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Hybrid Amplifier by Andrea Ciuffoli
To reduce the cost you can use a simple SRPP (Totem pole) of ECC88 powered at 270V as voltage stage or any other vacuum tube stage with a good voltage gain so also a 12AX7 followed by a 12AU7 like this:
http://www.audiodesignguide.com/av/input1.jpeg
Pcb available on Ebay store, I don't get money from this.
Hybrid Amplifier by Andrea Ciuffoli
To reduce the cost you can use a simple SRPP (Totem pole) of ECC88 powered at 270V as voltage stage or any other vacuum tube stage with a good voltage gain so also a 12AX7 followed by a 12AU7 like this:
http://www.audiodesignguide.com/av/input1.jpeg
Pcb available on Ebay store, I don't get money from this.
- no global feedback design
- passive components without any compromise on any section
- only few parts on the signal path, only 3 active components
- very short signal path
- very low output impedance (near to 0.1ohm) for high damping factor (near to 80)
- very high output current, output devices able to manage 60A
- high output power to drive any loudspeakers including ESL, 100W on 8ohm and 180W on 4ohm
- very good slew-rate
- large frequency band, near to 1MHz
- high separation from power supply for all the sections
- power supply separated for each channel
- zero noise
- ultra low distortion
- zero dc offset, dc servo loop
- no fuse on the signal path
- no relay on the output signal
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Well, the mention of Hi-End together with vacuum tube sound don´t really match. Either is the sound truly Hi-End, or else it has some sound signature such as vacuum tube sound. If the later is wanted, why then make a very complicated circuit just to sound as a tube circuit??? An all vacuum tube circuit would sound better, and if done right withouth any tube sound also 
Well, the mention of Hi-End together with vacuum tube sound don´t really match. Either is the sound truly Hi-End, or else it has some sound signature such as vacuum tube sound. If the later is wanted, why then make a very complicated circuit just to sound as a tube circuit??? An all vacuum tube circuit would sound better, and if done right withouth any tube sound also
+1
Many people think about the mosfet like a more linear components and more easy to drive if compared to transistor.
This is not true, mosfet are linear only if used in pure class A operation and the high input capacity limit the slew-rate and frequency band.
The transistor are more easy to drive because the input resistance is linear in all the frequency range, with only 2 stages like this you have very high input resistance so it is easy to drive.
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In my Amplifier End I have used mosfet but only because this is designed to have only 50W and there is a transformer to reduce the output impedance of the vacuum tube stage.
Amplifier End by Andrea Ciuffoli
The mosfet have also higher output impedance so to get the same performance of the 100W Extreme Hybrid probably I should use 6 or 8 mosfet and this mean 8 x 600pF (2sk1058) + 8 x 900pF (2sj160)= 10200pF, in this case is necessary a vacuum tube stage with very low output impedance.
Another factor to keep in mind is that the NPN transistors are very similar to the PNP while the complementary mosfet are never the same so they are worse in a push-pull without feedback
Amplifier End by Andrea Ciuffoli
The mosfet have also higher output impedance so to get the same performance of the 100W Extreme Hybrid probably I should use 6 or 8 mosfet and this mean 8 x 600pF (2sk1058) + 8 x 900pF (2sj160)= 10200pF, in this case is necessary a vacuum tube stage with very low output impedance.
Another factor to keep in mind is that the NPN transistors are very similar to the PNP while the complementary mosfet are never the same so they are worse in a push-pull without feedback
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Well, the mention of Hi-End together with vacuum tube sound don´t really match. Either is the sound truly Hi-End, or else it has some sound signature such as vacuum tube sound. If the later is wanted, why then make a very complicated circuit just to sound as a tube circuit??? An all vacuum tube circuit would sound better, and if done right withouth any tube sound also
I think tube sound means that most of the distortion is coming from the D3a (some 0.6-0.7% at full throttle without feedback)?
The only thing I don't like is the LC coupling. This is just my opinion. At the end of the day the LL1668 will cost 95 euros + the film coupling cap while the LL1692a will only cost 117 euros while providing (connected as ALT S) an easier load to the D3a, especially at low frequency.
I think the concept is fine, although one doesn't need a tube to recreate the identical harmonic spectra as a tube. There are a number of solid state devices that offer the same spectra and magnitudes when used with very low or no negative feedback in a single ended configuration. Personally though, I only like harmonics in the bass and sometimes the low midrange, below audibility elsewhere.
High End is a very vague term today that isn't attached to high fidelity's ideals. High end now is almost entirely subjective and based on names and visual preferences.
As audiodesign mentioned, Mosfets have very high capacitance and that makes the driver stage design more critical to overcome that limitation. But, many designs have overcome it, and not the most talked about brands. Whether or not they can be used is down to the design goals and skill level. I've heard some fantastic kit using Mosfets, but I'm sure there are some dogs out there.
The only thing at this moment that concerns me is the bandwidth of this amp - I think that 1MHz bandwidth is too wide, but I haven't simmed the tube section to know its upper F3 point or the solid state sections stability margin. I would think require an input filter, in this case the valve front end, is pretty much needed to prevent problems. Usually, what designers do is design a lower bandwidth or slew limited front end and couple it to a wide bandwidth high-slew rate output section, so that slewing is not impeded by high capacitance low-impedance loudspeakers. The final slew rate is the sum of the cumulative design, while the more limited front end/VAS stage prevents RF vunerability and instability.
Overall, I'm not going to get critical or pass judgement on this design. We have enough of that going on every day.
High End is a very vague term today that isn't attached to high fidelity's ideals. High end now is almost entirely subjective and based on names and visual preferences.
As audiodesign mentioned, Mosfets have very high capacitance and that makes the driver stage design more critical to overcome that limitation. But, many designs have overcome it, and not the most talked about brands. Whether or not they can be used is down to the design goals and skill level. I've heard some fantastic kit using Mosfets, but I'm sure there are some dogs out there.
The only thing at this moment that concerns me is the bandwidth of this amp - I think that 1MHz bandwidth is too wide, but I haven't simmed the tube section to know its upper F3 point or the solid state sections stability margin. I would think require an input filter, in this case the valve front end, is pretty much needed to prevent problems. Usually, what designers do is design a lower bandwidth or slew limited front end and couple it to a wide bandwidth high-slew rate output section, so that slewing is not impeded by high capacitance low-impedance loudspeakers. The final slew rate is the sum of the cumulative design, while the more limited front end/VAS stage prevents RF vunerability and instability.
Overall, I'm not going to get critical or pass judgement on this design. We have enough of that going on every day.
I think everyone has the right to spend his own money as he likes. Can you build a 100W tube amplifier without feedback that works and sounds just as well for few hundred euros?
A full tube amplifier can be both the best and the worst depending on what is driving. A solid state amplifier is not free from any signature on the sound either. The Pioneer A-09 that uses the same type of output stage is one of the best amplifiers ever made, IMHO, but is not perfect. So? "Hi-end", " tube sound" are just words to make a long story short. Nothing else.
The major component that determines the sound is the voltage amplifier stage.
The current amplifier affects very little in the characteristics of the sound if it can follow the voltage amplifier.
This concept was applied in my hybrid amplifiers since 1999 with this design:
the Power Folllower
Some person consider the sound of PF99 very near to a pure 300B amp. and for some aspect better.
In the Extreme Hybrid the current amplifier have a distortion 10 times lower than voltage amplifier also if D3a have only 0.6% thd at 40Vrms.
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