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:
http://secure.oatleyelectronics.com/files/K272X.pdf
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

 

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 properly 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 meager 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 the speaker impedance to give a certain sound which is actually sought after.. Also, certain hifi tube amps have interactions that make the more suitable for certain speakers. Solid state is a different story, and that is why some 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 should act almost exactly like triodes but apparently do not. That is why people buy tube amps.

 

[Phil Allison]

et...@whidbey.com wrote:

http://secure.oatleyelectronics.com/files/K272X.pdf
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?

** 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

 

[Tim Wescott]

On Wed, 16 Mar 2016 21:55:36 -0400, DJ Delorie wrote:

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.

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

 

[DJ Delorie]

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.

 

[George Herold]

On Thursday, March 17, 2016 at 3:19:24 PM UTC-4, et...@whidbey.com wrote:

On Wed, 16 Mar 2016 21:45:29 -0500, Tim Wescott <tim@seemywebsite.com
wrote:

On Wed, 16 Mar 2016 21:55:36 -0400, DJ Delorie wrote:

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.

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)
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

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.

 

On Wed, 16 Mar 2016 21:45:29 -0500, Tim Wescott <tim@seemywebsite.com>
wrote:

On Wed, 16 Mar 2016 21:55:36 -0400, DJ Delorie wrote:

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.

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)

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

 

On Wed, 16 Mar 2016 19:55:34 -0700 (PDT), jurb6006@gmail.com wrote:

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 properly 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 meager 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 the speaker impedance to give a certain sound which is actually sought after. Also, certain hifi tube amps have interactions that make the more suitable for certain speakers. Solid state is a different story, and that is why some 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 should act almost exactly like triodes but apparently do not. That is why people buy tube amps.

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