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Old 1st April 2009, 08:21 PM   #1
syn08 is offline syn08  Canada
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Default VSOP amp

Boards for the latest HPS 3.3 phono stage are on order (6"x8" so I decided to use an overseas house to keep down the costs), so meantime here's what's on the board...

But before that, here's an HPS update: National Semiconductor is now manufacturing LME49610, a high current (250mA) buffer, also supporting +/-22V. This, in combination with the relatively new LME49860 dual opamp, also supporting +/-22V, allows simplifying the power supply, without any sacrifice in headroom. Noise performance is of course the same... Details to follow soon!

I was looking for some time to further develop an as much as possible simpler power amp, preferable with a higher power spec (compared to PGP and YAP), without compromising to much the performance. The new schematic is attached. I'm currently feeding this new design with +/-60V (that's how much I can get from my 2 x 600W lab supplies), plan is to go up to +/-70V. Here are some key measured specs of the new VSOP (Very Simple Output Powerstage) amp. Pretty good for 12 small signal trannies, plus the output devices, and an opamp. No overcurrent protection, not sure if all that extra circuitry required to make the amp fool proof is really required, anyway the YAP "hifi thyristor" circuit works perfectly fine and has no impact on the performance. The protection cuts, in this incarnation, the D23/D24 zenners current feed, shutting down the folded cascode gain stage. Also, no special soft clipping circuitry, the amp sticks for well under 1uS.

- 15ppm THD20 (0.0015%) at 160W output into 8ohm
- 18ppm THD20 (0.0018%) at 320W output into 4ohm
- IMD 19+20KHz is around 20ppm (0.0020%)
- 110V/uS slew rate, 800nS rise time for +/-50V output, defined by the input filter. Yes, the input filter is dampening the overshoot .
- Output spectra shows mainly 2nd harmonic with some 3rd residual.
- Closed loop gain = 28
- 80 degree phase margin
- 0.01 degree closed loop phase shift
- Stable in whatever (capacitive) load I was able to provide.

Current feedback output stage with local loop. Low bias input stage, folded cascode gain stage, EF driver, 4 pairs of 2SK1530/2SJ162, two pole frequency compensation. Gain is set to 11, input stage is a THS4031 opamp, swinging about +/-5V, global gain is set to 28. Other +/-15V high speed opamps can be used, in fact it worked fine with almost any high speed unity gain stable opamp I was able to test. Using a low bias/low offset part guarantees a low output offset, no servo is required as in YAP. Only the EF drivers need on board heatsinks.

Costruction is a single board with local supply filtering 4x10,000uF/80V, board bolted to the 11"x5", 0.4C/W, heatsink, pretty much like the YAP output stage. Nothing really special regarding the PCB layout, this is not an ultra high ULG as YAP. Preliminary, I have connected two 1KW toroids, providing +/-72Vac, and the thing delivers (short time) 600W into a 2ohm load with 30ppm THD20. Not that I really need that kind of power, just making sure that the VSOP is going to drive even the most difficult 4ohm reactive loads without much of a problem.

I'll be back with further pictures, graphs, details, etc... meantime comments are welcomed.

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Old 1st April 2009, 08:43 PM   #2
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Seems like a grossly over complicated design for what it is supposed to do.

Too many transistors.
Too many smoothing caps across output transistors.
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Old 1st April 2009, 08:55 PM   #3
syn08 is offline syn08  Canada
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Quote:
Originally posted by nigelwright7557
Seems like a grossly over complicated design for what it is supposed to do.

Too many transistors.
Too many smoothing caps across output transistors.
Those are not filtering caps, ever heard about parasitic inductances and decoupling?

Now, if you can come up with a simpler design while keeping the same performance spec, I'm all ears
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Old 1st April 2009, 09:10 PM   #4
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Quote:
Originally posted by syn08


Those are not filtering caps, ever heard about parasitic inductances and decoupling?

Now, if you can come up with a simpler design while keeping the same performance spec, I'm all ears

A good amp doesnt need all that decoupling.

To me it looks like someone just kept throwing transistors at the design until no more would stick !
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Old 1st April 2009, 09:27 PM   #5
syn08 is offline syn08  Canada
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Quote:
Originally posted by nigelwright7557
To me it looks like someone just kept throwing transistors at the design until no more would stick !
Yep, I used my favorite pitchfork for that

May I suggest this design that I'm sure you will appreciate for its simplicity, performance and, above all, amazing sound?

http://www.diyaudio.com/forums/showt...194#post488194
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Old 1st April 2009, 09:37 PM   #6
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Quote:
Originally posted by syn08


Yep, I used my favorite pitchfork for that

May I suggest this design that I'm sure you will appreciate for its simplicity, performance and, above all, amazing sound?

http://www.diyaudio.com/forums/showt...194#post488194

I have built 3 high power amps for my disco and for playing guitar through.

They all had a LTP front end with the LTP served with a CCS.
The LTP fed into a VAS again with a CCS.
The bias was a Vbe multiplier of which each side feeds into 3 vertical MOSFETs.

Its by far not an ideal solution but it works and sounds good.

Sorry I cant supply a cct diagram as my CAD software doesnt have an export facility.
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Old 1st April 2009, 09:49 PM   #7
syn08 is offline syn08  Canada
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Quote:
Originally posted by nigelwright7557
They all had a LTP front end with the LTP served with a CCS.
The LTP fed into a VAS again with a CCS.
The bias was a Vbe multiplier of which each side feeds into 3 vertical MOSFETs.

Its by far not an ideal solution but it works and sounds good.
Sounds familiar The same o' same ancient Self topology that can be seen everywhere, anytime. Have you measured the performances? And how many small signal trannies? Am I wrong or its 8 or 10? Wow, that's much less than 12
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Old 1st April 2009, 10:15 PM   #8
wg_ski is online now wg_ski  United States
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I think he's seeing the triple cascode (with the last one folded) and an emitter follower to boot being overkill. Maybe, maybe not.

It does take a minimum two stages in between the output followers and the opamp if you want nested current feedback. If .01% THD is all you want, that's all you need. (You won't even need the local loop.) If you're building a PA amp with 20 output trannies the layout will limit you to that anyway regardless of OLG. I've found that I at least like the Vas to be some sort of compound (darlington or cascode), even with those limits so that's 3 stages instead of 4. Whoopee. Saving 2 small signal trannies is nothing when you start adding up all the outputs.

To get distortion in the ppm range, the circuit itself is only part of the battle.
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Old 1st April 2009, 10:15 PM   #9
andy_c is offline andy_c  United States
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Hi Ovidiu,

I noticed the gate resistors for the output devices are pretty large. Was this necessary for stability or were you just being conservative here?

Also, what's your take regarding the position of the gate protection zeners? I've considered putting them in the location shown in your schematic, but also worried that their max current isn't limited in that configuration. I've also seen (in Hafler designs) their cathodes hooked up to the the driver bases so the max current is limited by that of the VAS. OTOH, I guess one would not see them turn on even with a shorted output - only the case of parasitic oscillation of the output devices I suppose. So maybe it doesn't matter.
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Old 1st April 2009, 11:08 PM   #10
syn08 is offline syn08  Canada
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Quote:
Originally posted by andy_c
Hi Ovidiu,

I noticed the gate resistors for the output devices are pretty large. Was this necessary for stability or were you just being conservative here?

Also, what's your take regarding the position of the gate protection zeners? I've considered putting them in the location shown in your schematic, but also worried that their max current isn't limited in that configuration. I've also seen (in Hafler designs) their cathodes hooked up to the the driver bases so the max current is limited by that of the VAS. OTOH, I guess one would not see them turn on even with a shorted output - only the case of parasitic oscillation of the output devices I suppose. So maybe it doesn't matter.
Yes, they are conservative values. I may further tweak them, but there's no plan to push the ULG up, as for the YAP amp, at least in the first version. High ULG PCBs are barely available for the regular DIYer, the ultimate plan is to build VSOP on a single sided PCB.

Zeners have an interesting intrinsic property: they always get shorted, never open (and that's because the dynamic resistence at hih currents is usually smaller than the internal wiring, therefore the silicon melts first. OTOH, they always work in some sort of current limited environment, usually less than it takes to melt the internal wiring anyway. So I think connecting the zeners as close as possible to the MOSFET gates is better in terms of protection. The Hafler connection exposes the gates to drivers failures and, after all, worst case, who cares if the zeners die, they cost pennies anyway. Of course, ideally zeners should be connected directly to the gate of MOSFETS, but I think the advantage would be very small to pay for the board space required to accomodate 16 (for VSOP) DO-35 or DO-41 cases.
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