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"Estudiante" Hybrid (7370 + MOSFET)
"Estudiante" Hybrid (7370 + MOSFET)
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Old 17th October 2017, 12:30 PM   #21
twocents is offline twocents  South Africa
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Quote:
Originally Posted by xrk971 View Post
It's that opaque ampule shape in the middle. Two Raytheon compact tubes. Probably used on 1960's jet fighters.
Interesting - never seen it before. I thought they were some boutique or Russian caps.

Quote:
Originally Posted by JPS64 View Post
Listening not bad with high impedance headphones (HD600, DT880 600R), lets say more bass as Wayne´s HPA supplied with +/-18VDC?!
This is what I am interested in. I don't really need another headamp but would like to try this one sometime in future. I'm just drawn to simple schematics/designs that work/sound good - and this one seems to fit that bill.
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Old 17th October 2017, 03:29 PM   #22
xrk971 is online now xrk971  United States
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"Estudiante" Hybrid (7370 + MOSFET)
It is very simple and you can simplify further by replacing the active LM317HV CCS (which technically adds an opamp with 20 transistors to the circuit) with a simple 5w 220R wire-wound. There is possibility of quadrupling the output power (passively) by replacing the resistor CCS with a properly sized inductor. A large many tens of mH low DCR one (a big spool of magnet wire works) or an air gapped steel core one for compactness. The reactive nature of the inductor amplifies the amount of voltage swing for more power.
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Old 10th November 2017, 11:28 PM   #23
00940 is offline 00940  Belgium
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I just had a look at the amp in the op. With 20v at 130ma heaters, you could use them as load for the mosfets... That's what was done in the famous Melos sha1. It works surprisingly well.
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Old 11th November 2017, 01:39 AM   #24
Sodacose is offline Sodacose  United States
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The 7370 draws 260mA at 20V with the two heaters in series. You could ground the CT and give both halves of the heater 20V. I thought about trying this (just like Millet does it with the SSMH, too) but I was also keen on trying to direct couple the stages and was worried about how consistent the heaters would be. If I build another one of these, I'll probably give this a shot (maybe cap coupled). Could be a little more compact by eliminating heatsinks.

The 417A is a 20V 100mA heater and I thought about using two halves in parallel the same way, but the plate curves didn't look like it would like the low B+ (even in parallel).
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Old 11th November 2017, 12:45 PM   #25
xrk971 is online now xrk971  United States
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"Estudiante" Hybrid (7370 + MOSFET)
You would still need sinks on main MOSFETs. Would not the variying pulsing current in the heaters, in this way perhaps drive some unwanted feedback? I guess one could put such big caps to filter it and make it very steady though.
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Old 11th November 2017, 01:12 PM   #26
Sodacose is offline Sodacose  United States
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Yes, eliminating some of the heatsinks is what I should have written. Still needed for the mosfets.

As far as feedback, only trying it would give one a good idea. Heaters on indirectly heated tubes are frequently run AC silently but in some cases you can get some of the mains frequency in the output. Whether that is the heater/cathode interaction or just wiring best practice is another question.

In this hypothetical case, we would have an inverted signal at the cathode, so nfb. Not necessarily a deal killer even if there is some small effect.
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Old 17th November 2017, 11:37 AM   #27
JPS64 is offline JPS64  Germany
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Here, after successful test, 1AD4/IRF510 Headphone Amplifier.

I will order a batch of 5.

JP
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Old 17th November 2017, 05:17 PM   #28
xrk971 is online now xrk971  United States
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"Estudiante" Hybrid (7370 + MOSFET)
Nice work JP. What DC-DC step up are you using? Note that you need to use the LM317HV for the CCS of the SE Class A output stage. I tried regular LM317 and it did not work. Very finished design.
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Old 17th November 2017, 05:30 PM   #29
Sodacose is offline Sodacose  United States
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I bet with the 24V supply it will be ok.

I think the issue with the Estudiante and 'regular LM317 was at start up. The gate would see the full supply voltage because the tube heaters had not warmed up. This was essentially dumping all 48V across the CCS and the supply was probably cycling at the same time because of the heater current draw. I've seen builds work without the 'HV' but it probably comes down to the supply used.

I updated all of my docs to specify LM317HV. It was listed in places (that's the part I built my own with because I had them) but I didn't reinforce the importance.

At any rate, JP's project (awesome job, by the way) shouldn't have the same issue with the regular LM317. I'm interested to see the rest.

You know you could easily make it portable with just a couple of standard 12V car batteries in series
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Old 22nd November 2017, 06:46 AM   #30
xrk971 is online now xrk971  United States
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"Estudiante" Hybrid (7370 + MOSFET)
Or a big fat 10,000mAhr 3.7v LiPo pack and a small $5 DC-DC step up board.
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