I have some ECX10N20's around here somewhere and most of the parts and boards for the MoFo. But I don't think I want to sweeten things up anymore than what I have now, it seems just about right. With my previous SissySIT R.3, I always used a gain stage in front, either internal or external, to get the overall gain where I needed it for my system. And those gain stages were always "clean" to maintain that sweetness in the.... sweet spot..since the SS R.3 has it's own pleasing character. I listen to a wide variety of music and if the total distortion gets too high, I think things just start sounding too sloppy, particularly on complex material.
I really like this square law HGF. It just thrives into the lower impedance loads and sounds great. My speakers are essentially 5 ohms and not super efficient, so the amp has some nice synergy there..
I really like this square law HGF. It just thrives into the lower impedance loads and sounds great. My speakers are essentially 5 ohms and not super efficient, so the amp has some nice synergy there..
Breaking the “no building in the Spring or Summer” over here.
I want to say it’s an entry level DAC/streamer, but I cannot tell when I listen to it. My Lord, what a rabbit hole!
I want to say it’s an entry level DAC/streamer, but I cannot tell when I listen to it. My Lord, what a rabbit hole!
Looks fantastic! Don’t tease us with those pics… tell us more. Which DAC is it? I recently ordered an R2R dac as well, though it hasn’t arrived yet.
@william2001 I totally get what you are saying with too much H2. I don’t need a lot of power, so the single ended follower is just right for me. But a push-pull lateral follower would cancel most of the H2. Now that would be interesting. Of course, there are tons of amps with that config already. Would be fun to try that with SCG driving.
It's a ProtoDAC. TD1837 X 8. A NOS DAC HAT on top of a Raspberry Pi 5.
In its current incarnation, the PSU is shared between the Pi and the DAC, which is not the best scenario. But it sounds pretty great already, probably because the Pi only has 1 job: Running the RAAT bridge.
At some point in the future, I'll add a low noise linear supply for the HAT, but for now, I just need to clean up the build and put a lid on it.
That should be my last update on this.
I replaced all JFETs and used some old J175s made by Siliconix. These rate higher than anything else in terms of Vp far away from zero.
Put the board back in and the puttering was higher still. 🙄😳🤦
It was a setback.
But clearly all fingers were pointed at the PSU board once again.
I built a new PSU with higher rated parts, 3W 33R resistors, higher rated caps, 2W R6, no snubber parts, 1.5W 20V and 6.2V Zener diodes, etc. Everything overrated.
Put that PSU and…total silence.
Victory achieved!
The preamp makes 0.2% of H2, flat across the FR, at the output level that matters.
The preamp can also swing a little more than 40 Vpp before clipping.
I let that run 8 hours straight to ensure durable stability and it passed the test.
Subjectively, the sound is rich (H2), detailed and open. You can certainly hear all the layers in a mix.
What impresses me the most is the quality of timbre reproduction.
Bass rolloff is a tad above 2 dBr at 20 Hz, but nothing to be too concerned about.
Fantastic preamp! I can’t wait to find a way to put it in my main system.
Thanks again, Rahul!
Awesome! Great work! So good to hear. Chasing down the noise gremlins is always tough 
You could play with the Vd a bit and make the H2 go down if so desired. It acts like a triode and you can play with the Vd to adjust the harmonics.
A side note: I have been thinking about a couple more variations on the SCG theme. Just some ideas I want to try. Will post them here.
You could play with the Vd a bit and make the H2 go down if so desired. It acts like a triode and you can play with the Vd to adjust the harmonics.
A side note: I have been thinking about a couple more variations on the SCG theme. Just some ideas I want to try. Will post them here.
I did play with that. I dialed the supply voltage down to 62V from the original 70V. Then I played around with Vd from 25V to 30V. I settled on 28V, as I like the sound of that.
It’s really great that you can do some rolling like that by just turning the trimpots.
You could come full circle and drive some 300Bs. 😂 That could be too much H2 stacking, though.
I have upgraded my build of the original SCG design (FQP12P20/STP30N10F7) to JFET's (J111 and J175) according to ra7's notes in post #1348. I would like to use the SCG to drive to a SIT Mu Follower (Ben Mah's design) which requires a preamp with low output impedance and significant voltage swing to achieve full output. Would the JFET upgrade be suitable for this, or should I look at moving to the new design with buffer?
Thoughts and comments would be appreciated.
Thoughts and comments would be appreciated.
Driving Ben's SIT Mu Follower directly without the buffer would be fine, and since you built it already, just go for it. I would try it with the buffer also, it will probably drive it better. Some amps, like F4, already have a buffer on the input and with those you definitely don't need the buffer on the SCG.
Thanks for your comments. As you suggest, I'll go with the JFET upgrade version at present and build the new SE buffer version as well, since you have so kindly provided the Gerbers, BOM and a wealth of information regarding construction, tuning and mods. Many thanks for making these great designs available to the diyAudio community.
Rahul,
After listening without the buffers for many months I am curious to try the NEW buffer.
Would the selection process for the J113s be the same - using the 47R resistor?
I am thinking of doubling, maybe even quadrupling them. Would there be additional things to "match" in this case? Or would finding four that measured the same in your jig be enough?
Thanks and take care,
PS have to say what ACOURATE can do is close to astonishing.
After listening without the buffers for many months I am curious to try the NEW buffer.
Would the selection process for the J113s be the same - using the 47R resistor?
I am thinking of doubling, maybe even quadrupling them. Would there be additional things to "match" in this case? Or would finding four that measured the same in your jig be enough?
Thanks and take care,
PS have to say what ACOURATE can do is close to astonishing.
Beefy Buffer
Hi Rick,
If you are looking for a beefier buffer, there are a few options. Yes, you can double, triple, or quadruple the JFET in the buffer and get more current. But there is another way.
First, with the JFETs. When paralleling JFETs, I'd highly recommend using a separate source resistor per JFET and connecting the other end of the source resistor to ground. In this post, we are setting it to ~10 mA per JFET. So, you would have one 4k7 resistor per JFET when paralleling.
The JFETs to be paralleled should have as close Vgs as possible at the operating point. So, yes, use a 47R resistor in the source with a 9V battery and pick JFETs within < 0.1 Vgs (at worst) of each other when producing the same current. This might take a few parts to get good matches.
Another way, and what I would do, is to use a bigger part. I know you have some KSC3503 in your stash because we went through them during development. That part can take more heat and you can put a small heatsink on it. See the schematic below. There may be other small part changes on the schematic--ignore them--the things to focus on are Q1 and R12. If you adjust Vd for 40V, you will have about 40V - 0.7V (one diode drop), or about 40V, on the emitter pin of the KSC3503. So, then if you put a 1k resistor from the emitter to ground, you will get 40 mA through it. Plenty of juice. Be sure to put a heatsink on the KSC3503 and the 1k resistor will need to be properly sized for heat dissipation as well. If you put a 2k resistor there, you will get 20 mA through it--plenty for most applications.
In fact, this is how I have it configured. I have it at a little less than 2k for about 25-30 mA and it now has enough grunt to drive the Elmo. The sound is very close to the JFET buffer, very transparent.
I'll post an updated PCB with the footprint for a KSC3503.
Hi Rick,
If you are looking for a beefier buffer, there are a few options. Yes, you can double, triple, or quadruple the JFET in the buffer and get more current. But there is another way.
First, with the JFETs. When paralleling JFETs, I'd highly recommend using a separate source resistor per JFET and connecting the other end of the source resistor to ground. In this post, we are setting it to ~10 mA per JFET. So, you would have one 4k7 resistor per JFET when paralleling.
The JFETs to be paralleled should have as close Vgs as possible at the operating point. So, yes, use a 47R resistor in the source with a 9V battery and pick JFETs within < 0.1 Vgs (at worst) of each other when producing the same current. This might take a few parts to get good matches.
Another way, and what I would do, is to use a bigger part. I know you have some KSC3503 in your stash because we went through them during development. That part can take more heat and you can put a small heatsink on it. See the schematic below. There may be other small part changes on the schematic--ignore them--the things to focus on are Q1 and R12. If you adjust Vd for 40V, you will have about 40V - 0.7V (one diode drop), or about 40V, on the emitter pin of the KSC3503. So, then if you put a 1k resistor from the emitter to ground, you will get 40 mA through it. Plenty of juice. Be sure to put a heatsink on the KSC3503 and the 1k resistor will need to be properly sized for heat dissipation as well. If you put a 2k resistor there, you will get 20 mA through it--plenty for most applications.
In fact, this is how I have it configured. I have it at a little less than 2k for about 25-30 mA and it now has enough grunt to drive the Elmo. The sound is very close to the JFET buffer, very transparent.
I'll post an updated PCB with the footprint for a KSC3503.
ra7
Thanks for posting your beefy driver, three questions if I may:
1). In post 1863 you had a J113 below a DN2535 but in post 1913 you got rid of the J113 and replaced it with a resistor, is this for better sound?
2). In post 1913, I think R5 should be on the other side of the 10uF output cap, as drawn it is in parallel to the 1k resistor, lowering the resistance hence increasing the output current beyond your 40mA.
3). If I was to build your Elmo, how do I eliminate one capacitor by mating the SCG which has an output cap to Elmo which has an input cap and biasing arrangement without stuffing up the lateral's bias and input resistance etc?
Thanks for posting your beefy driver, three questions if I may:
1). In post 1863 you had a J113 below a DN2535 but in post 1913 you got rid of the J113 and replaced it with a resistor, is this for better sound?
2). In post 1913, I think R5 should be on the other side of the 10uF output cap, as drawn it is in parallel to the 1k resistor, lowering the resistance hence increasing the output current beyond your 40mA.
3). If I was to build your Elmo, how do I eliminate one capacitor by mating the SCG which has an output cap to Elmo which has an input cap and biasing arrangement without stuffing up the lateral's bias and input resistance etc?
1) That's just how my trials in Spice went. Result of multiple edits to the schematic in Spice. The J113 should be in the SCG CCS.
2) Yes, that's right. It's a remnant from a previous trial in Spice.
3) You can eliminate the cap in the SCG or the Elmo. If putting SCG and Elmo into the same chassis, this makes a lot of sense.
Here's where my standalone Elmo (when not driving with beefy SCG) development stands. This is for illustration purpose only. It WILL definitely change before calling it final. The point here is that you could direct couple the lateral FET driver (KSC3503) with the output stage, but the input will need a cap. This is easier to do when building the power amp section separate from the gain stage (standalone Elmo). Alternatively, you can direct couple the driver to SCG (as in post 1913) and put a cap between the driver and the lateral FET, thereby saving one choke (L2 below). The choke in the schematic below (L2) is simply so we don't need two power supplies for Elmo, otherwise to get symmetrical swing, we'd need a bigger supply on Q1 or a bipolar supply.
I just listened to Beefy SCG driving Elmo. It makes wonderful music!
2) Yes, that's right. It's a remnant from a previous trial in Spice.
3) You can eliminate the cap in the SCG or the Elmo. If putting SCG and Elmo into the same chassis, this makes a lot of sense.
Here's where my standalone Elmo (when not driving with beefy SCG) development stands. This is for illustration purpose only. It WILL definitely change before calling it final. The point here is that you could direct couple the lateral FET driver (KSC3503) with the output stage, but the input will need a cap. This is easier to do when building the power amp section separate from the gain stage (standalone Elmo). Alternatively, you can direct couple the driver to SCG (as in post 1913) and put a cap between the driver and the lateral FET, thereby saving one choke (L2 below). The choke in the schematic below (L2) is simply so we don't need two power supplies for Elmo, otherwise to get symmetrical swing, we'd need a bigger supply on Q1 or a bipolar supply.
I just listened to Beefy SCG driving Elmo. It makes wonderful music!