Headphones on Trancendent Grounded Grid preamp.

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> You can find the Ignore function under the User Control Panel at top of the screen.

Thank You!!
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> what a bad design for a headphone amp.

Not marketed as a headphone amp. Assuming "line output" (generally Power amps), it is a fine design.

> It's not a grounded-grid amplifier.

That bothered me at first. GG is "all wrong" for most hi-fi work.

It is of course a grounded plate feeding a grounded grid feeding something on top (SRPP?). The top-bit is grounded plate and will act as "Cathode follower" for some range of load impedance. GPGGGP? GPGGSRPP? Looks like a cat on the keyboard. Not a marketable name.

> short tailed pair

Tail voltage is ~~twice tube voltage. Pretty "long" by many standards.
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me> Say upper cathode internal impedance 1/Gm is 1K

12AU7 at 100V 2mA has Gm of 1,100uMho. 909 Ohms. Nominal voltage gain is 6. 12AU7 as a straight voltage-amp gives gain above 15. 6/15 is 0.4. 0.4 times 909r is 360-something. The SSRP(?) may add something to this. I could stimulate but it really is not important. It sure will drive 10K line loads with authority, but as hpeter says it "has not big enough balls to drive phones." 2mA max peak in the proposed 150 load is 0.3V peak, 0.21Vrms, 0.3 milliWatts totally clipped. Audible, but not satisfying.
 
DavesNotHere said:
and I think the 200 ohm is a typo too. Just by looking at this, even 2K might be stretching it. because of the cathode resistor being 3K.
200 may be stretching it, but it won't be as bad as 2k. The modified CF action of the upper valve will ensure a resonably low output impedance. Having a highish cathode resistor up there (3k) will reduce output impedance, provided that the valves still carry enough current - although I'm not sure they do in this design.

If I was asked to make a wild guess at the output impedance of this design I would go for 300-400 ohms. I can't be bothered to actually calculate it. It would be interesting to know on what basis they claim 200: measurement or calculation? If the latter, did they do it correctly? For example, did they take account of the fact that this is not a standard SRPP but modified by the cathode-coupled drive?

PRR said:
Tail voltage is ~~twice tube voltage. Pretty "long" by many standards.
Ah, I hadn't spotted the negative rail for the tail. So it is a long tail pair.
 
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I was curious and simulated the circuit. Current through each valve is about 2ma, anode voltage of the bottom valve is about 80V, gain is 3.8, output impedance @ 1KHz is 1.7 KOhm. The 2ma current moves the bias into a region with rather high rp which explains the high(ish) output impedance.
 
In the literature I have on this preamp, the designer says 300 ohms output impedance. This comes from the writeup he did in his book Audio Reality.

He also doesn't claim it as a SRPP, but rather the use of the extra triode in the envelope as a pseudo current source, noting it's not even very good at that purpose because it doesn't offer high enough impedance.

Specs as the designer has stated them:
  • bandwidth: flat from a few herts to 300 KHz.
  • Voltage gain: 12 dB
  • 20V RMS output (I assume that is into a typical tube amp load of say ~100K)
  • Feedback: 7 dB

By the way, there is no reason why the three terminal regulator used to power the filaments cannot be totally reliable and last for decades as long as the heat issues are addressed. It's not shown heat sinked on the schematic but he makes specific refererence to it in the text that it must be heat sinked.
 
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