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What's the point of a gain of 1 amp?

I ordered the headphone amp from Oatley Electronics and it arrived yesterday. Reading the instructions and the explanation of how it works has me a little confused. Refer to this link:

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The explanation says that there is a 7x gain output coming from the tubes. This output is next sent to the amplifier IC. The gain of this amp is set by two resistors, R10/R6. In the case of how this amp was supplied the gain is 1. So what's the point? Why not just avoid the amplifier IC alltogether and connect the headphones directly to the tube through a capacitor? I can understand that a person may want more gain in order to drive their headphones to eleven but why supply the kit with only a gain of 1? There is no provision for changing the gain except by unsoldering R6 and replacing it with a lower value for more gain. Obviously I am missing something which is why I'm posting here. So what's going on that I don't understand? By the way, I also ordered the O2 amp from JDS labs and their service is great. I have put both amps together except for plugging in the ICs. I need to check my work, especially on the O2 because it has way more parts and they are much more tightly packed, and there are specific testing procedures for the O2 supplied so I may as well follow them and learn a little more about how they work while at the same time checking my work. I'm looking forward to reporting here my impressions of each amp. Thanks for reading. Eric

Reply to
etpm
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My guess is that it's a case of matching impedance. Headphones are, effectively, a resistor (600 ohms or less). Hooking that resistor to the anode of the tube (even through a capacitor) might cause the tube stage to malfunction, whereas the op-amp is designed to handle that load.

So it's basically converting a 600 ohm load into a 1 Mohm load.

Reply to
DJ Delorie

To clarify, it has a _power_ gain significantly more than one, but a _voltage_ gain of 1.

(Driving low-impedance things from a gain-of-one amp that's driven by a high-impedance thing is fairly common, BTW)

--
www.wescottdesign.com
Reply to
Tim Wescott

The VOLTAGE gain is 1, the current gain is not. There is therefore a power gain.

The outputs in almost any normally designed stereo are simply current gain. The voltage gain is ahead of that. Ir you took off your speaker outputs at the voltage stage you might blow it, and it CERTAINLY will not work proper ly because it cannot supply the current for a low impedance load.

You would have to have a bunch of tubes in parallel to supply even the meag er current for dynamic headphones. An IC inherently has better quality than a transformer, unless you are looking for non linearity in power bandwidth or the voltage itself.. Old tube guitar amps do this and even react with t he speaker impedance to give a certain sound which is actually sought after . Also, certain hifi tube amps have interactions that make the more suitabl e for certain speakers. Solid state is a different story, and that is why s ome people like tube amps.

There are tube driven solid state amps, solid state driven tube amps, they all have their own characteristics. Your headphone amp is tube driven solid state. I am not exactly sure what is the allure. Depletion mode FETs shoul d act almost exactly like triodes but apparently do not. That is why people buy tube amps.

Reply to
jurb6006

** See the 180kohm resistors in the anode circuit of each valve? That is their best operating impedance, your headphones may well be 32 ohms so there is one hell of a mismatch. The amplifier IC is designed to drive 32ohm loads and deliver enough power to go quite loud - which the tubes alone could never do.

.... Phil

Reply to
Phil Allison

Thanks Tim, that makes sense. Does increasing the gain of the IC amp raise just the available current or does the voltage also increase with the current? Eric

Reply to
etpm

Thanks for the reply Jurb. So is part of the reason the xmfrs for tube amps are so expensive because they need to be wound a certain way to get good sound? Or is it all mostly hype? I have heard tube amps that sound very good to my ears but all my amps but the one I just built are all solid state and they sound very good too. Eric

Reply to
etpm

Amps are typically voltage controlling devices. A certain gain means that Vout/Vin = gain. An amp will also have a maximum current. So as you change the load on the amp you will see the voltage remain (approximately) constant, and the current change to meet whatever the load is. Until you reach the maximum current at which point the amp will typically output a reduced voltage... V = I_max*R_load.

George H.

Reply to
George Herold

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