In my and in Alcaid's case we both have a pure resistive load at high frequencies so the need for this resistor is less important. I can only speak for myself here but my headphone amp works very nicely with SMD parts and a groundplane pcb and rather low values of the feedback network.Christer said:
peranders said:
Which resistor are you talking about? It sounds like
you are mixing them up now. Anyway, I am not saying you
must have either of them. I just commented on what you
said about the input resistor being built in. According to the
datasheet there may still be useful to have an extra external
one.
Maybe an orAlcaid said:Now that's an interesting idea. An OPA627 poweramp, or even better, an OPA637 poweramp.
Mirand A1 or an Alexander amp
buffer distortion
Look what I stumbled on!
Buffer distortion measurements
--------------------------------------------------------------------------------
I made some distortion measurements of a number of different buffers that I'd like to share. I have measured not only the THD (total harmonic distortion) but also how many dB down from the fundamental 1kHz frequency each of the first 4 harmonics are. Typically even harmonics are less bothersome than odd harmonics. But first let me state that lower distortion doesn't necessarily mean that it sounds better, it is just one of several important factors that influence the sound quality. I've made the measurements with both a 30 ohm load and a 100 ohm load with a 1.0v RMS output level. The amp used as a test vehicle was a META42 with AD8610 op amps and a regulated and filtered 24v supply, with a gain of 9. There was no attempt to optimise any of the resistor values for any particular buffer. For ease of taking the data, both channels were driven but one channel had a 30 ohm load and the other a 100 ohm load. This means that there were two different sets of buffers used, one for each load. The loads were metal-film resistors. In general I didn't see any significant relative differences between buffers when using different frequencies or other voltage levels within a reasonable range.
With a 30 ohm load:
BUFFER, % THD, 2nd harmonic, 3rd , 4th, 5th harmonic
EL2001 x 1, 0.28%, -62, -53, -78, -59
EL2001 x 2, 0.21%, -66, -55, -80, -62
EL2001 x 3, 0.168%, -69, -56, -83, -64
HA5002, 0.129%, -81, -58, -89, -67
EL2002, 0.126%, -65, -59, -83, -67
EL2002 x 2, 0.067%, -70, -64, -85, -74
EL2002 x 4, 0.034%, -74, -72, -80, -81
BUF634, 0.336%, -63, -51, -71, -58
BUF634 WB, 0.045%, -70, -76, -84, -76
BUF634 WB x 2, 0.037%, -74, -72, -88, -87
EL2008, 0.034%, -74, -72, -87, -81
With a 100 ohm load:
BUFFER, % THD, 2nd harmonic, 3rd, 4th, 5th harmonic
EL2001 x 1, 0.151%, -70, -57, -84, -65
EL2001 x 2, 0.09%, -74, -61, -85, -71
EL2001 x 3, 0.068%, -78, -64, -87, -75
HA5002, 0.044%, -87, -69, -89, -79
EL2002, 0.043%, -73, -70, -87, -81
EL2002 x 2, 0.027%, -79, -80, -89, -85
EL2002 x 4, 0.027%, -83, -88, -88, -87
BUF634, 0.162%, -67, -57, -78, -64
BUF634 WB, 0.031%, -78, -78, -90, -85
BUF634 WB x 2, 0.028%, -82, -90, -90, -84
EL2008, 0.03%, -87, -77, -89, -84
An interesting data point, taken from the data sheets, is the static current draw (in mA) for these buffers:
EL2001 x 1, 1.3mA
EL2001 x 2, 2.6mA
EL2001 x 3, 3.9
HA5002, 8.3
EL2002, 5
EL2002 x 2, 10
EL2002 x 4, 20
BUF634 1.5mA
BUF634 WB (wide bandwidth mode), 15mA
BUF634 WB x 2, 30mA
EL2008, 13
The most obvious observations are that the higher current buffers generally have lower distortion, and using multiple buffers lowers the distortion, especially for a lower impedance load.
Which sounds better? Well, they don't all sound the same, but which one is better is a matter of opinion and right now I'm just presenting data.
--------------------------------------------------------------------------------
Last edited by KurtW : 01-04-2003 at 04:12 PM.
Look what I stumbled on!
Buffer distortion measurements
--------------------------------------------------------------------------------
I made some distortion measurements of a number of different buffers that I'd like to share. I have measured not only the THD (total harmonic distortion) but also how many dB down from the fundamental 1kHz frequency each of the first 4 harmonics are. Typically even harmonics are less bothersome than odd harmonics. But first let me state that lower distortion doesn't necessarily mean that it sounds better, it is just one of several important factors that influence the sound quality. I've made the measurements with both a 30 ohm load and a 100 ohm load with a 1.0v RMS output level. The amp used as a test vehicle was a META42 with AD8610 op amps and a regulated and filtered 24v supply, with a gain of 9. There was no attempt to optimise any of the resistor values for any particular buffer. For ease of taking the data, both channels were driven but one channel had a 30 ohm load and the other a 100 ohm load. This means that there were two different sets of buffers used, one for each load. The loads were metal-film resistors. In general I didn't see any significant relative differences between buffers when using different frequencies or other voltage levels within a reasonable range.
With a 30 ohm load:
BUFFER, % THD, 2nd harmonic, 3rd , 4th, 5th harmonic
EL2001 x 1, 0.28%, -62, -53, -78, -59
EL2001 x 2, 0.21%, -66, -55, -80, -62
EL2001 x 3, 0.168%, -69, -56, -83, -64
HA5002, 0.129%, -81, -58, -89, -67
EL2002, 0.126%, -65, -59, -83, -67
EL2002 x 2, 0.067%, -70, -64, -85, -74
EL2002 x 4, 0.034%, -74, -72, -80, -81
BUF634, 0.336%, -63, -51, -71, -58
BUF634 WB, 0.045%, -70, -76, -84, -76
BUF634 WB x 2, 0.037%, -74, -72, -88, -87
EL2008, 0.034%, -74, -72, -87, -81
With a 100 ohm load:
BUFFER, % THD, 2nd harmonic, 3rd, 4th, 5th harmonic
EL2001 x 1, 0.151%, -70, -57, -84, -65
EL2001 x 2, 0.09%, -74, -61, -85, -71
EL2001 x 3, 0.068%, -78, -64, -87, -75
HA5002, 0.044%, -87, -69, -89, -79
EL2002, 0.043%, -73, -70, -87, -81
EL2002 x 2, 0.027%, -79, -80, -89, -85
EL2002 x 4, 0.027%, -83, -88, -88, -87
BUF634, 0.162%, -67, -57, -78, -64
BUF634 WB, 0.031%, -78, -78, -90, -85
BUF634 WB x 2, 0.028%, -82, -90, -90, -84
EL2008, 0.03%, -87, -77, -89, -84
An interesting data point, taken from the data sheets, is the static current draw (in mA) for these buffers:
EL2001 x 1, 1.3mA
EL2001 x 2, 2.6mA
EL2001 x 3, 3.9
HA5002, 8.3
EL2002, 5
EL2002 x 2, 10
EL2002 x 4, 20
BUF634 1.5mA
BUF634 WB (wide bandwidth mode), 15mA
BUF634 WB x 2, 30mA
EL2008, 13
The most obvious observations are that the higher current buffers generally have lower distortion, and using multiple buffers lowers the distortion, especially for a lower impedance load.
Which sounds better? Well, they don't all sound the same, but which one is better is a matter of opinion and right now I'm just presenting data.
--------------------------------------------------------------------------------
Last edited by KurtW : 01-04-2003 at 04:12 PM.
Hi guys,
Great discussion.
I'm not so technically adept, but if I were to make the preamp to drive just a power amp, would it be difficult to bypass the buffer section using the circuit board shown?
Any interest in laying out a board with only the Op Amp and no buffer?
Thanks for the great discussion. Very cool stuff.
Best,
KT
Great discussion.
I'm not so technically adept, but if I were to make the preamp to drive just a power amp, would it be difficult to bypass the buffer section using the circuit board shown?
Any interest in laying out a board with only the Op Amp and no buffer?
Thanks for the great discussion. Very cool stuff.
Best,
KT
no more 627's for me...
a friend of mine has recently built a pimeta headphone amplifier incorporating a bunch of ad8610's (not cheap but good) and some buf634's. He built it on a commercial pcb. We compared it with my 627 / buf634 based pre and i mus say that i like the sound of his version better. as a source we used my (and his) most recent diy-usb-dac.
It sounds more powerful, it has a little more kick, more detail, especially in the lower regions, and a slightly warmer sound than my pre does. If you're in need of a buffered pre for not too much money i can reccomend getting a pimeta pcb and stacking it with some nice components.
http://www.tangentsoft.net/audio is where you'll find about everything including schematics of the pimeta headphone amplifier.
a friend of mine has recently built a pimeta headphone amplifier incorporating a bunch of ad8610's (not cheap but good) and some buf634's. He built it on a commercial pcb. We compared it with my 627 / buf634 based pre and i mus say that i like the sound of his version better. as a source we used my (and his) most recent diy-usb-dac.
It sounds more powerful, it has a little more kick, more detail, especially in the lower regions, and a slightly warmer sound than my pre does. If you're in need of a buffered pre for not too much money i can reccomend getting a pimeta pcb and stacking it with some nice components.
http://www.tangentsoft.net/audio is where you'll find about everything including schematics of the pimeta headphone amplifier.
I have a pimeta headphone amp and in reading a great deal of threads on this amp i found more discourgement on trying to use it as a pre-amp then not I.E. one of the co-designer's who goe's by Morsel I believe was against useing it for anything other than a headphone amp did you change or add anything to the circuit to make it's application into a preamp?
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