will have an equivalent noise resistance of 300R. 300R at room temp and a 20kHz bandwidth should generate about 0.25uV of noise (that's consistent with experience using ECC88 in phono stages). That noise is about 120dB below 1W into 8R. Ahem.
Sy,
Again "expanding" the thread of Fiat1 (hi, Fiat1, long time no hear - hope you are OK!).
You mention the tube noise only; what about that of the resistors in the circuit? OK, they are in parallel with tube rp's. Still, resistors used in ss circuits are quite lower, so the equivalent noise is?? I have never compared calculations (both can provide more than acceptable low noise), but am just wondering.
Ray,
Mmmmmm. Some say they want tube sound, but mosfets improve on tubes with output transformers, thus they imagine that should be better - at least less expensive per watt. Not something I fancy though. I am not even sure about this "tube sound" thing. I do not hear any good reason why that should exist. (Now please, not to start another topic here, unless Fiat1 changes it!)
Regards.
Again "expanding" the thread of Fiat1 (hi, Fiat1, long time no hear - hope you are OK!).
You mention the tube noise only; what about that of the resistors in the circuit? OK, they are in parallel with tube rp's. Still, resistors used in ss circuits are quite lower, so the equivalent noise is?? I have never compared calculations (both can provide more than acceptable low noise), but am just wondering.
Ray,
Mmmmmm. Some say they want tube sound, but mosfets improve on tubes with output transformers, thus they imagine that should be better - at least less expensive per watt. Not something I fancy though. I am not even sure about this "tube sound" thing. I do not hear any good reason why that should exist. (Now please, not to start another topic here, unless Fiat1 changes it!)
Regards.
ray_moth said:
Who is confused about classes and what does it have to do with tube front ends (or am I reading you wrong)?
Anyway, regarding tube front ends to SS power stages, here's a clue: transistors of any kind have non-linear input, transfer and output capacitances. While their effect can be minimized with carefull attention to topology, in the end they act in some form at the very input of a SS amplifier, forming a nonlinear filter with the source and feedback network impedances, or worse, get multiplied by miller effect. Tubes have vacuum capacitances which is about as linear as you can get, hence this problem is avoided.
Then, if you don't like feedback, you can get quite a lot more linearity from a tube than a solid state device, especially from something like a CCS loaded triode. On the other hand, driving amperes of current without a costly transformer that has many of it's own issues, tends to be very difficult with tubes...
Let's take a worst case, a tube in cascode so that rp has no bypassing effect. For a typical 47k plate load (I'm still talking about an ECC88 type), the Johnson noise voltage 20Hz-20kHz at 40 degrees C is about 4uV. Referred back to the input, that's 4/33uV (assuming worst case gain), so the tube still dominates. This ignores excess noise in that plate resistor, but that's still likely to be fairly small and, when divided by the tube gain to refer to the input, will still be negligible compared to tube noise.
SY,
No fine then. I was too lazy to calculate! I will compare this some time with my transistor job. Thanks.
Ilimzn,
Ray was commenting on the contents of that article that was quoted - not necessarily to do with our subject.
Regarding the non-linearities you mention, one normally tries to keep their effects out of the audio band. In that way you can get some < [0.1 - 0.05]% non-linear distortion out of a transistor front-end with good design and some local feedback. You will be hard put to get that with tubes. But since one can have inaudible distortion with both technologies, the question does arise, why the extra bother with tube power supplies etc. For me it will be a consideration of convenience, not really a substantial difference in performance.
No fine then. I was too lazy to calculate! I will compare this some time with my transistor job. Thanks.
Ilimzn,
Ray was commenting on the contents of that article that was quoted - not necessarily to do with our subject.
Regarding the non-linearities you mention, one normally tries to keep their effects out of the audio band. In that way you can get some < [0.1 - 0.05]% non-linear distortion out of a transistor front-end with good design and some local feedback. You will be hard put to get that with tubes. But since one can have inaudible distortion with both technologies, the question does arise, why the extra bother with tube power supplies etc. For me it will be a consideration of convenience, not really a substantial difference in performance.
So I am reviving this thread because I started thinking about using a semisouth jfet to drive a 300b. That is a bit cost prohibitive for me to experiment with personally but I am curious if there are any amps people have personally built that have a SS front end and tube output? If so how did it sound? If not, do you think this can be done while being directly coupled to the output tube? I like the idea of having no caps, or only one cap in the audio signal path.
You can drive a 300B with a more common power MOSFET as a source follower. If you're talking about those new SiC JFETs, you'd do much better to use them as the finals, with a hollow state front end. Also, these SiC JFETs need to operate at higher than usual voltages to keep the device capacitance from going squirrelly. (Attached) Otherwise, it looks like these are very linear if you get the voltage up, and the current down. (The other problem is excessive gate current if you operate at high drain currents.)
SS up front, with VTs on the back end is more for guitar amps: high, solid state gain up front, maybe some DSP, and the hollow state back end for the desirable, hollow state, distortion and crunch.
For music reproduction, I would prefer hollow state up front, and a couple of power MOSFETs (or those SiC JFETs, if they ever become something other than unobtanium for anyone who's not in the market to buy hundreds at a time, that is) on the back end.
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