John Curl's Blowtorch preamplifier part II

[bcarso]

Originally Posted by jneutron

Closest I found.

CentralSemi.com

ya gotta love them astericks.

jn

Yes, "special order". Dear me.

[a.wayne]

Originally Posted by Esperado

The fact that i'm not in concern with the sell of anything ? You share your opinion, when i don't agree, i share mine. Am-i not allowed ?
Anyway, i believe generalizations like "OPA not good for audio" are obviously suspect, like all generalizations.

I use to compare my preamp circuits with strait wires. When i'm not able to hear a real difference, i believe i do not have to worry anymore. And , as i said previously, i achieved that many time with various OPAs. Funny, never with OPA rated for audio.

Acceptance in a vacuum Esp ...

One like OPA's the other not, how are we to know who to know which is best. i can hear JC interpretation anytime i wish, it's available , how can we hear ESP's OPA setup , where are these great OPA pre-amps ..?


I'm not against , I would love to hear ......

[Esperado]

Originally Posted by a.wayne

where are these great OPA ?
I'm not against , I would love to hear ...

Find anything (they are soooo many) with a slew-rate >=300 V/µs witch fill your other needs (noise, PSU and current)
By example, on My Behringer DCX2498, i was obliged to use voltage feedback (cause differential input signals and i wanted to took the output from the out of the second stage)
I was very limited because +-15V, 8SOIC, and i don't wanted to modify too much the cms parts. I found this one:
THS4012 (310V/µs, Out:110mA, vn:7.5nV/rtHz)

But, when it is possible, i prefer to chose current feedback's ones (i have a stock of nice old OPA260, + some in my Rotel preamp).

During my researches on Google, in order to find a part, i was looking at people's reports about "the sound" of various OPA they have tested: When people compare parts, they are *always* rated in the order of their slewrate. N°1 is always the highest.

Originally Posted by a.wayne

how are we to know who to know which is best.

Doing like-me, compare with a strait wire ? If no difference nothing can be better.

[Waly]

Originally Posted by a.wayne

where are these great OPA pre-amps ..? I'm not against , I would love to hear ......

Meet THS4631:

- JFET input, +/15V
- 325MHz unity gain freq.
- 80dB open loop gain, -3db @ 30KHz (happens to meet JCs criteria of "open loop bandwidth")
- 1000V/uS slew rate
- 7.5nV/rtHz noise
- 0.01% distortion @G=5 into 100ohm at 5MHz and 2Vpp (much less distortions at audio frequencies).

[Esperado]

Originally Posted by Waly

Meet THS4631

Nice to met-you.
(it was a single, while i wanted a couple :-)
The best voltage feedback of the market, yes.

And, current-feedback, one of the best killers: LME49713

Supply Min (Min) (Volt) 10
Supply Max (Max) (Volt) 36
Special Features Current Feedback Audio Amp, AvCl>2
Vmax (V) 36
THD + N @ 1 kHz (%) 0.0004
Vmin (V) 10
Number of Channels 1
Slew Rate (Typ) (V/us) 1900
Iq per Channel (Max) (mA) 18
BW @ Acl (MHz) 132
BW @ G=+2 (MHz) 107
GBW (Typ) (MHz) 190
Vio (Max) (mV) 2
Offset Voltage Drift (+/-) (uV/Degrees Celsius) 0.29
Vn at Flatband (Typ) (nV/rtHz) 1.9
Noninverting Input Current Noise (pA/rtHz) 16
Iib (Max) (nA) 5000
Io (Typ) (mA) 140
Pin/Package 8TO-99, 8SOIC
Approx. Price (US$) 1.45 | 1ku

[Wavebourn]

Originally Posted by Waly

Meet THS4631:

- JFET input, +/15V
- 325MHz unity gain freq.
- 80dB open loop gain, -3db @ 30KHz (happens to meet JCs criteria of "open loop bandwidth")
- 1000V/uS slew rate
- 7.5nV/rtHz noise
- 0.01% distortion @G=5 into 100ohm at 5MHz and 2Vpp (much less distortions at audio frequencies).

Nice part. But needs attention when doing layout, easy to oscillate if to do it wrong.

[john curl]

Nice part, Waly!

[scott wurcer]

Originally Posted by jneutron

Closest I found.

CentralSemi.com

ya gotta love them astericks.

jn

Which basement in Hauppauge do these come from?

[RNMarsh]

Originally Posted by scott wurcer

These are all fashion design principles common to a narrow body of audio designers.

yes it is. But --- you get low thd+n that way also. -RNMarsh

[bcarso]

Originally Posted by Waly

Meet THS4631:

- JFET input, +/15V
- 325MHz unity gain freq.
- 80dB open loop gain, -3db @ 30KHz (happens to meet JCs criteria of "open loop bandwidth")
- 1000V/uS slew rate
- 7.5nV/rtHz noise
- 0.01% distortion @G=5 into 100ohm at 5MHz and 2Vpp (much less distortions at audio frequencies).

That looks like a nice part.

From the datasheet (see http://www.ti.com/lit/ds/symlink/ths4631.pdf ; too large to post directly here) it looks like 0.01% is a little optimistic. But the change in 2nd and 3rd distortions with lighter loading is very encouraging, particularly the 3rd behavior. Note that the figures 7 and 8 are for noninverting gain of 2, which means that common-mode distortion is not much of an issue. It would be interesting to see the curves for inverting mode and light loading. And any buffer in the feedback loop is going to have to be a very fast one.

The voltage range is refreshingly large for this fast a part as well.

Slew rates are reasonably equal, and 0.01% settling time is specified, so we know it is pretty well-behaved in that respect, which can also be partially inferred from the phase response.

Since the open-loop gain is only about 80dB we would not expect the frequency-dependent nonlinearities to be much further reduced with global feedback below the -3dB frequency, and if we want much gain we will have to live with more distortion. But still in all, a nice part. And the voltage noise corner is not too high.

The predominant 2nd suggests that if part-to-part matching is good, arrangements with multiple amps can be envisioned to mostly-cancel 2nd and leave almost-only 3rd.