1626 HPA

[Cressy Snr]

I also made up an adaptor plate to take a 9 pin socket, so that an EZ80 could be employed.
This small 90mA rectifier, is perfect for the low, 60 mA...ish current draw of the headphone amp.

Additionally the 6V heater, only draws 600mA; a lot less energy usage than the 2 and 3A, 5V octal rectifiers, which TBH were complete overkill for a headphone amp using 1626s.

EZ80PowerSupply.png (94.67 KiB) Viewed 11464 times

[Mike H]

Nice!

[Cressy Snr]

Cheers Mike.

[Cressy Snr]

And so, the never-ending saga of the headphone amp continues:

I have, I believe, got the best sounding version of the 1626 headphone amp so far. I've changed the input stage again; this time for a 12AX7 running at 1mA bias per triode.

This means my HPA is almost the same as the Ampsandsound 'Kenzie' which used a 12SL7 as its input stage, though when I last looked at their site, they had changed the front end to a 12AX7, superseding the older model. Obviously I don't know what their circuit is but according to their site, it is a Darling derivative.

In terms of my own amp, I had become tired of the sonic image washing around between my ears due to the shared cathode resistors. Yes, it was widescreen, but it was a bit diffuse and instruments/voices were difficult to place. Also, I was being driven slowly mad by the mechanical hum coming from the 230V toroid providing the HT supply.

I dug out an old OEP transformer, bought from Maplins, God knows how many years ago. The small wattage transformer has a 240-0-240V secondary, electrostatic screen and low power 6.3V heater winding; ideal for the EZ80 rectifier. The cathodes of the input and output stages were 'un-shared' and appropriate cathode resistors were installed, along with bypass caps. The LED bias was removed and a bulb for the indicator was re-installed.

IMG_0512.jpeg (142.79 KiB) Viewed 9864 times

This transformer is, thank goodness, virtually silent in operation. This is the power supply as of now:

Screenshot 2024-08-23 at 16.47.44.png (88.36 KiB) Viewed 9864 times

And the audio circuit:

Screenshot 2024-08-23 at 16.21.35.png (56.68 KiB) Viewed 9864 times

At the moment, I'm using a Psvane 'Art Series' 12AX7 a the voltage amp. the sound is sweet, dynamic, detailed and has treble to die for.
We're getting there and thoroughly enjoying ourselves into the bargain.

[Paul Barker]

Sounds like some good changes

[Cressy Snr]

Remember when I said earlier in this thread that with this headphone amp, hum was almost completely absent with no music playing? Well I've since discovered that with headphones right next to one's ears, virtual silence between tracks is not good enough; absolute silence is the requirement, nothing else will do, which is damned frustrating TBH.

So, the first requirement for silence is DC heaters on all the valves and I've achieved this with one of the Meanwell switching supplies off Ed's old SV811 breadboard, which I bought off our Ant for what he paid for it. Absolute stupid overkill with a 12V, 12A supply feeding only a couple of 1626 and a 12AX7 but I had nothing else to hand, so that was that:

Switcher.jpeg (122.69 KiB) Viewed 9236 times

Testing with no signal and a pair of headphones produced worse hum than with the AC heaters. The problem was solved by grounding one side of the 12.6V DC output. Now we were getting somewhere, but there was still some residual noise as soon as I plugged in the interconnects to the passive 10K peramp.

Much scratching of heads had ensued before eventually, I dug out a pair of Sowter 3575 10K 1:1CT input isolating transformers. There was nowhere to put them inside the amp, so I had to mount the cans on the front:

HPAwithIPT.jpeg (124.58 KiB) Viewed 9236 times

Anyway, the good news is that silence has been achieved and the aforesaid background silence has made a big difference to the degree of detail retrieval, the recovery of ambient cues and the length of reverb tails. No 100Hz component, merely a gentle, very low level hiss. Excellent!

I suppose if I want a completely black background, then a solid state HPA such as the Pass Labs Whammy kit is one of the obvious answers. But the valve HPA is more than liveable with, now there is nary a whisper of a 100Hz component breaking through.

All I need to do now is make a nice ventilated cover for the fugly switching supply sitting at the rear of the amp.

[Cressy Snr]

Revised diagrams:

IPT.png (66.4 KiB) Viewed 9224 times

Screenshot 2024-08-28 at 16.56.02.png (84.35 KiB) Viewed 9220 times

[Nick]

The pair of 100R to ground on the heater supply may still be worth doing.

[Cressy Snr]

The pair of 100R to ground on the heater supply may still be worth doing.

OK Nick,
I'll try it tomorrow and see what happens.

[Nick]

I would also try standing signal ground off safety ground with another 100R (maybe with a bipass cap), see if that removes the need for transformers at the front.

[Cressy Snr]

I would also try standing signal ground off safety ground with another 100R (maybe with a bipass cap), see if that removes the need for transformers at the front.

Thanks for that Nick. I no longer need the input transformers

I added a 100nF bypass cap (maybe it should be 10nF?) in parallel with the 100R resistor and, after reading Rod Elliott, a couple of 8A, 1KV diodes in anti-parallel across the resistor and cap in case as he said, anything nasty happens. It's all as quiet as the grave and without the complication of the input transformers.

Also the volume control has been moved on board the amp.

HPAwithgroundlift.png (67.84 KiB) Viewed 8698 times