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Designing my headphone amp
Designing my headphone amp
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Old 28th January 2021, 10:11 AM   #51
NickKUK is online now NickKUK  United Kingdom
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Join Date: Dec 2019
This is the main issue atm :/

Click the image to open in full size.

Obviously the substitute models aren't playing pretty..
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Old 29th January 2021, 09:31 PM   #52
NickKUK is online now NickKUK  United Kingdom
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Ignore the voltages, but I have been playing with a staggered start with a 30V cathode boost to ensure a -30 grid on startup. No voltage regulator or smoothing on this..

Front end - input valve
Click the image to open in full size.

Green B+ (no voltage regulator and only 470uF)
Red - cathode bias
Blue - headphone output
Cyan - current through the first valve

Then on the backend, just one ecc99 not the 3 or 4.

Click the image to open in full size.

Looking at the output stage:
Blue - headphone voltage
Green - headphone current through 32ohms
Red - is the cathode feedback into the initial stage
Pink - cathode line at output stage (before output cap)
Cyan - B+
Grey - Current across one valve.

I think this needs a timing change - the B+ hits and there's a minor current spike. Also I'll add a current limiter in.

This was to test the idea of forcing negative grid to prevent high current flow on startup through the valves. This seems to work nicely as a concept so worth looking at in the design.

Last edited by NickKUK; 29th January 2021 at 09:34 PM.
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Old 31st January 2021, 06:22 PM   #53
NickKUK is online now NickKUK  United Kingdom
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So I switched from ccs to normal resistors and the current balance is far better.

Almost ready to order bits
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Old 1st February 2021, 11:42 AM   #54
NickKUK is online now NickKUK  United Kingdom
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So I dropped the fancy gadgets, stuck with a 3 level RC filter, bias resistors and the voltage drop suffrage.

Click the image to open in full size.

This is a full model, both channels but with the two channels using a single power supply and filtering, it includes a 0.4sec soft start mains side and a transformer. I set the inputs - left at 10Khz 447mV and right at 1Khz 447mV to check for cross talk.
The amp model is setup to take +20dB 3.16V input without distorting, so in real life there's possibly a little more tweaking todo.

This is looking decent, and I'm happy enough with the current performance for the initial amp.

I'm just running a revised filter to target the cross talk through the B+ lines.

Last edited by NickKUK; 1st February 2021 at 11:48 AM.
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Old 1st February 2021, 12:28 PM   #55
NickKUK is online now NickKUK  United Kingdom
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Cross talk begone!

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Old 1st February 2021, 04:36 PM   #56
NickKUK is online now NickKUK  United Kingdom
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So I've tried modelling a snubber to get rid of the 100Hz+harmonic noise, seems that I'm testing LTspice's limits with a defcon warning and then with adding parasitics to the transformer coils, it's modelling at femtosecond/sec speed

I think this is one of those - sort out out when you have the hardware up and running.

Next steps are to look at RC filters and see if they can be reduced (some don't need to cope with the current so smaller uF and R could be used).

I want to put a current limiter in so I will have to sort out something. The model seems to shower the system is good, but this is 'incase' for any reason. Same with a TRIAC for a crowbar.. however I think I'm now ready to start ordering stuff which should be fun with Brexit!

I've not given up on the walnut top etc but under that I would have a metal case with side heatsinks and a sink at the back.

Looking at LC filters last night - 250mA 1H 230V are cheap enough.. but get to 4A and 5-10H and it's $$$ and the need for structural house supports! (plus they won't fit into the space!)

I'll put together a bill of materials and costing as part of this.

It did occur to me.. that it may be cheaper to buy a new pair of higher impedance headphones. Certainly with OTL it would reduce the power requirements. Perhaps that's a discussion when I blow up my old headphones!
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Old 2nd February 2021, 09:03 AM   #57
NickKUK is online now NickKUK  United Kingdom
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Click the image to open in full size.

looking at the cathode of the output tubes - fast slow square wave, the slight sloping back shows a low frequency rolloff. The amp is still starting up so not at full power.

This is likely to be the coupling cap to grid of the output tubes needing a tweak.
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Old 3rd February 2021, 05:05 PM   #58
NickKUK is online now NickKUK  United Kingdom
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So my rough estimation for the no-frills (no case) cost will be about 300-350 (inc VAT). This has no gadgets other than:
* inrush thermistor - on a manual bypass switch
* NPN+PNP current limiter, offering a normal limit and a separate dead short limit per channel and one for the startup of the filaments.

Initial plan is to use a SMPS brick supply I already have for the heater elements so a current limiter is needed to stop it from tripping the dead short protection initially. Alternative is to run a 230:12V and fully rectify it and then drop it down to 12.6V.

The idea is that this will be tweaked over time so things will be changed and improved (and the total cost will increase).
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Old 4th February 2021, 07:32 AM   #59
NickKUK is online now NickKUK  United Kingdom
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Naturally I don't advise copying this design, if you do so you do at your own risk. This is my first. This design has not be built before.

This is the current configuration. Some notes:
* For my build I'll be adding a a current limiter per channel between R48 and C13.
* fuses will be added between C13 & R48.
* The low pass is pretty basic targeting 1Hz multiple times
* there is possible way to reduce noise further by moving the ground point.
* The 12.6V I've pretty much ignored, so I will not be implementing, it's kept to add current drain to the transformer. 5V digital line will be missing too from the initial build but is there for calculating the thermistor Joules required.
* Right channel is a duplicate including the filter C13/C12/R34. If you don't have one for each channel you'll get cross talk through the B+ line.
* R12, R9, R48, R20 will be variable resistors initially until the system is tuned. It is highlighly likely that R11,R17 and R25 / R15,16,18 will also be variable as the ecc99s are not matched and may vary.
* U1 may need some matching but I'm hoping that the balanced tubes will at least be closer than the ecc99s.
* There is no current limiter shown for the heaters
* There is no negative grid bias mechanism - careful as the U2 anode voltage may rise faster than the e99 cathode voltages, this I've not checked since removing the voltage regulators.
* no bleed resistors show (100K) for each of the caps. I need to see if this changes the filter and adjust the resistor values according if it does.
Click the image to open in full size.

Power supply section
* the hard power switch, fuse are not shown
* the bypass will be a manual switch, literally next to the power switch - switch main power, count switch the other. The Thermistor should be specced for at least 4A sustained and 490J inrush so I will be using a 10A capable 600J (about 9).
* Again no bleed resistor across the 1000uF cap.
Click the image to open in full size.
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Old 4th February 2021, 10:47 AM   #60
NickKUK is online now NickKUK  United Kingdom
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Just noted that one of the 300ohm resistors didn't get moved over hence the first gets a sees not 300ohm but something different so I'll need to look at this.
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