This project, like most DIY, is based on the work of others. Nelson, Zen Mod, Ben, Mikey and Lynn, were all a huge influence on this project.
Starting with Nelsons 2015 Sony VFET front end, further modified by Lynn for his version of the XA25. I have been using this front end on many amps as it gives a nice 20dB gain and has an adjustable harmonic spectrum. I normally set it for minimum H2 and let the output stage add the honey. I have this frontend driving one of ZM’s DEF R3 output stages, a big SIT-Mu Follower, a ZM Redneck DEF with a Pass SIT1 and now the Choke Loaded Follower. This has helped me in evaluating the output stages as they all have the same input/gain stage.
This Choke Loaded Follower is following Nelson and Mikey’s work and then all of Bens excellent posts. It does incorporate a buffer for the SIT. At first I was concerned about this additional stage but it worked so well in ZM’s DEF R3 that I figured I would give it a try. It’s a good thing. IMHO It add additional speed and transparency over direct dive of the SIT and the bias is nice and stable.
The reason for this amp is for the summer when I need something that dissipates a bit less heat. The Big SIT-Mu Follower is excellent sounding amp but at 185W per channel heats up the listening room. Ben said that he liked the sound of the Choke Loaded Follower and at less than half the heat dissipation it seemed like a good thing.
The caveat with the Choke Loaded Follower is that adjusting the VDS is problematic. You can change the bias and this changes the drop across the choke. I played with different supply voltages and the higher you are, up to about 37v the lower the distortion. 32 volts seemed like a nice compromise and that gave about 30V across the SIT at 2.5Amps. I have included a graph to see how bias changes the distortion with a fixed 32v supply.
Lastly I added Nelsons De-Thumper, just because. I’m sure my vintage Altec 515’s will appreciate it.
Oh I like to see my work, (modern art?) so it’s all in the open. It also makes it nice for adjusting and testing.
I hope it inspires people to try the Choke loaded Follower. It sounds great!
Jim and ZM like pictures so here are a bunch…
Starting with Nelsons 2015 Sony VFET front end, further modified by Lynn for his version of the XA25. I have been using this front end on many amps as it gives a nice 20dB gain and has an adjustable harmonic spectrum. I normally set it for minimum H2 and let the output stage add the honey. I have this frontend driving one of ZM’s DEF R3 output stages, a big SIT-Mu Follower, a ZM Redneck DEF with a Pass SIT1 and now the Choke Loaded Follower. This has helped me in evaluating the output stages as they all have the same input/gain stage.
This Choke Loaded Follower is following Nelson and Mikey’s work and then all of Bens excellent posts. It does incorporate a buffer for the SIT. At first I was concerned about this additional stage but it worked so well in ZM’s DEF R3 that I figured I would give it a try. It’s a good thing. IMHO It add additional speed and transparency over direct dive of the SIT and the bias is nice and stable.
The reason for this amp is for the summer when I need something that dissipates a bit less heat. The Big SIT-Mu Follower is excellent sounding amp but at 185W per channel heats up the listening room. Ben said that he liked the sound of the Choke Loaded Follower and at less than half the heat dissipation it seemed like a good thing.
The caveat with the Choke Loaded Follower is that adjusting the VDS is problematic. You can change the bias and this changes the drop across the choke. I played with different supply voltages and the higher you are, up to about 37v the lower the distortion. 32 volts seemed like a nice compromise and that gave about 30V across the SIT at 2.5Amps. I have included a graph to see how bias changes the distortion with a fixed 32v supply.
Lastly I added Nelsons De-Thumper, just because. I’m sure my vintage Altec 515’s will appreciate it.
Oh I like to see my work, (modern art?) so it’s all in the open. It also makes it nice for adjusting and testing.
I hope it inspires people to try the Choke loaded Follower. It sounds great!
Jim and ZM like pictures so here are a bunch…
Attachments
Thanks Patrick,Gorgeous work, as always. Interesting (at least to me) to note that the distortion rises with increased bias. Inspiring to see great measurements. Love the near perfect representation of the negative phase 2nd harmonic distortion.
Congratulations on another fantastic amplifier.
The reason the distortion goes up with increased bias is because the VDS is going down and the voltage drop across the choke increases. You can control this with the Big SIT-MU Follower to see only one factor changing at a time.
Very nice. I like your input buffer and bias arrangement. The DN2540 is great when large voltage swings and a simple buffer is needed. I have a similar buffer in my 2SK79 preamp, actually two - input and output. Thanks to Mikey since he had a DN2540 buffer in his Luminaria design. And of course Zen Mod's SIT bias arrangement.
Great job!
Great job!
the choke maybe ?Gorgeous work, as always. Interesting (at least to me) to note that the distortion rises with increased bias.
Thank you! I had to try and wrap my head around that for a bit, but it now makes sense. One of the values of watching people with more experience take a few 'pieces' and put together a full puzzle of their own is getting a bit more of an understanding of each piece and also how they work together to make a whole. Also seeing how people go through the process of testing an amp at various operating points is fascinating. This one was particularly interesting to me b/c it (now for an understood reason) goes against some of the overall adage re: higher bias => good.
Yep!
Very nice build as always. You are a true craftsman.
The DIY Sony VFET amp you helped me bias and send off on its first start powering your speakers still sounds fantastic.
I have a pair of Tokin THF-51S SITs waiting for a home in one of these Pass DIY variants, and a Wayne’s BA 2018 line stage to build. Thing is, I’m still enjoying my DIY Sony VFET amp and an Aleph J I built during the worst of the pandemic in 2020.
The DIY Sony VFET amp you helped me bias and send off on its first start powering your speakers still sounds fantastic.
I have a pair of Tokin THF-51S SITs waiting for a home in one of these Pass DIY variants, and a Wayne’s BA 2018 line stage to build. Thing is, I’m still enjoying my DIY Sony VFET amp and an Aleph J I built during the worst of the pandemic in 2020.
Thanks Richard,
The front end gain stage of this amp is very similar to the DIY version you are using. The output stage just has a lot more 2nd harmonic, this gives it a bit of sugar.
Quick Update, I have been running these amps all summer, they sound great!
I did have one issue, a close lightning strike to the house blew out the gates of the input buffer DN2540. I replaced them and all is well. I also added a gate protection zener (D3) to hopefully prevent it from happening again.
I did have one issue, a close lightning strike to the house blew out the gates of the input buffer DN2540. I replaced them and all is well. I also added a gate protection zener (D3) to hopefully prevent it from happening again.
