Gareth, it's a good opportunity to investigate the issue and finally make the design "bullet proof". I have ordered the OS's Slewmaster PCBs in order to build it and try to reproduce the exact setup Terry is struggling with (as close as possible). Hopefully, it will oscillate (fun to wish so, huh?
). There are still some tricks I did not try yet - one of them is grid stoppers. Simulation shows, together with grid capacitance, they influence the overall phase response above 10MHz. Also, as you suggest - more aggressive killing of OLG at HF may help, but I would better test it myself, instead of waiting for somebody else frying the output devices. Looking forward to getting the PCBs and finding the exact cause of the problem
Cheers,
Valery
The reason I am trying a "different OPS" is because this was offered in the Slewmaster thread as viable IPS. Were this just offered here as a complete system with Valery's OPS I probably wouldn't be bothering you.
Hi Valery,
I tried the two solutions. The first one worked well. Offset was low at 12mv. I was able to set the output bias. I even played some sinewaves and square waves through it. They looked pretty good. Then I tried the second solution and it looked pretty good. Everything came up pretty good on the lightbulb. I was able to set the bias but the offset was about 66mv. I let it warm up for a while and then I hooked up the scope again and played some sinewaves through it. The sine wave looked pretty thick so I think there was some oscillation. The offset began to grow so I tried to hook up a jumper to short the input but one of the clips slipped off leaving the input open. Instantly, the fuses blew. Measuring, I now have some shorted outputs. Fun stuff.
We are heading in the right direction I think.
Hi Valery,
I tried the two solutions. The first one worked well. Offset was low at 12mv. I was able to set the output bias. I even played some sinewaves and square waves through it. They looked pretty good. Then I tried the second solution and it looked pretty good. Everything came up pretty good on the lightbulb. I was able to set the bias but the offset was about 66mv. I let it warm up for a while and then I hooked up the scope again and played some sinewaves through it. The sine wave looked pretty thick so I think there was some oscillation. The offset began to grow so I tried to hook up a jumper to short the input but one of the clips slipped off leaving the input open. Instantly, the fuses blew. Measuring, I now have some shorted outputs. Fun stuff.
We are heading in the right direction I think.
Ufff... Terry, sorry about the shorted outputs
Meanwhile, I did some additional research and simulation. Adding 220 pF cap between Q11, Q16 emitters (set on top of R22, R28 pins) increases phase margin further on. With my OPS it's rock stable, reproducing perfect sine and square waves and sounding delicious.
I attach the final schematic for reference.
Waiting now for Slewmaster PCBs... I actually had it in mind as OPS while designing the front-end, but looks like there's something I don't know about it...
Attachments
Hi Valery,
As much as I am desiring to learn, this is just too much over my head and I am going to run out of output at this rate. I know you are calling this the final schematic but I have a feeling once you have the OS OPS we will be seeing a new "Final" version.
This seems to work until you feed the OPS NFB into the IPS. Hopefully having the Slewmaster OPS will give you what you need to get to the bottom of this.
As much as I am desiring to learn, this is just too much over my head and I am going to run out of output at this rate. I know you are calling this the final schematic but I have a feeling once you have the OS OPS we will be seeing a new "Final" version.
This seems to work until you feed the OPS NFB into the IPS. Hopefully having the Slewmaster OPS will give you what you need to get to the bottom of this.
Hi Valery,
Yes I will wait. I was lucky and only blew one output and got even luckier as it was the first one I pulled. All this trouble had me worried so I hooked up a couple of the other IPS and they worked perfectly so I feel assured that there is just something going on with the way the Low TIM hybrid and the Slewmaster work together. I feel confident you will find the issue. I really do want to hear how this thing sounds.
Blessings, Terry
Yes I will wait. I was lucky and only blew one output and got even luckier as it was the first one I pulled. All this trouble had me worried so I hooked up a couple of the other IPS and they worked perfectly so I feel assured that there is just something going on with the way the Low TIM hybrid and the Slewmaster work together. I feel confident you will find the issue. I really do want to hear how this thing sounds.
Blessings, Terry
Hi Valery,
I swapped out the resistor and cap. When I tried it, it never kicked in. I waited over 5 minutes and the LED just kept flashing. I then switched the the 220K resistor back to the 68K. Now it comes on after about 24 seconds. That feels like a long time but may be what is needed. The Low TIM Hybrid does take a long time to settle. A couple things have me puzzled about this circuit. I used a little 2A adjustable power supply that I have an it takes about 3VDC to trip the protection circuit and that only if you have the - to the ground and + to the speaker out. If you switch that so the - is on the speaker out and the + is on the ground it takes about 7.5VDC to trip it. This is testing with no speaker or amp connected so that may change with a load or input. Does this sound about right?
Thanks, Terry
Hi Terry,
24 seconds look fine.
3VDC to trip the protection is acceptable, 7.5VDC looks pretty high to me, however I think this is a peculiarity of DC "sensor" used in this circuit (2 transistors at the very left of the schematic) - it is less sensitive for negative offset than to the positive one. Not sure we can do something about it without significant changes...
Cheers,
Valery
24 seconds look fine.
3VDC to trip the protection is acceptable, 7.5VDC looks pretty high to me, however I think this is a peculiarity of DC "sensor" used in this circuit (2 transistors at the very left of the schematic) - it is less sensitive for negative offset than to the positive one. Not sure we can do something about it without significant changes...
Cheers,
Valery
Hi Valery,
Yeah, probably not a big deal. 7V won't hurt the speaker. Main thing is to protect them from rail voltage. If it goes to 7V it is probably on it's way to rail anyway and will trip.
Initially, I was hoping to use the DC protection alone but if it kicks in at 7V I will see a pretty good thump. Better to use the 24 second delay so it will be close to 0V before it connects the speakers.
So far, all of the Slewmaster IPS/OPS combinations I have tried only go to about 2v offset if one fuse blows. Hopefully this one will be the same once we get the other issues sorted out.
BTW, the schematic I posted has a typo. It shows 100K on the L in but they are both 10k. It should look like the one attached here.
Blessings, Terry
Yeah, probably not a big deal. 7V won't hurt the speaker. Main thing is to protect them from rail voltage. If it goes to 7V it is probably on it's way to rail anyway and will trip.
Initially, I was hoping to use the DC protection alone but if it kicks in at 7V I will see a pretty good thump. Better to use the 24 second delay so it will be close to 0V before it connects the speakers.
So far, all of the Slewmaster IPS/OPS combinations I have tried only go to about 2v offset if one fuse blows. Hopefully this one will be the same once we get the other issues sorted out.
BTW, the schematic I posted has a typo. It shows 100K on the L in but they are both 10k. It should look like the one attached here.
Blessings, Terry
Attachments
Tested with SlewMonster
OK. Finished soldering SlewMonster. Got 3 output pairs so far - more transistors to come in a couple of weeks.
Connected everything, switched on, set the bias, and... no problem
No stability issues. 20KHz square wave response is perfect, no sign of ringing. Excellent sound - same great bass, natural mids, airy highs...
SlewMonster is a great power section - it does not actually influence neither frequency, nor phase response, at least within 1MHz (as far as I can see on my analyzer). I use 33 pF caps at pre-drivers and do not use optional 68 pF caps at the drivers.
Terry, let's think what can be different in your setup. Power supply? Wiring? Tubes? Are you sure you connect PD+ and ND- wires correctly? I mean, at my front-end they are "mirrored" relatively to SlewMonster - PD+, being "at the top" on front-end PCB, is placed "at the bottom" on SlewMonster. Another thought - grounding. I have G1 and G2 directly connected with a wire at my prototype. Is it the same at yours?
I experimented with increasing LTP degeneration and adding the grid stoppers (cutting the traces and soldering them right on top of the gap). Also increase the VAS cascode load (decreased R26, R27), slightly decreasing VAS gain and ensuring less dependency on the power section input impedance.
Those changes potentially further improve stability, but there was no problem without them as well. I've tried to run very fast single impulses with high amplitude, fast-front square waves - I've tried everything. Solid performance. Attached schematic is the one I have on a test bench at the moment.
Thimios, did you progress with building your copy of the head-end? It would be interesting to have more "statistics".
Cheers,
Valery
OK. Finished soldering SlewMonster. Got 3 output pairs so far - more transistors to come in a couple of weeks.
Connected everything, switched on, set the bias, and... no problem
No stability issues. 20KHz square wave response is perfect, no sign of ringing. Excellent sound - same great bass, natural mids, airy highs...
SlewMonster is a great power section - it does not actually influence neither frequency, nor phase response, at least within 1MHz (as far as I can see on my analyzer). I use 33 pF caps at pre-drivers and do not use optional 68 pF caps at the drivers.
Terry, let's think what can be different in your setup. Power supply? Wiring? Tubes? Are you sure you connect PD+ and ND- wires correctly? I mean, at my front-end they are "mirrored" relatively to SlewMonster - PD+, being "at the top" on front-end PCB, is placed "at the bottom" on SlewMonster. Another thought - grounding. I have G1 and G2 directly connected with a wire at my prototype. Is it the same at yours?
I experimented with increasing LTP degeneration and adding the grid stoppers (cutting the traces and soldering them right on top of the gap). Also increase the VAS cascode load (decreased R26, R27), slightly decreasing VAS gain and ensuring less dependency on the power section input impedance.
Those changes potentially further improve stability, but there was no problem without them as well. I've tried to run very fast single impulses with high amplitude, fast-front square waves - I've tried everything. Solid performance. Attached schematic is the one I have on a test bench at the moment.
Thimios, did you progress with building your copy of the head-end? It would be interesting to have more "statistics".
Cheers,
Valery
Attachments
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Maybe you could mark up a schematic with VR and VBE readings so I could have something to compare to. I had hoped that maybe I had the PD, VD reversed. You used all red wires and it is too hard to try to follow them but I just got home and rechecked and the PD is definitely closest to the V+ and ND is closet to the V- so I had it hooked up correctly. Weird thing is that it works fine if I tie the NFB from the IPS to it'self but when I try and tie the NFB from the OPS it looses it. I'm doing that again until I can be certain that the IPS is running where it is supposed to. I'll need those readings from you to do that.
Thanks, Terry
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