and now for something completely different part 3

[Pars]

If it were me, I'd source 2 quads of Toshiba 2SK170/2SJ74 from punkydawgs on ebay, and just do the SS bal/ubal board with JFET input. But that's me. It seems like someone on HF has a tube input CFA3, but I didn't see any pics or direct posts by the person who supposedly has it. I know dukei has been talking about it. Kevin would probably know. A lot of the HF crowd seems to run a tube preamp into the CFA3 and use that to get their "tube" sound. I guess you could do that if you wanted.

I used to have a tube pre, a Counterpoint SA5.1, which used 6922s and sounded good. It had a slight hum to it which took me about 6-7 years to find (PSU board layout error), and I sold it right after that and never listened to it after I fixed it, other than to verify it was now dead quiet.

I feel like I need a shower now after being on HF for a few minutes

[kevin gilmore]

there are a few units with the tube front end that have been built. less than 5 i think.

boosting the front end high voltage will increase the power in the current mirror that follows the tube. so you might have to change some of those parts.

[Pars]

Hmmm, ebay seller punkydawgs doesn't seem to have anything available right now, but says he's out until May 23. There is another seller there, ash_win, who is selling curve matched pairs of K170/J74s that looks to be the real deal. Austin, TX.

[audiostar]

20 hours ago, Pars said:

I would think it would be OK to run all of these thru a single umbilical. Most people use Amphenol circular connector, MIL spec.

The Amphenol's don't have removable pins, so no crimping possible and also no variable size pins available as well. It only has one 18 AWG pin (and all the others are max 20 AWG wire pins), so not really suited for a high current class A amp.

A much better connector is the AMP CPC shell Size 17, p/n 206043-1, 206044-1 (PSU side), 183040-1, 183039-1 (amp side), as used on the BHSE or the Dark Star. Pins are removable and all sorts of pins for crimping (as well as soldering if you wished) and sizes for 16...26 AWG wire are available for the same size 17 connector. Very nice modular and serviceable system. It is HV rated and the 16-18 AWG pins would be perfect for the beefy GRLVs. Really nice. 

Edited May 16, 2023 by audiostar

[Satyrnine]

1 hour ago, kevin gilmore said:

there are a few units with the tube front end that have been built. less than 5 i think.

boosting the front end high voltage will increase the power in the current mirror that follows the tube. so you might have to change some of those parts.

Sounds like something I don't want to mess with.  Thanks Kevin.

1 hour ago, Pars said:

If it were me, I'd source 2 quads of Toshiba 2SK170/2SJ74 from punkydawgs on ebay, and just do the SS bal/ubal board with JFET input. But that's me. It seems like someone on HF has a tube input CFA3, but I didn't see any pics or direct posts by the person who supposedly has it. I know dukei has been talking about it. Kevin would probably know. A lot of the HF crowd seems to run a tube preamp into the CFA3 and use that to get their "tube" sound. I guess you could do that if you wanted.

I used to have a tube pre, a Counterpoint SA5.1, which used 6922s and sounded good. It had a slight hum to it which took me about 6-7 years to find (PSU board layout error), and I sold it right after that and never listened to it after I fixed it, other than to verify it was now dead quiet.

I feel like I need a shower now after being on HF for a few minutes

Just chatted with Miroslav/Dukei, and he had great things to say about the sound/performance of the tube bal/ubal input. I think I'm going to go for it. This will likely be my endgame amp, so why not go all the way.

Edited May 16, 2023 by Satyrnine

[audiostar]

11 minutes ago, Satyrnine said:

Just chatted with Miroslav/Dukei, and he had great things to say about the sound/performance of the tube bal/ubal input. I think I'm going to go for it. This will likely be my endgame amp, so why not go all the way.

Think about keeping it modular and separate the tube pre from the power amp. Same as you separated the PSU already and not stuff everything into a single like 6U 800mm deep chassis 

Edited May 16, 2023 by audiostar

[Pars]

Just an FYI, the diyaudio store has Toshiba matched quads and octets of J74/K170. Octet is US $90 or $100 depending upon IDSS. These are BL devices.

Good luck on the tube input, and if that is what you really want, I'd agree; go for it. I'm sure Dukei will be much help here.

[Satyrnine]

1 hour ago, audiostar said:

Think about keeping it modular and separate the tube pre from the power amp. Same as you separated the PSU already and not stuff everything into a single like 6U 800mm deep chassis 

So you you suggest three different enclosures? Even with the tube pre in the power amp chassis, it would be only DC coming into the power amp chassis. Would put the tube pre powersupply in the PSU chassis. Are you concerned more about the 6v and 100v or some sort of other interference/issue I'm overlooking? So far, everything should fit in two 2U chassis, (one a pessante, one a dissipante) with plenty of room. Thank you!

[Satyrnine]

Here's one of Dukei's he sent me. Primo work as usual: 

Edited May 17, 2023 by Satyrnine

[JoaMat]

Inspired by Satyrnine's project and an attempt to recover from my Tube input CFA3smd failure some time ago I’ve been busy with this today.

I call it Cascode Tube Input Something - intended for my CFA3smd something. +220/+15/-15V and filament needed. Based on ubaltoblatubeschem2, thank you Kevin.

Edited May 17, 2023 by JoaMat

[Satyrnine]

18 hours ago, JoaMat said:

Inspired by Satyrnine's project and an attempt to recover from my Tube input CFA3smd failure some time ago I’ve been busy with this today.

I call it Cascode Tube Input Something - intended for my CFA3smd something. +220/+15/-15V and filament needed. Based on ubaltoblatubeschem2, thank you Kevin.

So is this still the balanced/current mirror design of kevins but cascoded 6922's? I love it. Cascoded 6SN7's is one of my fav recipes. I have a SE pre setup that way, and it sounds great. I'm shooting for balanced all the way through for this build though. 6SN7's ideally should get more B+ vs 6922 obv, not sure how far down a that rabbit hole I want to go. Tempting as heck.

[JoaMat]

Yes, Kevin’s design. Just added one 6922 and two resistors. I have a lot of 6922 doing nothing.

[Satyrnine]

On 11/24/2017 at 6:37 PM, Pars said:

What would the sonic effect be of running a 25K pot with this? It is what I have (TKD) in my DynaHi/DynaFET and I was planning on running these boards in that. I know you usually spec a 10K.

Thanks!

On 11/25/2017 at 5:47 AM, kevin gilmore said:

Slight DC offset unless you match input parts. So a whoosh as you turn the knob

 

Just rereading some of this thread. So uh... lets say someone ordered a Goldpoint in 25k. How much of a problem is that. Asking for a friend. 😑

Edited May 20, 2023 by Satyrnine

[audiostar]

I would stick to 10k

[audioguru1]

On 5/16/2023 at 3:24 PM, Pars said:

Just an FYI, the diyaudio store has Toshiba matched quads and octets of J74/K170. Octet is US $90 or $100 depending upon IDSS. These are BL devices.

Good luck on the tube input, and if that is what you really want, I'd agree; go for it. I'm sure Dukei will be much help here.

 

Thanks for the heads up! Thoughts on which LDSS value might be most appropriate? 6-8 or 8-11ma?

[audiostar]

1 hour ago, audioguru1 said:

Thanks for the heads up! Thoughts on which LDSS value might be most appropriate? 6-8 or 8-11ma?

I would go with the higher.

[Pars]

On 5/19/2023 at 11:40 PM, Satyrnine said:

Just rereading some of this thread. So uh... lets say someone ordered a Goldpoint in 25k. How much of a problem is that. Asking for a friend. 😑

I haven't had any issues with the 25K, but I'm using JFET input on the DynaFET (only option). If you use the THAT BJT quads on an SS input stage, that would probably be a problem as those require low input impedance. CFA3 with JFETs is probably not a problem, though Kevin could clarify.

[Satyrnine]

14 hours ago, audiostar said:

I would stick to 10k

10 minutes ago, Pars said:

I haven't had any issues with the 25K, but I'm using JFET input on the DynaFET (only option). If you use the THAT BJT quads on an SS input stage, that would probably be a problem as those require low input impedance. CFA3 with JFETs is probably not a problem, though Kevin could clarify.

Thanks for the input guys. As a testament to their great customer service, Goldpoint is letting me exchange my (unused) 25k quad for a 10k quad, for cost of shipping only. Their shipping is prompt too. I haven't used one of their pots yet, but so far so great. My config will be just 4x cfa2 at first, and then the tube ubaltobal input once I get boards/bom/iron ordered up and together. Since it's a free exchange for the 10k, might as well take advantage I figure.

As mentioned earlier my legit dealer sourced 15031's range from 70 to 120hfe, approx evenly distributed. If it even matters, what would be the best way to distribute this range across my 4x cfa2 boards? If I recall correctly, one of the 7 sinked devices on each board doesn't get as hot as the main 3/3 set, should I put outliers in that position, or...???

[Satyrnine]

Fired up first two cfa2's last night. A couple issues:

Board 1) Without servo, I'm seeing around 400mv (!) offset on output with input shorted. If I plug in the opamp, it slowly comes down to 1-2mv over 30sec or so. 

I started with bias pot all the way counter clockwise. Not seeing anything across the 1R resistors after a good 8 turns clockwise, do I just need to keep going, or should I be seeing some idle current by that point?

Board 2) LED's not lighting. It was late, and I gave up before investigating further.

[audiostar]

400mV is around 10x too much for the offset.

8 rotations are normal for the bias pot until you start noticing the voltage increase. The pot has around 30 turns or so. 150 to 200mV would be in range. 

Edited May 24, 2023 by audiostar

[Satyrnine]

5 hours ago, Satyrnine said:

Fired up first two cfa2's last night. A couple issues:

Board 1) Without servo, I'm seeing around 400mv (!) offset on output with input shorted. If I plug in the opamp, it slowly comes down to 1-2mv over 30sec or so. 

I started with bias pot all the way counter clockwise. Not seeing anything across the 1R resistors after a good 8 turns clockwise, do I just need to keep going, or should I be seeing some idle current by that point?

Board 2) LED's not lighting. It was late, and I gave up before investigating further.

Board 1) Butterfingered probing one of the 15030's trying to diagnose the high negative DC offset on this board. Shorted base to connector momentarily, 1R's went nuclear and started melting my probe tips within 1 sec. I powered down a second later, but I'm guessing I have some sand and resistors to replace now. Any suggestions on what to replace? Neat. Stupid mistake.

Board 2) Remind me not to trust mouser to always get their parts picking correct. Could have been my mess up too I suppose. There was a 10k in place of a 10R near the input (R9 or R10) which I deduce was causing the LED's not to light. Replaced that resistor w correct value, LED's light and this board is now showing around 40mv offset without opamp and zero offset with opamp, which seems about right. Biased to low around 80ma for now, just had to adjust trimpot further. This board seems to be good to go.

3 hours ago, audiostar said:

400mV is around 10x too much for the offset.

8 rotations are normal for the bias pot until you start noticing the voltage increasing. The pot has around 30 turns or so. 150 to 200mV would be in range. 

Indeed too much. Now that I messed this board up, may be replacing a bunch of sand anyway. I double checked my N's and P's and resistor values, and polarities before it went south, everything seemed to be in the right place. 

[dip16amp]

Just replace both 1 ohm resistors and the one bad 15030. Everything else should be fine.

Measure resistance across the 15031 and 15030 to find a shorted one.

I've only had a bad 15031.

Measure resistance across both resistors may be 1.5 ohms instead of 2 ohms.

Measure these in circuit with power off.

 

Edited May 22, 2023 by dip16amp
added info

[ang728]

so after some fiddling around , now my CFA3 working perfectly.I replaced all my old stock BC546/556 to newer parts with roughly matching,offset still 40mV w/o Servo but no big difference switching back to 300ohm 

maybe my problem comes from some crooked old BC parts...? I dunno but fixed anyway lol

On 5/13/2023 at 1:54 AM, ang728 said:

Finished building CFA3 Ver1.21 recently.But offset reading seems off ,about 0.8V relative to gnd and 0.2V  after solving oscillating issue ,VO+ to  VO- also high as 0.2V

and bias went huge bump when switching ss to zf with oscillation on scope.I was quickly solving the problem by pulling 600pf,20pf caps form the mentions in this thread.

After some wild goose chase then I noticed a slight difference between schematic and board silkscreen.R211,R222,161,R262 value at current mirror is 680ohms but showing 300ohm on PCB.

now I have about 40mV offset w/o servo at each side with 680ohm resistor,haven't done any further test but I think I can have a good sleep tonight

Scrolling through the thread and it seems nobody mentioned .Not sure if that's a issue but just a heads up

 

Edited May 23, 2023 by ang728

[Satyrnine]

On 5/22/2023 at 11:19 AM, dip16amp said:

Just replace both 1 ohm resistors and the one bad 15030. Everything else should be fine.

Measure resistance across the 15031 and 15030 to find a shorted one.

I've only had a bad 15031.

Measure resistance across both resistors may be 1.5 ohms instead of 2 ohms.

Measure these in circuit with power off.

 

Thank you! This is great to hear. Was concerned it may have taken out more, I will measure everything! The one blown 1R is currently about 250ohms, haha! Will replace both though, as they both got to warm.

[audiostar]

On 5/23/2023 at 9:20 AM, ang728 said:

so after some fiddling around , now my CFA3 working perfectly.I replaced all my old stock BC546/556 to newer parts with roughly matching,offset still 40mV w/o Servo but no big difference switching back to 300ohm 

maybe my problem comes from some crooked old BC parts...? I dunno but fixed anyway lol

 

Those large boards on top look interesting.