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 13th June 2018, 11:09 PM #21 jdrouin   diyAudio Member   Join Date: Feb 2012 Location: Tulsa, OK OK, the formula for figuring out voltage to dB is dB = 20 x Log(volts1/volts2) So in my case, that's dB = 20 x Log(6.2/281.9) dB = -33.15 Or on the calculator: * 6.2 / 281.9 * Hit enter * Hit log10 button * Multiply by 20 * = -33.15 The ripple at the rectifier output is -33.15 dB down from Edc. If we want to go for the standard -90 dB, that's another -56.85 dB. Let's round to 57. To figure out the smoothing factor required to achieve that, it's: Fs = 10^(Fs[dBv]/20) On the calculator * 57/20 = 2.85 * Hit 10^x button * 707.95 So let's say I need a smoothing factor of 708 to reach a target ripple of -90 dB below Edc. First of all, do these calculations seem correct? Second, do you think that would be enough PSRR for a headphone amp? Cans are Grado SR125 (32 ohm, 98 dB SPL for 1mV input). Last edited by jdrouin; 13th June 2018 at 11:11 PM.
jdrouin
diyAudio Member

Join Date: Feb 2012
Location: Tulsa, OK
Now to get back to what PRR said in Post #4 about peak current being limited by transformer resistance, the rather high 419 ohm DCR of the PT secondary seems to be a major help. In PSUD2, I'm able to model the PS using a 20uF input cap (which the rectifier would see with two 10uF units after the rail split, as I mentioned in post #1) and 100uF caps after the choke and the RC filter, and it does just fine. If I increase the 100uF caps to 200uF, it still does just fine. So unless I'm doing something wrong here, it's OK to exceed that 10uF input cap limit on the datasheet and to use some larger caps throughout the PS network.

Adjusting the end resistor value in PSUD2 to 2.5k, at rongon's suggestion, we get a nice curve up to stability, with no ringing, settling right in at 140V (first screenshot).

If we allow a 30 second delay and run the sim for 10 seconds, it sits at 140V with a ripple of 152.82uV (second screenshot).

I'm sure there are ways to fine-tune this, but for now I'm glad to know that I can safely make a first iteration of this amp using parts I have (unless someone sees any mistakes I'm making).
Attached Images
 Screen Shot 2018-06-13 at 9.54.34 PM.png (218.6 KB, 164 views) Screen Shot 2018-06-13 at 9.52.10 PM.png (200.1 KB, 159 views)

Last edited by jdrouin; 14th June 2018 at 03:03 AM.

 14th June 2018, 04:22 AM #23 jdrouin   diyAudio Member   Join Date: Feb 2012 Location: Tulsa, OK Calculating ripple reduction manually with LCRC sections as above (10H -- 100uF -- 1.8K -- 100uF): LC Section Fs1 = (2 x pi x (2 x 60Hz))^2 x 10H x 100uF -1 = 567.5 RC Section Fs2 = 2 x pi x (2 x 60Hz) x 1800R x 100uF +1 = 136.48 Fst = Fs1 x Fs2 = 77547.74 77547.74 seems like a really high ripple reduction factor to me. In dBv, it translates to 20 x Log(77547.74) = -97.79 dBv That also leaves out whatever ripple attenuation would be provided by the 10uF input cap. So, more than the target of -90 dB In terms of voltage reduction, I calculated 281.9V output from the rectifier. Target B+ is 146V, so a drop of 143.1V is required. In a split-rail PS and 18mA on the 5842s, that would require a total resistance of R = V/I = 143.1/.018 = 7.95K in each channel. That means that the series resistance of the network would need to be much greater than what is shown in the PSUD2 model (which admittedly might have been set up incorrectly). If I kept the 2-stage design, one channel would look like: 10uF -- 10H/414R -- 100uF -- 7.5K -- 100uF DHTRob's "Classic" schematic (post #7) does show a 10K 5watt resistor in that position, so the value is not necessarily extreme (the fallacy of appeal to authority). On a hunch, I went back to PSUD2 and changed the load to current sink @ 18mA, on rongon's suggestion. To get 146V without an error message (current sink causing negative voltage on startup), I have to reduce the 100uF caps to 50uF and use a 9.4K resistor -- very close to the 10K in Rob's "classic" schemo. That's 2K more than my manual calculation, but closer than the 7.6K difference from the earlier PSUD sim with a resistive load (and 1.8k resistor in the last position). So, which do I trust? My calculations or PSUD2? Last edited by jdrouin; 14th June 2018 at 04:49 AM.
jdrouin
diyAudio Member

Join Date: Feb 2012
Location: Tulsa, OK
Target B+ of 146.6V. PSUD2 screenshot and hand calculation schematics attached. Difference is 9.4K final resistor in PSUD2, 7.1K final resistor in hand calc. Think I'm leaning toward hand calc.

Do these seem reasonable or ill-advised?
Attached Images
 Screen Shot 2018-06-14 at 12.40.38 AM.png (198.6 KB, 152 views) 5842-ps-calc.jpg (448.5 KB, 146 views)

Last edited by jdrouin; 14th June 2018 at 06:02 AM.

jdrouin
diyAudio Member

Join Date: Feb 2012
Location: Tulsa, OK
Another possibility is to have a single rail PS, placing the chokes in series with lower value caps, and splitting into separate channels before the final dropping resistors.

10uf -- 10H/414R -- 10H/414R -- (channel split) -- 5.8K -- 10uF (x2)

B+ is 146.6V in each channel; reduction factor is 356,688.58 or -111 dB, the best value so far.

I had wanted the split right after the rectifier so that the two supply rails could respond more dynamically to the music in each channel. Not sure if it would really make an audible difference.
Attached Images
 Screen Shot 2018-06-14 at 10.27.43 AM.png (313.2 KB, 119 views) 5842-ps-calc-lower-c.jpg (475.4 KB, 38 views)

 19th June 2018, 04:03 AM #26 jdrouin   diyAudio Member   Join Date: Feb 2012 Location: Tulsa, OK So, I'm using the Raphaelite OP5.5K5A OPT (Raphaelite 5W single-ended output transformer OP5.5K5A 6V6 EL84 sealed | eBay), which is 5.5K into 8 or 32ohm. I need to wire it for 32ohm and am wondering how to do that. It has two 16 ohm secondaries. As far as I can tell there is no owner's manual available. Pinout groupings are as follows: Pin 2 -- B+ Pin 10 -- Plate Pin 6 -- Ground Pin 5 -- 16ohm Pin 4 -- 0 Pin 8 -- 16ohm Pin 7 -- 0 For 32 ohm, I'd need to wire the two 16 ohm secondaries in series, so how would that work? Would I simply wire the two secondary groupings like this? 5 --> 4 --> 8 --> 7, where 5 goes to the signal tab on the headphone jack and 7 goes to the common tab? Last edited by jdrouin; 19th June 2018 at 04:10 AM.
 19th June 2018, 04:41 PM #27 stocktrader200   diyAudio Member   Join Date: Jan 2008 Location: Calgary cap multiplier is delivering -112 db of ripple from a voltage tripler. 68uf main cap ( 149v ) 2 * 2N5551 and 2 10uf 250v caps. 140.5v out .004 mv ripple 26ma. 4 triodes at 5.5 to 6.5 ma each it is lower then the 6922 gain noise.
jdrouin
diyAudio Member

Join Date: Feb 2012
Location: Tulsa, OK
Here's a diagram to show what I have in mind for wiring the secondaries to get 32 ohm. Does this seem right?

stocktrader: thanks for the suggestions. I'm only using 2 triodes but I'll try modeling that in PSUDII.
Attached Images
 opt-diagram.jpeg (503.4 KB, 41 views)

jdrouin
diyAudio Member

Join Date: Feb 2012
Location: Tulsa, OK
I've got everything wired up and running at good voltages for a starting point, but am not getting any sound. My guess is either that I miswired the OPT secondary or the headphone jack.

B+ = 148V
Va = 142V
Vk = 2V

Here's a pic of the OPT secondary wiring. It follows the diagram in the previous post. Seem OK?
Attached Images
 IMG_1407.jpg (562.0 KB, 31 views)

 22nd June 2018, 03:56 PM #30 jazbo8 diyAudio Moderator     Join Date: Jan 2011 Location: In Transient You shorted out the seconary windings - you need to disconnect 4-5, and 7-8. Pin 7 should also be grounded.

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