F4 based integrated amp without jfet buffer

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A friend was just listening at my tube preamp coupled to a F4 and really liked it, as I do. This is just a wonderful combination.
We just decided to build him an integrated amp with the F4 power stage and a triode preamp inside. As the JFETs are hard to source nowadays, the following alternatives may be applied:
  1. Replace the JFET buffer of the F4 by a opamp buffer. I did some simulations in LTspice and it looks quite promissing. By using a high voltage opamp (e.g., OPA445, OPA453) this buffer may be supplied by the F4 rails directly (+/-24V).
  2. A bipolar transistor diamond buffer like I have seen a schematic replacing the JFET buffer in a F5.
  3. Removing the JFET buffer (like having a BA complementary output stage) and adding a source or cathode follower to the tube preamp.
Did anyone try one of these suggested approaches or does anyone has another idea?
 
Personally I would leave the j-fet buffer as configured. The j-fets can be purchased easily at our store. I have built an op-amp buffer to compare with the j-fet buffer. J-fet easily won. I recently built the F4 and I have a number of different pres both SS and tube. I like the option of choosing a different sound. Very tube like sound with a good tube pre and very quick and dynamic sounding with a good SS pre. Depending on mood I like both.

There is a reason the Master uses them. You may find something close but I doubt better.
 
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I used the HA5002 high speed buffer for my F6 it has the ability to drive most amplifier output stages as it can supply up to 200mA and plus minus 20Volt swing again with the supply from the main PSU via regulators or zeners of your choice. I soon will be doing the same for the F4 Beast which I will be building shortly.
 
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The original JFET buffer was included for those with high source impedances.

Generally I don't use these as I have an XP-30 with SE and balanced outputs
having low impedance and high voltage swing that was designed to
accommodate this sort of thing. Makes it easier to evaluate output stage
designs by themselves.
 
Yes I will indeed be driving it with my own preamp and besides I have plenty of the HA5002 chips lying around but do not have any of the JFets. As these chips are high input impedance and low output impedance so are suited to driving and testing output stages such as the hockey pucks in the F4 Beast.
 
I used the HA5002 high speed buffer for my F6 it has the ability to drive most amplifier output stages as it can supply up to 200mA and plus minus 20Volt swing again with the supply from the main PSU via regulators or zeners of your choice. I soon will be doing the same for the F4 Beast which I will be building shortly.

This is a nice buffer with a relatively high GBP. Nevertheless, the absolute maximum suppy rating of 44V makes some regulators necessary. I intended to avoid this by using a high voltage opamp. Otherwise the full power of the output stage is limited by the output swing of the buffer.
 
The j-fets can be purchased easily at our store.

Yes there are some available, but not the matched pairs and the price is way to high for my friend. The plan is to substitute this unavailable/expensive JFET pair by something affordable and technically equivalent.

Another option would be to replace the N-/P-JFET pair by a N-JFET buffer like the DC-coupled B1 shown here: http://www.diyaudio.com/forums/pass-labs/145201-building-symmetrical-psu-b1-buffer.html#post1844308
Then, the 2SK170 may be replaced by a BF862, BF256, or something similar.
 
I did some simulations on the dc-coupled B1 frontend and will modify some old F4 boards missing the complementary JFETs. Let's see how it sounds. :D
 

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The Burr-Brown (from TI) OPA604 can be supplied with +/-24Vdc rails and should easily swing +/-12V, if one wanted to make the F4 an integrated amplifier.

The datasheet for that op-amp also states in can take an input of +/-13V, if one wanted to continue to use a dedicated preamp. I know the F4 needs to have a 20V swing to reach the maximum power of 25 watts, but for the cost and ease of designing with op-amps it may be a fair compromise.

I would like to see how using two N Jfets sounds!

I reluctantly bought a pair of matched Toshiba 2SJ74 + 2SK170 and the amp sounds great. Before I bought the matched pair from ebay, I used a J176 and J113... it sounded like garbage! I am not sure if it was because I did not match the fets or because they aren't meant for audio application.

Rail-to-rail op-amps will typically not linearly output to it's rail voltage also.
 
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