Sometimes it is tempting to try parts that have recently gone out of production. However, I would prefer to focus on parts that will be around for a while. (The big SITs are an exception; I ordered a few pairs while they were still available.)
The FQH44N10 were still in production when I first started using them. They were also used by a few ACA builders, and found to perform very well in that application. I might try the IXTQ75N10P in a pair of ACAs. I have fresh boards that can be used in a couple of my old Hafler chassis. The big Vandersteens can be bi-amped.
The FQH44N10 were still in production when I first started using them. They were also used by a few ACA builders, and found to perform very well in that application. I might try the IXTQ75N10P in a pair of ACAs. I have fresh boards that can be used in a couple of my old Hafler chassis. The big Vandersteens can be bi-amped.
I am currently enjoying music through my original channel boards with the FQH44N10 devices in place. I have switched to ceramic insulators and found that the bias current adjustment needed to be increased to reach a given current setting.
The ceramic insulators seem to provide better thermal coupling between the outputs and the heatsinks. With this in mind, I left the setting at a point that achieved 1.5A bias current. The amp only needs a half hour to warm up now, much shorter than previously. And the amp sounds great this way, so a win all around.
The ceramic insulators seem to provide better thermal coupling between the outputs and the heatsinks. With this in mind, I left the setting at a point that achieved 1.5A bias current. The amp only needs a half hour to warm up now, much shorter than previously. And the amp sounds great this way, so a win all around.
I have you followed, so I can go back and try all your, tweak/mods I have found them to be thoughtful and mostly right on with my thoughts before and after listening. The exception is I tend to like a little less in the high ranges due to sensitivity in this area. Keep doing what you are doing... I've a, honey do, list of stuff to try yet.
Thanks,
JT
Why? I don't know. Likely just an error.
Where do you see something labeled "-IN" either on the schematic or on the PCB?
You may have noticed that I am currently working with a Threshold CAS-2. This wonderful 40 year old amp has been something of a revelation, and keeps sounding better as I continue to selectively replace the old capacitors. The CAS-2 uses different techniques to keep its input stages less sensitive to power supply ripple. While the most recent upgrade of the main supply caps cut the ripple approximately in half, the sonic benefit was more subtle. (Still very worthwhile when incremental improvements are difficult to achieve.)
I am learning more as I go, and have noticed a common feature with with my preferred input stage, the Austin-style diamond buffer, and the CAS-2. Current sources are used to keep the input bipolar junction transistors under more constant operating conditions. Seems to work well in both amps. The CAS-2 also makes full use of a cascode circuit topology to keep its output transistors in more ideal operating conditions.
I’m thinking of ways to apply what I’ve learned from the CAS-2 to my F6.
I am learning more as I go, and have noticed a common feature with with my preferred input stage, the Austin-style diamond buffer, and the CAS-2. Current sources are used to keep the input bipolar junction transistors under more constant operating conditions. Seems to work well in both amps. The CAS-2 also makes full use of a cascode circuit topology to keep its output transistors in more ideal operating conditions.
I’m thinking of ways to apply what I’ve learned from the CAS-2 to my F6.
Regarding the F6 and its performance in the upper frequency range..
The F6 has used a number of different Mosfets in its output stage, beginning with the now unobtainable SemiSouth devices. The version that is sold in the diyAudio store uses IRFP240 devices. These have significantly different input capacaticance and Gate charge figures. The schematics that accompanied the store PCBs show 47 Ohms as the value for the Gate stopper resistors. The values of these resistors may be tweaked to one's preference. I started with 121 Ohms in my build, and have considered using lower values to adjust the frequency response. I am now considering switching to 75 Ohms, as this amp could use a slightly brighter sound to match my speakers and my ears. Those who might prefer slightly lower output in the high ranges might want to try 121 Ohms or 100 Ohms in place of 47 Ohms, depending on their current build. This is a small adjustment, but may be just right for sensitive ears.
The F6 has used a number of different Mosfets in its output stage, beginning with the now unobtainable SemiSouth devices. The version that is sold in the diyAudio store uses IRFP240 devices. These have significantly different input capacaticance and Gate charge figures. The schematics that accompanied the store PCBs show 47 Ohms as the value for the Gate stopper resistors. The values of these resistors may be tweaked to one's preference. I started with 121 Ohms in my build, and have considered using lower values to adjust the frequency response. I am now considering switching to 75 Ohms, as this amp could use a slightly brighter sound to match my speakers and my ears. Those who might prefer slightly lower output in the high ranges might want to try 121 Ohms or 100 Ohms in place of 47 Ohms, depending on their current build. This is a small adjustment, but may be just right for sensitive ears.
AG- negative on board. Obviously there isn't -IN on the schematic. So not the same as Aleph then?
Correct. Not the same.
"IN-" on the Aleph J (and most anything else noting a differential signal) indicates the input for the inverted signal or negative signal vs. the "IN+" for the normal / positive signal.
For single-ended operation with the Aleph J, you short the connection for the inverted phase signal to ground in order for the circuit to operate properly. For people that know they'll never use a differential input, sometimes they do it permanently on the amp PCB. For those that equip their AJ with both XLR inputs for a differential signal and RCAs for SE, they may short pin 1 to Pin 3 of their XLR using a temporary jumper wire / shorting plug when they use the RCA inputs.
Since the F6 is single-ended only; that's why you don't see any notes for an IN- on the schematic or on the boards.
Based on my recent experiences with the Threshold CAS-2, I made a few changes to my F6.
Bias current for the FQH44N10 Mosfets has been set to 1.6 Amps. This value lets the amp reach thermal equilibrium faster that the 1.85A that I was using earlier. The amp is ready to play after one hour and sounds fantastic after two hours or more. This seems to be optimum for my speakers.
The gate stopper resistors were changed from 121 Ohms to 68 Ohms. This something I have been planning to do for a long time. The original F6 article by Nelson Pass used 47Ω gate stoppers. I initially chose 121Ω because that is what I had in my ACA builds. In my various conversations with 2picoDumbs, he suggested using a lower value. I went with 68Ω because I had a good stash of those. I am unsure if this change is part of the sonic improvements I've heard with the amp, but they are in there now.
The Source resistor network below Q1 (the upper output Mosfet) was changed to Bourns thick film 0.05Ω, 1% + 0.40Ω, 1% resistors with the C1 capacitor connected to the junction of the two thick film resistors. The source resistor bellow Q2 remains a Bourns 0.40Ω, 1% thick film resistor. This is also a change I have been planning to do for a long time. The 0.05Ω part replaces a parallel combination that I arrived at through experimentation and listening. I have been using 1% thick film resistors rather than the more common 5% metal oxide resistors mainly for their precision. This way I am more confident in equal bias currents among all output devices in both channels. The network at Q1 (0.05Ω + 0.40Ω) provides the best sound in my system, and the new thick film parts are noticeably better sounding than the metal oxide parts they replaced.
The CL-60 thermistors on the SLB boards used for ground lift between audio and chassis ground are now bypassed with 220 nF film capacitors. I had left the locations for these caps empty, as I was unsure if they were necessary. After listening to the CAS-2 for some time, I wanted to try something that connected the audio grounds of the two channels together. The 220 nF caps (0.22 uF) give a solid AC connection between the audio grounds. I believe this is part of the overall improvement I've been hearing.
The F6 is a remarkably uncomplicated design, as far as the number of components is concerned. The interplay between those few components must demand the highest quality of the devices that are used. In the end we are mainly listening to the Jensen JT-123-FLPCH signal transformer, buffered at its input and amplified at its output. This is one of the best transformers that Jensen makes, and is commonly used for sensitive amplification in professional recording systems. I think the best that any power amplifier can do with the Jensen transformer is to translate its sound from the low level studio mixing application to driving a pair of speakers in a large listening space in a way that the recording engineer would have heard in the studio.
My F6 has been sounding really great since I arrived at its previous configuration a couple years ago. With the latest tweaks, it has been transformed into an amp that renders my favorite recordings in a sublime fashion. When I wrote of the CAS-2 (different thread) ability to make my big Vandersteens disappear, I was also wondering if my F6 met a similar level of performance. Now it does. In the realm of increasing effort to achieve small but significant returns, this upgrade to my F6 has been especially gratifying.
Bias current for the FQH44N10 Mosfets has been set to 1.6 Amps. This value lets the amp reach thermal equilibrium faster that the 1.85A that I was using earlier. The amp is ready to play after one hour and sounds fantastic after two hours or more. This seems to be optimum for my speakers.
The gate stopper resistors were changed from 121 Ohms to 68 Ohms. This something I have been planning to do for a long time. The original F6 article by Nelson Pass used 47Ω gate stoppers. I initially chose 121Ω because that is what I had in my ACA builds. In my various conversations with 2picoDumbs, he suggested using a lower value. I went with 68Ω because I had a good stash of those. I am unsure if this change is part of the sonic improvements I've heard with the amp, but they are in there now.
The Source resistor network below Q1 (the upper output Mosfet) was changed to Bourns thick film 0.05Ω, 1% + 0.40Ω, 1% resistors with the C1 capacitor connected to the junction of the two thick film resistors. The source resistor bellow Q2 remains a Bourns 0.40Ω, 1% thick film resistor. This is also a change I have been planning to do for a long time. The 0.05Ω part replaces a parallel combination that I arrived at through experimentation and listening. I have been using 1% thick film resistors rather than the more common 5% metal oxide resistors mainly for their precision. This way I am more confident in equal bias currents among all output devices in both channels. The network at Q1 (0.05Ω + 0.40Ω) provides the best sound in my system, and the new thick film parts are noticeably better sounding than the metal oxide parts they replaced.
The CL-60 thermistors on the SLB boards used for ground lift between audio and chassis ground are now bypassed with 220 nF film capacitors. I had left the locations for these caps empty, as I was unsure if they were necessary. After listening to the CAS-2 for some time, I wanted to try something that connected the audio grounds of the two channels together. The 220 nF caps (0.22 uF) give a solid AC connection between the audio grounds. I believe this is part of the overall improvement I've been hearing.
The F6 is a remarkably uncomplicated design, as far as the number of components is concerned. The interplay between those few components must demand the highest quality of the devices that are used. In the end we are mainly listening to the Jensen JT-123-FLPCH signal transformer, buffered at its input and amplified at its output. This is one of the best transformers that Jensen makes, and is commonly used for sensitive amplification in professional recording systems. I think the best that any power amplifier can do with the Jensen transformer is to translate its sound from the low level studio mixing application to driving a pair of speakers in a large listening space in a way that the recording engineer would have heard in the studio.
My F6 has been sounding really great since I arrived at its previous configuration a couple years ago. With the latest tweaks, it has been transformed into an amp that renders my favorite recordings in a sublime fashion. When I wrote of the CAS-2 (different thread) ability to make my big Vandersteens disappear, I was also wondering if my F6 met a similar level of performance. Now it does. In the realm of increasing effort to achieve small but significant returns, this upgrade to my F6 has been especially gratifying.