I'd picked a schematic to build, and lo and behold it's one of the earliest (if not the original) incarnations of Ultra-Linear topology, as invented by Hafler and Keroes of later Dynaco and Acrosound fame.
Here's a nice copy of the schematic:
http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/Schematics/UL-H&K-Fig3.pdf
I've got a matched quad of Sovs (the WXT+ kind), and matched pairs of Sov 6SN7GT's and 6SL7GT's. My power supply is a well-regulated (~10mV) CLC, where the C's are two 200uF sections of a big 500V LCR can cap, and the L is a Hammond 193K for 2.6H and 21 ohms. I also stuck a 470uF 400V Rubycon cap parellel to the first C. I get about 364V 1/2A out of this (I have a beefy AnTek 300VA toroid at 275-0-275). Chassis is a Hammond aluminum 16x8x3.
Anyway, the output transformer that should arrive sometime next week is from Edcor, 6600 primary impedance and a nice healthy 60W power tolerance, only has an 8-ohm tap, besides ground. Seeing as the NFB on this amp is connected to the 16-ohm tap, what should I do? What effect would connecting the NFB to the 8-ohm tap do? I'll post pics of my layout as all the stuff arrives.
Here's a nice copy of the schematic:
http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/Schematics/UL-H&K-Fig3.pdf
I've got a matched quad of Sovs (the WXT+ kind), and matched pairs of Sov 6SN7GT's and 6SL7GT's. My power supply is a well-regulated (~10mV) CLC, where the C's are two 200uF sections of a big 500V LCR can cap, and the L is a Hammond 193K for 2.6H and 21 ohms. I also stuck a 470uF 400V Rubycon cap parellel to the first C. I get about 364V 1/2A out of this (I have a beefy AnTek 300VA toroid at 275-0-275). Chassis is a Hammond aluminum 16x8x3.
Anyway, the output transformer that should arrive sometime next week is from Edcor, 6600 primary impedance and a nice healthy 60W power tolerance, only has an 8-ohm tap, besides ground. Seeing as the NFB on this amp is connected to the 16-ohm tap, what should I do? What effect would connecting the NFB to the 8-ohm tap do? I'll post pics of my layout as all the stuff arrives.
The NFB resistor value will set the gain. Most "classic" amps have gain that is much too high, so you might think about reducing that resistor's value. Now, you'll probably have to adjust the compensation cap value to make up for this, but with a new output transformer, you'd have needed to do that anyway.
Double-check that this schematic is accurate. The coupling RC time constants don't look right- they should be staggered at least 5:1, if not 10:1, for stability.
Double-check that this schematic is accurate. The coupling RC time constants don't look right- they should be staggered at least 5:1, if not 10:1, for stability.
sorenj07 said:What I'd heard about the NFB was that with an 8-ohm tap, I should use about 8.5K instead of 12, and raise the capacitance to about 170pF. I have no clue what you mean by the coupling contants, though.
That will get you in the ballpark- in reality, the exact gain is not that important. That's why you have a volume control. The capacitor value will not scale- since you're using a different output transformer, you'll have to adjust that value on the bench.
Without getting into the Mysterious Bode and his Imaginary Poles, stability with feedback depends very much on how the open loop gain rolls off on each end, most especially the slope.
Now C2 and R6 (also C3 and R7) form a high pass filter (i.e., roll off low frequencies). The time constant is C2*R6, in this case about 0.1 s. Likewise, C4 and R13 form a time constant, C4*R13, about 0.067 s. That's a 1.33 ratio. If there are no other time constants around, that would be stable. A third rolloff nearby would cause things to get rather tipsy. But unfortunately there are a bunch of other LF rolloffs: the output transformer, the bypass cap on the input stage cathode, the power supply decoupling... you really want the RC time constants to be further apart.
The best way, IMO, to do this is to reduce C4 (and C5), to perhaps 0u047 or 0u033. This will restrict the bottom end, but the feedback will partially make up for this. And you've got no business using a 25 watt amp to drive a subwoofer anyway.
A perfectly acceptable alternative would be to bring C2(3) up by a factor of 2 or 3, then reducing C4(5) by a factor of 2 or 3. That might also require stiffening the supply a bit, but you ought to have a good solid supply regardless.
Hmm. First off, if I were to stick a volume control in there, what rating should it be? I have a smallish 6SN7-based SRPP linestage that I could use to adjust volume (it has 4 input selection too) but I wonder if it'd be too much gain with what I assume to be an integrated amp. I don't suppose I could could simplify things by cutting out one of the gain/phase split stages? Anyway probably the easiest solution for me would be to slap another .22uF cap alongside c2 and c3 as I have a couple left over. It _is_ all right that I'm using .22 and not .2, right? I couldn't really find any of those.
Thanks for the info on the filtering, however, as it's provided me with some sweet insight into how schematics work. They used to look like random lines and symbols a few months ago, and already I'm beginning to make sense of them.
Thanks for the info on the filtering, however, as it's provided me with some sweet insight into how schematics work. They used to look like random lines and symbols a few months ago, and already I'm beginning to make sense of them
.
A few things.
First, a general note- you might want to use one of Hafler's later designs as your starting point.
Specifics: the LF rolloffs from the RC coupling are still only spaced by a factor of 3. You really want them much further apart. Five times should be a minimum, ten times is best. For my first shot, I'd set the second rolloff at 20-30 Hz, the first one at 2-3 Hz. It's better to make the second rolloff the dominant one because of the way this phase splitter operates.
Also, you've got almost 700u as the input to the pi filter in the power supply. That will cause unneccessarily large ripple currents, and very likely cause the 4007s to pop. Something like 60u would be more appropriate.
You'll likely have to revisit the decoupling. The time constants are too short for comfort. Bigger decoupling caps would help; regulation would be even better.
Check out the later Hafler article for his adaption of the Williamson. It's a much better starting point.
BTW, my volume control comment was directed at your preamp.
First, a general note- you might want to use one of Hafler's later designs as your starting point.
Specifics: the LF rolloffs from the RC coupling are still only spaced by a factor of 3. You really want them much further apart. Five times should be a minimum, ten times is best. For my first shot, I'd set the second rolloff at 20-30 Hz, the first one at 2-3 Hz. It's better to make the second rolloff the dominant one because of the way this phase splitter operates.
Also, you've got almost 700u as the input to the pi filter in the power supply. That will cause unneccessarily large ripple currents, and very likely cause the 4007s to pop. Something like 60u would be more appropriate.
You'll likely have to revisit the decoupling. The time constants are too short for comfort. Bigger decoupling caps would help; regulation would be even better.
Check out the later Hafler article for his adaption of the Williamson. It's a much better starting point.
BTW, my volume control comment was directed at your preamp.
SY said:
Well, I can put in a bigger cap than two .22uF, maybe I should put one .47uF cap and then later a .047? Nice, wide spacing? In any case, what do you mean about ripple currents? PSUDII didn't say that the 4007's would pop, but if need be, I can always leave out the extra 470u, or move it to parallel the second 200uF rather than the first, if that's better. I just had it lying around and figured it could help. I don't have a whole lot of room, ideally I'd have less capacitance and another choke, so this is kind of a quick'n'dirty approach
. I kind of made a mistake in the PT I got as well, if it was 360-0-360 I'd have tried very hard to fit another choke (or even two) under the hood for well-filtered awesomeness. Ah well, such is life. As for Hafler's later designs, I might be stuck with this one unless the later ones can use the 364V B+, the 6L6's and the 6600 ohm 60W OPT's? I don't mind changing the non-power tubes, but it means more hassle and ordering different parts.
Hmm. I know the power transformer's good for a half amp for the HV, and I'm not drawing the maximum of 5A for the filaments (probably only 3.6) so I think it'll be all right. I'll try sticking the extra 470u capacitor after the choke to start off, but would really rather have reduced hum, even if there should be some ripple currents. The worst that can happen is it's a bit noisy, or maybe I blow a couple 1N4007s, right? After either of which, I'll definitely change some things.
You don't want a big cap before the choke, and you really don't need a particularly big one after. Hafler showed a 10uF input choke in the original; that would be OK, but anything up to 50-60uF or so will be fine. The cap following the choke doesn't need to be huge- you hit the laws of diminishing returns pretty quickly, and the amp circuit has pretty good power supply rejection. Maybe 100-200uF at most.
BTW, Hafler's adaption of the Williamson, including a discussion of staggering the low frequency rolloffs, appears in the same volume of Audio Anthology (vol 2) as the circuit you're posting. It's an article well worth reading.
BTW, Hafler's adaption of the Williamson, including a discussion of staggering the low frequency rolloffs, appears in the same volume of Audio Anthology (vol 2) as the circuit you're posting. It's an article well worth reading.
I ran PSUDII with a 47uF cap to start and a 150uF after the choke, and got a hum of about 1/8V, which is quite acceptable, but a voltage of about 261V as opposed to 264. I'm hesitant about deviating any further from what I think the schematic calls for, which is 385V, but on the other hand, it's not going to make that much of a difference, right?
261 versus 264 is not significant. 264 versus 385 is very significant. If the sim is accurate, you need to use a different power transformer. But I have doubts regarding the sim- it's difficult to believe that the voltage could be that low if you really have a beefy 275-0-275 transformer. If you give me primary and secondary DCR, I can try the sim myself.
sorenj07 said:
I assume you mean 364V? I would not sweat this, since your line voltage will vary substantially regardless. SY has a very good point about the caps ... The recent tube amps I've had in my system have not had any caps bigger than 40uF and they have each been dead silent (they are choke-input, however). You can have a fair amount of ripple on the output stage B+ and never hear it in a push-pull amp. The important part is having low ripple and good isolation for the first stages of the amp, which is much easier to achieve. In fact you might get more satisfactory results using smaller value film capacitors than monster electrolytics. Something like a 40uF first cap and 80uF second cap would be more than sufficient for silent operation, and would give you the option to add tube rectification at a later date. if you feel like spending a bit of $$$ down the road, you can pick up some ebay motor-run film/oil caps in these values for $10-12/each...which would be a tremendous upgrade from electrolytic. I swapped out a 40/40/20/20 multisection for a 50/50uF LCR electrolytic and a pair of 12/12uF film caps... gigantic improvement in clarity / neutrality.
yeah, i did mean 364/361V. The nice thing is, I don't have to go out and buy a holesaw to chassis-mount the beast LCR cap I had, which was 200x200. It's about 1.75" thick and 4" long 0_o. Unfortunately, now I have no reason to use the thing. And it doesn't compare to the 3 550uF 385V can caps I had left over from another project I'm rebuilding . I picked up two 47uF 450V's and have a couple pulls, namely 100uF and 150uF at 400V, so I tihink I'm good to go for both this power amp PSU and the preamp. Now, to wait.
. I picked up two 47uF 450V's and have a couple pulls, namely 100uF and 150uF at 400V, so I tihink I'm good to go for both this power amp PSU and the preamp. Now, to wait.
Also, for my next project, I've discovered the GLORY of "motor start" or "motor run" capacitors - they're oil, from what I can tell, and VERY CHEAP - a simple ebay search of "motor capacitor" will show you what I mean. If only I'd found these things before buying that stupid 200x200uF 500V can, I'd be a lot better off
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